Relationship between nanohardness and microstructures in high-purity Fe–C as-quenched and quench-tempered martensite

The relationship between the nanohardness and the microstructures in the Fe–C martensite was studied to understand the contributions of the matrix and the grain boundary to the macroscopic strength. As-quenched martensite was examined for five kinds of Fe–C alloys with various carbon contents in the range of 0.1–0.8 mass%, while quench-tempered martensite was investigated for an Fe–0.4% C alloy. The ratio of the nanohardness to the macrohardnessH_n/H_vwas much smaller for the Fe–C martensite than those for the single crystals, indicating that there is a significant grain-boundary effect for the martensite. The ratioH_n/H_vof the as-quenched martensite decreased with an increase in the carbon content since the size of the block structure decreased with increasing carbon content. For the quench-tempered specimens, a significant reduction of the grain-boundary effect occured at the tempering temperature of 723 K. It is mainly due to the depression of the locking parameter caused by the disappearance of the film-like carbides on the boundaries.