Transformation plasticity significantly affects the stress distribution of a forging after heat-treatment. In this study, the transformation plasticity in a multi-phase transformation was measured experimentally. Two kinds of low-alloy steels were used: ASME SA508 and JIS SCM420. The materials were heat treated under different conditions to vary the phase fraction of ferrite. Then the transformation plastic strains of bainite were experimentally identified from the deviation between the stress-free total strain value and the strain value with an applied stress. After the tests were finished, an orientation microscopy was performed using EBSD. The grain average misorientation (GAM) values of the bainite phase become higher than those of the ferrite phase due to the bainite sub-unit. The volume fractions of ferrite were defined as the area lower than the threshold value of the GAM. The transformation plastic strain during the bainite transformation decreased with increasing volume fraction of ferrite. Although there is a quantitative difference in the transformation plastic strains between SA508 and SCM420, the normalized values give the same tendency in both materials. Furthermore, the calculated transformation plasticity using a multi-phase model shows a good agreement with the measurements. This result suggests that the transformation plasticity behavior of the bainite transformation is the same as between a single transformation (bainite) and a double transformation (bainite and ferrite).
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