Improvement of mechanical properties for weld heat affected zones of low-carbon ultra-high strength steel by multi-structured Cu-rich precipitation via Al adding

In order to investigate the influence of Al addition on the precipitation behavior of multi-structured Cu-rich particles and strength-toughness for the HAZs of low-carbon ultra-high strength steel, the HAZs of Al-free and Al-bearing steels subjected to peak temperatures ranging from 780°C to 1350°C were systematically investigated through microstructural characterization, mechanical properties testing and PFM simulation. The addition of Al significantly promoted the precipitation of multi-structured Cu-rich particles, such as B2-Cu and 9R-Cu, in ‌ICHAZ and FGHAZ of Al-bearing steel. The size and volume fraction of B2-Cu were 2-3 nm and 1.4-1.8%, respectively. PFM results shows that Al atoms were expelled from Cu-rich particles during the continuous cooling process of HAZ, which accelerated the enrichment and nucleation of Cu atoms. The Al addition increased the α-Fe transformation start temperature, broadening the temperature range for the Cu-rich precipitation. The precipitation of multi-structured Cu-rich particles in ICHAZ and FGHAZ contributed the strength enhancement of 424 MPa and 758 MPa, respectively. The yield strength and -40°C impact toughness of HAZs were higher than 1000 MPa and 60 J, achieving the good balance between strength and toughness.