Precipitation mechanism of nanoscale Cu-rich particles in restrained welding conditions and its effect on the mechanical properties of HAZs

•Constraint tensile stress increased the α-Fe transformation start temperature of HAZs.•Constraint tensile stress increased the volume fraction and average size of Cu-rich particles in HAZs.•The increment of α-Fe transformation start temperature promoted the precipitation of Cu-rich particles in α-Fe matrix.•The tensile strength of FGHAZ increased from 970 MPa to 1032 MPa due to the nanoscale Cu-rich precipitation strengthening.