Effect of peak temperature on nanoscale Cu-rich re-precipitation behavior and strength-toughness of welding heat affected zones for Cu-bearing high strength steel

To clarify the effect of peak temperature on the Cu-rich nanoscale re-precipitation behavior and strength -toughness for welding heat affected zones (HAZs), Gleeble-3500 was utilized to simulate the HAZs for Cu -bearing high strength steel with the peak temperature of 780-1350 degrees C. 9R-Cu with the size of 5 nm precipi-tated in FGHAZ. Multi-structured Cu-rich particles precipitated in ICHAZ, including 9R-Cu with the size of 4 nm and 3R-Cu with the size of 12 nm. Phase field simulation showed that the re-precipitation behavior of Cu-rich particles was related to alpha-Fe transformation temperature. When the peak temperature was above Ac3, the increment of peak temperature decreased the alpha-Fe transformation temperature and inhibited the re-precipitation of Cu-rich particles. When the peak temperature was between Ac1 and Ac3, the increase of peak temperature promoted the re-precipitation of 9R-Cu and suppressed the coarsening of 3R-Cu in ICHAZ. Re-precipitation of Cu -rich particles provided precipitation strengthening of 132 MPa and 237 MPa for ICHAZ and FGHAZ, respectively. The high cooling rate during welding increased the grain boundary density of HAZs, which improved the strength of matrix and crack propagation energy. Therefore, the strength and toughness of HAZs for Cu-bearing steel were both higher than base metal.