Effect of Prior Austenite Grain Size on Strain Distribution of As-Quenched Martensitic Steel

Abstract

Strain distribution developed during tensile deformation in as quenched martensitic steels with various prior austenite grain sizes (PAGS) was visualized by using digital image correlation method, and then the effect of PAGS on strain distribution was discussed based on the results obtained from SEM/EBSD measurements. The martensitic steels with PAGS ranging from 8 to 110 μm were fabricated by controlling the solution-treated temperature. Carbides were observed in all specimens, suggesting that the auto-tempering occurred during the water quenching. The amount and size of carbide increased with increasing the PAGS. Inhomogeneous microscopic strain distribution in a unit of block was developed by tensile deformation in all specimens, and the strain distribution became rather inhomogeneous with increasing the PAGS. The blocks with the high Schmid factor of habit plane slip system tended to exhibit high strain. It was observed that there were blocks with low Schmid factor but high strain, and such blocks had numerous carbide precipitates, indicating that those blocks were well-tempered by auto-tempering. The nano-indentation hardness in the blocks with well-tempered blocks tended to be lower than that in the less-tempered blocks. The well-tempered blocks exhibited high strain independent of Schmid factor of habit plane slip system. Thus, the auto-tempering is one of the influential factors on the inhomogeneous strain distribution in as quenched martensitic steel. It could be reasonably understood that the increase of PAGS led to the increase of Ms temperature and the promotion of auto-tempering, resulting in rather inhomogeneous strain distribution in coarse-PAGS specimen.