TiC-induced nanoscale Cu-rich re-precipitation mechanism under welding thermal cycles and its effect on the strength-toughness of heat affected zones

In order to solve the softening problem of welding heat affected zone (HAZ) for Cu-bearing steel caused by the dissolution of Cu-rich particles, TiC was utilized to induce the Cu-rich re-precipitation. With the peak temperature increased from 780°C (ICHAZ) to 950°C (FGHAZ), the size of 9R-Cu increases from 4nm to 5nm, accompanied with the increment of volume fraction from 0.6% to 1.0%. Phase field simulations indicate that TiC repels Cu atoms into γ-Fe matrix and promotes the formation of Cu-rich clusters. Then, Cu-rich clusters provide more nucleation sites for Cu-rich particles, which contributed to the re-precipitation of refined 9R-Cu. Due to the disappearance of coarse grain boundary Cu-rich particles and formation of fresh lath bainite, re-precipitated Cu-rich particles provided precipitation strengthening value of 237MPa and 298MPa without decreasing the low temperature toughness for ICHAZ and FGHAZ. The yield strength and absorbed energy at -40°C of HAZs exceed 1000MPa and 120J, respectively.