Austenite formation during thermochemical treatment of eutectoid steel
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then isothermally transformed at 675 ~ for 15 hours and furnace cooled. Material of thickness 0.4 cm was either cold-rolled 75 pct to a thickness of 0.1 cm (designated 75-740-t) or machined to samples 0.1 cm thick x 1 cm x 4 cm (designated 0-740-t), where t represents the time of an up-quench and hold at 740 ~ Quantitative metallography was used to determine the area fraction of martensite in samples quenched after brief up-quenches to 740 ~ for times from 5 to 60 seconds. The final fraction of martensite present was indicative of the fraction of austenite formed during the up-quench. Both 75 pct cold-rolled and isothermally transformed steel samples were studied. Figure 1 shows the sigmoidal transformation curves for both conditions. Clearly, the cold-rolled material (75-740) transformed much faster than the isothermally transformed material for any given upquench time at 740 ~ The volume fraction of austenite,fv, is also plotted (data points) as In In [1/(1 -fv)] vs In t in Figure 2. The solid lines through the experimental points have slopes of 3.1 for the TMT (75-740-t) and 3.7 for the undeformed pearlite (0-740-t), respectively. Cahn 4 has shown that a slope of 3 is consistent with early saturation of grain boundary corner sites. The nucleation rate does not enter the kinetic equations in this case. Therefore, the Avrami equation becomes fv = 1 - e x p ( - ~ N / ~ 3 t 3)
Austenite Formation during Thermochemical Treatment of Eutectoid Steel
[1]
where N is the number of nuclei per unit volume, and/~ is the average rate of migration of the austenite/pearlite interface. Austenite nucleated preferentially at the interfaces between pearlite colonies for the isothermally transformed and unrolled pearlite, as in the work of Speich et a1.5 The only locations for austenite nucleation which could readily be identified by optical metallography in the deformed samples were the inclusion-matrix interfaces. In order to test Eq. [ 1],
PO-WE KAO and J. G. BYRNE The formation of austenite in heavily cold-rolled pearlite during very brief excursions above the AI temperature was examined. This is the basis of a number of interesting thermomechanical treatments (TMT) involving the retransformation of that austenite on subsequent air-cooling or quenching. 1'2'3 The rate of austenite formation from undeformed pearlite is, not surprisingly, much slower than it is from deformed pearlite. However, what may be surprising is the fact that the growth rate of the austenite/pearlite interface seems to be smaller in the case of deformed material as will now be shown. The alloy, provided by the Bethlehem Steel Co., was consumable electrode vacuum arc remelted (CEVAR) eutectoid steel of the following composition: 0.85 C, 0.77 Mn, 0.002 P, 0.008 S, 0.17 Si, 0.2 Mo, 0.002 V, 0.02 Ni, 0.02 Cu, 0.002 Sn, and 0.005 A1 in wt pct. The as-received material was austenitized at 1090 ~ for 1 hour,
PO-WE KAO is Associate Professor and Director, Institute of Materials Science and Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan, Republ
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