Effects of High-Power Laser Heat Input on the Mechanical Properties of GPa-Grade Advanced High Strength TRIP Steel

Abstract

Welding and local heat treatment with a high-power laser for GPa-grade transformation-induced plasticity (TRIP) steel were studied. When the focal length and scan speed were controlled under a fixed laser power, defect-free full penetration conditions were achieved at various heat inputs. The effects of laser heat input on the mechanical properties of the steel were evaluated. The results showed that a softened zone was lowered to 350 HV of hardness by tempered martensite under all laser conditions. As the heat input increased, the zone became wider. When an appropriate heat input of 20-60 mm/J was irradiated, the tensile strength and elongation were improved as compared with the base material. Under these conditions, the softened zone was significantly narrowed by the TRIP effect under tensile stress, and base metal fracture occurred. In addition, the laser increased the hardness in the weld metal and further activated the TRIP effect of the retained austenite. However, at a heat input of 90 J/mm, the TRIP effect was limited in the tensile test due to the wide softened zone and the increased hardness of sub-critical heat affected zone during the laser process. In this study, the effective heat input of the laser formed a full-penetration joint and locally enhanced strength and elongation simultaneously.