Insights into microstructure evolution and fracture mechanisms of welded joints of high strength steel patchwork components

The hot roll bending patchwork components of B1500HS uncoated ultra-high strength steel processed at industrial scale were welded with filler wire. The microstructure, grain orientation, mechanical properties and fracture behavior of the joint were investigated. The results showed that the weld metal (WM) was mainly composed of lath martensite and lower bainite (LB). Various microstructure appeared in different positions of the HAZ, among which martensite-austenite (M-A) constituents in the incompletely quenched zone and tempered martensite (TM) in the tempered zone were the key factors determining the performance of the joint. The welded joint exhibited complex structure and anisotropic mechanical response due to the decarburization and rolling texture of the original base material (BM). In the tempered zone, lath merge and boundaries disappear lead to the annihilation of dislocations, carbide precipitation leads to reduction of mobile dislocations, and recrystallization and recovery eliminate work hardening. The reduction of the effect of dislocation strengthening results in the formation of a softening belt at 6 mm from the center of the weld, with a minimum hardness of 266 HV. The tensile strength of the joint in the rolling direction (RD) was 983 MPa, and the fracture occurred in the softening belt. Both brittle and ductile fracture features were observed. The reason why the fracture occurs in the high temperature tempered zone instead of the incompletely quenched zone was the difference in the effect of "constraint strengthening". Fracture occurred through delamination cracking along the "weak interface" owing to precipitated high-hardness cementite particles and the unfavorably oriented ferrite cleavage planes.