Ultrastructure of precipitates in Cu-modified Q&T steels and its effect on tensile responses

Abstract

The quenching and tempering schedule has been implemented in Cu added microalloyed steels, (LCu-0.6 wt.% Cu and HCu-1.1 wt.% Cu) to investigate the role of Cu on the structure-property relationship. The samples were quenched from 920 °C after soaking for 0.5 h followed by tempering at 650 °C for 0.5–1 h. Detailed high-resolution electron microscopic evidences along with thermo-kinetic simulations were employed to represent the evolution of Cu precipitates during tempering. The ultrastructure, and the detailed unit cell configuration of different Cu precipitates have been discussed thoroughly in the present study. The gradual formation of stable and incoherent FCC-Cu variant has evolved through the ledge mechanism from the unstable 9R-Cu variant. In both the steels, delayed nucleation along with the incomplete transformation of Cu precipitates were observed. Engineering stress-strain curves demonstrated the superior strength for HCu samples, whereas the tensile toughness were found to be superior for LCu samples. Also the work hardening characteristics were linked to their microstructural counterparts for both the steels. Eventually, a theoretical correlation has been established between the ultrastructure of different Cu precipitates with their mechanical responses, which depicted an excellent fit with the experimental findings. © 2024 Acta Materialia Inc.