Engineering Materials Heat Treatments of Ferrous Alloys Annealing Processes The term annealing refers to a heat treatment in which a material is exposed to an elevated temperature for an extended time period and then slowly cooled. Ordinarily, annealing is carried out to (1) relieve stresses; (2) increase softness, ductility, and toughness; and/or (3) produce a specific microstructure. Internal residual stresses may develop in metal pieces in response to the following: (1) plastic deformation processes such as machining and grinding; (2) non-uniform cooling of a piece that was processed or fabricated at an elevated temperature, such as a weld or a casting; and (3) a phase transformation that is induced upon cooling wherein parent and product phases have different densities. Distortion and warpage may result if these residual stresses are not removed. A variety of annealing heat treatments are possible; they are characterized by the changes that are induced, which many times are microstructural and are responsible for the alteration of the mechanical properties. Any annealing process consists of three stages: (1) heating to the desired temperature, (2) holding or soaking at that temperature, and (3) cooling, usually to room temperature. Process Annealing is a heat treatment that is used to negate the effects of cold work that is, to soften and increase the ductility of a previously strain-hardened metal. It is commonly utilized during fabrication procedures that require extensive plastic deformation, to allow a continuation of deformation without fracture or excessive energy consumption. Recovery and recrystallization processes are allowed to occur. Ordinarily a fine-grained microstructure is desired, and therefore, 1
the heat treatment is terminated before appreciable grain growth has occurred. Surface oxidation or scaling may be prevented or minimized by annealing at a relatively low temperature (but above the recrystallization temperature) or in a non-oxidizing atmosphere. Stress Relief annealing heat treatment in which the piece is heated to the recommended temperature, held there long enough to attain a uniform temperature, and finally cooled to room temperature in air. The annealing temperature is ordinarily a relatively low one such that effects resulting from cold working and other heat treatments are not affected. Annealing of Ferrous Alloys Several different annealing procedures are employed to enhance the properties of steel alloys. However, before they are discussed, some comment relative to the labeling of phase boundaries is necessary. Figure (1) shows the portion of the iron carbon phase diagram in the vicinity of the eutectoid. The horizontal line at the eutectoid temperature, conventionally labeled A 1, is termed the lower critical temperature, below which, under equilibrium conditions, all austenite will have transformed into ferrite and cementite phases. The phase boundaries denoted as A 3 and A cm represent the upper critical temperature lines, for hypoeutectoid and hypereutectoid steels, respectively. For temperatures and compositions above these boundaries, only the austenite phase will prevail. Note that other alloying elements (if exist) will shift the eutectoid and the positions of these phase boundary lines. 2
Figure (1). The iron carbon phase diagram in the vicinity of the eutectoid, indicating heattreating temperature ranges for plain carbon steels. Normalizing Steels that have been plastically deformed by, for example, a rolling operation, consist of grains of pearlite (and most likely a proeutectoid phase), which are irregularly shaped and relatively large, but vary substantially in size. An annealing heat treatment called normalizing is used to refine the grains (i.e., to decrease the average grain size) and produce a more uniform and desirable size distribution; fine-grained pearlitic steels are tougher than coarse-grained ones. Normalizing is accomplished by heating at least (55 C) above the upper critical temperature that is, above A3 for compositions less than the eutectoid, and above A cm for compositions greater than the eutectoid as represented in Figure(1). After sufficient 3
time has been allowed for the alloy to completely transform to austenite a procedure termed austenitizing the treatment is terminated by cooling in air. Full Anneal A heat treatment known as full annealing is often utilized in low- and medium carbon steels that will be machined or will experience extensive plastic deformation during a forming operation. In general, the alloy is treated by heating to a temperature of about (55 C) above the A 3 line (to form austenite) for compositions less than the eutectoid, or, for compositions in excess of the eutectoid, (55 C) above the A 1 line (to form austenite and Fe 3 C phases)., as noted in Figure (1). The alloy is then furnace cooled; that is, the heat-treating furnace is turned off and both furnace and steel cool to room temperature at the same rate, which takes several hours. The microstructural product of this anneal is coarse pearlite (in addition to any proeutectoid phase) that is relatively soft and ductile. Coarse and Fine Pearlite The thickness ratio of the ferrite and cementite layers in pearlite is approximately 8 to 1. However, the absolute layer thickness depends on the temperature at which the isothermal transformation is allowed to occur. At temperatures just below the eutectoid, relatively thick layers of both the α-ferrite and Fe 3 C phases are produced; this microstructure is called coarse pearlite, and the region at which it forms is indicated to the right of the completion curve on figure (2). At these temperatures, diffusion rates are relatively high, such that during the transformation from austenite to pearlite the atoms of carbon can diffuse relatively long distances, which results in the formation of thick lamellae. 4
With decreasing temperature, the carbon diffusion rate decreases, and the layers become progressively thinner. The thin-layered structure produced in the vicinity of 450 C is termed fine pearlite; this is also indicated in figure (2). Spheroidizing Medium- and high-carbon steels having a microstructure containing even coarse pearlite may still be too hard to conveniently machine or plastically deform. These steels, and in fact any steel, may be heat treated or annealed to develop the spheroidite structure. Spheroidized steels have a maximum softness and ductility and are easily machined or deformed. The spheroidizing heat treatment, during 5
which there is a coalescence of the Fe 3 C to form the spheroid particles, can take place by several methods, as follows: Heating the alloy at a temperature just below the eutectoid [line A 1, or at about (700 C)], spheroidizing times will ordinarily range between 15 and 25 h. Heating to a temperature just above the eutectoid temperature, and then either cooling very slowly in the furnace, or holding at a temperature just below the eutectoid temperature. Heating and cooling alternately within about ±50 C of the A 1 line of Figure (1). To some degree, the rate at which spheroidite forms depends on prior microstructure. For example, it is slowest for pearlite, and the finer the pearlite, the more rapid the rate. Also, prior cold work increases the spheroidizing reaction rate. Quenching of Steels Martensite is formed when austeniteized iron carbon alloys are rapidly cooled (or quenched) to a relatively low temperature (in the vicinity of the ambient). Martensite is a non-equilibrium single-phase structure that results from a diffusionless transformation of austenite. It may be thought of as a transformation product that is competitive with pearlite and bainite. The martensitic transformation occurs when the quenching rate is rapid enough to prevent carbon diffusion. Any diffusion whatsoever will result in the formation of ferrite and cementite phases. Large numbers of atoms experience cooperative movements, in that there is only a slight displacement of each atom relative to its neighbors. This occurs in such a way that the FCC austenite experiences a polymorphic transformation to a body- 6
centered tetragonal (BCT) martensite. A unit cell of this crystal structure (figure 3.A) is simply a body-centered cube that has been elongated along one of its dimensions; this structure is distinctly different from that for BCC ferrite. All the carbon atoms remain as interstitial impurities in martensite; as such, they constitute a supersaturated solid solution that is capable of rapidly transforming to other structures if heated to temperatures at which diffusion rates become appreciable. Martensite grains take on a plate-like or needle-like appearance, as indicated in figure (3.B). The white phase in the micrograph is austenite (retained austenite) that did not transform during the rapid quench. As already mentioned, martensite as well as other microconstituents (e.g., pearlite) can coexist. 7
Being a nonequilibrium phase, martensite does not appear on the iron carbon phase diagram. The austenite-to-martensite transformation is, however, represented on the isothermal transformation diagram [time temperature transformation (or T T T) plots]. Since the martensitic transformation is diffusionless and instantaneous, it is not depicted in this diagram as the pearlitic and bainitic reactions are. The beginning of this transformation is represented by a horizontal line designated M (start), figure (4). Two other horizontal and dashed lines, labeled M(50%) and M(90%), indicate percentages of the austenite-to-martensite transformation. The temperatures at which these lines are located vary with alloy composition but, nevertheless, must be relatively low because carbon diffusion must be virtually nonexistent. The horizontal and linear character of these lines indicates that the martensitic transformation is independent of time; it is a function only of the temperature to which the alloy is quenched or rapidly cooled. A transformation of this type is termed an athermal transformation. The presence of alloying elements other than carbon (e.g., Cr, Ni, Mo, and W) may cause significant changes in the positions and shapes of the curves in the isothermal transformation diagrams. These include (1) shifting to longer times the nose of the austenite-to-pearlite transformation (and also a proeutectoid phase nose, if such exists), and (2) the formation of a separate bainite nose. These alterations may be observed by comparing figures (4) and (5), which are isothermal transformation diagrams for carbon and alloy steels, respectively. 8
Tempered Martensite In the as-quenched state, martensite, in addition to being very hard, is so brittle that it cannot be used for most applications; also, any internal stresses that may have been introduced during quenching have a weakening effect. The ductility and toughness of martensite may be enhanced and these internal stresses relieved by a heat treatment known as tempering. 9
Tempering is accomplished by heating a martensitic steel to a temperature below the eutectoid for a specified time period. Normally, tempering is carried out at temperatures between 250 and 650 C; internal stresses, however, may be relieved at temperatures as low as 200 C. This tempering heat treatment allows, by diffusional processes, the formation of tempered martensite, according to the reaction: where the single-phase BCT martensite, which is supersaturated with carbon, transforms to the tempered martensite, composed of the stable ferrite and cementite phases, as indicated on the iron carbon phase diagram. The microstructure of 10
tempered martensite consists of extremely small and uniformly dispersed cementite particles embedded within a continuous ferrite matrix. This is similar to the microstructure of spheroidite except that the cementite particles are much, much smaller. An electron micrograph showing the microstructure of tempered martensite at a very high magnification is presented in Figure (6). Tempered martensite may be nearly as hard and strong as martensite, but with substantially enhanced ductility and toughness. The dependence of tensile and yield strength and ductility on tempering temperature for an alloy steel is shown in figure (7). Bainitic Steel In addition to pearlite, other microconstituents that are products of the austenitic transformation exist; one of these is called bainite. The microstructure of bainite is composed of a ferrite matrix and elongated particles of Fe 3 C, and thus diffusional processes are involved in its formation; the microstructural details of bainite are very fine that it can be seen only using electron microscopy, not an optical 11
microscopy. Because bainitic steels have a finer structure, they are generally stronger and harder than pearlitic ones; yet they exhibit a desirable combination of strength and ductility. The time temperature dependence of the bainite transformation may also be represented on the isothermal transformation diagram (figure 1). It occurs at temperatures below those at which pearlite forms; begin-, end-, and half-reaction curves are just extensions of those for the pearlitic transformation, as shown in figure (1), the isothermal transformation diagram for an iron carbon alloy of eutectoid composition that has been extended to lower temperatures. All three 12
curves are C-shaped and have a nose, where the rate of transformation is a maximum. As may be noted, whereas pearlite forms above the nose (i.e., over the temperature range of about 540 727 C to), at temperatures between about 215 and 540 C, bainite is the transformation product. It should also be noted that pearlitic and bainitic transformations are really competitive with each other, and once some portion of an alloy has transformed to either pearlite or bainite, transformation to the other microconstituent is not possible without reheating to form austenite. Review of Phase Transformations for Iron-Carbon Alloys Figure (8) summarizes the transformation paths that produce these various microstructures. Here, it is assumed that pearlite, bainite, and martensite result from continuous cooling treatments; furthermore, the formation of bainite is only possible for alloy steels (not plain carbon ones). Furthermore, microstructural characteristics and mechanical properties of the several microconstituents for iron carbon alloys are summarized in Table 1. 13
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Example 1: Microstructural Determinations for Three Isothermal Heat Treatments. Using the isothermal transformation diagram for an iron carbon alloy of eutectoid composition (figure 4), specify the nature of the final microstructure (in terms of microconstituents present and approximate percentages) of a small specimen that has been subjected to the following time temperature treatments. In each case assume that the specimen begins at 760 C and that it has been held at this temperature long enough to have achieved a complete and homogeneous austenitic structure. (a) Rapidly cool to 350 C, hold for 10 4 s, and quench to room temperature. (b) Rapidly cool to 250 C, hold for 100 s, and quench to room temperature. (c) Rapidly cool to 650 C, hold for 20 s, rapidly cool to 400 C, hold for 10 3 s, and quench to room temperature. Solution: The time temperature paths for all three treatments are shown in figure (9). In each case the initial cooling is rapid enough to prevent any transformation from occurring. (a) At 350 C austenite isothermally transforms to bainite; this reaction begins after about 10 s and reaches completion at about 500 s elapsed time. Therefore, by 10 4 s, as stipulated in this problem, 100% of the specimen is bainite, and no further transformation is possible, even though the final quenching line passes through the martensite region of the diagram. 15
(b) In this case it takes about 150 s at 250 C for the bainite transformation to begin, so that at 100 s the specimen is still 100% austenite. As the specimen is cooled through the martensite region, beginning at about 215 C, progressively more of the austenite instantaneously transforms to martensite. This transformation is complete by the time room temperature is reached, such that the final microstructure is 100% martensite. (c) For the isothermal line at 650 C, pearlite begins to form after about 7 s; by the time 20 s has elapsed, only approximately 50% of the specimen has transformed to pearlite. The rapid cool to 400 C is indicated by the vertical line; during this cooling, very little, if any, remaining austenite will transform to either pearlite or bainite, even though the cooling line passes through pearlite and bainite regions of the diagram. At 400 C, we begin timing at essentially zero time (as indicated in figure 9); thus, by the time 1000 s have elapsed, all of the remaining 50% austenite will have completely transformed to bainite. Upon quenching to room temperature, any further transformation is not possible inasmuch as no austenite remains; and so the final microstructure at room temperature consists of 50% pearlite and 50% bainite. 16
Homework: 1- Using the isothermal transformation diagram for an iron carbon alloy of eutectoid composition (figure 10), specify the nature of the final microstructure (in terms of microconstituents present and approximate percentages of each) of a small 17
specimen that has been subjected to the following time temperature treatments. In each case assume that the specimen begins at 760 C and that it has been held at this temperature long enough to have achieved a complete and homogeneous austenitic structure. (a) Cool rapidly to 350 C, hold for 1000 s, then quench to room temperature. (b) Rapidly cool to 625 C, hold for 10 s, then quench to room temperature. (c) Rapidly cool to 600 C, hold for 4 s, rapidly cool to 450 C, hold for 10 s, then quench to room temperature. (d) Reheat the specimen in part (c) to 700 C for 20 h. (e) Rapidly cool to 300 C, hold for 20 s, then quench to room temperature in water. Reheat to 425 C for 1000 s and slowly cool to room temperature. (f ) Cool rapidly to 665 C, hold for 1000 s, then quench to room temperature. (g) Rapidly cool to 575 C, hold for 20 s, rapidly cool to 350 C, hold for 100 s, then quench to room temperature. (h) Rapidly cool to 350 C, hold for 150 s, then quench to room temperature. 2- Make a copy of the isothermal transformation diagram for an iron carbon alloy of eutectoid composition (figure 10) and then sketch and label time temperature paths on this diagram to produce the following microstructures: (a) 100% coarse pearlite (b) 50% martensite and 50% austenite (c) 50% coarse pearlite, 25% bainite, and 25% martensite 18
Figure (10) complete isothermal transformation diagram for an iron carbon alloy of eutectoid composition. 19