Austenitic to Ferritic Transformation

2 Isothermal Time-Temperature-Transformations 14.2.1 Austenitic to Ferritic Transformation 

Fe-FesC phase diagram. The eutectoid point at 727°C and 0.76 wt% FeiC is the focal point for steel making. (From Massalski, T.B., Senior Editor, Handbook of Binary Alloy Phase Diagrams, Vols. 1-3, American Society for Metals, 1990. Reprinted with permission of ASM International. All rights reserved.) 

Austenite is a solid solution of 7-Fe and C atoms that reside in the interstities of the fee strueture. There is a very limited region of solid solubility of C in a-Fe (0.022 wt% C). Reeall from Chapter 3 that the interstitial sites in the fee lattiee are eonsiderably larger than in the bcc lattice, which account for the much greater terminal solid solubility of C in austenite. 

Of particular importance to steel making is the eutectoid point at 727°C 0.76 wt% C. When austenite of this composition is cooled to this temperature, it undergoes euctectoid reaction y a + FeaC. Further cooling produces lamellas of alternating a-Fe and cementite called pearlite because its appearance resembles mother-of-pearl. The spacing of the lamella can be controlled by the cooling rate as discussed in Chapter 12. 

Steels with less than 0.76 wt% C are called h) oeutectoid steels. Before they reach the euctectoid temperature, some proeutectoid a-Fe is formed. Upon further cooling to the eutectoid temperature, the remaining 7 is transformed to pearlite, composed of euctectoid a and cementite. The final sfruefure fhen consists of proeutectoid a and pearlite. The relative amounts of each phase are defermined by the starting composition and can be found by applying fhe lever rule. 

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Many phase transformations such as nucleation and crystal growth from a melt, solid-state recrystallization during annealing, austenitic to ferritic transformation in steels, etc. are stochastic events because they involve either liquid- or solid-phase nucleation and therefore must be treated in terms of probabilities as done in Chapter 11. Let Vq be the total volume of the material, Vj be the volume of the transformed material, and Vu be the volume of material that has not yet been transformed. Recall from Poisson statistics that the probability of no events occurring in a given time is exp (expected number of events in that time). In this case the expected volume to be transformed is kt" where k and n may be specified by models of the transformation mechanism or may be determined empirically. Therefore, the fraction of trarrsformed matter at time t can be written as... 

The phase diagram tells us only what phases can exist in equilibrimn. The final micro-structure is controlled by the rate of transformation. A sketch of a typical isothermal time-temperature-transformation (often called a T-T-T diagram) map for the austenitic to ferritic transformation is shown in Figure 14.3. 

The fcc-bct conversion, known as the Bain transformation, is a diffusionless process. That is, unlike the previous high-temperature conversions we saw earlier e.g., austenite to ferrite), martensite can form at temperatures significantly below room... 

Gam] Gamsjaeger, E., Svoboda, J., Fischer, F.D., Austenite-to-ferrite Phase Transformation in Low-Alloyed Steels , Comput. Mater. Sci., 32, 360-369 (2005) (Phase Relations, Calculation, 16)... 

The diffusion-dependent transformations of austenite (to ferrite, pearlite, and bainite) compete with the martensitic transformation such that the volume fraction available for the latter will decrease as the volume transformed by the former increases. This transformation kinetics of the diffusional phase transformations is strongly dependent on alloy composition. 

In Fig. 21.9, the driving force for transformation from austenite to ferrite is indicated as well. It can be calculated by ... 

The temperature at which transformation of austenite to ferrite or to ferrite plus cementite is completed during cooling... 

The eutectic of the iron-carbon system, the constituents of which are austenite and cementite. The austenite decomposes into ferrite and cementite on coohng below the temperature at which transformation of austenite to ferrite or ferrite plus cementite is completed. 

A cross-section of a modified surface is shown in Fig. 13.4. The schematic of the modifying process is shown in Fig. 13.5 and Figs 13.6 and 13.7 show the iron carbon diagram for austenitic T91 at 1050°C and 0.1%C, and the iron-silicon phase diagram with phase transformation from austenitic to ferrite steel lattice with growing Si content, respectively. 

Iron-siiicon phase diagram with phase transformation from austenitic to ferrite steel lattice with growing Si content from 0% to 5% Si, y-Fe section [17]. 

Many stainless steels, however, are austenitic (f.c.c.) at room temperature. The most common austenitic stainless, "18/8", has a composition Fe-0.1% C, 1% Mn, 18% Cr, 8% Ni. The chromium is added, as before, to give corrosion resistance. But nickel is added as well because it stabilises austenite. The Fe-Ni phase diagram (Fig. 12.8) shows why. Adding nickel lowers the temperature of the f.c.c.-b.c.c. transformation from 914°C for pure iron to 720°C for Fe-8% Ni. In addition, the Mn, Cr and Ni slow the diffusive f.c.c.-b.c.c. transformation down by orders of magnitude. 18/8 stainless steel can therefore be cooled in air from 800°C to room temperature without transforming to b.c.c. The austenite is, of course, unstable at room temperature. Flowever, diffusion is far too slow for the metastable austenite to transform to ferrite by a diffusive mechanism. It is, of course, possible for the austenite to transform displacively to give... 

Times to failure for various stainless steels tested in MgClj have been shown to increase with increasing proportions of martensite present Perhaps the role of martensite under anodic dissolution conditions is comparable to that of ferrite in duplex stainless steels where the enhanced dissolution of one phase prevents crack initiation in the other. There is, of course, another aspect of martensitic transformation that should be mentioned, i.e. the transformation of austenite to martensite either in the bulk material or at a growing crack tip that can give increased susceptibility to... 

The Fe-based matrix would be austenitic at 1000°C but transform to ferrite below 8S7°C, thus giving a reason for the observed martensitic structure. 

When austenite is transformed to ferrite and pearlite below 727 °C, the composition of the pearlite and the amount of proeutectoid ferrite depend on the transformation temperature. The reason for this can be understood by extrapolating below 727 °C the line that represents the solubility of carbon in austenite, as shown in Figure 7.7. In a steel that contains less than 0.77% C, proeutectoid ferrite must form before any pearlite forms. Ferrite formation enriches the carbon content of the austenite. Pearlite can form only when the austenite has been enriched enough so that it is saturated with respect to carbon. This happens at 0.77% C if the transformation occurs at 727 °C. At temperatures below 727 °C,... 

The transformation of austenite to lower temperature transformation products (martensite, bainite, pearlite, ferrite etc) in a steel which is being steadily cooled, as in a weld HAZ, as opposed to being... 

The change of crystal structure which occurs in some materials in which different crystal structures are stable over different temperature (and pressure) ranges. In ferritic steels, the most important transformation is from the high temperature form, austenite, to lower temperature transformation products, such as ferrite, pearlite, bainite, martensite and, in weld metals, acicular ferrite and ferrite with aligned second phase. 

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