Strengthening and toughening mechanism of weld metal of 690 MPa-grade high-strength steel

Two different types of weld metals with a yield strength of 690 MPa were prepared. The microstructure analysis and mechanical property tests were carried out, and the strengthening and toughening mechanism of the weld metals was revealed. The results show that the nickel equivalent (Ni_eq) of the traditional 690 MPa-grade weld metal is small, and the segregation of alloy elements in the inter-dendritic region can be negligible. The weld metal forms a relatively uniform structure dominated by coarse grain bainite (GB). Because the coarse GB has little resistance to crack propagation, its low temperature toughness is poor. A new type of 690 MPa-grade weld metal is prepared by increasing Ni_eq. The Ni_eq is relatively large, and due to the significant segregation of Mn and Ni in the inter-dendritic region, the stability of undercooled austenite in the inter-dendritic region is greater than that in the dendrite region. Therefore, a complex phase structure with acicular ferrite (AF) in the dendrite and lath martensite (LM) in the inter-dendritic region is developed. In this structure, AF is the main toughening phase, and LM is the main strengthening phase. The yield strength of the weld metal is 738 MPa, and the impact absorption energy at −50 ℃ is 122 J, which achieves a good matching between strength and toughness.