Austenitic transformation

For maximum temperatures below 800°F, suitable ferritic steels are usually good selections. Above 800°F their loss of strength must be considered carefully and balanced against their lower thermal expansion. It should be recognized that if they are heated through the ferrite to austenite transformation temperature their behavior will become more complex and the results probably adverse. 

All the remarks so far made have been concerned with conditions of cyclic reheating. When an alloy is held at a steady temperature above the critical range, some growth will arise from graphitisation, partly offset by the contraction involved in the ferrite-austenite transformation, but most of the growth will be due to oxide penetration. 

Finally, at even lower transformation temperatures, a completely new reaction occurs. Austenite transforms to a new metastable phase called martensite, which is a supersaturated solid solution of carbon in iron and which has a body-centred tetragonal crystal structure. Furthermore, the mechanism of the transformation of austenite to martensite is fundamentally different from that of the formation of pearlite or bainite in particular martensitic transformations do not involve diffusion and are accordingly said to be diffusionless. Martensite is formed from austenite by the slight rearrangement of iron atoms required to transform the f.c.c. crystal structure into the body-centred tetragonal structure the distances involved are considerably less than the interatomic distances. A further characteristic of the martensitic transformation is that it is predominantly athermal, as opposed to the isothermal transformation of austenite to pearlite or bainite. In other words, at a temperature midway between (the temperature at which martensite starts to form) and m, (the temperature at which martensite... 

Time-temperature-transformation (T-T-T) diagrams are used to present the structure of steels after isothermal transformation at different temperatures for varying times. The T-T-T diagram for a commercial eutectoid steel is shown in Fig. 20.48a. Also shown on the curves are the points at which the microstructures illustrated in Figs. 20.46 and 20.47 are observed, and the thermal treatments producing these structures. When a steel partially transformed to, say, pearlite, is quenched from point a in Fig. 20.48a to below nif, the untransformed austenite transforms to martensite. 

Continuous cellulosic fibers, 20 557 Continuous compression filters, 77 379-381 Continuous conveyors, 9 119 Continuous cooling, in austenite transformation, 23 282-283 Continuous-cooling transformation (CCT) diagrams, 77 16 23 280 Continuous copper-drossing process, 74 745-747... 

The phase diagram also contains a peritectic point, at the highest temperature of the austenite phase region, 1493 °C. At this peritectic point, austenite transforms into liquid and 5-ferrite on heating ... 

At lower temperatures the austenitic phase decomposes into ferrite and cementite. The lowest point of stability of austenitic crystals is at a concentration of 0.8% carbon and a temperature of 723 °C. At this point the austenite transforms to a eutectoid mixture of ferrite with 0.02% carbon and cementite with 6.67% carbon. This eutectoid point of the solid solution is adequate to the eutectic point of a melt where the melt decomposes into a eutectic mixture. The eutectoid mixture of ferrite and cementite is called pearlite. 

These steels contain chromium (12-25%) and nickel (8-25%). The high nickel content stabilizes the austenitic modification. After a heat treatment of the region of austenite transformation and rapid cooling the steels remain in the austenitic form. They are nonmagnetic. The most famous example is thel8/8 chromium-nickel steel. 

Vit] Vitek, J.M., Vitek, S.A., David, S.A., Numerical Modeling of Diffusion-controlled Phase Transformations in the Ternary Systems and Application to flic Ferrite/Austenite Transformation in the Fe-Cr-Ni System , Metall. Mater. Trans. A, 26(8), 2007-2025 (1995) (Calculation, Kinetics, 28)... 

Vit] Vitek, J.M., Kozeschnik, E., David, SA., Simulating the Ferrite-to-Austenite Transformation in Stainless Steel Welds , Calphad, 25(2), 217-230 (2001) (Caleulation, Kineties, 20) [2002His] Hishinuma, A., Takaki, S., Abiko, K., Reeent Progress and Future R D for High-Chromium Iron-Base and Chromium-Base Alloys , Phys. Status Solidi A, 189(1), 69-78 (2002) (Eleetro-chemistry, Interface Phenomena, Meehan. Prop., Phys. Prop., Review, 24)... 

Sht2] Shtansky, D.V., Nakai, K., Ohmori, Y, Pearlite to Austenite Transformation in an Fe-2.6Cr-lC Alloy , Acta Mater, 47(9), 2619-2632 (1999) (Morphology, Phase Relations, Thermodyn. Calculation, Experimental, Kinetics, 40)... 

Lan] Lange, H., Mathieu, K., About Evolution of the Austenite Transformation in Supercool State of Iron-Nickel-Carbon Alloys (in German), Mitt. K. W. Inst. Eisenforschung, 20, 125-134 (1938) (Experimental, Phys. Prop., Morphology, 17)... 

Numerous investigations have been done regarding the liquidus surface, die isothermal sections and the vertical sections in the stable and metastable systems. The other investigations on die ternary system concern the solubility measurements of carbon in the "y and liquid phases which go always widi activity measme-ments, the determination of the phase diagram under high pressures and die kinetics studies of die austenite transformation in martensite or bainite because these phases are important in die forecast of mechanical properties of steel. The main experimental investigations on crystal structure, phase equilibria and thermodynamics are gathered in Table 1. 

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