Effect of tempering process on microstructure and hardness of Cu–Cr–Ni weathering steel for bridge construction

The influence of tempering process on the microstructure and hardness of a Cu–Cr–Ni weathering steel for bridge construction was investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and hardness testing. The results indicate that as the tempering temperature increases from 450 °C to 600 °C, the M/A islands continuously decompose, accompanied by a reduction in both their size and volume fraction. Concurrently, the proportion of polygonal ferrite increases due to recovery and recrystallization of the matrix, while the hardness generally exhibits an initial increase followed by a decrease. With the extension of holding time at 570 °C from 15 min to 120 min, the hardness also shows a trend of first increasing and then decreasing, reaching a peak value of 251 HV after holding for 1 h. This peak hardness is attributed to the pronounced strengthening effect induced by the precipitation of a high density of nano-sized Cu-rich particles. During the tempering process, the precipitation of abundant nano-sized Cu-rich particles and the fine carbides resulting from the decomposition of M/A islands are the primary factors retarding the decrease in hardness or increase the strength. In contrast, the reduction in dislocation density due to matrix softening and the decomposition of the hard M/A promote softening. The competition between these two mechanisms governs the overall evolution of hardness. • The consistent hardness obtained at the optimal tempering temperature for different holding times reflects the stability of the heat treatment process. • The evolution of precipitate size follows the Ostwald ripening mechanism, revealing the thermodynamically driven growth behavior. • The synergistic strengthening between Cu-rich phase precipitation and M/A island decomposition reveals the microstructural mechanism responsible for the increase in hardness.