Strengthening and toughening mechanism of coarse-grained heat-affected zone for the Cr-/Mo-free Cu-bearing HSLA steel under high heat input welding conditions

In this study, Cr-/Mo-free Cu-bearing steel with the addition of 1.5 wt% Cu was designed to improve the strength and toughness of the coarse-grained heat-affected zone (CGHAZ) at high heat input. A comparative test with traditional EH690 steel was conducted to analyze the strengthening and toughening mechanism. As heat input increased from 50 to 200 kJ/cm, microstructure of the CGHAZ for Cu-bearing steel transformed from lath/granular bainite to quasi-polygonal ferrite, while EH690 steel contained granular bainite under these conditions. The phase field simulation results showed that the increase in heat input promoted the diffusion of Cu and precipitation of nanoscale Cu-rich particles. At heat inputs of 100 kJ/cm and 200 kJ/cm, Cu-rich particles with sizes of 3.8 nm and 7.5 nm precipitated in the CGHAZ, providing precipitation strengthening increment of 47 MPa and 167 MPa, respectively. The precipitation strengthening of nanoscale Cu-rich particles compensated for the strength loss caused by the ferritic transformation of Cu-bearing steel. Low carbon and Cr-/Mo-free for Cu-bearing steel reduced the content of M-A constituents, leading to the increase in crack propagation energy and toughness. Under heat input of 200 kJ/cm, tensile strength, yield strength and impact energy at -40 °C of CGHAZ for Cu-bearing steel were 975 MPa, 837 MPa and 63.7 J, respectively, exhibiting the good mechanical properties under high heat input condition.