On the isolated retained
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W.C. JEONG, formerly Graduate Student, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), E O. Box 131, Chongryang, Seoul, Korea, is Senior Researcher, Cold-Rolled Steel Sheets Group, Technical Research Laboratories, Pohang Iron and Steel Company, Ltd., E O. Box 36, Pohang 680, Korea. C.H. KIM is Professor, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, P. O. Box 131, Chongryang, Seoul, Korea. Manuscript submitted August 12, 1986. METALLURGICALTRANSACTIONSA
the austenite particle on cooling. All specimens for optical microscopy were etched with 2 pct nital solution and stained with a boiling (about 118 ~ alkaline chromate solution (8 g CrO3 and 40 g NaOH in 72 ml distilled water). With this treatment, martensite appeared black, retained ferrite appeared gray, and transformed ferrite, white. 1~ Figures l(a) through (c) show microstructures of specimens cooled at a rate of 5.6 ~ to 650,590, and 470 ~ respectively, and then water-quenched. Transformed ferrite preferentially grows from retained ferrite into edges of the
Fig. 1 --Transformed ferrite growth of specimens cooled at 5.6 ~ to (a) 650, (b) 590, and (c) 470 ~ followed by water-quenching. Etched with 2 pct nital and stained with boiling alkaline chromate solution. M: martensite, TF: transformed ferrite, RF: retained ferrite. VOLUME 18A, MAY 1987--933
austenite (martensite in water-quenched condition) particle. It should be noticed that a large number of small particles are isolated in ferrite grain as a consequence of local growth of transformed ferrite. In specimen quenched at 650 ~ there are no observable differences in etching response between the martensite and small isolated particles as shown in Figure l(a). However, as quenching temperature is decreased to 590 or 470 ~ the number of small isolated particles increases and they are etched lighter than the martensite particles. These small, isolated particles being etched lighter than the martensite particles can be identified by the following experiment. Figure 2(a) shows the optical microstructure of a specimen cooled at a rate of 5.6 ~ to 410 ~ followed by water-quenching. As already shown in Figures l(b) and (c), it is also observed here that small isolated particles appear to be etched lighter than the martensite particles. In contrast, the optical microstructure of a specimen tensile-strained to uniform strain (14 pct) after the same heat treatment as the specimen of Figure 2(a) reveals that there are no etching contrast differences between small isolated particles and martensite particles. It is well known that retained austenite is mechanically unstable and thus undergoes a strain-induced transformation to martensite. J.3.4.12Eldis 3 reported that the retained austenite content is reduced to less than 0.5 pct after 2 pct plastic strain. On the basis of various results, 3'5 it is believed that the
amount of tensile strain used in the present study is sufficient to transform most retain
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