Metallurgy

Metallurgy is the study of metals and is the oldest science devoted to the study of engineering materials. The growth of metallurgy has led to its division into three well-defined groups:

• Extractive

• Mechanical

• Physical

Extractive Metallurgy

Extractive metallurgy, the study of the extraction and purification of metals from their ores, is conducted in several steps. Each step increases the purity of the metal by removing unwanted impurities. For example, the route from ore to refined metal may follow one of three paths:

1. Pyrometallurgy: melting the ore in a furnace to release the metal.

2. Hydrometallurgy: dissolving the metal from the ore and recovering it as a powder.

3. Electrometallurgy: dissolving the metal from the ore with the use of electricity, plate the metal out of solution.

Mechanical Metallurgy

Mechanical metallurgy is the study of the techniques and mechanical forces that shape and make the finished forms of metal. This includes studying the effects of stress, time and temperature.

Physical Metallurgy

Physical metallurgy is the study of the structure of metals. Properties of metals are intimately related to their structures. Careful modification of the structure can produce more desirable and useful properties in a metal. For example, the structure of metals can be changed by modifying chemical composition, alloys and heat treatments.

Metals may be defined as substances that are good conductors of heat and electricity. They are generally malleable and ductile. They occur naturally in ores in the form of chemical compounds such as sulphides or oxides. With the exception of the noble metals, such as platinum and gold, metallic materials tend to react chemically with the environment and revert to compound forms, a process known as corrosion.

A detailed discussion of the extraction of metals from their ores, their subsequent refinement, and the principles of property modification through alloying, are beyond the scope of this chapter; rather, this material covers fundamental concepts.

Many non-ferrous metals are used in power plant construction (e.g. copper and copper-based alloys for condenser tubing and alternator windings, tin-based alloys for bearings, and aluminum for bus bars), but the majority of power plant components are made from iron-based metals. This chapter largely concerns these ferrous materials.

This chapter discusses:

• the principles underlying the behaviour of ferrous metals

• the assessment criteria used for material selection

• the material requirements for fabrication into components, and

• the effects the operating environment has on metals.

Atomic Structure of Metals

Metals, like other matter, can exist in three common physical states—solids, liquids, and gases—depending on the temperature and pressure. For practical purposes, we only work with metals in the first two states, solids and liquids, making temperature our only concern.

If we could view the atomic structure of a solid metal as it was heated, we would see the atoms of the material increasingly vibrate as the temperature increased. At a certain temperature, the substance would melt, and the cohesive bonds holding the individual atoms together would break, releasing them from their captive positions and allowing them to travel freely throughout the liquid. At the melting point, as with water, heat energy is absorbed without a further increase in temperature until all the metal is in the liquid state. This transition phase is called the latent heat of fusion. Every metal and alloy has a fixed melting point, and the internal arrangement of the atoms in the solid state can also be temperature sensitive as we will see in the next objective. Our main concern in metallurgy is to deal with and understand the complexity of the solid-to-liquid-to-solid phase changes metals go through as they are refined, and the chemical and physical properties exhibited by them during these transitions.

Figure 1 shows a simple cooling curve for pure copper.

As metal solidifies at the freezing point “a”, a rigid atomic structure forms within the solid, holding individual copper atoms in fixed configurations known as unit cells. In most metals, the geometry of the atoms in these unit cells takes one of three basic structures:

• Face-centered cubic (FCC)

• Body-centered cubic (BCC)

• Close packed hexagonal (CPH)

Face-Centered Cubic (FCC) Structure

Fourteen atoms form the FCC unit cell, shown in Figure 2. This configuration gives the metals that solidify in this pattern the properties of high ductility, low shear, and low tensile strength but good thermal and electrical conductivity. Examples of metals that are FCC in the solid state are gold, aluminum, silver, lead, nickel, and gamma iron (iron between the temperatures of 910°C and 1390°C).

Body-Centered Cubic (BCC) Structure

Nine atoms are contained in the BCC cubic structure shown in Figure 3. Metals with this configuration in the solid state exhibit high strength, low ductility, and are very resistant to shear deformities. Metals included in this group are chromium, tungsten, molybdenum, vanadium, alpha iron (iron in the solid state below a temperature of 910°C), and delta iron (iron above 1390°C).

Close Packed Hexagonal (CPH) Structure

Seventeen atoms make up the CPH unit cell structure, shown in Figure 4. This configuration gives the metals that comprise this group intermediate strength and ductility. Metals that fit in this group include zinc, magnesium, cadmium, and titanium.

Crystalline Structure of Metal

When molten metal cools and solidifies, unit cells become packed together to form three-dimensional crystals that occupy a space lattice. Further growth of these crystals forms dendrites, which look like the branches of an evergreen tree that extend throughout the molten matrix until they contact neighbouring dendrites. These contact surfaces become the crystal or grain boundaries. Any impurities that are not soluble in this solid solution are pushed ahead of the growing crystals and become trapped at the grain boundaries and between the limbs of the dendrites. Figure 5 (a) shows the side view of a growing metallic crystal dendrite while Figure 5 (b) is a top view.

Grain size determines important properties of metals. As a rule, smaller grain size increases tensile strength and ductility while larger grain size tends to resist creep and deformation under constant loading but may be more prone to cracking. At the atomic level, the shear strength of metals is determined primarily by the type of unit cell structure exhibited.

Figure 6 shows the high-atom packed density in a FCC structure in which the top layer is easier to pull than in the BCC structure. This type of structure is found in lead which is ductile with a low shear strength.

Figure 7 shows the low-atom packed density in a BCC structure. To move, the top line of atoms has to jump over the atoms below them. This requires a large force. This type of structure is found in iron, which is hard with a high shear strength.

Polymorphism

Polymorphism is defined as the ability of a metal to change to a different unit cell structure depending on its temperature. Most metals and alloys exhibit polymorphism, including iron.

Allotropy of Iron

The change in unit cell structure, dependent on temperature, is known as the allotropy of iron. The metal can exist in different physical forms that affect its melting point, hardness, metal solubility, and alloying chemistry. This is very important in determining the way iron reacts with carbon to form steel and cast iron.

Iron-Carbon Equilibrium Diagram

Pure iron is a relatively soft, ductile element with low strength that possesses few of the noble properties commonly associated with steel. In large part, the science of steelmaking has been an attempt to understand and control the phase changes of iron, in the presence of carbon and other alloying metals, in order to make the high-strength steels that modern industry relies on. Quality control and materials testing must meet strict and exacting engineering specifications.

Carbon steels are produced by adjusting the carbon content in iron. Carbon steels are alloys containing 2% or less carbon, while cast iron contains from 2% to 6% carbon. Carbon steel is divided into three grades:

• Low (maximum carbon content 0.30%)

• Medium (maximum carbon content 0.30 to 0.60%)

• High (maximum carbon content 0.60 to 1.00%)

The wide range of physical properties exhibited in carbon steels depends on

• the form that carbon assumes in the iron matrix (single atoms, graphite flakes, spheres, or molecular combinations such as cementite (Fe3C) a compound where the carbon atoms fit in the interstitial spaces between the iron atoms),

• the carbon atom’s final resting place (inside the unit cell structures or in the exterior intra-granular spaces), and

• the carbon concentration.

In steel, hardness and brittleness increase as the carbon content increases. Softness, ductility and weldability increase as carbon content decreases.

Iron–carbon equilibrium in steel is determined by the:

• Initial concentration of carbon in the ferrite mixture

• Temperature

• Cooling rate

• Presence of other alloying metals

Figure 8 is an iron-carbon equilibrium or “phase” diagram showing the phase changes that occur as carbon content and temperature vary for carbon concentrations ranging from 0% to 6.5% in a pure iron solvent.

All phases below the solidus line are solids, and the phase above the liquidus line is molten. The areas between these two lines are pasty state phases where the mixture exists in a solid-liquid state at the corresponding temperature. To use this graph, draw a vertical line through the diagram to indicate the carbon content to be studied and follow the line up or down as the temperature changes. Refer to the definitions below.

Austenite

Austenite is the structural name of iron in a unit cell of face-centered cubic (FCC) form (called gamma iron), that can contain dissolved carbon, up to 2%. All quenching heat treatment procedures must begin from this phase.

Cementite

Cementite is the common name for iron–carbon in the form of molecular iron carbide (Fe3C).

Eutectic Reaction Point

Eutectic reaction point occurs when the liquid alloy changes directly into solid austenite and cementite without going through a pasty state phase. As the diagram indicates, this only occurs with an alloy composition of 4.0% carbon at a temperature of 1130°C; the lowest melting point of any composition of an iron-carbon mixture.

Ferrite

Ferrite is the structural name for iron in the body-centered cubic (BCC) form. The maximum amount of carbon atoms that can be dissolved in ferrite is 0.025% at 723°C.

NOTE: the term “ferrite” describes a unit cell structure, and not a chemical composition.

Lower Critical Change Line

Lower critical change line is the temperature at which an iron alloy of any carbon composition returns to a body-centered cubic unit cell structure. The diagram indicates this temperature at 723°C.

Pearlite

Pearlite consists of a layered structure (microscopically) of ferrite and cementite. It appears dark-grained in colour and forms in iron-carbon alloys below the lower critical change line at 723°C.

Peritectic Reaction

Peritectic reaction is the point where liquid delta iron, in the body-centered cubic form, changes directly into solid austenite without going through a pasty state phase. This occurs at 1492°C.

At room temperature, the low and medium carbon steels (below 0.8% carbon) always have a ferrite component that makes the steels tough and ductile and an iron carbide influence from the pearlite that still makes them relatively hard. These steels have the scientific name “hypoeutectoid steels.” Steel having 0.8% carbon would, as it cools to room temperature, change into 100% pearlite. This type of steel is used for railway rails and is called “eutectoid steel.”

When the carbon content exceeds 0.8%, steel exhibits hardness and high tensile strength and is used in tools such as axes and chisels. At 723°C these steels become a mixture of cementite and pearlite and have the scientific name “hypereutectoid steels.”

Heat Treatment of Steels

Heat treatment in steelmaking is a large secondary industry. Its importance is commensurate with the costs and resources spent on the processes used to manufacture steel products with a vast array of different mechanical and physical properties. Definitions of these processes can be better understood by referring to the partial iron-carbon diagram shown below in Figure 9. In general, the purpose of heat treating a metal is to force a physical and/or chemical transformation in the alloy and then cool it at a rate, and in such a manner, that it retains the desired properties. For power engineers and inspectors, the main application of heat treatment is for the purpose of stress relieving.

The transformation lines, shown on the previous iron-carbon diagram (Figure 8), shift with the rate of heating (transformation line rises) and the rate of cooling (transformation line falls). The effect is shown below (Figure 9) on a section of the iron-carbon phase diagram.

Note: The suffixes “r” and “c” are from the French words “refroidissant” (cooling) and “chauffant” (heating).

Annealing Processes

Annealing processes are heat treatments that produce pearlitic microstructures (ferrite, pearlite, and cementite). They are performed to homogenize the microstructure, increase ductility, remove residual stresses, and improve machinability.

Full Annealing

Annealing processes heat the steel to just above the transition temperature required to produce austenite (Ac3 for < 0.8% carbon steels and A1 for > 0.8% carbon steels), hold it at that temperature to allow uniform crystal restructuring, and then cool it very slowly to room temperature at a maximum rate of 38°C/hour. The steel is usually left in the furnace with the heat shut off or packed in sand or another material that is a good heat insulator.

Annealing high carbon steels can induce brittleness by allowing larger grain formation; this reduces toughness and ductility. Heat soaking for too long a period encourages grain enlargement in any annealing procedure and increases brittleness in the metal.

Sub-critical annealing or process annealing is a similar process, but the steel is heated to just below its austenite transformation temperature, then cooled slowly to reduce internal stresses in the metal.

Normalizing

Carbon steels containing less than 0.8% carbon are normalized to:

• Relieve internal stresses embedded in the metal from welding, machining, or forging

• Refine grain size and promote uniform composition to increase strength and toughness

• Improve machinability

Normalizing raises the temperature of the steel to approximately 55°C above the upper transition temperature into the austenite region. The steel is held at that temperature just long enough to ensure even heating throughout. It is then allowed to cool in still air at a rate not exceeding approximately 100°C/hour. If a furnace is used, the furnace should have a reducing atmosphere (no free oxygen present) to prevent oxide scale forming on the surfaces.

Referring to the iron-carbon phase diagram in Figure 8, the normalizing process produces finer and more consistent pearlite layers in the ferrite matrix. Because all high-strength, tough steels have less than 0.8% carbon in their alloy, no transformation products other than pearlite and ferrite are produced by normalizing. The importance of normalizing, which produces tougher steels than any other heat-treatment process, can be appreciated by the fact that the ASME Codes require normalized and tempered materials in many of their specifications for steel forgings and castings. Normalizing low carbon steels makes the steels just hard enough to machine freely, leaving the surface free of tears.

Spheroidizing

Spheroidizing refers to any process of prolonged heating and cooling of steel, similar to annealing, that converts the carbide content of the matrix into a rounded or spheroid structure. Metal in this form is the softest and most workable.

Hardening Processes

Hardening processes involve heating mild steel to a temperature above its transformation range (austenizing), and then cooling it quickly to increase hardness by the formation of martensite. Martensite is a structure of fine carbide needle-like grains that are extremely hard and are formed during the transformation from austenite. If the temperature is dropped quickly, the carbon in the austenite does not have time to precipitate as pearlite but instead forms distorted needle-like grains of carbide in the ferrite matrix. The cooling rate varies with the material and is called the critical cooling rate. Cooling is typically done in water, brine, oil, or air and is promoted by the agitation of the liquid or the sample.

Case Hardening

Case hardening is a type of heat treatment process that produces martensite in the outer layer only, leaving the interior to retain a tough ferrite-pearlite composition.

Metals parts surface hardened by these methods include: bearings, machine tools, crankshafts, cams, valves, gears, rollers, and hand tools.

Two important thermochemical case hardening processes for low alloy steels are carburizing and nitriding.

Carburizing is achieved by heating the part to its transformation temperature in an atmosphere of carbon monoxide (CO). Carbon diffuses into the skin of the metal increasing martensite formation in this area when the part is later quench hardened.

Nitriding is carried out in a furnace at a temperature below the transformation range of iron (approximately 500°C to 600°C) in an atmosphere of ammonia (NH3). Ammonia dissociates at this temperature into nitrogen and hydrogen. Atomic nitrogen diffuses into the surface layer of the metal forming iron nitrides which are extremely hard. This process, unlike carburizing, does not require subsequent quench hardening.

These two processes can be combined into one operation called carbonitriding when a source of carbon and nitrogen is introduced into the furnace at a temperature above the transformation range of the steel. A less severe quench hardening step is required after this operation (than with carburizing), but the resulting hardening effect is comparable. Nitriding, alone, produces the hardest surface.

Quenching

Quenching is the rapid cooling of a heated metal. This process is performed to obtain the desired transformation products. Quenching a metal increases strength and hardness and decreases toughness and ductility. Steel with a carbon content over 0.8% is heated above the upper transformation temperature and held there to allow the formation of austenite. The steel part is then quickly cooled by immersing it in a liquid such as water, brine, or oil.

Tempering

Tempering refers to the process of heating quenched steels to a specific temperature below their lower transformation ranges, which forces the saturated carbon in the martensite to form back into a stable iron carbide (cementite) and ferrite mixture (Figure 8), and then cooling the sample to room temperature at a rate that prevents martensite reformation. The primary purpose of tempering is to improve the mechanical properties of the steel. The goals are to increase ductility and toughness with slightly reduced hardness. A sword that has been quench hardened and tempered will not shatter, but will retain a hard sharp edge.

Metal Specifications

The American Society of Mechanical Engineers specifies which metals may be used to construct pressure vessels, pressure piping, and process piping systems. The ASME makes this determination by adopting material specifications developed by the American Society for Testing and Materials (ASTM), and listing the reviewed and approved materials in the ASME Boiler and Pressure Vessel Code (BPVC) Section II - Materials. Because of this reliance on the ASTM, ASME material specifications are identical (or nearly identical) to the ASTM specifications.

After development, the ASME codes are approved by the American National Standards Institute (ANSI). The extensive complexity of these codes are intended for professional engineers who design power and processing plants, and may appear to rest outside the scope of responsibility of operating power engineers. A career in power engineering, however, will almost certainly include exposure to construction projects where the onus may fall on a power engineer to monitor the suitability of the materials used. A chief engineer in a power plant may be responsible for purchasing piping, fittings, and equipment to replace or repair plant systems. To this extent, all candidates should be familiar with basic metal specifications.

It should be noted that under no circumstances should an unidentified metal ever be used in a plant application or a careless substitution of materials ever be made. Material specification and selection should be left to the design engineer.

One should also be aware of a problem with counterfeit products in the marketplace that are stamped to suggest compliance with ASME code specifications but are, in fact, inferior and have metallurgical compositions different than that required by the code. These products are forgeries and are dangerous to use. Ordering materials from reputable suppliers helps avoid this situation.

In general, piping, boiler and exchanger tubing, fittings, and structural materials can be categorized as being manufactured from one of the following types of metals:

• Low carbon or mild weldable steels (carbon content < 0.30%)

• Medium carbon steels (carbon content > 0.3% but < 0.60%)

• High carbon steels (carbon content > 0.60% but < 1.5%)

• Cast irons (carbon content between 2% and 6%)

• Alloy steels (low carbon steels alloyed with manganese, chromium, vanadium, nickel, molybdenum, tungsten, and other elements). Alloy steels include stainless steel which contains 12% to 20% chromium.

• Non-ferrous metals (copper, aluminum, brass, nickel, tungsten and zirconium)

Most piping in a power plant is made from low carbon or alloy steels. Choosing a metal for a particular job is done on the basis of safety and metal survivability. The engineer has to ensure the material can withstand the most extreme environment it will be used in and still retain a factor of safety.

An engineering piping specification will call for an ASTM “A” or “B” number material of specified grade, dimension, and schedule (wall thickness). “A” material represents carbon steels and alloys while “B” material represents non-ferrous material. ASME Codes B31.1 (Power Piping) and B31.3 (Process Piping) offer a complete choice of carbon steel and alloy piping suitable for defined applications in power plants and hydrocarbon processing plants. Compatible flanges and fittings for a particular choice of piping can be found in ASME Code B16.5 (Pipe Flanges and Fittings).

Table 1 shows an example of the most commonly used carbon steel pipe.

Material Test Reports

Material test reports, (MTRs), are a good resource for monitoring material specifications. Also referred to as certificates of testing, these reports are made available by the vendor to the purchaser upon request. MTRs originate in the smelter where the metal was made, and give a comprehensive chemical and physical analysis of the metal used to manufacture the pipe or fitting. Molten metal is sampled from every ladle after the smelting stage. Each batch of metal (or alloy) is given a heat number which identifies the batch and all the products made from that batch. The heat number follows products made from these materials throughout their lifetime. The heat numbers are so important that if a length of pipe is cut, the heat number must be copied from the original length onto the newly created, unmarked length.

An MTR identifies the pipe or fitting and specifies the heat number of the metal it was made from. It documents the chemical composition, the ASME or ASTM specification number, grade, schedule, tensile strength, and yield point of that metal. The results of any specialized testing, such as a Charpy test for brittleness, and any heat treatment processes that part underwent are also recorded. An example of a material test report is shown in Table 2.

Metal Selection

Plant operating engineers have many responsibilities, but in strict terms, these do not usually include designating metal specifications for plant pressure piping, vessels, or structural steels. This is the responsibility of professional design engineers. However, close association with professional engineering and construction personnel during construction and the duty to safely operate and maintain the plant afterward put the onus on power engineers to become familiar with the procedures, specifications, and jargon of the trade.

An outline for a construction project might take the following form, as shown in Figure 10.

An operating engineer would usually become involved in this project as the owner’s inspector or be included in an operations staff group responsible for monitoring construction progress and compliance. Operating engineers have to become familiar with the operating systems and design philosophy. As illustrated in Figure 10, all designations for pressure piping and fittings use a Canadian Standards Association (CSA) or ASME material specification.

Regulations controlling the design, construction, and installation of boilers and pressure vessels require, in part, the submission of material specifications, size, schedule, and primary service rating of all piping and fittings used in that construction to the local boiler authority, for approval and registration.

The American National Standards Institute has established minimum requirements for manufacturers to identify pipes, fittings, and flanges:

• Size

• Wall thickness

• Schedule designation

• Material type

• Manufacturer’s identity stamp

Size

“Size” refers to nominal pipe size (NPS) in inches, or diamètre nominal (DN) in mm. This designates the nominal outside pipe diameter or the inside flange diameter.

Wall Thickness

“Wall thickness” refers to the thickness of piping, steel plate, vessel walls, and pipe fittings, in inches or millimetres.

Schedule Designation

“Schedule” refers to the thickness of a pipe or fitting based on standard criteria. The greater the schedule number, the thicker the material, and the greater its ability to withstand internal pressure, corrosion, or erosion. On piping, the schedule number is stenciled along the entire length of pipe, to ensure it can be identified when cut into shorter lengths during construction.

Flanges

Flanges are always referred to by the pressure rating class of ANSI schedule 150, 300, 400, 600, 900, 1500, or 2500. As per ASME Code B16.5 (Pipe Flanges and Fittings), forged or cast flanges are identified in a material group and under a nominal designation (alloy composition) by “A” numbers and grade. They have different maximum allowable working pressures depending on their class number (ANSI 150 through 2500) and maximum service temperature. This can be more easily understood by referring to the following chart taken from ASME B16.5.

For example, a common choice for a vapour line flange from the top of a propane chiller in a gas processing plant might be a 12 inch, A 350 LF2, ANSI 300. “ANSI 300” ensures that the flange meets the requirements for the maximum allowable working pressure of the vessel with an included safety factor. Grade “LF2” allows a minimum process temperature of –45°C. The ANSI class number “A” is commonly referred to as the schedule number. Remember, flanges that comply with ASME B16.5 must be stamped “B16.” If not, they are unacceptable for pressure equipment.

Piping

There are numerous tables available that show the dimensions of various piping schedules, including one in the PanGlobal Academic Supplement. An excerpt from such a table has been reproduced in Table 4 (below).

For example, a 2-inch schedule 40 pipe has a wall thickness of 3.91 mm. Note that commercial pipe sizes 6 -inches diameter and larger are only manufactured in even inch diameters. In all pipe sizes, the outside diameter (OD) is constant, and varying wall thickness determines the resulting inside diameter of the pipe.

Although only two common pipe schedules are listed above (40 and 80), pipe schedules range from schedule 10 through schedule 160 with increasing wall thickness. Above schedule 40, the schedule designation is incremented by 40 (e.g. schedule 40, 80, 120, and 160).

Table 5 shows a material selection of commonly used piping and fittings that applies to power and processing plants complying with the following codes: ASME B31.1 (Power Piping), ASME B31.3 (Process Piping), and ASME B31.4 Pipeline Transportation Systems for Liquids and Slurries.

In these carbon-steel systems, low temp means a limiting design temperature range from –45°C to an upper specified value < 425°C, while high temp means a limiting range from –29°C to an upper specified value < 425°C. These limits are the lowest temperatures expected from either the processing fluid or the external environment that will affect a particular piping system. These design specifications are based on acceptable Charpy impact testing results that demonstrate these metals do not become brittle within these ranges. Cryogenic systems are common in certain processes; stainless steels are used because of their resistance to fracture at very low temperatures.

In general, selection of a metal for a pressure piping system has to take into account its suitability when exposed to the stresses of its operating environment, which may include consideration of the following variables:

• Internal or external maximum pressures

• Maximum and minimum temperatures

• Tensile and compressive stress loads

• Vibration and cyclic stress loads

• Chemical corrosion exposure

• Temperature gradients and thermal expansion stress factors

• Nozzle loadings (on flange connections)

• Impulse stresses (liquid slugging, hydraulic shock, physical collisions)

• Heat conductivity

Cost saving will always be a consideration in the selection of a material if the design engineer has a range of metals that can successfully meet a given specification.

Copper

The symbol for copper (Cu) comes from the Latin word cuprum, which means “from the island of Cyprus.” The Romans obtained copper by smelting malachite, a hydroxylated copper carbonate (Cu2CO3(OH)2). Although elemental copper can be found free in nature, the most important sources of this metal are the mineral ores chalcopyrite, cuprite, malachite, and bornite. Approximately 90% of the world’s primary copper occurs in the sulphide ores chalcopyrite (CuFeS2) and bornite (CuFeS4). Most copper refining processes use acid leaching, smelting, or electrolysis to separate the metal from its ores. Pure copper metal is reddish brown in colour and weighs 20% more than iron. It is tough, malleable, and ductile. It can be forged into sheets and bars and drawn into wire. Other properties that make copper a desirable engineering material are its excellent electrical and heat conductivity and its high resistance to corrosion. Industry uses this metal in its pure form as conductors in electrical switchgear and related equipment, and as electric wire. Copper and its alloys are used in the manufacture of heat exchangers, tubing, piping, valves, pipefittings, sheet roofing, and as feed stock for the production of brass and bronze alloys.

Copper Alloys

At the beginning of the 21st century, the world production of copper exceeded 16 million tonnes per year with more than half of this refined supply used to produce electric wire and conductors and the remainder used for copper alloy production. Recycling scrap copper in the form of used wire, tubing, and exchanger cores accounted for almost 40% of the North American supply.

Copper, when mixed with other elements, produces a myriad of alloys having special and superior properties to the virgin metal itself. The most commonly used copper alloys are the various brasses and bronzes. There are well over 300 different copper alloys commercially available.

Bronze

Bronze is an alloy of copper and tin but may also contain phosphorus, lead, silicon, nickel, zinc, and aluminum. It is used to make many groups of cast and wrought bronze alloys including phosphor bronzes, leaded-tin bronzes, nickel-tin bronzes, aluminum, and silicon-bronzes each having distinctive properties. Overall, these alloys have a high resistance to corrosion and show better machinability and increased strength over the parent metals.

Aluminum bronzes, containing up to 12% aluminum, have superior wear resistance and anti-galling properties, are very corrosion resistant and strong, and are used to construct bearings, bushings and machine parts. Phosphor bronze (90% copper, 9.75% tin, and 0.25% phosphorus) is very strong, hard, and resilient and is used to make bushings and high grade springs. Bronze has long been in artistic demand for casting statues, bells, and coinage.

Brass

Brass is essentially an alloy of copper and zinc with small amounts of other metals including tin, manganese, lead, nickel, iron, aluminum, and silicon. Ductility maximizes with a 70/30 ratio of copper zinc. This characteristic can be used, for example, in cartridge brass where expansion sealing in the breech is required. Naval brasses are alloys with small amounts of tin that result in a metal that shows superior resistance to saltwater corrosion, and are used extensively in marine applications. Iron-tin brasses have high strength and hardness and are used in the manufacture of bearings, valves, fittings, and naval propeller castings. Brasses have good machinability and can be welded.

Lead

Lead is dense, malleable, soft, highly corrosion resistant, and has a low melting point. Lead is extracted from lead sulphide ore (galena). The recycling of scrap from batteries, sheet, cable, bearings, and solder is also a major source of lead. Lead is usually used as an alloy in applications such as lead-acid batteries, ammunition, cable sheathing, bearings, construction and electrolytic refining and plating. Battery grids are the largest single use of lead.

Babbit and White Metals

Tin-based and lead-based bearing metals, called babbitt, are alloys used to line machine bearings. Babbitt compositions vary, from 80% lead (Pb) and less than 5% tin (Sn) to greater than 80% tin and 0% lead. The metals antimony and copper are usually present as a small percentage. Other metals which may be used to add strength are cadmium, nickel, bismuth, arsenic, zinc, and tellurium. Babbit is one form of white metal.

White metal is the name given to a variety of alloys made from varying combinations of lead, tin, antimony, bismuth, silver, and zinc. In industry, white metals are used chiefly for bearing materials (babbit) because of their low melting point, which allows easy casting of bearing shells. This metal shows sufficient strength and ductility not to crack and squeeze out under heavy loads. In addition, white metals are soft enough to contour to a shaft, preventing high points on bearing surfaces, and because of their good thermal conductivity, heat is readily dissipated away from operating bearing surfaces.

Tin-based babbit (eg. 89% tin, 7.5% lead, 3.5% copper) is widely used for higher speed, heavier load bearing applications. Lead-based babbit (eg. 75% lead, 15% antimony, and 10% tin) is used for bearing applications at lower speeds and lighter loads. Lead-based babbits are tougher but less ductile than the tin-based babbits.

Tin

Tin is extracted from oxide ores. It is a soft, white metal with good corrosion resistance and lubricity. Tin is mainly used in tin plate and as an alloying addition. Tin plate production is the largest single use of tin. Tin plate is used to make containers and packaging for food and also non-food items.

Tin-Lead Solders

Solders are divided into tin-lead solders and other solders based on various metals. The tin-lead group includes high-lead, general purpose, and high-tin solders.

High-lead solders, containing 80% lead, are used for joining tin-plated containers and automobile radiators. General purpose solders vary from 25% tin/25% lead to 50% tin/50% lead.

The tin-lead phase diagram (Figure 11) indicates that a wide range of solder alloys are possible, depending on the melting range (pasty stage) required. For example, wiping a lead pipe to lead pipe joint requires a lengthy pasty stage, but electronic components require the lowest melting point and no pasty stage (eutectic) to minimize damage to printed circuit boards.

Aluminum

Pure aluminum metal has a density of 2699 kg/m3. In the useful lightweight metal class, it is only surpassed by magnesium at 1738 kg/m3. In comparison, iron has a density of 7870 kg/m3. Light weight, with the additional properties of very good heat and electrical conductivity, make this metal highly valued.

Aluminum has only been commercially produced in the last century because of the difficulty of separating the pure metal from its abundant ores (which comprise over 8% of the earth’s crust). Bauxite, a reddish-brown ore, with abundant sources located in Australia and Jamaica, is a complex mixture of minerals containing mostly aluminum hydroxides (Al2O3•3H2O). Through a caustic leaching operation called the Bayer process, alumina (Al2O3) is produced from bauxite. Alumina is then fed to an electrolytic cell in the presence of the mineral cryolite (Na3AlF6). An electric current fuses these minerals together forming molten aluminum which settles and is tapped off the bottom of the bed. This final refining step is known as the Hall-Heroult process.

Elemental aluminum has a relatively low tensile strength and is very malleable and ductile. However, as with other metals, its alloys have properties that are far superior, and it is these alloys that are widely used in industry.

Aluminum Alloys

Aluminum is combined with other metals such as copper, silicon, manganese, zinc, nickel, magnesium, chromium, and lithium to produce hundreds of important alloys that show a remarkable range of strength, fatigue resistance, toughness, and light weight. These properties make them ideal for use in aircraft and spacecraft construction, automobile parts, industrial plant equipment, military hardware, and domestic implements.

Aluminum, combined with copper, is a good example of a useful common alloy. This alloy is produced by heating aluminum above 550°C and then saturating the crucible with copper to form a solution that contains about 5% copper by weight. Since this matrix is not positively soluble in the solid state, the copper tends to be rejected by the aluminum during cooling. But, if the mixture is quenched with cold water, the copper does not have time to precipitate into copper crystals and instead forms an intermetallic compound (CuAl2). This process is called precipitation hardening and produces an alloy that is 5 to 6 times stronger than pure aluminum.

The CuAl2 micro-particles are extremely hard and interfere with the surrounding aluminum atoms by preventing easy slippage of their atomic-plane structures, resulting in a hard, strong alloy. Carbon atoms in an iron matrix react very much the same way in converting iron into steel.

Recently, strong interest has been shown in aluminum-lithium alloys and aluminum metal matrix composites (MMC) which have very high strengths, are heat resistant, durable, and light weight. They are used to manufacture machine parts, such as diesel engine pistons and high load-bearing components for the aerospace industry. Metal matrix composites are made by solidifying molten alloys that have been reinforced with boron or ceramic fibres. Further research in this direction holds promise for many new applications.

Aluminum has another remarkable property. It is unique in being the only metal known that increases in tensile strength as its temperature decreases. This property is utilized in the construction of large plate-fin, aluminum-alloy heat exchangers, called cold boxes, with multiple internal flow paths that display incomparable heat exchange efficiency in cryogenic process plants. A pair of these cold box heat exchangers installed in an ethane extraction plant under construction is shown in Figure 12. Temperatures drop to –100°C in the coldest sections. Note the number of flanged connections; each represents an associated internal passage.

Aluminum and its alloys are also used extensively in the electrical field for conductors in large electric transmission lines, switchgear equipment, and motor control centre main bus bar connectors. Aluminum conductors do not need additional surface protection because they develop a thin, passivating, corrosion-resistant oxide surface layer. This layer protects the underlying base metal from further corrosion. For this reason, and because of its high electrical conductivity, strength, and light weight, most high-voltage transmission systems use strand-twisted aluminum alloy conductor cable that, in systems above 250 kV, can exceed 28 mm in diameter. Because of the mass reduction when compared to copper-clad steel cable, utility companies can place the transmission towers much farther apart and realize large cost savings when constructing these lines using aluminum wire cable.

Cross-sectional views of modern conductor cables used in large electric transmission systems are shown in Figure 13. The conductor is made from an aluminum-magnesium-silicon alloy with high electrical conductivity and, due to its corrosion resistance properties (no steel core), it is usually installed in areas that have severe, corrosive environments such as coastal regions. This type of cable is also used in urban areas, where supports are close together, omitting the need for higher strength steel core cable which is more expensive.

The conductor shown in Figure 14 is made with a solid or stranded steel core surrounded by aluminum stranded wire. It can be manufactured with a wide range of tensile strengths that vary with the size of the inner steel core. They are used for river crossings and other long-span installations.

An advantage of this type of conductor is its high tensile strength.

Disadvantages are:

• A greater diameter is required to carry a given electrical load

• The inner steel core is prone to corrosion

• They are comparatively costly

In an attempt to increase tensile strength without sacrificing load carrying capacity, an innovative manufacturing method uses a continuous carbon fibre-polymer resin composite to replace the steel inner core of the ACSR conductor. The aluminum conductor composite core (ACCC), shown in Figure 15, is 75% lighter than steel with comparable tensile strength for a given wire diameter and is completely unaffected by corrosion.

1. a) Explain the differences between face-centered cubic, body-centered cubic and close-packed hexagonal unit cell structures in metals and give examples of each.

b) What is the allotropy of iron and why is this process important to understand?

c) A metal is prone to cracking under stress if its grain boundaries are contaminated with impurities. Explain how this can occur and give an example.

d) At the atomic level, what generally determines the shear strength of a metal?

2. a) Define the austenite and cementite structures in steel.

b) What occurs at the lower critical change line with a dropping temperature in an alloy of 3% carbon in iron that is significant in forming cast iron?

c) Why is cast iron brittle?

3. a) Which heat treatment process produces the maximum toughness in mild steels? Describe this process.

b) How are steels hardened? Why are some steels only case hardened? Give five uses for case hardened steel.

c) Tempering and quenching steel is a process used to control the diffusion and precipitation of which element in the matrix?

4. a) What information is given in a material test report?

b) What organizations establish the metal specification of power plant steam piping?

5. a) What term generally refers to the wall thickness in a steel pipe? The pressure rating of a steel flange?

b) What minimum information must be permanently stencilled on piping, fittings, and flanges?

6. a) List two main supply sources of copper.

b) Explain why copper is an important metal.

c) List five industrial uses of pure copper.

d) How do the composition and the properties of brass and bronze differ?

e) What is white metal? Name the most important alloy of white metal and give its uses.

7. a) Describe the process that produces our major supply of aluminum.

b) State three main uses of aluminum and explain why aluminum alloys are so important.

Self-Assessment

Question 1

What type of steel is used to make small tools, is strong and hard, can only be welded with special welding techniques and has a carbon content of 0.60 - 1.00%?

mild

high carbon

very high carbon

medium carbon

zero carbon

Question 2

Certain metals are __________ which means that their atoms can rearrange from one crystal structure to another at certain critical temperatures with each structure being stable within definite limits of temperature and pressure and having distinct properties.

allotropic

alloys

critical

cementite

ferrite

Question 3

The strength of steel __________ as the carbon content increases but the ductility __________.

increases, decreases

increases, increases

decreases, decreases

decreases, increases

remains the same, increases

Question 4

The process used to soften steel by heating above the transformation range and then cooling it slowly is called:

stress relieving.

case hardening.

annealing.

tempering.

softening

Question 5

This non-ferrous metal has good corrosion resistance, is an excellent heat conductor, is very malleable and ductile, has low density and low tensile strength:

aluminum.

bronze.

copper.

lead.

tin