Characterizing local distribution of microstructural features and its correlation with microhardness in resistance spot welded ultra-low-carbon steel: Experimental and finite element characterization

Abstract

This study investigated the influence of microstructural features on the heterogeneous distribution of hardness in resistance spot welded (RSWed) 340BH steel. The microstructure and texture across the weld region was investigated using optical microscopic and electron backscattered diffraction (EBSD) techniques. Finite element analysis (FEA) was used to simulate the RSW process, which helped in predicting the nugget dimensions as well as the cooling rate in different weld regions. Rapid cooling in the fusion zone (FZ) led to the formation of martensite with typical packet and block substructures. Numerical reconstruction of the prior austenite grain (PAG) was employed to understand the orientation relationship between martensite and austenite. The results showed that child laths obey a Kurdjumov-Sachs orientation relationship (K-S OR) with respect to the austenite matrix with a slight deviation (~3° angular deviation) from a conventional K-S OR. Moreover, microhardness was measured across the weld zone. An increase in the hardness was observed throughout the weld zones. The hardness increase in the heat-affected zone (HAZ) was due to a finer (smaller) grain size and an increase in the dislocation density, whereas in the FZ, the improvement in hardness was attributed to the phase change during the solidification process. The microhardness of martensite formed during RSW was strongly dependent on the block size according to the Hall-Petch relationship. • A heterogeneous distribution of microstructure and hardness in RSWed 340BH steel was characterized using EBSD and FEA. • Lower ZHP values and fast kinetics of dynamic recrystallization resulted in fine structure of ferrite grains in ICHAZ. • FZ hardness was mainly driven by martensite formation, while grain size distribution and dislocation density were mainly responsible for higher hardness in HAZ. • The block size in the martensite substructure followed Hall-Petch type relationship with microhardness of lath martensite.