Influence of heat input on the changes in the microstructure and fracture behavior of laser welded 800MPa grade high-strength low-alloy steel

Abstract

High-strength low-alloy (HSLA) steels with a yield strength of 800 MPa and a thickness of 8 mm were subjected to laser welding to address the problem of loss of strength in the welded joint. The influence of the heat input of the laser on the microstructure and the resultant mechanical properties of the joint were thoroughly investigated. The results show that the phases present in the fusion zone (FZ), the morphology and amount of martensite/austenite constituents, and the grain size and width of the heat affected zone (HAZ) vary with respect to the heat input, which consequently influences the mechanical properties of the joint. In this work, an excellent tensile strength of the base metal of 98 % is obtained at a heat input of 0.39 kJ/mm, whereas a low tensile strength of the base metal of 77 % is obtained at a heat input of 0.78 kJ/mm. The location of fracture changes from the FZ to the HAZ when the heat input is reduced from 0.78 kJ/mm to 0.39 kJ/mm. In addition, a large amount of martensite/austenite constituents with a long chain morphology is observed in the FZ for a moderate heat input, which results in the low impact toughness of the joint, which is 44.1 J at a heat input of 0.56 kJ/mm. In comparison, reducing the heat input to 0.39 kJ/mm or increasing it to 0.78 kJ/mm tends to increase the impact energy to beyond 54.0 J. Therefore, a low heat input is necessary for laser welding HSLA steel with 800 MPa yield strength.