Characterization of Mechanical and Metallurgical Notch Effects of DP980 Steel Weld Joints in Fatigue Performance

This work investigates the use of 980 MPa grade dual-phase (DP) steel joints with pulsed current gas metal arc welding (P-GMAW) for reliable weld performance by comparing actual welded joints with thermo-mechanical simulations. We focus on the weldment characteristics that can cause mechanical and metallurgical notch effects, which impact the fatigue performance of welds. For this purpose, we conduct experiments investigating the use of two types of filler metals and three heat inputs as variables. We find that HAZ regions in the actual weldments have varied microstructures caused by different phase transformations of martensite corresponding to various peak temperatures experienced. We confirm that improved weld geometry plays a key role in enhancing fatigue performance. We identify and discuss the degradation of mechanical properties arising from increasing heat input, in terms of transformation kinetics investigated by means of HAZ thermo-mechanical simulations and thermodynamic calculations. This study provides an overall understanding of the welded joints of DP steel and suggests a way to improve weldability by adjusting the mechanical and metallurgical parameters of various regions in the weldments.