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

The effect of block size on the reduction rate of 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 became finer as the prior austenite grain size became finer. The dislocation density decreased with increase in the holding time at 873 K. In the quenched martensite steel, the dislocation density was independent of the prior austenite grain size. On the other hand, in the martensite steel tempered at 873 K for 3.6×105 s, the dislocation density decreased as the prior austenite grain size became finer. The recovery coefficient m was adopted to discuss the reduction rate of dislocation density. It has been found that m increases as the block size becomes finer and that m increases linearly with the inverse of the block size.