This document summarizes the austenite to pearlite transformation process. It describes how pearlite forms via nucleation and growth, with the rate of transformation dependent on the rates of nucleation and growth. Nucleation occurs primarily at grain boundaries and is structure sensitive, while growth is structure insensitive. The morphology of pearlite involves the alternating growth of ferrite and cementite plates. Alloying elements generally retard the austenite to pearlite transformation by decreasing nucleation and growth rates.
This document summarizes the austenite to pearlite transformation process. It describes how pearlite forms via nucleation and growth, with the rate of transformation dependent on the rates of nucleation and growth. Nucleation occurs primarily at grain boundaries and is structure sensitive, while growth is structure insensitive. The morphology of pearlite involves the alternating growth of ferrite and cementite plates. Alloying elements generally retard the austenite to pearlite transformation by decreasing nucleation and growth rates.
Original Description:
Explica como se da un cambio de fase tipo difusional.
This document summarizes the austenite to pearlite transformation process. It describes how pearlite forms via nucleation and growth, with the rate of transformation dependent on the rates of nucleation and growth. Nucleation occurs primarily at grain boundaries and is structure sensitive, while growth is structure insensitive. The morphology of pearlite involves the alternating growth of ferrite and cementite plates. Alloying elements generally retard the austenite to pearlite transformation by decreasing nucleation and growth rates.
This document summarizes the austenite to pearlite transformation process. It describes how pearlite forms via nucleation and growth, with the rate of transformation dependent on the rates of nucleation and growth. Nucleation occurs primarily at grain boundaries and is structure sensitive, while growth is structure insensitive. The morphology of pearlite involves the alternating growth of ferrite and cementite plates. Alloying elements generally retard the austenite to pearlite transformation by decreasing nucleation and growth rates.
2 A u s t e n i t e - p e a r l i t e transformation
8.3.2.1 Nucleation and growth of pearlite
If a homogeneous austenitic specimen of eutectoid composition were to be transferred quickly to a bath held at some temperature between 720~ and 550~ decomposition curves of the form shown in Figure 8.19a would be obtained. These curves, typical of a nucleation and growth process, indicate that the transformation undergoes an incubation period, an accelerating stage and a decelerating stage; the volume transformed into pearlite has the time-dependence described by the Avrami equation (7.44). When the transformation is in its initial stage the austenite contains a few small pearlite nodules each of which grow during the period A to B (see curve obtained at 690~ and, at the same time, further nuclei form. The percentage of austenite transformed is quite small, since the nuclei are small and their total volume represents only a fraction of the original austenite. During the B to C stage the transformation rate accelerates, since as each nodule increases in size the area of contact between austenite and pearlite regions also increases: the larger the pearlite volumes, the greater is the surface area upon which to deposit further transformation products. At C, the growing nodules begin to impinge on each other, so that the area of contact between pearlite and austenite decreases and from this stage onwards, the larger the nodules the lower is the rate of transformation. Clearly, the rate of transformation depends on (1) the rate of nucleation of pearlite nodules, N (i.e. the number of nuclei formed in unit volume in unit time), and (2) the rate of growth of these nodules, G (i.e. the rate that the radius of the nodule increases with time). The variation of N and G with temperature for a eutectoid steel is shown in Figure 8.19b.
The rate of nucleation increases with decreasing
temperature down to the knee of the curve and in this respect is analogous to other processes of phase precipitation where hysteresis occurs (see Chapter 3). In addition, the nucleation rate is very structure sensitive so that nucleation occurs readily in regions of high energy where the structure is distorted. In homogeneous austenite the nucleation of pearlite occurs almost exclusively at grain boundaries and, for this reason, the size of the austenite grains, prior to quenching, has an important effect on hardenability (a term which denotes the depth in a steel to which a fully martensitic structure can be obtained). Coarse-grained steels can be hardened more easily than fine-grained steels because to obtain maximum hardening in a steel, the decomposition of austenite to pearlite should be avoided, and this is more easily accomplished if the grain boundary area, or the number of potential peadite nucleation sites, is small. Thus, an increase in austenite grain size effectively pushes the upper part of the TTT curve to longer times, so that, with a given cooling rate, the knee can be avoided more easily. The structure-sensitivity of the rate of nucleation is also reflected in other ways. For example, if the austenite grain is heterogeneous, pearlite nucleation is observed at inclusions as well as at grain boundaries. Moreover, plastic deformation during transformation increases the rate of transformation, since the introduction of dislocations provides extra sites for nucleation, while the vacancies produced by plastic deformation enhance the diffusion process. The rate of growth of pearlite, like the rate of nucleation, also increases with decreasing temperature down to the knee of the curve, even though it is governed by the diffusion of carbon, which, of course, decreases with decreasing temperature. The reason for this is that the interlamellar spacing of the pearlite also decreases rapidly with decreasing temperature, and because the carbon atoms do not have to travel so far, the carbon supply is easily maintained. In contrast to the rate of nucleation, however, the rate of growth of pearlite is quite structure-insensitive and, therefore, is indifferent to the presence of grain boundaries or inclusions. These two factors are important in governing the size of the pearlite nodules produced. If, for instance, the steel is transformed just below A~, where the rate of nucleation is very low in comparison with the rate of growth (i.e. the ratio N/G is small), very large nodules are developed. Then, owing to the structureinsensitivity of the growth process, the few nodules formed are able to grow across grain boundaries, with the result that pearlite nodules larger than the original austenite grain size are often observed. By comparison, if the steel is transformed at a lower temperature, just above the knee of the TTT curve where N/G is large, the rate of nucleation is high and the pearlite nodule size is correspondingly small. 8.3.2.2 Mechanism and morphology of pearlite formation The growth of pearlite from austenite clearly involves two distinct processes: (1) a redistribution of carbon (since the carbon concentrates in the cementite and avoids the ferrite) and (2)a crystallographic change (since the structure of both ferrite and cementite differs from that of austenite). Of these two processes it is generally agreed that the rate of growth is governed by the diffusion of carbon atoms, and the crystallographic change occurs as readily as the redistribution of carbon will allow. The active nucleus of the pearlite nodule may be either a ferrite or cementite platelet, depending on the conditions of temperature and composition which prevail during the transformation, but usually it is assumed to be cementite. The nucleus may form at a grain boundary as shown in Figure 8.20a, and after its formation the surrounding matrix is depleted of carbon, so that conditions favour the nucleation of ferrite plates adjacent to the cementite nucleus (Figure 8.20b). The ferrite plates in turn reject carbon atoms into the surrounding austenite and this favours the formation of cementite nuclei, which then continue to grow. At the same time as the pearlite nodule grows sideways, the ferrite and cementite lamellae advance into the austenite, since the carbon atoms rejected ahead of the advancing ferrite diffuse into the path of the growing cementite (Figure 8.20c). Eventually, a cementite plate of different orientation forms and this acts as a new nucleus as shown in Figures 8.20d and 8.20e. Homogeneous austenite, when held at a constant temperature, produces pearlite at a constant rate and with a constant interlamellar spacing. However, the interlamellar spacing decreases with decreasing temperature, and becomes irresolvable in the optical microscope as the temperature approaches that corresponding to the knee of the curve. An increase in hardness occurs as the spacing decreases. Zener explains the dependence of interlamellar spacing
on temperature in the following way. If the
interlamellar spacing is large, the diffusion distance of the carbon atoms in order to concentrate in the cementite is also large, and the rate of carbon redistribution is correspondingly slow. Conversely, if the spacing is small the area, and hence energy, of the ferrite-cementite interfaces become large. In consequence, such a high proportion of the free energy released in the austenite to pearlite transformation is needed to provide the interfacial energy that little will remain to provide the 'driving force' for the change. Thus, a balance between these two opposing conditions is necessary to allow the formation of pearlite to proceed, and at a constant temperature the interlamellar spacing remains constant. However, because the free energy change, A G, accompanying the transformation increases with increasing degree of undercooling, larger interfacial areas can be tolerated as the temperature of transformation is lowered, with the result that the interlamellar spacing decreases with decreasing temperature. The majority of commercial steels are not usually of the euctectoid composition (0.8% carbon), but hypo-eutectoid (i.e. <0.8% carbon). In such steels, pro-eutectoid ferrite is first formed before the peadite reaction begins and this is shown in the TTT curve by a third decomposition line. From Figure 8.18b it can be seen that the amount of pro-eutectoid ferrite decreases as the isothermal transformation temperature is lowered. The morphology of the precipitated ferrite depends on the usual precipitation variables (i.e. temperature, time, carbon content and grain size) and growth occurs preferentially at grain boundaries and on certain crystallographic planes. The Widmanst~itten pattern with ferrite growing along {1 1 1} planes of the parent austenite is a familiar structure of these steels. 8.3.2.3 Influence of alloying elements on pearlite formation With the exception of cobalt, all alloying elements in small amounts retard the transformation of austenite to pearlite. These elements decrease both the rate of nucleation, N, and the rate of growth, G, so that the top part of the TTT curve is displaced towards longer times. This has considerable technological importance since in the absence of such alloying elements, a steel can only transform into the harder constituents of bainite or martensite if it is in the form of very thin sections so that the cooling rate will be fast enough to avoid crossing the knee of the TTT curve during the cooling process and hence avoid pearlite transformation. For this reason, most commercially heat-treatable steels contain one or more of the elements chromium, nickel, manganese, vanadium, molybdenum or tungsten. Cobalt increases both N and G, and its effect on the pearlite interlamellar spacing is contrary to the other elements in that it decreases the spacing. With large additions of alloying elements, the simple form of TTT curve often becomes complex, as shown in Figure 8.18c. Thus to obtain any desired structure by heat treatment a detailed knowledge of the TTT curve is essential.