Microstructural Evolution and Fatigue Behavior of Laser-Welded Joints in Air-Cooled Steel

Air-cooled steel is a high-strength steel widely used in automotive subframe applications. In this study, the microstructural evolution and fatigue performance of air-cooled steel welded joints subjected to a one-step heat treatment and an additional two-step heat treatment were systematically investigated. The results indicate that, after the one-step heat treatment, the microstructure of the welded joints transformed from coarse lath martensite to a mixture of tempered martensite and newly formed martensite. After the two-step heat treatment, the microstructure of the welded joints evolved from coarse lath martensite to newly formed martensite, M-A islands, bainitic ferrite, and a small amount of polygonal ferrite. Under both heat treatment conditions, the fatigue limits of the welded specimens were lower than those of the base metal, which can be attributed to the reduced overall deformation compatibility induced by the welded joints. In addition, the welded joints exhibited superior crack propagation resistance compared with the base metal after both heat treatment processes, which is likely related to the enhanced ability of dislocation structures and grain boundaries to impede crack propagation.