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Level 43

27 - HEAT TREATMENT OF METALS


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Goal of heat-treating
Metals are heat-treated to effect metallurgical changes that beneficially alter properties
Annealing
Annealing consists of heating the metal to a suitable temperature, holding at that temperature for a certain time, a step called soaking, and slowly cooling. It is performed on a metal for any of the following reasons: (1) to reduce hardness and brittleness, (2) to alter microstructure so that desirable mechanical properties can be obtained, (3) to soften metals for improved machinability or formability, (4) to recrystallize cold-worked (strain-hardened) metals, and (5) to relieve residual stresses induced by prior processes.
Reasons for annealing
The purposes of annealing include (1) to control properties, (2) to reduce brittleness and improve toughness, (3) to recrystallize cold-worked metals, and (4) to relieve stresses from prior metalworking.
Most important heat treatment
The most important heat treatment for steels is martensite formation by heating steel into the austenite region and quenching.
Martensite
Martensite is a hard, brittle phase that gives steel its unique ability to be strengthened to very high levels. The extreme hardness of martensite results from the lattice strain created by carbon atoms trapped in the Body Centered Tetragonal structure, thus providing a barrier to slip.
Ferrite
At room temperature, ferrite is body centered cubic structure, relatively soft, magnetic, and has almost 0% carbon content.
Austenite
Austenite exists at higher temperatures than ferrite. At low carbon levels, steel contains mostly ferrite. As the temperature increases, more of structure converts to austenite. Austenite is a face-centered cubic structure, whereas ferrite is a body centered cubic structure.
Strengthening mechanisms in heat-treating
When steel containing carbon is heat-treated, martensite is formed which is a hard and brittle non-equilibrium phase of steel. The extreme hardness of martensite results from the lattice strain created by carbon atoms trapped in the body-centered tetragonal structure, thus providing a barrier to slip.
Time-temperature-transformation curve
The TTT, time-temperature-transformation curve indicates what phases in the iron-carbon phase diagram will be produced under various conditions of cooling.
Heat treatment process of steel
The heat treatment to form martensite consists of two steps: austenitizing and quenching. These steps are often followed by tempering to produce tempered martensite.
Austenizing
Austenitizing involves heating the steel to a sufficiently high temperature that it is converted entirely or partially to austenite. The transformation to austenite involves a phase change, which requires time as well as heat. Accordingly, the steel must be held at the elevated temperature for a sufficient period of time to allow the new phase to form and the required homogeneity of composition to be achieved.
Quenching
The quenching step involves cooling the austenite rapidly enough to avoid passing through the nose of the TTT curve, thus allowing formation of martensite. The cooling rate depends on the quenching medium and the rate of heat transfer within the steel workpiece. Various quenching media are used in commercial heat treatment practice: (1) brine—salt water, usually agitated; (2) fresh water—still, not agitated; (3) still oil; and (4) air. Quenching in agitated brine provides the fastest cooling of the heated part surface, whereas air quench is the slowest. Trouble is, the more effective the quenching media is at cooling, the more likely it is to cause internal stresses, distortion, and cracks in the product. The rate of heat transfer within the part depends largely on its mass and geometry.A large cubic shape will cool much more slowly than a small, thin sheet. The coefficient of thermal conductivity, k, of the particular composition is also a factor in the flow of heat in the metal, for example plain low carbon steel might have a three times higher thermal conductivity than highly alloyed steel.
Tempering
Martensite is hard and brittle. Tempering is a heat treatment applied to hardened steels to reduce brittleness, increase ductility and toughness, and relieve stresses in the martensite structure. It involves heating and soaking at a temperature below the austenitizing level for about 1 hour, followed by slow cooling. This results in precipitation of very fine carbide particles from the martensitic iron–carbon solution, and gradually transforms the crystal structure from BCT to BCC. This new structure is called tempered martensite. A slight reduction in strength and hardness accompanies the improvement in ductility and toughness. The temperature and time of the tempering treatment control the degree of softening in the hardened steel, because the change from untempered to tempered martensite involves diffusion.
Goal of tempering
Tempering involves heating and soaking of martensite for about one hour at a temperature below the austenitizing region, followed by slow cooling to reduce brittleness, relieve stresses, and increase toughness and ductility.
Hardenability
Hardenability is the relative capacity of a steel to be hardened by transformation to martensite. It is a property that determines the depth below the quenched surface to which the steel is hardened, or the severity of the quench required to achieve a certain hardness penetration. Steels with good hardenability can be hardened more deeply below the surface and do not require high cooling rates. Hardenability does not refer to the maximum hardness that can be attained in the steel; that depends on the carbon content.
Increasing hardenability
The hardenability of a steel is increased through alloying. Alloying elements having the greatest effect are chromium, manganese, molybdenum (and nickel, to a lesser extent). The mechanism by which these alloying ingredients operate is to extend the time before the start of the austenite-to-pearlite transformation in the TTT diagram. In effect, the TTT curve is moved to the right, thus permitting slower quenching rates during quenching. Therefore, the cooling trajectory is able to follow a less hastened path to the Ms line, more easily avoiding the nose of the TTT curve.
Testing hardenability
The most common method for measuring hardenability is the Jominy end-quench test. The test involves heating a standard specimen of diameter of 25.4 mm (1 inch) and length of 102 mm (4 inches) into the austenite range, and then quenching one end with a stream of cold water while the specimen is supported vertically. The cooling rate in the test specimen decreases with increased distance from the quenched end. Hardenability is indicated by the hardness of the specimen as a function of distance from quenched end.
Hardenability elements
Important hardenability elements are chromium, manganese, molybdenum, and nickel.
Hardenability and the TTT curve
The hardenability alloying elements operate by pushing the nose of the TTT curve to the right, thereby permitting slower cooling rates for conversion of austenite to martensite.
Precipitation hardening
Precipitation hardening is a heat treatment in which very fine particles (precipitates) are formed so that dislocation movement is blocked and the metal is thus strengthened and hardened. It is the principal heat treatment for strengthening alloys of aluminum, copper, magnesium, nickel, and other nonferrous metals. Precipitation hardening can also be used to strengthen certain steel alloys. When applied to steels, the process is called maraging (an abbreviation of martensite and aging), and the steels are called maraging steels.
Conditions for precipitation hardening
The necessary condition that determines whether an alloy system can be strengthened by precipitation hardening is the presence of a sloping solvus line, a line which separates a homogeneous solid solution from a field of several phases which may form by exsolution or incongruent melting.
Process of precipitation hardening
The precipitation hardening heat treatment processes consists of the following three stages: (1) solution treatment, in which the alloy is heated to a temperature Ts above the solvus line into the alpha phase region and held for a period sufficient to dissolve the beta phase; (2) quenching to room temperature to create a supersaturated solid solution; and (3) precipitation treatment, in which the alloy is heated to a temperature Tp, below Ts, and held there for a specific time in order to cause precipitation of fine particles of the beta phase. This third step is called aging, and for this reason the whole heat treatment is sometimes called age hardening. However, aging can occur in some alloys at room temperature, and so the term precipitation hardening seems more precise for the three-step heat treatment process under discussion here. When the aging step is performed at room temperature, it is called natural aging. When it is accomplished at an elevated temperature, as in our figure, the term artificial aging is often used.
Temperature and time in precipitation hardening
The combination of temperature and time during the precipitation treatment (aging) is critical in bringing out the desired properties in the alloy. At higher precipitation treatment temperatures, the hardness peaks in a relatively short time; whereas at lower temperatures, more time is required to harden the alloy but its maximum hardness is likely to be greater than in the first case. As seen in the plot, continuation of the aging process results in a reduction in hardness and strength properties, called over-aging. Its overall effect is similar to annealing.
Surface hardening
Surface hardening refers to any of several thermochemical treatments applied to steels in which the composition of the part surface is altered by addition of carbon, nitrogen, or other elements. The most common treatments are carburizing, nitriding, and carbonitriding. These processes are commonly applied to low carbon steel parts to achieve a hard, wear-resistant outer shell while retaining a tough inner core. The term case hardening is often used for these treatments.
Carburizing
Carburizing adds carbon to the surface of low-carbon steel, thereby transforming the surface into high-carbon steel up to a depth of 4mm , by heating a low carbon steel part in a carbon-rich environment up to temperatures of 900 degrees, well into the austenitic region, heating the part in environments full of charcoal or coke, or using hydrocarbon fuels to get some carbon into the steel in a process o pack carburizing. Carburizing followed by quenching produces a case hardness of around HRC of 60.
Nitriding
Nitriding is a treatment in which nitrogen is diffused into the surfaces of special alloy steels to produce a thin hard casing without quenching. To be most effective, the steel must contain certain alloying ingredients such as aluminum (0.85% to 1.5%) or chromium(5%or more).These elements form nitride compounds that precipitate as very fine particles in the casing to harden the steel. Nitriding methods include: gas nitriding, in which the steel parts are heated in an atmosphere of ammonia or other nitrogen rich gas mixture; and liquid nitriding, inwhich the parts are dipped in molten cyanide salt baths. Both processes are carried out at around 500 degrees. Case thicknesses range as low as 0.025 mm and up to around 0.5 mm , with hardnesses up to HRC 70.
Carbonitriding
Carbonitriding is a treatment in which both carbon and nitrogen are absorbed into the steel surface, usually by heating in a furnace containing carbon and ammonia. Case thicknesses are usually 0.07 to 0.5mm, with hardnesses comparable with those of the other two treatments
Selective surface-hardening
SELECTIVE SURFACE HARDENING uses different methods are used to heat only the surface of the work, or local areas of the work surface. They differ from surface-hardening methods in that no chemical changes occur. Here the treatments are only thermal - a flame, a magnetic field, a high frequency current, an electron beam, or a laser heat a part of a part and then the part goes through quenching and tempering. The selective surface hardening methods include flame hardening, induction hardening, high-frequency resistance heating, electron beam heating, and laser beam heating.
Selective surface-hardening methods
The selective surface-hardening methods include: [1] flame hardening, [2] induction hardening, [3] high-frequency (HF) resistance heating, [4] electron beam (EB) heating, and [4] laser beam (LB) heating.