Microstructural analysis of cold-drawn medium-carbon low-alloy martensite: effect of deformation and tempering temperature

Abstract

Microstructure of cold-drawn (≈ 0.9 effective deformation) medium-carbon steel containing Cr and Mo, was investigated in samples tempered at 923 K (tempering at high temperature [HTT]) and 723 K (tempering at intermediate temperature [ITT]) using transmission electron microscopy, X-ray diffraction and microhardness. The study revealed distinct features in lath martensite. HTT samples showed reduced dislocation density and larger precipitates compared to ITT samples. After cold drawing, ITT samples exhibited lamellar dislocation cells and cell blocks, while HTT specimens primarily showed cell block structures. Analysis of precipitates revealed changes in orientation after drawing, as well as alterations in both inter- and intragranular precipitate density. ITT samples also displayed higher post-tempering hardness. This analysis provides quantitative insights into the microstructural evolution and mechanical behaviour after tempering and cold-drawing, which can help optimise processes for achieving ultrafine-grained structures.