Effect of Block Size on the Reduction Rate of Dislocation Density during Tempering Treatment of Lath Martensite in Low Alloy Steel

Abstract

The effect of the block size on the reduction rate of the dislocation density in quenched and tempered martensite steels was investigated. Three different prior austenite grain sizes were chosen by applying different heat treatment patterns using 0.5mass%C-1mass%Cr-0.7mass%Mo steel. The block size of lath martensite decreased as the grain size of prior austenite decreased. The dislocation density decreased with an increase in the holding time at 873 K. In quenched martensite steel, the dislocation density was independent of the prior austenite grain size. Conversely, in the martensite steel tempered at 873 K for 3.6 × 105 s, the dislocation density decreased as the prior austenite grain size decreased. The recovery coefficient m was adopted to determine the reduction rate of the dislocation density. It was found that m increased linearly as the block size decreased, and increased linearly with the inverse of the block size.