Effect of strain rate on martensitic transformation during uniaxial testing of AISI- 304 stainless steel
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men gage length, respectively, at the time of fracture. Similar results of rise in temperature have also been reported, t4-9] Due to nonhomogeneous deformation along the gage length and heat conduction from the center toward the ends of the specimen, a temperature gradient develops along the gage length of the specimen. The temperature gradient along the specimen gage length affects the stability of austenite in AISI-304 stainless steel. Variation in strain and temperature along the gage length would affect the extent of martensitic transformation. Measurements show that the volume fraction of martensite increases from the end of gage length to the center at all strains and strain rates encountered in the present study. As expected, the magnitude of martensite decreases with increasing imposed strain rate at a given level of strain. From finite element analysis, we find that effective strain also increases sharply on traversing toward the center of the specimen. Thus, in spite of increase in temperature, the volume fraction of martensite increases from the end of the specimen to the center. The volume fraction of martensite was computed with the following equation: f a ' = 1 - exp {-/311 - exp ( - ~ e ) ] " }
and by taking values of parameters ce and/3 at different temperatures from Hecker curves. {3] While the experimental and computed values exhibit good agreement at the end of the gage length where temperature rise is limited, a reverse trend is observed as we move toward the center of the specimen. This discrepancy apparently arises since the temperature rise toward the center has not been considered in estimating values of a and /3. The latter parameters are shown to taper off to zero at the relatively low temperature of nearly 80 ~ (Figure 10). Since martensite continues to form [3~ with increasing strain even in the specimen deformed at very high strain rate (103/s), where the temperature of 80 ~ would be achieved at a very early stage of deformation, it is evident that the values of ot and/3 have been underestimated by Olson and Cohen, as well as Hecker, and should be suitably
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ASHOK KUMAR, Principal Research Engineer, and L.K. SINGHAL, General Manager, are with the Research and Development Centre for Iron and Steel, Steel Authority of India Ltd., P.O. Hinoo, Doranda, Ranchi-834 002 Bihar, India. Manuscript submitted June 17, 1988.
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Fig. 1 - - V o l u m e fraction of martensite along the specimen gage length at different strains (strain rate - 2 . 1 • 10-3/s). VOLUME 20A, DECEMBER 1989--2857
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