Prediction of Transformation Strain Using Microtexture Data
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inite or martensite transformations, which are displacive in nature, manifest shear and volume strain. It is observed that when these transformations occur without any external stress, the overall transformation strain becomes equivalent to volume strain and the shear strains of individual variants cancel each other. However, when stress is applied during transformation, a few specific variants are preferentially formed and the transformation strain becomes significant.[1–6] The nature of transformation strain is also affected by the type of stress being applied. Using a mathematical model, it has been shown in the past that the nature and magnitude of transformation strain can be predicted.[7] However, in the model, it was assumed that a fixed number of variants form in each austenite grain and all such variants have the same volume fraction. These assumptions are far from reality. In the present work, actual fractions of variants were determined using electron backscattered diffraction (EBSD) data and are used in two different mathematical models to calculate the transformation strain. To the best of the authors’ knowledge, this is the first instance where data from individual variant levels is used for successfully
SUSHIL K. GIRI, PINAKI BISWAS, and SAURABH KUNDU are with the R&D Division, Tata Steel Ltd., Jamshedpur-831001, India. Contact e-mail: [email protected] Manuscript submitted January 30, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS A
predicting transformation strain. The calculated transformation strain is then compared with the transformation strain experimentally measured by Mangal et al.[8] In this experiment, subsize tensile specimens with 5 mm 9 5 mm cross section were used, and they were machined out of forged material with chemical composition of 0.79C-1.56Si-1.98Mn-0.002P-1.01Al-0.24Mo-1.01Cr-1.51Co. For experiments under tensile stress and no stress conditions, the same specimen geometry was used. The details of the specimen geometry can be obtained from the work by Kundu et al.[2] The specimens were heated to 1273 K (1000C) and held for 10 min followed by cooling to 573 K (300C). Bainite transformation was induced holding the specimen under an elastic tensile stress of 220 MPa for 4 h at the same temperature. In another specimen, the same thermal profile was maintained, but no stress was applied during transformation. Using a thermomechanical simulator, dilation was measured in the transverse direction during transformation in both specimens. The EBSD measurements with a step size of 0.5 lm were carried out in a SUPRA* 25 FEG scanning *SUPRA is a trademark of Carl Zeiss AG, Oberkochen, Germany
electron microscope on these two specimens at locations close to the position where thermocouples were welded. Microtexture data were acquired using CHANNEL-5** **CHANNEL-5 is a trademark of Oxford instrument PLC., Abingdon, Oxfordshire
software, but in this work, the analysis was carried out using OIM EBSD TSL software after the data was
OIM is a trademark of EDAX/TSL, Draper, UT
converted int
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