Microstructure evolution during tempering of martensitic Fe–C–Cr alloys at 700 °C

Abstract The microstructure evolution of two martensitic alloys Fe–0.15C–(1.0 and 4.0) Cr (wt%) was investigated, using X-ray diffraction, electron backscatter diffraction, electron channeling contrast imaging and transmission electron microscopy, after interrupted tempering at 700 °C. It was found that quenching of 1-mm-thick samples in brine was sufficient to keep most of the carbon in solid solution in the martensite constituent. The high dislocation density of the martensite decreased rapidly during the initial tempering but continued tempering beyond a few minutes did not further reduce the dislocation density significantly. The initial martensitic microstructure with both coarse and fine laths coarsened slowly during tempering for both alloys. However, a clear difference between the two alloys was distinguished by studying units separated by high-angle boundaries (HABs). In the low-Cr alloy, M₃C precipitates formed and coarsened rapidly, thus they caused little hindrance for migration of HABs, i.e., coarsening of the HAB units. On the other hand, in the high-Cr alloy, M₇C₃precipitates formed and coarsened slowly, thus they were more effective in pinning the HABs than M₃C in the low-Cr alloy, i.e., coarsening of HAB units was minute in the high-Cr alloy.