Effect of Heat Treatment Process on Microstructure, Precipitates, and Mechanical Properties of Cu-Cr-Ni-Mo Weathering Steel for Bridge Construction

This study compares the strengthening and toughening mechanisms of the experimental steel in the as-rolled, tempered, and quenched and tempered (Q&T) conditions. It also explores, from a microscopic viewpoint, how heat treatment processes influence the structural evolution of Cu-rich phases under these conditions. The findings indicate that the as-rolled experimental steel exhibits a yield strength exceeding 500 MPa. However, the presence of coarse M/A islands in its microstructure causes stress concentration, which leads to a low impact energy of only 95 J at -40 °C. Upon tempering at 500 °C for 1 h, the specimens not only exhibit a yield strength of 565 MPa, a yield ratio of 0.85, and an impact energy of 184 J at -40 °C, but also achieve an excellent combination of toughness and strength. The microstructure of the Q&T experimental steel is predominantly composed of tempered sorbite. As the tempering temperature increases from 500 °C to 550 °C, the lath-like characteristics gradually disappear. Meanwhile, both the strength and low-temperature toughness are remarkably improved. Specifically, the yield strength exceeds 860 MPa, and the impact energy at -40 °C is greater than 245 J. However, it exhibits a high yield ratio ( YR ), reaching 0.97. In the as-rolled condition, the tempered state at 500 °C, and the Q&T state after tempering at 550 °C, the lattice structures of the Cu-rich phases are body-centered cubic (BCC) clusters, B2 ordered structures, and 9R ordered structures, respectively. This structural evolution occurs because as the tempering temperature rises, the Cu-rich phase structure progressively evolves towards a more stable state.