Metal Testing - Introduction

Metals for manufacturing pressure equipment must be tested to establish and document their mechanical properties under various temperature-dependent, time-dependent, or cyclical loading service conditions. Testing is also performed after construction or repairs, to determine whether welding, forming, or cutting procedures have maintained, improved, or diminished the mechanical properties of the material. Testing is used to determine whether a vessel material’s mechanical properties, over its normal service life, have diminished due to corrosion and cracking. Testing can locate defects in the parent material, weld joints, and the areas adjacent to weld joints. These defects may cause premature or drastic failure of the vessel pressure boundary, and include conditions such as laminations, porosity, cracks, or localized loss of ductility. Tests are performed not only the parent metal, but also on weld joints and their immediate vicinity.

There are two main categories of material testing:

Destructive testing involves applying a test procedure that purposely causes damage to the tested metal, thereby destroying its integrity and rendering it unusable.

Non-destructive testing, also called non-destructive examination (NDE) involves test procedures that do no damage to the tested metal, allowing it to retain its integrity and its usefulness.

Objectives 1 and 2 examine four methods of destructive testing:

• Tensile tests

• Hardness tests

• Impact tests, and

• Proof tests

Objective 3, as a precursor to non-destructive testing, examines common weld defects.

Objectives 4 through 10 examine seven methods of non-destructive testing.

• Visual

• Magnetic Particle

• Liquid Penetrant

• Ultrasonic

• Radiographic

• Acoustic Emission and

• Hydrostatic and Pneumatic Leak Testing

Objective 11 examines the testing of metals for creep, fatigue, and corrosion.

Tensile Test

A tensile test (sometimes called a tension test) is a relatively simple and inexpensive test, used to determine how a metal will react when tension forces are applied. The test involves applying tension to a test piece, thereby producing tensile stress in the metal. As the material is stressed, the elongation and stress of the material are determined. To ensure valid results, consideration is given to the specimen’s shape and dimensions plus the choice of grips and faces.

Specimen

The specimen is the piece of metal that is tested. The shape of the specimen is defined by the standard or specification being utilized in the test (e.g. ASTM E8 or D638). The shape is important as it ensures the break occurs within the “gauge length” section; the cross sectional area or diameter of the specimen is reduced throughout the gauge length.

Figure 1 shows a specimen prior to testing. The testing mechanism (called an “extensometer”) is illustrated in Figure 2.

Figure 3 shows how the specimen physically deforms as more stress is applied to it. As the diameter reduces within the gauge length, the stress becomes greater because stress is inversely proportional to the cross-sectional area under load:

stress (σ) = $\frac{Load}{crosssectionalarea}$

In a tensile test, a sample is extended (stretched) at constant rate, and the load needed to maintain this is measured. The stress (σ) (calculated from the load), and the strain (ε) (calculated from the extension), can be plotted as stress against strain as shown in Figure 4.

Referring to Figure 4, the stress points on the graph are defined as:

(A) Proportional Limit

This is the stress point at which slip (or glide) due to dislocation movement occurs. The graph is linear up to this point, above which the transition from elastic to plastic deformation begins. Depending on the material being tested, this point is either poorly defined (as in Figure 4) or well defined.

(B) Yield stress

This is the stress at which yielding occurs across the whole specimen. The stress required for slip in a particular metal grain depends on how the grain is oriented, so points A and B are not generally coincident in a polycrystalline sample. A polycrystalline material is composed of variously oriented, small individual crystals. At this point, the deformation is purely plastic (non-reversible).

(C) Proof stress

This point is sometimes described as the yield stress of the material. This is the point at which the specimen has undergone a certain (arbitrary) value of permanent strain, usually 0.2%. The stress at this point is then known as the 0.2% proof stress. This is used because the precise positions of A and B are often difficult to define and depend to some extent on the accuracy of the testing machine.

(D) Ultimate tensile strength (UTS)

The UTS is the point at which plastic deformation becomes unstable and a narrow region (a neck) forms in the specimen. It is the peak (maximum) value of nominal stress during the test. Deformation will continue in the necked region until fracture occurs.

(E) Final instability point

This is the failure point, at which fracture occurs.

Strain

There are two main types of strain:

• Elastic

• Plastic

Elastic Strain

Elastic strain involves the stretching of a material within its elastic limit (i.e it will return to original dimensions when stress is removed). Elastic strain can be defined by Hooke’s Law, which states that the stress in a solid is directly proportional to the resulting strain. In formula this is:

Note: The value of Young’s Modulus (available in engineering tables) has been individually defined for specific metals. The value for each specific metal also varies with temperature.

Plastic Strain

Plastic strain, or plastic flow, is irreversible deformation of a material. The material has exceeded its elastic limit. There is no equation to relate the stress to plastic strain.

The elongation (amount of stretch) that a specimen undergoes during a tensile test can also be found. This can be expressed as an absolute measurement of the change in length or as a relative measurement. Strain can be expressed in two different ways: Engineering strain and True strain

Engineering Strain

Engineering strain is probably the most easily understood and the most common expression of strain. It is the ratio of the change in length to the original length:

True Strain

True strain is similar, but is based on the instantaneous length of the specimen as the test progresses.

Ultimate Tensile Strength

One of the properties determined about a material is its ultimate tensile strength (UTS). This is the maximum load the specimen sustains during a tensile test. The UTS may or may not equate to the strength at fracture. This depends on whether the tested metal is brittle, ductile, or exhibits both properties. Sometimes a metal may be ductile when tested in a lab, but when placed in service and exposed to extreme cold temperatures it may become more brittle.

Hardness Testing

Hardness is a measurement of the resistance of a material to surface indentation. A number of different tests have evolved for determining hardness, and each has its own scale for measuring hardness. These scales are arbitrary and express hardness purely quantitatively. This module describes four hardness tests, which are based on a metal’s resistance to indentation.

Indentation hardness, or the resistance of a material to indentation, can be measured in two ways. The first method utilizes a specific force and measures either the penetration depth of the indentation or the area the indentation. The second method measures the load (i.e force) that is applied to the indentor. The most important criteria for an indentor is its ability to provide indentations that are geometrically similar and well defined. The four common hardness testing methods, based on indentation, are known as the:

• Brinell

• Rockwell

• Vickers

• Knoop

Brinell Hardness Test

Brinell hardness testing equipment includes both laboratory and portable testing systems. Testers use several methods to apply test loads, including dead weight, pneumatic, spring, hydraulic, and impact methods. A hydraulic Brinell hardness tester, shown in Figure 5, uses the principle of a constant load applied for a period of time, using an indentor with a predetermined diameter.

The test is performed in the following sequence:

1. The specimen is placed on the anvil.

2. The load is applied.

3. The indentor penetrates the specimen for 10 to 30 seconds.

4. The diameter of the round indentation is measured twice, with the second measurement taken 90 degrees from the first.

5. The mean diameter of the indentation is determined.

6. The Brinell hardness number is either calculated or determined from an appropriate table.

The following formula may be used to calculate the Brinell Hardness Number (BHN).

Where:

However, this calculation is not usually done. Instead, tables are available that provide Brinell Hardness Numbers, based on the load and the mean diameter of the indentation. The tables reference loads that range from 500 kg to 3000 kg. Each table also applies to a specific indentor ball diameter (typically 10 mm).

The diameter of the indentation is measured with the aid of a microscope. Errors in determining the actual diameter are a result of incorrect instrument readings, a poorly defined indentation boundary, or both. The error in reading a Brinell microscope should not exceed 0.01 mm over the entire 7 mm scale.

The definition (clarity) of the indentation boundary depends on the characteristic of the tested material. Some materials develop a raised ridge around the indentation, as shown in Figure 6(a). Other materials have no sharp line of demarcation between the surrounding surface and the indentation, due to the rounding and unevenness of the boundary, as shown in Figure 6(b). Some materials exhibit a clearly defined “edge”, as shown in Figure 6(c).

Rockwell Hardness Test

Rockwell hardness testing is the most widely used method for determining hardness because it is simple to perform. Rockwell hardness testing equipment is used in laboratories, in automated systems for high production, and portable test units. The testers use either a 120° diamond cone for the Rockwell C test, or a 1.6 mm diameter ball as the indentor for the Rockwell B test.

The Rockwell hardness test involves applying two loads to a specimen and measuring the difference in depth of penetration between the light (minor) load and the heavy (major) load. The minor load is 10 kg and the major load is 60, 100, or 150 kg, regardless of the type of indentor used. The minor load is applied first to zero the setting on the dial depth gauge; the major load is then applied for a specific time and then released. The hardness number is indicated on a dial gauge.

Rockwell test results are described using a numerical value (related to the depth of indentation) and a letter scheme that describes the conditions of the test. The letters describe the scale symbol, type of indentor, and the major load. For example, every Rockwell test value is followed by the letter H for hardness, then R for Rockwell and, finally, the scale used, either “B” or “C.”

Figure 7 illustrates the measurement of penetration depth using a diamond cone indentor.

Vickers Hardness Test

The Vickers hardness test employs a 136° diamond pyramid as an indentor (i.e. a square based pyramid with an angle of 136° between faces). The loads applied to the diamond indentor (for a period of 10 to 15 seconds) range from 1 to 120 kg, with standard loads of 5, 10, 20, 30, 50, 100, and 120 kg.

The Vickers hardness number (HV) is provided by the following ratio:

The indented area is calculated using the average readings of both diagonals. The diagonals of the indentation are measured using a micrometer microscope.

The Vickers hardness number is determined using a table for the appropriate load, identifying the average diagonal value, and reading the appropriate hardness value.

Vickers hardness numbers are reported with the hardness value and the load used. For example, if hardness of 820 is determined using a 50 kg load, the hardness is reported as “820HV50”.

Vickers hardness testing has several advantages over other hardness tests:

• The method is accepted world wide

• The indentor is accurate and shape is not distorted under loads

• The impressions made are small, and thus limit damage to finished products

• Light loads of 1 kg allow for the testing of thin materials

• There is only one scale for the hardness of all materials

• Damage to indentors is readily apparent as viewed under the microscope

• The diagonal of a square impression can be measured more accurately than the diameter of a ball impression

Vickers Hardness Testing Guidelines

• The full test load must be applied for sufficient time; 15 seconds is recommended.

• The spacing of indentations is important. A rule of thumb is that the distance between indentations or from the edge of the test specimen should be at least two and one half (2½) times the length of the diagonal of the indentation.

• Diagonals of indentations should be measured with the indentation perfectly centered within the viewing field of the microscope

• The micrometer microscope should be calibrated against a stage micrometer

Table 1 shows a comparison of Brinell, Vickers and Rockwell hardness values.

Knoop Microhardness Testing

The Knoop microhardness test utilizes a rhombic-based pyramidal diamond that produces a diamond shape with a long diagonal of seven times the short diagonal. The expected depth of the indentations is about 1/30th of the long diagonal axis. Typical testing loads are less than 500 g. Knoop microhardness testing is strictly confined to laboratory applications.

Hardness Testing Categories

Hardness testing is described in the following three categories:

1. Microhardness testing, using up to 200 g of load

2. Low-load hardness testing, using 200 g to 3 kg of load

3. Macrohardness, using greater than 3 kg of load

Brinell hardness testing falls in the macrohardness testing category. It can also be described as bulk hardness testing, since it averages out small imperfections.

Rockwell hardness testing is a macrohardness test based on minor and major loads greater than 3 kg. The diameters of the indentations are significantly smaller than those with the Brinell indentor.

The Vickers hardness test is considered either a low-load microhardness test or a macrohardness test. Test loads can range from 1 g to 120 kg. Because of the diamond test point, the Vickers hardness test is used to test hard materials with high loads and to measure the hardness of small areas.

The Knoop hardness test is a microhardness test. It tests for the hardness of micro-constituents of a matrix. Knoop microhardness testing is utilized in failure analysis of components and for the testing of extremely brittle materials.

Industrial Applications of Hardness Testing

Hardness testing is a simple and inexpensive test for evaluating material properties during production.

Manufacturing processes influence the hardness of materials. Hardness testing ensures uniformity of heat treated components. Hardness tests are done on materials that have been cold worked, quenched and tempered, or precipitation hardened. Mill test reports provide hardness readings and are used to determine the suitability of pressure component materials for their intended use.

Indentation hardness testing is used in the fabrication and repair of pressure components to identify the misuse of welding consumables or to identify fabrication techniques that require post-weld heat treatment. Brinell portable hardness tests are completed in the shop or in the field on pressure components.

In welding applications, hardness tests determine if the strengths of the base and weld metals are matched, and also provide indications of any effects the welding process may have on the heat affected zone (HAZ).

To effectively test the hardness of a weld, it is advisable that the Welding Procedure Specification (WPS) be supported by a Procedure Qualification Report (PQR) that has had hardness tests completed by the Rockwell Superficial Hardness or Vickers Diamond Pyramid Hardness testing technique.

For butt welds, traverse hardness testing from the base metal through the HAZ, weld metal, and ending in the adjoining base metal, is recommended. The spacing of the indentations is as described in the guidelines for testing with the Rockwell or Vickers hardness techniques. A number of traverses across the cross-section of the weld at different elevations are necessary to provide a representative sample of the weld.

The acceptance criteria are established by the end user’s specifications. For carbon steels that are subject to sulphide stress cracking (SSC), the recognized acceptance criteria is the National Association of Corrosion Engineers MR-0175 Standard, which states a maximum hardness of 22 HRC for carbon steel welding procedure specifications.

Impact Testing

One property of metals that must be tested is the impact toughness, which is the ability of a metal to resist fracture when subjected to shock loading. It is the energy required to break a piece of metal of standardized shape with a cross-sectional area of 1 cm2. Two common tests for toughness are the Charpy Test and the Izod Test.

The metal be tested is formed into a rectangular bar, with a 45 degree V-shaped notch removed. This specimen is carefully placed into the apparatus’ anvils with precision tongs. Then the bar is struck with a striker, mounted on a pendulum, causing the specimen to break. The height to which the pendulum rises after the strike is compared to the height from which the pendulum originally fell. The difference between the potential energy before and after the strike is a measure of the energy absorbed in the fracture.

Figure 8 illustrates the Charpy and Izod test arrangements. In the Charpy test, the specimen is held at each end and is struck in the middle, at the notch location. In the Izod test, the specimen is held on one side of the notch and is struck on the other side of the notch.

Precise details of the required specimens and procedures for these tests are provided in written standards. Unfortunately, there are several different standards: the ISO (International Organization for Standardization) standard; the ASTM (American Society for Testing and Materials) standard; plus several national European standards. The parameters of the striker, machine and test pieces are slightly different between these standards so the results from the tests are subtly different and, therefore, difficult to compare.

The notch behaviour of the face-centred cubic metals and alloys, a large group of nonferrous materials and the austenitic steels can be estimated from their common tensile properties. If they are brittle in tension, they will be brittle when notched. If they are ductile in tension, they will be ductile when notched, except for unusually sharp or deep notches (i.e. much more severe than the standard Charpy specimens). Even low temperatures do not alter the notch behaviour of these materials.

In contrast, the behaviour of the ferritic steels under notch conditions cannot be predicted from their properties. Some metals that display normal ductility in the tension test may break in brittle fashion when tested or used in the notched condition. Notched conditions include restraints to deformation in directions perpendicular to the major stress, or triaxial stresses, and stress concentrations. It is in this field that the Charpy tests prove useful for determining the susceptibility of steel to notch-brittle behaviour.

Introduction

For the most part, ASME Section I uses experience-based design methods, known as design-by-rule. Other sections of the ASME Code, namely Section III, Subsection NB, and Section VIII, Division 2, use a newer method, known as design-by-analysis. Design-by-rule is a process requiring the determination of loads, the choice of a design formula, and the selection of an appropriate design stress for the material to be used. Rules for this kind of design are found throughout Section I, with most being in Part PG (the general rules). Other design rules are found in those parts of Section I dealing with specific types of boilers or particular types of construction.

The principal design rules are found in Part PG, paragraphs PG-16 through PG-55. There are formulas for the design of cylindrical components under internal pressure (tube, pipe, headers, and drums), heads (dished, flat, stayed, and unstayed), stayed surfaces, and ligaments between holes. Rules are also provided for openings or penetrations in any of these components, based on a system of compensation in which the material removed for the opening is replaced as reinforcing in the region immediately around the opening, called the limits of compensation (see PG-36). All of these formulas involve internal pressure except for the rules for support and attachment lugs of PG-55, for which the designer chooses the design loads on the basis of the anticipated mass or other loads to be carried.

Proof Test (Hydrostatic Deformation)

A method of design permitted by Section I is the proof, or hydrostatic deformation, test (PG-18 and Appendix A-22). This is an experience-based method used to establish a safe design pressure for components for which no rules are given or when strength cannot be calculated with a satisfactory assurance of accuracy. In this test, a full-size prototype of the pressure part is subjected to a slowly increasing hydrostatic pressure until yielding or bursting occurs (depending on the test). The maximum allowable working pressure is then established by an appropriate formula, which includes the strength of the material and a suitable safety factor.

ASME Sections I and VIII allow a burst test to be stopped before actual bursting occurs, when the test pressure justifies the desired design pressure. In this case, the tested component itself may never be used for Code Construction because it was on the verge of failure.

Proof testing may NOT be used if Section I has design rules for the component and, in practice, such testing is seldom employed. However, it can be a simple and effective way of establishing an acceptable design pressure for unusual designs, odd shapes, or special features that are difficult and costly to analyse, even with the latest computer-based methods.

Tests used to establish the maximum allowable working pressure of pressure parts must be witnessed and approved by an Authorized Inspector, as required by ASME Section I, Appendix A-22.10. The test report becomes a permanent reference to justify the design of such parts if the manufacturer wants to use that design again for other boilers.

Applications of Proof Testing

This information can be found in ASME Section IV Part HLW-Potable Water Heaters. A proof test may be applied to determine the MAWP on the water heater. Hydrostatic pressure is applied to a full-sized sample of a water heater. On the other hand, one sample vessel may be tested to establish the MAWP for a series of water heaters. Water heater vessels are in series under the following conditions:

• The heads are of the same geometry and thickness

• The cylindrical shell and tubes, differ only by length

• The openings are the same size and type as those on the proof-tested vessel

Test Procedure

Before the Proof Test is applied, the outer surface of the vessel must be thoroughly cleaned and a brittle coating applied to the entire surface. The purpose of this coating is to indicate any displacement (movement) in the vessel surface caused by the internal hydrostatic pressure; the coating will flake off wherever displacement occurs.

After the coating is applied, according to HLW-502.1(c),

The hydrostatic pressure shall be increased gradually until approximately one-half the anticipated MAWP is reached. Thereafter, the test pressure shall be increased in steps of approximately 1/10 or less of the anticipated MAWP. The pressure shall be held stationary at the end of each increment for a sufficient time to allow the observations required by the test procedure to be made and shall be released to zero to permit determination of any permanent strain or displacement after any pressure increment that indicates an increase in strain or displacement over the previous equal pressure increment, as evidenced by flaking of the brittle coating, or by the appearance of strain lines. The application of pressure shall be stopped when the intended test pressure has been reached, or at the first sign of yielding.

Test Based on Yield Strength

For portable water heaters constructed to ASME IV, the average yield strength is determined (for use in the formulas for P), as noted in ASME IV HLW-502.1. After completion of the test, three specimens are cut from the tested part. The average yield strength from these three specimens is used to calculate P by the following formula:

Test Based on Tensile Strength

If the test is stopped before any yielding, the MAWP is calculated using one of the following given formulas:

For carbon steel with a maximum tensile strength of 480 MPa:

For any other material:

Test Gauges

A gauge is connected directly onto the water heater for indicating hydrostatic pressure. If the indicating gauge is not clearly visible to the operator, an additional gauge is furnished. Also, a recording gauge is installed for larger water heaters. The dial range of the indicating gauge is 1.5 times the intended maximum test pressure, and both the indicating and the recording gauge are calibrated against the master gauge.

Collapsing of the Parts

The water heater parts should withstand (without major deformation) a hydrostatic test pressure that is a minimum of 3 times the desired MAWP.

Test Records

The Manufacturer’s designated person is required to witness the proof tests to establish the MAWP of the water heaters. The authorized inspector also witnesses and accepts the tests. All results are recorded on Form HLW-8 (Manufacturer’s Master Data Report Test Report for Water Heaters or Storage Tanks). The Manufacturer’s designated person certifies the completed form, which is kept on file.

Hydrostatic Test

A hydrostatic test, in which the pressure is 1.5 times the MAWP, is required to be performed on all water heaters. The MAWP is marked at a suitable location on the water heater vessel. While the water heaters are under hydrostatic test pressure, all joints and connections are inspected for leakage. The test pressure is kept under control so that it cannot be exceeded by more than 69 kPa.

Welding Discontinuities

The objective of good welding practice is to produce weldments with the integrity to ensure they perform adequately for the service intended. However, even with the very best of efforts, discontinuities occur.

Weld discontinuities, which may lead to defects, are caused by one or more of the following:

• Departures from qualified procedures

• Altering weld designs

• Substituting or using defective materials

• Substituting or using defective electrodes

Any one of these factors produces weld defects which cause a structure to fail in service. This has costly and sometimes disastrous results for the owner/operator and may jeopardize public safety.

The identification, evaluation, and disposition of weld discontinuities is an important welding activity. Visual identification of imperfections in welds is the first step in welding inspection and can usually reveal 80% of weld imperfections. With the aid of non-destructive techniques such as dye penetrant, magnetic particle, radiographic, and ultrasonic testing, the location and size of most defects the human eye cannot detect are determined.

To evaluate weld imperfections, the inspector judges whether or not the discontinuity is likely to become a defect leading to failure. This requires experience and knowledge of the flaws associated with a particular welding process and procedure, the metallurgy of the base metals and filler metals used, and the design of the weld joint. Evaluation also involves the use of specifications, standards, and codes which provide acceptance and rejection criteria for types of weld discontinuities.

Evaluation of weld discontinuities leads to disposition or a decision as to whether the weldment is acceptable, requires rework, or is unsuitable for the service intended and must be removed. Personnel involved in inspection may not be involved in the final disposition decisions, especially when these decisions are of a critical nature requiring an engineering assessment.

Welding Terminology

The communication of information from the personnel involved in the inspection and evaluation of weld discontinuities to the personnel who make decisions on redesign, repair, and rework of welded structures is made in a common technical language that all understands and accept. Industry specific and shop language are still used but may not be technically precise. Welding specialists and those involved with evaluating welds should use standard terminology.

Organizations and authorities involved in welding technology and science have developed terminology and classification systems to identify weld discontinuities and to describe the criteria used to evaluate and dispose of weld discontinuities.

Although there is some variation associated with industry specific terminology, there is broad agreement in the welding industry on the need for standardization of welding terms. Most welding codes use terminology that is standard for the industry. Most codes include a definitions section where departures from standard terms are identified and described.

The definitions and terms used are referenced to the ANSI/AWS Standard Welding Terms and Definitions AWS A3.0M/A3.0 except where noted. The definitions provided in this module are those which the pressure containing and piping industries accept as the norm.

The terms defect, discontinuity, fault, flaw, and imperfection require definition. They all refer to inconsistencies in weld integrity, but are often misunderstood and used incorrectly.

Defect: A discontinuity or flaw whose size, shape, type, location, or orientation creates a substantial chance of material failure. The discontinuity is detrimental to the integrity of the pressure equipment.

Discontinuity: Any local variation in material continuity; including changes in geometry, properties of composition or structure, holes, cavities, or cracks.

Fault: The word “fault” is often used to denote defect. However, it can also be interpreted to mean an imperfection or flaw. The word fault is not included in the ANSI/AWS Standard Welding Terms and Definitions. Because the term lacks definition in a welding sense, it should not be used. Rather, the terms discontinuity, defect, and flaw are precise and should be used.

Flaw: An imperfection in the material that may or may not be harmful.

Imperfection: This term is used extensively in some codes; for example, Chapter V of ANSI/ASME B31.3, Process Piping. It is not defined in the ANSI/AWS Standard Welding Terms and Definitions, but because it is used in a major code it should be understood to have the same meaning as flaw.

Classes of Discontinuities

Weld discontinuities are usually grouped into broad categories. One method is to group discontinuities according to causes. One major authority relates weld defects to one of the following three causes:

• Weld procedure

• Weld design

• Metallurgical causes

However, before examining causes of defects, first identify and describe discontinuities. For this purpose, it is useful to group discontinuities into types that provide an organized approach to their identification. This chapter uses the system of dividing discontinuities into three broad classes:

• Dimensional

• Structural

• Base metal properties

Dimensional Discontinuities

Dimensional discontinuities relate to any inconsistencies or departures from specified dimensions in the weld, weld joint, or parent metal. Also included in this general category are welds with imperfect shapes or unacceptable contours, including undercut, underfill, and overlap.

Structural Discontinuities

Structural discontinuities are flaws in the weld deposit or heat affected zone. The flaw’s potential for failure is directly related to its shape and location in the weld. Planar defects, such as cracks and lack of fusion, are sharp and pointed and create severe notching and high potential for failure. Pores and non-metallic inclusions are usually rounded and pose less potential for failure.

Base Metal Discontinuities

Base metal discontinuities are deficiencies in the chemical, physical, or mechanical properties of the base metals which may contribute to a defect in the weldment. Base metal discontinuities arise mainly from the production of the metal and its subsequent processing and manufacturing. When a metal is produced, all of the data relating to its chemical composition, method of manufacture, heat treating, mechanical properties (such as tensile, yield, and impact properties), relative hardness, and ductility are given on a document to verify its standard of quality.

Even though production quality standards are high, metals that do not meet the required chemical composition standards can sometimes be delivered to the fabricator. Heat treating reduces impact strength or other mechanical properties. In the rolling, forging, and casting operations which follow the production of the metal, base metal imperfections such as laminations, laps and seams, or casting defects find their way into the product and cause defects in the weldment.

Defects in Weld Metals

Discontinuities that occur in welds create conditions that lead to failure in service. Weld strength is the property most affected by weld defects. Planar-type defects can cause rapid propagation of cracking through a weldment.

Discontinuities that create notching (abrupt changes in the contour of the weld where it fuses to the base metal) can be sites for fatigue failure. Fatigue failure in welded joints is associated with cyclical loading of the joint, such as that caused by vehicular traffic over a bridge, which creates many reversals of stress at the weld. Notches can reduce the fatigue resistance of welded joints and cause failure even though the yield strength of the original base metal was never exceeded. Spherical type defects, although not as serious, do create voids, displace weld metal, and reduce the intended volume of deposited weld metal. Failure to fill a joint or melting away of the base metal causes a reduction of the through thickness of the base metal.

Inappropriate chemical properties can cause a weldment to lose impact and tensile strength in a very hot or cold environment. Failure to match base metal and electrodes correctly can cause a loss of resistance to corrosion at the surface of a metal.

Defects in Weld Joints

There are five basic weld joints:

• Butt

• Tee

• Lap

• Edge

• Corner

Even though the weld deposit forming the joint may be sound, a weld failure may occur because the joint has some undesirable features. Highly restrained tee and corner joints, unless properly designed, are sites for weld failure. Misaligned butt joints can also be areas of high stress. Steep transitions between thick and thin lapped material can create areas of high stress.

The degree of weld quality possible is not the same as the degree of weld quality necessary. The function imposed on the weldment determines the required degree of weld quality. Welds designed for the pressure containing industry must conform to high standards because of potential danger to the public from failure in service.

Non-destructive Examination (NDE) is the testing of materials without destroying the integrity of the material or lowering its ability to perform its primary function. The pressure equipment industry uses a wide variety of NDE techniques that test the properties of materials. The most common NDE techniques are:

• Visual Inspection

• Magnetic Particle Testing

• Liquid Penetrant Testing

• Ultrasonic Testing

• Radiographic Testing

• Acoustic Emission

• Leak Testing

ASME Section V, Non-destructive Examination is a reference code that supports construction codes such as ASME Section I, III, IV, VIII, and X. Some non-ferrous materials are used in power plant construction. For example, copper-based alloys for condenser tubing, copper for alternator windings and tin-based alloys for bearings, but the majority of components are made from iron-based or ferrous materials. The tests covered in this chapter are for ferrous materials.

NDE Definitions

In NDE, the misuse of the terms defect, discontinuity, flaw, indication, interpretation, and evaluation can create much confusion.

The following definitions are used in this chapter:

• Flaw: An imperfection in the material that may or may not be harmful

• Indication: A noticeable response to an NDE test that requires interpretation to determine its significance

• Discontinuity: Any local variation in material continuity including changes in geometry, properties or composition, holes, cavities, cracks, or structure

• Defect: A discontinuity or flaw, whose size, shape, type, location, or orientation creates a substantial chance of material failure. The discontinuity is detrimental to the integrity of the pressure equipment

• Interpretation: A study to determine the cause of an indication (labelling or naming the discontinuity, i.e. cold lap, fatigue crack)

• Evaluation: A judgment made based on codes, standards or engineering assessment on the significance of an indication. The judgment decides whether the indication is detrimental to the service life of the component under test

NDE Applications

In the pressure equipment industry, NDE techniques are used for testing materials for three types of defects:

1. Inherent defects created during the initial production of the material

2. Processing defects created during the processing/manufacturing of the pressure equipment

3. Service defects created in the equipment during service

NDE Benefits

The following are the four main benefits to NDE techniques.

Safety

• Prevent accidents, damage to property, injuries, and loss of life

Cost Savings

• Eliminating faulty raw material reduces scrap, manpower required for rework and prevents waste

• Repeat business is increased due to customer satisfaction

• Manufacturing control provides continuous improvement because it monitors the process to pinpoint sources of trouble

• Eliminating delivery of faulty products reduces servicing costs

• Insurance costs are reduced by eliminating risks of equipment failure

Service Reliability

• Faults can be sized for critical engineering assessment

• Remaining life span of used equipment can be estimated - faults can be located before failure occurs

• Service and operating costs are lower as testing can often be completed on line

• Inspection intervals can be established for maximum benefit

• Prevents unscheduled downtime

Material Verification

• Differences in chemical properties can be identified

• Differences in metallurgical properties can be identified

• Material can be identified for sorting

• Differences in zones of heat treating can be identified

• Differences in physical properties can be identified

Any form of inspection is based upon an initial visual assessment of the geometry of the component and the type and nature of the defect which is likely to be present. Visual inspection is the most widely used technique for surface inspection, alignment of mating surfaces, and evidence of leaking.

Visual inspection ranges from the use of the naked eye to remote visual examination with electronic video systems. For remote visual examination, the system used must have a resolution capability at least equivalent to that obtained with direct visual observation. Direct visual examination may be completed with the aid of mirrors, magnifying lenses, and artificial illumination. The criteria for conducting a visual examination are:

• Access allowing placement of the eye within 610 mm of the surface being examined

• The angle of view must not be less than 30 degrees to the surface being inspected

• A minimum illumination of 162 lux (15 foot candles)

• For study of small anomalies a minimum illumination of 538 lux (50 foot candles) is required

• Personnel capable of reading standard J-1 letters on a standard Jaeger eye test chart

The ASME Section V Article 9: Visual Examination sets out the procedure requirements for an authorized inspector to follow if the Code Section requires a visual examination to be completed on the component.

Requirements

When the referencing Code sections require, the examination is performed in accordance with a written procedure the manufacturer prepares. The manufacturer makes available to the authorized inspector the procedure and a list of the examinations to be performed. The procedure includes at least the following:

• How the examination is performed

• Type of surface condition and criteria for surface cleaning

• Cleaning instructions or reference to a cleaning procedure

• Methods and tools used for surface preparation

• Whether direct or remote viewing is used

• Special illumination, instruments, or equipment used

• Sequence of performing the operation, when applicable;

• Data to be tabulated

• Report forms or statement of examination results

Procedures may be general or specific for a certain application. The procedure contains or refers to a report of the test method used to demonstrate the procedure’s adequacy.

Techniques used are direct viewing, remote viewing, and translucent examination. Specific requirements for each technique are provided in the ASME Section V, Article 9.

When the ASME Code Section requires, a written report is filled out and maintained.

Magnetic Particle Inspection

This method is valuable for the detection of cracks present at the surface of a component made from a ferromagnetic material. The range of ferromagnetic materials includes cast irons, carbon steel, and all kinds of steel alloys (with the exception of austenitic steels). The method is based on the fact that the faces of a crack tend to form north and south poles if a magnetic flux is established in the component. This flux may be induced using a permanent magnet or an electromagnet, causing current flow through the component or by wrapping coils around it and then passing a current through the coils. If the system is arranged so that the crack interrupts the flux lines, the application of either dry iron powder or iron powder in a liquid suspension may reveal the crack. The particles of iron are attracted to the poles formed at the crack, which is delineated as a black line.

AC transformers are often used to supply the necessary flux when searching for surface defects. Heavy DC currents, which produce a flux below the surface of the material, can be used to indicate subsurface defects to a depth of approximately 4 mm.

ASME Code Section V Article 7 describes the requirements and methodology for the performance of the magnetic particle examination test method. Magnetic particle examination is widely used by the ASME Code and is referenced as a requirement in many Code Sections. Article 25 contains the reference standard SE-709 for magnetic particle examination. Users should consult it when establishing their test procedures. Also, when a referencing Code Section specifies Article 7, the requirements of Article 1 apply. In some cases, the referencing Code Section alters the Article 1 and Article 6 requirements. It is important to review the referencing Code Section requirements when establishing the test procedure. Article 7 has four Mandatory Appendices:

• Mandatory Appendix I covers examination of coated ferritic materials using the AC yoke technique

• Mandatory Appendix II covers the terms used in magnetic particle examination

• Mandatory Appendix III covers the use of the yoke technique with fluorescent particles in an undarkened area

• Mandatory Appendix IV covers the qualification of light sources for fluorescent particle excitation, when alternate wavelengths are specified.

Magnetic Particle Procedures

Article 7 specifies magnetic particle examination procedures, which includes the following:

• Magnetic particles

• Surface conditioning

• Examination techniques

• Acceptance criteria

Magnetic Particles

Dry, wet, or fluorescent particles are used in accordance with the applicable technique selection. When using fluorescent particles, the examination is performed using an ultraviolet light (black light) in a darkened area.

The black light has an intensity of 1000 µW/cm2 at the surface of the part. The light intensity is measured using a black light meter at least once every 8 hours and whenever the work station is changed. It is important to maintain records of the intensity measurements and frequency for subsequent audit verifications. Pretest requirements for use of the black light include a warm-up period of five minutes. The inspector must be in the darkened area for five minutes. before starting the examination to enable the inspector’s eyes to adapt to dark viewing. Photosensitive eyeglass lenses are not permitted.

Surface Conditioning

Surface conditioning is normally not necessary, and satisfactory results are obtained when surfaces are, for example, in the as-welded, as-rolled, as-forged, or as-cast condition. However, surface preparation using any mechanical means may be required if the surface irregularities can mask indications. Before the examination, the surface and adjacent areas within 25 mm are cleaned with any suitable means to ensure removal of extraneous materials that can interfere with the examination. In Article 7, paragraphs T-741.1 and T-741.2 provide additional requirements regarding cleaning and use of surface contrast enhancement coatings for enhancing particle contrast.

Examination Techniques

At least two separate examinations are performed on each test area. For the second examination, the lines of flux are perpendicular to those used for the first examination. The examinations are conducted with sufficient overlap to ensure 100% coverage. Article 7 describes five magnetization techniques that can be used on ferromagnetic materials to detect cracks and other discontinuities on or near the surface of the material. The five magnetization techniques are:

• prod

• longitudinal

• circular (direct contact and central conductor methods)

• yoke

• multidirectional

1. Prod Technique

The prod technique uses portable prod contacts pressed against the test surface in the area to be examined. Care is taken to prevent arcing. The heat produced from arcing can create local hard spots that can cause service problems. Article 7, T-752 has the recommended magnetizing spacing for current and prod.

2. Longitudinal Magnetization Technique

This technique uses coils wrapped around the surface being examined. The current, passing through the coils, produces a longitudinal magnetic field parallel to the axis of the coil. The required field strength is based on the length to diameter ratio of the part being examined. Transverse cracks are revealed using the longitudinal magnetization technique.

3(a). Circular Magnetization Direct Contact Method

This method produces circular magnetic fields perpendicular to the part being tested. Generally, this method is used for small parts with no openings through the interior. Contact electrodes introduce the current into the test piece which behaves like a current carrying conductor. The circular shaped magnetic field established around the test piece develops stray fields across defects lying in the same direction as the connecting line between the current contacts, thus revealing longitudinal cracks.

3(b). Circular Magnetization Central Conductor Method

With tube or ring-shaped parts, magnetization is achieved without contact using a current carrying conductor. The test piece is surrounded by the circular magnetic field so that both longitudinal cracks together with star-shaped cracks running from the centre to the outside of the plane surfaces can be detected.

4. The Electromagnetic Yoke Technique

The electromagnetic yoke technique uses a coil wound around a U-shaped core of soft iron. The part being examined becomes the path completing the magnetic circuit. A permanent magnetic yoke works on the same principle. The lifting power of yokes is checked annually or when the yoke has been damaged. AC yokes must have a lifting power of 4.5 kg while DC permanent yokes must have a lifting power of 20.4 kg at maximum pole spacing. With yoke magnetization, a magnetic field from a coil system is generated over the pole of an iron core and then transmitted into the test object. The iron core and the workpiece form a closed magnetic circuit. The magnetic field lines flow in the test piece in a direct connection line between the poles enabling the detection of transverse cracks. Longitudinal cracks are not detected.

5. Multidirectional Magnetization Technique

This technique uses high amperage power packs operating up to three circuits. The circuits energize one at a time in rapid succession producing an overall magnetization of the part in multiple directions. This method requires only one processing step.

Acceptance Criteria

The acceptance criteria are as determined by the Code of Construction. It is suggested that the student read ASME Section V. Article 25, SE-709 “Standard Guide for Magnetic Particle Examination.”

Typical Test Procedure

1. Observe guidelines as per Code of Construction.

2. Preparation of the test part (for example: cleaning, degreasing, descaling, rust removal).

3. Visual examination of readily apparent cracks or other surface conditions.

4. Check inspection conditions (ambient light, UV light intensity).

5. Measure the applied magnetic field, in turn adjusting for the appropriate magnetic field intensity.

6. Clamp the material to be inspected or apply hand yoke magnet.

7. Switch on the magnetizing field.

8. Spray the part with the test medium containing the magnetic particles.

9. Switch off the magnetizing field.

10. Visually evaluate the surface for defect indications.

11. Repeat #5 to #10 at 90 degrees to original test orientation.

12. Demagnetize the part (if required).

13. Document the indications (position, size, number, orientation).

14. Classify the inspected part (acceptable, reject, possible rework).

Liquid Penetrant Test

Liquid penetrant testing is one of the oldest non-destructive testing methods in existence. This method enhances the visibility of material surface cracks that are open to the surface of the specimen under inspection. The material to be inspected may be magnetic or non-magnetic such as steel, aluminium, magnesium, titanium, glass, ceramic or plastic. The flaws to be detected must be open to the inspected part’s surface.

The principle of liquid penetrant inspection is based on a liquid with a low surface tension that is spread on the surface of the material. The liquid penetrant is allowed to soak into any cracks that are open at the surface. After a period of time, the excess liquid penetrant fluid is removed and a developer is applied to the test surface to draw the liquid penetrant remaining in the cracks back to the surface and make it visible. The surface flaws become increasingly more visible to the human eye because the liquid penetrant contains a dye indication that broadens the trace of the surface crack. The dye indicator is coloured either red or blue on a white background or appears greenish yellow on a dark violet background when the surface is illuminated by an ultraviolet lamp.

Detection

Detection of flaws depends on the general condition and finish of the surface of the material. Defects and surface conditions which limit the effectiveness are:

• Materials which are porous

• Subsurface defects

• Very wide and shallow defects

• Contaminated surfaces that have not been thoroughly cleaned

• Insufficient penetrant dwell time

• Surface temperature too high or too low

Testing Procedure

Test Preparations

Test guidelines or specifications are considered (Code of Construction, company specifications, special customer specifications). If there are no know applicable specifications, the test is performed according to ASME Section V. Article 6 and a test procedure written.

The procedure includes:

• The inspection conditions (ambient light and ultraviolet (UV) illumination conditions)

• The penetrant, penetrant remover, emulsifier and developer to be used

• The pre-cleaning and post-cleaning required

• The penetrant dwell time, removal of excess and the drying of the surface before the application of the developer

• The developer dwell time before interpretation

Pre-Cleaning

Pre-cleaning of the specimen is an important part of the dye penetrant inspection process. The care taken in this initial step determines the level of inspection success. The pre-cleaning process insures the surface of the specimen is free of all dirt, scales, oil, finger prints and all surface residues so the penetrant medium can penetrate into surface defects. Layers of paint or galvanic corrosion are removed chemically or mechanically.

Care is taken while grinding the specimen surfaces because grinding can roll material over an exposed surface crack and prevent the crack from opening up to the surface.

Dye Penetrant Application

Covering the entire surface of the specimen with the dye penetrant liquid medium initiates the dye penetrant process. This is accomplished by dipping, coating, spraying, immersing, or electrostatically applying the liquid. The appropriate method depends on the dimensions and the location of the part to be inspected.

The test liquid medium is applied within a temperature range between 15°C and 50°C. The penetrant dwell time depends on the specific test medium, the specimen material, the ambient and material temperature, and the desired defects detection sensitivity. The dwell time is between 5 and 30 minutes depending upon the above mentioned parameters.

Interim Cleaning

During the interim cleaning process, the residual dye penetrant medium is removed from the surface. The test medium can be dissolved or re-emulsified so that it can be washed with water. During this interim cleaning, the surface is checked for residual penetrant. With fluorescent dye penetration media, the interim cleaning is executed under UV light.

A low pressure water spray is applied so that the penetrant soaked into the surface cracks is not agitated out of the cracks. This can lead to crack washouts that can reduce the test sensitivity or completely remove the penetrant from the crack.

Drying

The surface of the specimen is dried. This can be done with very low pressure air drying or oven drying at a maximum temperature of 50°C.

Developing Procedure

The developing procedure causes the residual penetrant medium to be drawn to the surface through the application of wet or dry developers, enhancing the detected crack indication.

The application of the developer is done with spraying (aerosol cans, low pressure spraying systems, spraying pistols or compressed air) or electrostatic pistols. Brushing on or painting the developer on the material is not acceptable.

Through the lateral expansion of the penetrant medium within the crack, the crack width is enlarged, and the visibility of even the smallest defects and hair line cracks is ensured. The developing time is similar to the penetrant dwell time, normally 5 to 30 min. Austenitic steels may require more than 60 min.

Inspection

During the visual inspection of the part surface, the operator controls the inspection parameters, making sure that ambient light intensity and UV intensity are constant. To make sure that the specimen has not been overwashed, the interim cleaning allows a low-level coloured background to remain after the developing procedure. The negative indications remain in clear contrast to the background color.

The measurements are documented with reference to position, size, number and location. Typical indications are:

• Continuous line

• Intermittent line

• Rounded areas

• Small dot

• Diffuse

• Brilliance

According to typical test specifications and procedures, the inspection status of the inspected specimen is categorized as acceptable, reject, or re-work required.

Final Cleaning

If the inspected surface must be free of developer (for subsequent visual checks, further processing, coloring, anodizing), a final post-cleaning process is necessary. The developer coating can be removed with water, an air-liquid mixture, or in a liquid solvent immersion tank.

Quality Control

ASME Section V. Article 6 deals with maintaining the highest quality for liquid dye penetrant processes to provide consistent inspection results in accordance with ASTM Standards, referenced in ASME V Article 24, SE-165. These standards include the following:

• E 1208 Test Method for Fluorescent Liquid Penetrant Examination Using the Lipophilic Post-Emulsification Process

• E 1209 Test Method for Fluorescent Liquid Penetrant Examination Using the Water-Washable Process

• E 1210 Test Method for Fluorescent Liquid Penetrant Examination Using the Hydrophilic Post-Emulsification Process

• E 1219 Test Method for Fluorescent Liquid Penetrant Examination Using the Solvent-Removable Process

• E 1220 Test Method for Visible Penetrant Examination Using the Solvent-Removable Process

Ultrasonic Testing

Mechanical vibrations can be propagated in solids, liquids and gases. The particles of matter vibrate, and if the mechanical movements of the particles have a regular motion, the vibration is assigned a frequency in cycles per second, measured in hertz (Hz), where 1 Hz = 1 cycle per second. If this frequency is within the approximate range 10 to 20 000 Hz, the sound is audible. Above about 20 kHz, the “sound” waves are called ultrasound or ultrasonic.

The ultrasonic principle is based on the fact that solid materials are good conductors of sound waves. The waves are not only reflected at the interfaces but also by internal flaws (such as material separations and inclusions).

Piezoelectric Transducers

The conversion of electrical pulses to mechanical vibrations, and the conversion of returned mechanical vibrations back into electrical energy is the basis for ultrasonic testing. The active element is the heart of the transducer as it converts the electrical energy to acoustic energy, and vice versa. The active element is a piece of polarized material (i.e. some parts of the molecule are positively charged, while other parts of the molecule are negatively charged) with electrodes attached to two of its opposite faces.

When an electric field is applied across the material (Figure 9), the polarized molecules align themselves with the electric field producing induced dipoles within the molecular or crystal structure of the material. This alignment of molecules causes the material to change dimensions. This phenomenon is known as electrostriction. In addition, a permanently polarized material such as quartz (SiO2) or barium titanate (BaTiO3) produces an electric field when the material changes dimensions as a result of an imposed mechanical force. This phenomenon is known as the piezoelectric effect.

When a disc of piezoelectric materials is attached to a block of steel (Figure 10), either with cement or a film of oil (couplant), and a high-voltage electrical pulse is applied to the piezoelectric disc, a pulse of ultrasonic energy is generated in the disc and propagates into the steel. This pulse of waves travels through the metal. The waves are reflected or scattered at any surface or internal discontinuity such as an internal flaw in the specimen. This reflected or scattered energy is detected using a suitably-placed second piezoelectric disc on the metal surface. A pulse of electrical energy is generated in the second disc.

The time interval between the transmitted and reflected pulse is a measure of the distance of the discontinuity from the surface. The intensity of the return pulse is a measure of the size of the flaw. This is the basic principle of the ultrasonic flaw detector and the ultrasonic thickness gauge. The piezoelectric discs are the “probes” or “transducers”. Sometimes it is convenient to use one transducer as both transmitter and receiver. In an ultrasonic flaw detector, the transmitted and received pulses are displayed in a scan on an oscilloscope as shown in Figure 11. Time is the X axis on this type of graph while Y is the intensity of the pulse from the defect.

Transducers

Transducers may be purchased in various shapes and sizes to suit an application. Cylindrical crystal wafers are most commonly used. Small diameter, high frequency transducers are used to locate small discontinuities. Large transducers are capable of generating more energy, permitting the inspection of thicker specimens.

To detect defects quickly in a specimen with a large surface area, a paintbrush transducer up to 150 mm wide is used. If a defect is identified, a smaller transducer is used to find the specific location and size.

The transducer may have one or two crystals. In a single crystal design, the crystal acts as both the sending and receiving unit. Units with two crystals in the same probe allow one to act as the sender and the other as the receiving unit.

Transducer Orientation

The orientation of the transducer determines the angle at which the pulse strikes the specimen. For longitudinal wave testing, the transducer is located flat on the specimen. To create a shear wave, the pulse must enter the specimen at the desired angle. To accomplish this, the transducer is mounted on a plastic wedge which is part of the probe.

Generation of Pulse Waves

When a transducer is placed on the surface of a specimen and the pulse wave travels directly into the specimen, a longitudinal or compression wave is produced. It travels into the specimen at 90° to the surface and, if the far surface is parallel, returns directly to the transducer as shown in Figure 12(a) below. This system is used for measuring the thickness of materials and is capable of locating discontinuities directly under the transducer.

If the transducer is placed on a wedge-shaped plastic shoe, the pulse wave strikes the surface of the specimen at an angle. Just as light is refracted into the colours of the rainbow when it passes through a prism, refraction occurs when the ultrasonic pulse enters the specimen from the plastic shoe. The longitudinal (compression) and shear (transverse) waves are refracted at different angles and begin to separate as shown in Figure 12(b).

As the angle of the wedge is increased, the separation of the two waves increase until, at some point, the longitudinal wave travels parallel to the surface as in Figure 12(c). This is known as the critical angle. As long as the angle of the wedge is larger than the critical angle, only shear waves are being used to locate discontinuities.

If the angle of the wedge is increased to the point where the shear wave is refracted to the surface of the specimen, a surface wave is produced. This wave travels along the surface of the specimen until it strikes a discontinuity and an echo is generated. The surface wave only locates defects within a few millimeters of the surface.

Frequency

The size of the discontinuities to be located determines the frequency selection. To locate small discontinuities, short wavelengths are used. The shorter the wavelength, the higher the frequency required. For a high frequency, a thinner (and more fragile) crystal is used.

Couplant

For efficient testing, the ultrasonic pulses must be able to travel freely between the transducer and the specimen. If air is located between the two, its low acoustic impedance (low ability to conduct sound) causes most of the energy to be reflected from the surface of the specimen with little or no energy entering to examine the specimen. To remove the air, a liquid or paste couplant with higher acoustic impedance is used. This allows a larger percentage of the energy to travel into the specimen.

One solution to the low impedance is to immerse the specimen in a water tank. The water bath acts as the couplant. Adding wetting agents and removing any air trapped in the water increases efficiency. It is not practical to immerse larger specimens such as pressure vessels in water baths. In these cases, the specimen surface is covered with oil or grease that serves to bond the transducer to the specimen. The couplant used for these purposes is a substance that does not react with or contaminate the specimen, is easy to remove, and does not leak away during the test.

For relatively flat, smooth surfaces, a mixture of glycerin and water is used as a couplant. For rough surfaces, light motor oil with a wetting agent is used. As the surface temperature increases, heavier oils are used.

Signal-to-Noise Ratio

The detection of a defect involves many factors other than the relationship of wavelength and flaw size. For example, the amount of sound that reflects from a defect is dependent on the acoustic impedance mismatch between the flaw and the surrounding material. A void is generally a better reflector than a metallic inclusion because the impedance mismatch is greater between air and metal than between metal and another metal.

Often, the surrounding material has competing reflections. A good measure of detectability of a flaw is its signal-to-noise ratio (S/N). The signal-to-noise ratio is a measure of how the signal from the defect compares to other background reflections (categorized as “noise”). A signal-to-noise ratio of 3 to 1 is often required as a minimum. The absolute noise level and the absolute strength of an echo from a “small” defect depend on a number of factors such as:

• The probe size and focal properties

• The probe frequency, bandwidth and efficiency

• The inspection path and distance (water or solid)

• The interface (surface curvature and roughness)

• The flaw location with respect to the incident beam

• The inherent noisiness of the metal microstructure

• The inherent reflectivity of the flaw which is dependent on its acoustic impedance, size, shape, and orientation

Cracks and volumetric defects can reflect ultrasonic waves quite differently. Many cracks are “invisible” from one direction and strong reflectors from another. Multifaceted flaws tend to scatter sound away from the transducer.

Advantages and Limitations

Advantages of ultrasonic inspection include:

• It is sensitive to both surface and subsurface discontinuities

• The depth of penetration for flaw detection or measurement is superior to other NDE methods

• Only single-sided access is needed when the pulse-echo technique is used

• Its high accuracy in determining reflector position and estimating size and shape

• Minimal part preparation required

• Electronic equipment provides instantaneous results

• Detailed images can be produced with automated systems

• It has other uses such as thickness measurements in addition to flaw detection

As with all NDE methods, ultrasonic inspection also has its limitations, which include:

• Surface must be accessible to transmit ultrasound

• Skill and training required is more extensive than with some other methods

• It requires a coupling medium to promote transfer of sound energy into test specimen

• Materials that are rough, irregular in shape, very small, exceptionally thin or not homogeneous are difficult to inspect

• Cast iron and other coarse grained materials are difficult to inspect due to low sound transmission and high signal noise

• Linear defects oriented parallel to the sound beam may go undetected

• Reference standards are required for equipment calibration and characterization of flaws

ASME Section V

ASME BPVC V has two articles that cover ultrasonic examination. Article 4 covers ultrasonic examination methods for welds, whenever weld examination using ultrasound is specified or permitted in other Sections of the ASME BPVC. Article 5 provides requirements for selecting and developing ultrasonic examination procedures for parts, components, materials, and thickness determinations. Each article references the applicable Code Section for specific requirements, including:

1. Extent of examination and volume to be scanned

2. Personnel qualification

3. Certification requirements

4. Examination system characteristics

5. Acceptance criteria

6. Necessary records and documentation

7. Report requirements

8. Procedure requirements

Written Procedure Requirements

Ultrasonic examinations are performed according to a written procedure. The procedure includes the following:

1. Ultrasonic instrument types

2. Description of calibration including blocks and techniques

3. Technique (straight and angled beam, contact, and or immersion)

4. Search unit type with frequency and transducer size

5. Special search units (wedges, shoes or saddles)

6. Angles and modes of wave propagation in the material

7. Directions and extent of scanning

8. Couplant type and brand name

9. Weld and material types

10. Configurations to be examined (thickness dimensions) and form (casting, forging, plate)

11. The surfaces from which the examination is completed

12. Condition of the surface

13. Data to be recorded

14. Alarms

15. Rotating, revolving or scanning mechanisms

16. Post examination cleaning

Materials

Article 5 applies to the following:

1. Plate

2. Forgings and Bars

3. Tubular Products

4. Castings

5. Bolting Materials (studs and nuts)

6. Pumps and Valves

7. Inservice Examination

Each form of material has a section devoted to it describing the equipment, calibration, and examination to be used. Article 23 covers the requirements for examinations. It contains standards for ultrasonic examination such as SA-435 “Standard Specification for Straight-Beam Ultrasonic Examination of Steel Plates.”

Reports

For each ultrasonic examination a report is required. The documentation is according to the code of construction. Typical information required on reports is as follows:

1. identification and location of weld or volume scanned

2. surfaces from which examination was conducted, including surface condition

3. non-rejectable indications found

4. rejectable indications found

5. map or record of rejectable indications detected or areas cleared

6. areas of restricted access or inaccessible welds

7. examination personnel identity and, when required by referencing Code Section, qualification level

8. date of examination

9. identification of procedure (and revision) followed

10. ultrasonic instrument identification (including manufacturer’s serial number)

11. search unit or units identification (including manufacturer’s serial number, frequency, and size)

12. beam angle or angles used

13. couplant brand name or type used

14. search unit cable or cables used, including type and length

15. special equipment used (search units, wedges, shoes, automatic scanning equipment, recording equipment)

16. computerized program identification and revision used

17. calibration block identification

18. simulation block or blocks and electronic simulator identification (if used)

19. instrument reference level gain and damping and reject settings (if used)

20. calibration data (including reference reflectors, indication amplitudes, and distance readings)

21. data correlating simulation blocks and electronic simulators, with initial calibration (if used)

Radiographic Testing

Radiographic testing (RT), often called radiography, is an NDE technique used for detecting flaws that are internal or on an inside surface. It is one of the oldest NDE techniques used in the pressure equipment industry.

The use of industrial radiographic testing is legislated provincially through the Boiler and Pressure Vessel Act or the equivalent Act that specifies “Codes of Construction” for pressure vessels and pipelines.

Electronics and computers allow technicians to capture images digitally. Filmless radiography captures an image, digitally enhances it, and sends the image anywhere in the world. Digital images do not deteriorate with time. Technological advances have provided industry with small, light, and portable equipment that produces high quality x-rays. Linear accelerators generate extremely short wavelength, highly penetrating radiation. The technology has evolved to allow radiography to be widely used in numerous areas of inspection.

The principle behind RT techniques is that in the presence of flaws there is a differential absorption of penetrating radiation. Variations in density, composition and thickness result in the component being radiographed while absorbing different amounts of penetrating radiation. The unabsorbed radiation passes through the test component and exposes a film. The exposed film indicates the varying amounts of radiation passing through the component and gives a permanent record of the test.

Penetrating radiation can be x-rays or gamma rays. These sources of radiation differ primarily in the manner in which they are produced. X-rays are produced by high-speed electrons striking a metal target, causing a transfer of energy. An x-ray tube in an x-ray machine produces the high-speed electrons. Gamma rays are emitted from radioisotopes (radioactive materials), such as Cobalt 60 and Iridium 192, as they decay (disintegrate). The maximum penetration in steel for the various sources is shown in Table 2.

Radiography can be used on all materials. RT is best suited for detecting three-dimensional internal flaws. RT is useful for determining if something is inside a pressure component (an object stuck in a pipe or elbow or liquid trapped between double-walled expansion joints). Material thickness measurements can also help to determine corrosion rates. Another application in the pressure equipment industry is the NDE testing of welds.

Discontinuities that can be detected are:

• Voids

• Porosity

• Incomplete penetration

• Cupping

• Incomplete fusion

• Internal Bursts

• Thickness variations

• Corrosion, thinning and pitting

• Shrinkage cracks

• Slag inclusions

Radiography can only detect cracking when cracking is oriented parallel to the radiation beam.

When completing an RT there are four essential steps:

• Source selection

• Set-up

• Exposure of test component to the radiation source.

• Film development

It is interesting that approximately sixty percent of the time is spent on set-up. Of all the NDE techniques, RT requires constant attention to safety. Large doses of x-rays or gamma rays kill human cells and massive doses can cause severe disability or death. Safety is not only a concern for the operators of the RT equipment but for any individual in areas where RT is used. The safety personnel review the NDE company’s safety program and reviews dosages RT workers have received.

Workers wear a device call a dosimeter (Figure 13) to measure their exposure to radiation. A dosimeter is a pen shaped device that measures the cumulative dose of radiation it receives. Referring to Figure 13, the dosimeter is a precision instrument consisting of an ionization chamber (1) which is sensitive to radiation. It also consists of a quartz fibre electrometer (2) to measure the charge and a microscope (3) to read the shadow of the fibre on a reticle (4). A reticle consists of a network of dots, wires, crosshairs or fine lines in the focal plane of an optical instruments eyepiece. The electrometer contains two electrodes, one of which is a movable quartz fibre. When the electrometer is charged to a predetermined voltage, the electrodes assume a calibrated separation.

When the dosimeter is exposed to a radiation source, ionization occurs in the surrounding chamber which decreases the charge on the electrodes in proportion to the exposure. The deflection of the movable quartz fibre is then projected, by a light source, through an objective lens (5) to the calibrated reticle and read through a microscope eyepiece (6).

Illumination for the optical system is obtained by pointing the dosimeter at any convenient light source. Light passes through the clear glass bottom seal (7) to illuminate the reticule. The button is sealed by a bellows (8) which contains an insulated charging pin (9).

When charging, the charging pin moves up to contact the electrometer, thereby closing the circuit. Sufficient voltage is applied to recharge the system. The entire dosimeter is hermetically sealed in a protective barrel (10). The dosimeter is usually clipped to worker’s clothing to measure the actual exposure to radiation. For personal use, this is the most useful device to measure radiation because biological damage from radiation is cumulative.

When selecting an RT technique, the following points are considered:

• The size and geometry of the component being tested

• Typical defect type, size, location, and orientation

• Whether shop or field testing is necessary

• Assurance of personnel safety

• Size of discontinuity anticipated. Generally flaws must be at least as large as 2% of the penetration thickness to be detected

• Whether both sides of the component can be accessed. Access to both sides is required to place the film.

In addition to producing high quality radiographs, the radiographer is also skilled in radiographic interpretation. Interpretation of radiographs takes place in three basic steps which are:

1. Detection

2. Interpretation

3. Evaluation

All of these steps make use of the radiographer’s visual acuity. Visual acuity is the ability to detect a spatial pattern in an image. The lighting condition in the place of viewing, and the experience level for recognizing various features in the image affects an individual’s ability to detect discontinuities.

Discontinuities

Discontinuities are interruptions in the typical structure of a material. These interruptions may occur in the base metal, weld material or heat affected zones. Discontinuities, which do not meet the requirements of the codes or specifications used to invoke and control an inspection, are referred to as defects. The following discontinuities are typical of all types of welding.

Cold lap is a condition where the weld filler metal does not properly fuse with the base metal or the previous weld pass material (interpass cold lap). The arc does not melt the base metal sufficiently and causes the slightly molten puddle to flow into base material without bonding. This is illustrated in Figure 14.

Porosity is the result of gas entrapment in the solidifying metal. Porosity can take many shapes on a radiograph but often appears as dark round or irregular spots or specks appearing singularly, in clusters or rows. Sometimes porosity is elongated and may appear to have a tail. This is the result of gas attempting to escape while the metal is still in a liquid state and is called wormhole porosity. All porosity is a void in the material having a radiographic density more than the surrounding area. Porosity is illustrated in Figure 15.

Cluster porosity (Figure 16) is caused when flux coated electrodes are contaminated with moisture. The moisture turns into gases when heated and becomes trapped in the weld during the welding process. Cluster porosity appears like regular porosity in the radiograph, but the indications are grouped close together.

Slag inclusions are non-metallic solid material entrapped in weld metal or between weld and base metal. An example is shown in Figure 17. In a radiograph, dark, jagged asymmetrical shapes within the weld or along the weld joint areas are indicative of slag inclusions.

Incomplete penetration (IP) or lack of penetration (LOP) occurs when the weld metal fails to penetrate the joint. It is one of the most serious weld discontinuities. Lack of penetration creates a stress riser from which a crack may propagate. The appearance on a radiograph is a dark area with well-defined, straight edges that follows the land (root face) down the centre of the weldment. Incomplete penetration is shown in a weld and on a radiograph in Figure 18.

Incomplete fusion is a condition where the weld filler metal does not properly fuse with the base metal. Appearance on a radiograph is shown in Figure 19. It appears as a dark line or lines oriented in the direction of the weld seam along the weld preparation or joining area.

Internal concavity or suck back is condition where the weld metal has contracted as it cools and has been drawn up into the root of the weld. On a radiograph it looks similar to lack of penetration (Figure 20), but the line has irregular edges and it is often quite wide in the centre of the weld image.

Internal or root undercut is an erosion of the base metal next to the root of the weld. In the radiographic image it appears as a dark irregular line offset from the centreline of the weld as shown in Figure 21. Undercutting is not as straight edged as LOP because it does not follow a ground edge.

External or crown undercut is an erosion of the base metal next to the crown of the weld. In the radiograph, it appears as a dark irregular line along the outside edge of the weld area (Figure 22).

Offset or mismatch is a condition where two pieces being welded together are not properly aligned. On the radiographic image (Figure 23) there is a noticeable difference in density between the two pieces. The difference in material thickness causes the difference in density. Failure of the weld metal to fuse with the base metal causes the dark, straight line.

Inadequate weld reinforcement is an area of a weld where the thickness of weld metal deposited is less than the thickness of the base material. It is easy to determine by radiograph if the weld has inadequate reinforcement. The image density in the area of suspected inadequacy is darker than the image density of the surrounding base material as in Figure 24.

Excess weld reinforcement is an area of a weld, which has weld metal added in excess of that engineering drawings and codes specify. The appearance on a radiograph is a localized, lighter area in the weld. A visual inspection determines if the weld reinforcement is in excess of that the individual code used for the inspection specifies. An example is shown in Figure 25.

Cracking can be detected in a radiograph only if the crack is propagating in a direction that produces a change in thickness parallel to the x-ray beam. Cracks appear as jagged and often very faint irregular lines. Cracks can sometimes appear as “tails” on inclusions or porosity, as shown in Figure 26.

Discontinuities in Tungsten Inert Gas (TIG) Welds

The following discontinuities are peculiar to the TIG welding process. These discontinuities occur in most metals welded using TIG, including aluminium and stainless steels. The TIG method of welding produces a clean homogeneous weld which, when radiographed, is easily interpreted.

Tungsten Inclusions

Tungsten is a brittle and dense material, used as the electrode in gas tungsten arc welding (GTAW or “TIG”). Tungsten inclusions may occur if:

• the tungsten electrode accidentally contacts the weld puddle

• the tungsten electrode contacts the filler rod

• the current settings are excessive, causing the tungsten electrode to overheat

• the electrode is improperly installed in the gun

• the electrode is damaged (internal cracks)

• the improper shielding gas is used

• the shielding gas supply is interrupted or inadequate.

Since tungsten is denser than aluminum or steel, on a radiograph it appears as a lighter area with a distinct outline. See Figure 27.

Oxide Inclusions

Oxide inclusions are usually visible on the surface of material being welded (especially aluminum).Oxide inclusions are less dense than the surrounding materials and appear as dark irregularly shaped discontinuities in the radiograph (Figure 28).

Acoustic Emission Testing (AET)

Most materials and structures emit energy in the form of mechanical vibrations (acoustic emission) as a result of sudden change or movement. This is usually due to a defect-related phenomenon such as cracking or plastic deformation. These acoustic emissions propagate from the source throughout the structure. The technique of electronically “listening” to these acoustic emissions detects and locates defects as they occur, across the entire monitored area, providing early warning of pending failure in a timely and cost-effective manner. An example is volcano and earthquake seismic [acoustic] monitoring.

All types of structures and production processes undergo continuous loading and stressing. On pipelines and vessels, the process itself, both temperature and pressure, supply the stress. In production processes, machines apply stresses to materials as they are being formed, shaped and joined (e.g. in welding). These stresses eventually cause defect growth (e.g. cracking) in weaker or fatigued areas of the structure. Acoustic emission (AE) is unique to all other non-destructive test (NDT) methods because it detects the defect growth, in real time, as it is occurring. It is important to have such a non-destructive test method that can detect and locate flaws as early as possible. As a result, the structure can be repaired or replaced long before a catastrophe occurs, thereby preventing loss of life, environmental damage, and costly repairs.

ASME Section V Articles 11, 12 and 13 address acoustic emission examination methodology. To date, no Code Section has adopted any of these articles. A few Code Case requests have been allowed for very specific applications.

Acoustic Emission Procedures

Acoustic emissions (AE) are transient elastic waves, generated by the rapid release of energy from localized sources within a material. Figure 29 shows a material cracking under stress, producing acoustic emission waves.

The material cracking emits acoustic waves that emanate in all directions from the source. A piezoelectric acoustic emission sensor is in contact with the material being monitored. This senor detects the mechanical shock wave and converts it into an electronic signal, which is amplified and processed by the AE instrument. Mechanical stress is a requirement for the production of acoustic emissions. In many applications, acoustic emissions are generated naturally, due to normal process conditions. Other cases may require the application of an external force to produce an emission. The applied stresses must be non-destructive and well below the expected defect tolerance of the material.

AE systems operate with frequencies ranging from 1 to 2000 kHz. Background noises such as friction, outside impacts, or process generated signals, tend to mask acoustic emissions. The frequency of these noises determines the lower frequency limit. Signal attenuation (the gradual reduction of signal strength) determines the upper frequency limit. A critical part of the AE application process is the selection of a suitable frequency range for AE detection and signal processing. The frequency range must be above the background noises, and provide the range and sensitivity to detect acoustic emissions. This is achieved by careful selection of AE sensors (operating in various narrow- or wide-band frequency ranges), and electronic signal filtering.

Acoustic emission is transient in nature, occurring in discrete bursts. AE systems process these bursts as AE “hits.”

Figure 30 shows an AE hit waveform and a few of the AE features that are processed by the AE system. “Time of hit,” “rise time,” “AE amplitude,” “AE counts,” “duration,” “frequency content,” and the waveform itself are AE features that can be analyzed to help identify the source of AE as noise or defect-related.

Acoustic Emission Systems

AE systems range from simple single-channel, single purpose devices to complex multi-channel, multi-processing systems.

The basic AE system consists of one or more AE sensors and one preamplifier per channel, connected to an AE processor. The processor receives amplified signals from the AE sensors, as well as signals from external sensors or control inputs. These sensors follow the process, stress, or load applied to the materials under test.

The AE processor combines these inputs to form outputs indicative of the detected AE activity. The outputs correlate to the process or stress measured. These outputs can be a pass/fail signal for:

• control purposes

• indicators and annunciators, or

• graphical outputs.

Graphical outputs can show trending, or the relationship of acoustic emissions to the structure’s load or stress. A display might display clusters of AE sources to help operators determine areas of concern.

Location of Sensors

Acoustic emission sensors may be located some distance from the source and still detect a signal. If multiple sensors are used, the time difference for the signal to arrive at each sensor can be determined. Then, using triangulation, the precise location of the signal source can be determined.

To determine the source location in one dimension (in a single line), two sensors are required. To determine the location in two dimensions (over a surface or plane), three sensors are required. To determine location in three dimensions, a minimum of four sensors are required. Monitoring multiple AE events in the same area and applying cluster analysis methods can establish the severity of an acoustic emission source.

The ability to determine the location of an acoustic emission source an extremely powerful tool. A relatively large structure may be monitored with a small number of sensors. This is a great advantage in the case of in-service pressure vessels, especially when insulated, because very few access holes are needed for placement of AE sensors. All other NDT techniques require all the insulation to be removed for full inspection, making them much more expensive than AE examination techniques.

Acoustic Emissions Applications

Crack Detection

One of the oldest and most successful applications of acoustic emission has been in manufacturing. Here, crack detection may be performed during bonding, forming, or pressing operations. AE systems may be interfaced to programmable controllers, and set to monitor cracking during the high-stress manufacturing processes. When cracks occur, the AE system provides a “failure” output signal, and the part can be rejected. Many AE crack detection systems provide continuous real-time monitoring and inspection.

Vessel Inspection

AE examination of pressure vessels is a sensitive and cost-effective inspection method. The petrochemical industry has successfully used AE pressure vessel monitoring for some time. The entire pressure boundary of a vessel can be monitored by an array of sensors. Typically, pressures 10 percent greater than normal operating pressure (well below the MAWP) are applied incrementally to the vessel. After each incremental pressure increase, the pressure is held for a period of time. The AE activity during each pressure increment is monitored and recorded.

Leak Detection

Acoustic emissions are also created by the turbulent (or cavitational) flow through cracks, valves, seals, and orifices. AE sensors can detect the signals associated with these leaks. Acoustic energy is transmitted through the fluid, air, or structure to piezoelectric sensors. The signal is processed, filtered, and compared to a leak profile. The emission source is located using triangulation. AE leak detection is used in utility and petrochemical plants, to detect leaks in pipelines, boilers, vessels, and valves. AE leak detection systems may monitor multiple sensors throughout a plant. The systems can be operated in stand-alone mode, interfaced to PLCs, or tied into plant-wide distributed control systems. They can display plant piping systems and vessels on control room monitors, and pinpoint leak locations.

Pulp and Paper Industry

Within the paper industry, AE systems have located cracks in pressure vessels (such as steam-heated rollers and paper machine dryers), and in other rotating equipment such as felt rolls, reel spools, calendar rolls, and suction rolls. AE has also been used to identify delamination of bonded materials, including thermal spray metal coatings and rubber roll covers.

Leak Testing

Leak testing is used to verify the integrity of a completed pressure vessel or fitting before it is placed into service or to determine if a vessel currently in service may continue to be used. Leak testing identifies discontinuities in the vessel pressure boundary. For example, it shows cracks and pinholes in welds, but does not show porosity within a weld or small cracks that do not penetrate the vessel wall.

Pressure Testing

The ASME BPVC requires that boilers, pressure vessels, and pressure piping be pressure tested when complete. The minimum test pressure ranges from 1.25 to 1.3 times the maximum allowable working pressure. This test serves two purposes:

• To prove that the vessel can withstand the pressure for which it was designed

• To ensure no leaks are present.

Pressure testing is also used on piping and vessels that have already been in service, particularly when they are subject to corrosion or cracking. The test verifies that the vessel or piping can still safely withstand the operating pressure with a proven margin of safety and that no through cracks (that is, cracks that penetrate the vessel wall) or holes have developed. Pressure testing is especially useful if, due to the design of the equipment, the inspector does not have adequate access to conduct a visual inspection.

Many jurisdiction inspectors, as well as owners, require the organization performing the pressure test to provide written test procedures. Reviewing and approving these procedures prior to the test averts potential problems.

The first step in leak testing is to determine whether, in fact, leaks are present. The following types of pressure tests are used to aid in the detection of a leak:

• Hydrostatic

• Pneumatic

Hydrostatic Testing

Hydrostatic tests are generally carried out by filling the vessel with water and pressurizing the fluid to the required pressure. The vessel is then examined for leaks.

Preparation

In a new vessel, all connections such as flanges and couplings are closed off with blind flanges and plugs. A drain valve is located at the low point of the vessel to allow removal of the water after the test. The filling connection must have an isolating valve, and a vent valve is required at the top to allow all air to be expelled.

For testing of in-service vessels, more preparation is required. If the fluid the vessel normally handles is toxic or flammable, the vessel is cleaned and purged. The vessel is isolated from the rest of the system. Existing valves are used if they are in good condition. Otherwise, piping connections at the vessel are opened and blind flanges installed. Control line connections to the vessel are removed and closed off. The procedures listed for the preparation of new vessels are also carried out.

If a vessel is filled with water during normal operation, the hydrostatic test should not pose a problem. However, large vessels that normally contain a gas may not be designed to hold water. In such cases, additional supports may be required for the vessel, and the foundation must be inspected to ensure it can carry the additional weight.

The test pressure is usually monitored on a calibrated pressure gauge. As an additional precaution, two gauges are used. There are cases on record where vessels have been permanently deformed because a defective pressure gauge indicated a pressure lower than that applied.

Fluid Selection

The most common fluid for hydrostatic testing is water. It is inexpensive and readily available. However, if the test is to be performed when ambient conditions are near or below freezing, use of a glycol/water mixture or methanol should be considered. These are more expensive than water, and disposal poses environmental problems, but their use avoids the possibility of frozen drain lines when the test is completed.

In some cases, it is not desirable to have water in the equipment because of its effect on the process fluid. With the approval of the owner and jurisdiction inspector, another more compatible fluid may be used.

Temperatures and Pressures

For new vessels, the code of construction dictates the test pressure. For example, the ASME Code, Section VIII, Division 1 requires a hydrostatic test of 1.3 times the maximum allowable working pressure. For used equipment, the owner or jurisdiction inspector may determine the test pressure, but it should not exceed the test pressure used for testing when the vessel was new. If the inspector is only concerned with leaks that may have developed in service, a hydrostatic test at the operating pressure is acceptable.

The temperature of the test fluid is also important. ASME BPVC I requires boilers to be subjected to pressure tests using water at not less than ambient temperature, but in no case below 20°C. This will prevent brittle fracture during the test. ASME BPVC VIII-1 recommends that the metal temperature during hydrostatic test be maintained at least 17°C above the minimum design metal temperature, to minimize the risk of brittle fracture. To prevent burn hazards, if the test temperature exceeds 48°C, inspection of the vessel should be delayed until the temperature falls to 48°C or less.

Testing

After the vessel is filled with liquid and all air is vented, a pump is used to increase the pressure within the vessel to the required value. For low pressure tests, this is generally carried out in one step. For higher pressure tests, the pressure is increased in steps with a visual inspection conducted at the end of each step.

For pressures below 3000 kPa, the vessel may be inspected at the test pressure. If test pressures exceed 3000 kPa, water jets issuing through a pinhole or crack become a danger to the inspector. In these cases, the vessel is pressurized to test pressure, which is held there for a specified time while the pressure gauges are observed. The pressure is then reduced to the normal operating pressure before inspecting is done.

The visual inspection involves examining the entire surface of the vessel, looking not only for wet areas that indicate a problem, but also for small drops of liquid. A large defect causes a significant amount of liquid to escape from the vessel. However, a small crack may allow only a few drops to escape.

Another indication that defects are present is a drop in pressure during the hydrostatic test, since escaping liquid causes the pressure to drop. However, if pressure does remain constant, it is not proof that the vessel is leak-free. A very small leak may not produce a noticeable pressure drop. Also, an increase in the temperature of the test liquid can create sufficient expansion to compensate for any losses due to leakage. When testing is complete, the pressure is slowly released.

Safety Concerns

Liquids are used for testing because they do not expand significantly when the pressure drops. If a failure occurs during the test, only a small, harmless flow of water will be released. However, if not all of the air has been vented from the vessel, this air will be compressed. Therefore, in the event of a failure, there is still danger to personnel in the vicinity. Blind flanges whose bolts failed during a hydrostatic test have been known to travel up to 20 meters; plugs have blown out of couplings with deadly force. For this reason, all non-essential personnel should leave the area when pressure tests are conducted.

Advantages

Though hydrostatic testing requires skilled and knowledgeable personnel, it is inexpensive and relatively easy to perform, provided the vessel is properly prepared. By demonstrating that a vessel is capable of withstanding pressures in excess of its maximum allowable working pressure, a degree of confidence in vessel integrity is attained.

Pneumatic Testing

Pneumatic testing involves the pressurization of a vessel or piping system with a compressible gas, such as air or nitrogen, to determine if any leaks are present. The air or nitrogen may be the only suitable test substance if water can damage the interior of the vessel, as in the case of refractory linings or catalyst beds. Because pneumatic testing may be hazardous, it should only be used when other methods are impractical.

Preparation and Gas Selection

As with the hydrostatic test, the preparation for pneumatic testing involves isolating the vessel from the system and controls. Using a gas as a test medium does not place a significant amount of static head in the vessel. For general applications, air is an inexpensive and readily available test medium. However, if a possibility of combustion exists, an inert gas such as nitrogen may be used.

Temperatures and Pressures

Because of the inherent danger in testing with compressed gases, some codes allow the use of a lower test pressure than that used for hydrostatic tests. If the pneumatic test is used as an additional test to the hydrostatic test or as a leak test prior to the hydrostatic test, pressures below the operating pressure may be acceptable.

With this test, it is even more important that the proper test temperature be maintained. If there is any possibility of brittle failure of the test vessel, another method of testing is used. As the testing gas is usually delivered to the test site as a liquid or at a very high pressure, a pressure reducing system is used to produce the required test pressure. The pressure reduction causes a refrigerating effect which can cool the vessel or piping enough to affect the ductility of the metal. Heaters are used to maintain a minimum gas temperature.

If the supply pressure of the test fluid is higher than the test pressure, overpressure protection must be provided to prevent the test vessel from being overpressured during the test.

Testing

As with high pressure liquid testing, the pressure is increased in stages, with inspection occurring at each stage. The method of inspection to locate leaks can vary from the basic soap test to mass spectrometry. These techniques are discussed later.

Safety Concerns

The primary hazard in pneumatic testing is the amount of energy stored in the compressed fluid during the test. If a failure should occur, the results can be catastrophic. Pneumatic testing should be done with all non-essential personnel removed from the danger zone.

Advantages

Pneumatic testing is useful in determining whether leaks exist in a piping system prior to carrying out the hydrostatic test. At low pressures, the danger is reduced and a soap test can be done to locate leaks. The test does not create a mass loading on the item being tested and does not involve cleanup after the test because the gas may be vented to atmosphere. Pneumatic testing may be the only acceptable test method in cases where the interior of the vessel is lined with material that liquids can damage.

Location of Leaks

Once it has been determined that leaks are present or suspected, other tests are used to pinpoint the exact location of the leaks. ASME Section V Article 10 “Leak Testing” has ten mandatory appendices to cover nine specific types of tests, as follows:

• Appendix I Bubble Test (Direct Pressure Technique)

• Appendix II Bubble Test (Vacuum Box Technique)

• Appendix III Halogen Diode Detector Probe Test

• Appendix IV Helium Mass Spectrometer (Detector Probe Technique)

• Appendix V Helium Mass Spectrometer (Tracer Probe and Hood Technique)

• Appendix VI Pressure Change Test

• Appendix VIII Thermal Conductivity Detector Probe Test

• Appendix IX Helium Mass Spectrometer Test (Hood Technique)

• Appendix IX Ultrasonic Leak Detector Test

Bubble Leak Testing

The bubble test, sometimes called the soap test, is a basic method of locating a leak. It is used when the pressurizing fluid is a gas and access is available to the surface where the leak is suspected. This test method is quick, inexpensive, and does not require operator training.

The early test method involved brushing or pouring a liquid soap solution over the pressurized vessel or piping system. Escaping gas would form bubbles in the soap solution, providing a clearly visible indication of where a leak was occurring. The number and size of bubbles indicates the size of the leak.

While this type of test is acceptable in most applications, the soap leaves a film when it dries. If hard water is used, the soap tends to curdle rather than bubble. In some cases, impurities in the water can contaminate the material being tested. For example, chlorides in the water can have an effect upon stainless steel.

Instead of soap, special solutions with enhanced surface tension, viscosity, and film retention properties are now commonly used. These solutions enable the inspector to locate smaller leaks than is possible when using soap and water. A more involved method of bubble testing, called the immersion method, involves placing the pressurized specimen in a water bath. The escaping gas forms a trail of gas bubbles as it rises to the surface of the liquid.

Vacuum Testing

In vacuum testing, rather than pressurizing the interior of the vessel, a vacuum is used to create the pressure difference necessary for leakage detection. Because the best possible vacuum is 101 kPa below atmospheric pressure, the pressure differential is limited. However, the danger of an implosion is also reduced.

To perform the test, the specimen is placed in a vacuum chamber, or a vacuum chamber is attached to the side of the vessel. A pump is used to remove air from the chamber, causing the pressure to drop. Because a pressure difference exists between the interior of the vessel or sample and the vacuum chamber, leakage occurs through any cracks or other openings. Any of the methods described later in this module may then be used to locate the leaks.

Vacuum testing is commonly used for testing electrical equipment and in the laboratory. It is seldom used in the pressure vessel industry. It is sometimes used for testing the floor plates of vertical tanks.

Dye Tracer Leak Testing

In the standard hydrostatic test, observing formation of water droplets on the vessel surface detects leaks. When it is desirable to locate very small leaks, or visibility is a problem, a fluorescent dye is added to the water inside the vessel. The dye is similar to the fluorescent dye used in liquid penetrant inspection. The dye leaks through cracks or openings to the outside of the vessel. By examining the external surface of the vessel with an ultraviolet light, the dye is seen more readily than water.

Halogen Leak Testing

This testing method involves Freon or halogen gases and a halogen detector probe to detect leaks. Although the method is sensitive to leaks, the use of halocarbon gases poses environmental problems and it will probably be discontinued in the future.

This method is commonly used for leak testing refrigeration systems. However, filling a pressure vessel with halide for the purposes of a test is very expensive compared to other available methods.

Helium Mass Spectrometer Leak Testing

Mass spectrometers are the most sensitive leak detectors available. They introduce a tracer gas such as helium into the test vessel and move the detector “sniffer” hose over the surface to locate leaks. Because helium atoms are smaller than most other atoms, they penetrate through smaller cracks, providing a more sensitive test. The detector itself is able to detect one part of helium in 10 million parts of air. Helium is an inert gas that does not react with other gases or materials of construction. The amount of helium in the atmosphere is not significant and does not interfere with leak testing.

Creep

Creep is the tendency of a solid material to deform slowly, but permanently, under the influence of long-term exposure to stress well below its elastic limit. Creep tendency is increased as temperature increases. Creep can eventually cause metal to break, or rupture under pressure, even though the load applied is considerably lower than that required to cause rupture in the short term or in a tensile test. Creep deformation (strain) accumulates as a result of long-term stress. The factors that affect creep are the temperature, time, material properties, and the applied load.

A creep rupture test is used to determine the rate of deformation and the estimated time until rupture, at a given temperature. A test piece, at constant temperature, is subjected to a fixed tensile load. The deformation of the test sample is measured during the test and the time to rupture is determined. The test duration may range from 1000 to greater than 10 000 hours. A graph of the observed length of the specimen versus elapsed time is often of the form shown in Figure 31.

In Figure 31, the curve representing creep is divided into three stages. It begins after the initial extension (0-A), which is simply the measure of deformation caused by the initial loading. This initial extension depends on test conditions, usually increasing with increases in temperature and load. During the first stage of creep (from A to B), called primary creep, there is a decreasing rate of deformation.

The second stage (B to C), called secondary creep, has an extremely small variation in rate of deformation. This period is has an essentially constant rate of creep. The third stage (C to D), called tertiary creep, has an accelerating rate of deformation, leading up to fracture. Some alloys display little or zero secondary creep and spend most of their test life in the tertiary creep stage.

To simplify the practical application of creep data, it is customary to establish, under laboratory conditions, two values of stress (at a specific temperature) that produce two corresponding rates of creep: 1.0% per 10 000 and 100 000 hours, respectively.

For any specified temperature, several creep rupture tests are run under different loads. The creep rate during the period of secondary creep is determined from these curves and is plotted against the stress. When this data is plotted on logarithmic scales, the points for each specimen often lie on a line with a slight curvature. The minimum creep rate for any stress level can be obtained from this graph; also the curve can be extrapolated to obtain creep rates for stresses beyond those for which data are obtained. The shape of the creep curve depends on the chemical composition and microstructure of the metal as well as the applied load and test temperature.

In general, rapid rates of elongation indicate a transgranular (ductile) fracture and slow rates of elongation indicate an intergranular (brittle) fracture. As a rule, surface oxidation is present when the fracture is transgranular, and visible intercrystalline oxidation may or may not be present when the fracture is intergranular. Because intercrystalline oxides produce discontinuities, the time to rupture at a given temperature-load relationship may be noticeably less.

A complete creep rupture test program for a given steel consists of a series of tests at constant temperature, with each specimen loaded to a different level. Tests are not normally conducted for more than 10 000 hours, so extrapolation is used to determine longer rupture times. The ASME Boiler and Pressure Vessel Code provides several extrapolation methods. These depend on the behavior of the particular alloy for which design values are being established and on the extent and quality of the available database.

Design data is usually provided as a series of curves for constant creep strain (0.01 - 0.03%, etc.), relating stress and time for a given temperature. It is important to know whether the data used is for the secondary stage only or also includes the primary stage.

In designing plants that work at temperatures well above atmospheric temperatures, the designer must consider carefully what possible maximum strains can be allowed and what the final life of the plant will likely be. The permissible amounts of creep depend largely on the component and service conditions. Typical examples for steel are shown in Table 3.

Monitoring for Creep Damage

Creep is a function of temperature, stress and operating time. Higher operating temperature and other damage mechanisms (such as erosion and corrosion) that cause tube wall thinning and increased stresses reduce the creep life of superheater tubes. Excessive stresses associated with thermal expansion and mechanical loading can also occur leading to tube cracks and leaks, independent of predicted creep life.

Water-cooled tubes operating at or below saturation temperature are not subject to significant creep. Monitoring of the superheater tubes includes visual inspection, ultrasonic thickness testing and tube sample analysis. Problems due to erosion, corrosion, expansion, or excessive temperature can generally be located with visual examination.

High temperature steam headers (such as superheater and reheater outlets operating above 485°C) have a finite creep life and a high replacement cost. These headers experience creep under normal conditions, as well as thermal and mechanical fatigue. Creep stress, when combined with thermal and mechanical fatigue stress, leads to premature failure.

Creep fatigue in superheater and other high temperature headers is affected by steam flow, combustion, and boiler load. Because the firing rates in multiple burner boilers is not uniform (nor is the flow of combustion air and combustion products), heat distribution within a boiler is not uniform. The lack of heat transfer uniformity is aggravated by fireside surface fouling due to ash and slag deposition. The overall effect is variations in heat input to individual superheater and reheater tubes. When combined with steam flow variations within individual tubes, significant variations can occur in the steam temperature entering the header. Changes in boiler load further exacerbate the temperature differences between individual tubes and the header. When the boiler load increases, the firing rate increases to maintain pressure. As a result, the boiler temporarily over-fires to compensate for the increasing steam flow and the decreasing pressure. When boiler load decreases, the firing rate decreases slightly faster than steam flow in the superheater, which results in a decrease in tube outlet temperature relative to header temperature. These temperature gradients can cause localized stresses in the header, resulting in ligament cracks.

External stresses due to header expansion and piping loads also contribute to cracking. For units that cycle, thermal expansion can cause fatigue cracks at header support attachments, tube stub weld attachment points, and torque plates. Steam piping, as it expands and contracts, can transmit excessive loads to the header outlet nozzle. These stresses cause cracks at the outlet nozzle to header saddle weld.

In Situ Monitoring

Superheater tubes and headers should be monitored using a combination of NDE techniques, targeted at the welds where cracks are most likely to develop (stress risers.) Stress risers are flaws that can amplify an applied stress in a particular location.

Creep causes the header diameter increase. To monitor the creep rate, the header diameter is measured at several locations on the header and the outlet nozzle. In order to find creep damage, metallographic replication is performed. This is the most effective test for this purpose, and is done on any high-temperature header. Ideally, the evaluation is made at the hottest location along the header.

Metallographic Replication

Metallographic replication is used to study the grain microstructure of a component without taking samples from the component. This method is used when repeated observations are required. One method used for metallographic replication is illustrated in Figure 32.

Metallographic replication attempts to faithfully reproduce the surface topography of a metal specimen on an acetate film, which can be examined under a microscope. The surface of the specimen is cleaned of oxides to bare metal. The metal is polished with a series of grits and diamond pastes, leaving the surface scratch-free. Then, the surface is etched with 2 – 5% Nital, which is a 5% solution of nitric acid in alcohol. A cellulose acetate film is softened by soaking one side of the film with acetone. The softened film is immediately applied to the etched surface and pressed firmly into place. After about 20 minutes, the film hardens. Then, it is carefully peeled from the metal for microscopic examination. The side of the film that was not in contact with the metal is placed against a black surface, to improve the contrast. The grain boundaries are studied to determine the amount of carbide spheroidization and cavities that have formed. The films are kept and compared at each outage. Figure 33 illustrates typical cavity assessments from microstructure.

Action required from the diagram.

A – No action required at this stage

B – Schedule regular metallographic replication testing

C – Limit the service of the component and schedule repairs

D – Repair immediately as failure is imminent

Limitations of this method are:

• The area to be monitored must be accessible

• The same area of the tube or header must be monitored each time to ensure an accurate record

• The surface temperature of the tube or header must be comfortable to the touch. If it is too hot, the cellulose acetate film dries too fast and does not have time to penetrate between the grains. If it is too cold, the water content in the Nital affects the surface

Fatigue

Metals undergoing high temperature service may also be subject to fatigue. Failure may arise after exposure to cycles of alternating stress, with or without the superimposition of a mean stress. Although it is the most common cause of metal failure in general engineering, it is rare in a power plant. This is because it is possible to design away from high levels of alternating stress, and that the predominant failure mechanism at high temperatures is creep and not fatigue. In a power plant, it is possible to encounter situations that are classified as thermal fatigue. In these the frequency of straining is given by the number of stops and starts endured during the full life of the plant, (say 5 000 to 10 000). Figure 34 is an example of the original design of reinforcing ring and a replacement preformed drum end forging to eliminate cracking found in a boiler drum.

Corrosion

Good mechanical designs that minimize cracks crevices and high stress zones reduce the likelihood of accelerated corrosion attack on the waterside of a boiler. Corrosion occurs in a boiler and is allowed for in the design.

Most metals form an oxide or hydroxide corrosion film when exposed to water. This oxide layer acts as a coating on the metal and protects the metal from most types of corrosion. Boiler water treatments are designed to stabilize the protective oxide films so corrosion decreases with time. The metal losses associated with protective oxide films are uniform and occur at a predictable rate. This known rate of metal loss is the corrosion allowance designed in the vessel.

Corrosion of metal in industrial systems is complex and takes many different forms. The result of all corrosion is the loss of strength of the material and the structure. Understanding the various forms and combinations of corrosion is essential to determining the importance of each and to finding the most appropriate technologies for detection and characterization of corrosion.

Technology Applications

The areas where corrosion occurs, the materials in which it occurs, and the conditions under which it occurs combine to make the inspection for and detection of corrosion a difficult matter. All industrial systems experience some sort of corrosion and there are certain known problem areas.

The typical process of finding and identifying corrosion begins with visual inspection. Any damage that can be observed by visual means requires closer inspection. Field inspection using other means usually entails eddy current inspection, ultrasonic inspection, or both. These inspections can be accomplished during routine maintenance without impacting operational availability. If additional inspection is necessary, specialists conduct the inspection under controlled conditions, such as in a protected space or in an NDE laboratory.

Factors affecting corrosion are:

• the type of material selected for the application

• the heat treatment of the material

• the environment of the application, and

• the presence of any contaminants in the material itself.

Table 4 summarizes the types of corrosion and their characteristics.

Corrosion Detection

Corrosion detection is a subset of the larger fields of Non-Destructive Evaluation. Many of the technologies of NDE lend themselves to the detection, characterization and quantification of corrosion damage. Table 5 summarizes the major advantages and disadvantages of corrosion detection technologies.

It is also important to know if corrosion does not exist. If deep corrosion could be detected reliably and efficiently, the substantial costs associated with shutdowns and inspections would be dramatically reduced. Maintenance plans typically call for shutting down and inspection of equipment to determine their condition. If an NDE method could accurately determine the level of corrosion, including the probability that there is no corrosion present, then the huge costs associated with shutdowns could be avoided. Providing an accurate assessment of the condition of equipment supports the concept of condition-based maintenance.