Inhomogeneity of Plastic Deformation after Yielding in Lowcarbon Martensitic Steels

The effect of tempering on uniform elongation was investigated for fully lath martensitic steels with 0.2% C by mass. Stress-strain curves were obtained from specimens tempered at 100, 200, 300 and 400°C. Although total elongation rarely depends on the tempering temperature, the uniform elongation decreased while local elongation increased with the tempering temperature. It was found, using precision markers drawn on the specimen surface, that the distribution of the equivalent plastic strain developed during uniform deformation was inhomogeneous in the specimen tempered at 200°C and relatively homogenous in the specimen tempered at 300°C. Finite element analysis suggests that the uniform elongation depends on the volume fraction that continues elastic deformation even after macroscopic yielding. Therefore, the experimental results showing a decrease in the uniform elongation with tempering temperature can be explained by the decrease in the volume fraction that continues elastic deformation immediately after the macroscopic yielding, which is a deformation character specific to low temperature tempered martensitic steels. The range of variation in nanohardness decreased with the tempering temperature, suggesting that the variation in the local yield stress also decreases with the tempering temperature.