The Role of HAZ characteristics on Low-cycle Fatigue Properties of Welded DP780 Steels

Abstract

Mechanical properties of a welded part usually are deteriorated due to its susceptibility to crack formation, especially in the heat affected zone (HAZ). Therefore, it is important to control the size, shape and the microstructure of the HAZ in order not to degrade the mechanical properties of the welded structures. Microstructure and hardness of the HAZ are greatly influenced by various factors such as heat input capacity, cooling rate and initial microstructure of the steel. The aim of this study is to evaluate the effect of welding methods on the tensile and low-cycle fatigue properties of the HAZ in welded DP780 steels. In this study, three welding methods with different heat input capacity were utilized: Laser welding, tungsten inert gas (TIG) welding, and metal active gas (MAG) welding. In all welded materials, fatigue resistance was inferior to that of the base material. Fatigue life was comparable in Laser-welded and TIG-welded specimens, and was greater in both than in MAG-welded specimens. Fatigue cracks of Laser-welded specimen initiated in the base metal far from the welded zone because the HAZ was very thin and hardness of the HAZ was similar to that of the base metal. Fatigue cracks of TIG-welded and MAG-welded specimens were observed to initiate in the thick HAZ region, where hardness was much lower than that of the base metal.