Regulation of Cu precipitation by intercritical tempering in a HSLA steel

A multistep heat treatment process consisting of intercritical tempering between quenching and conventional tempering contributed to the development of a ferrite–martensite dual-phase structure in a Ni- and Cu-containing high-strength low-alloy steel. By using electron backscatter diffraction and scanning transmission electron microscopy, the microstructures were found to have an elongated lathlike morphology with carbide and Cu precipitates located especially at the boundaries of ferrite and martensite crystals. Atom probe tomography reveals at atomic scale the existence of solute-diluted ferrite and solute-rich martensite, and the later phase was considered to be transformed from the reverse austenite that was formed during intercritical tempering. Cu precipitation greatly correlates with the microconstituents, resulting in different distributional characteristics of Cu precipitates within these two phases and at their boundaries. It is a promising process to utilize Cu precipitation strengthening and phase transformation toughening simultaneously in alloy steels.