Low-temperature effects on mechanical properties of Q690E butt-welded joints

An experimental investigation was conducted to investigate the low-temperature (ranging from −80 °C to 25 °C) effects on the mechanical performance of Q690E butt-welded joints fabricated with various heat inputs (ranging from 1.01 kJ/mm to 2.02 kJ/mm). The effects of heat input during welding and subsequent exposure to low temperatures on mechanical characteristics were systematically evaluated through tensile tests. Optical microstructural (OM) and environmental scanning electron microscopy (ESEM) characterizations were systematically performed to reveal the fracture mechanisms of the Q690E butt-welded joint considering low-temperature and heat-input influence, respectively. The experimental results demonstrated that enhanced heat input resulted in a notable degradation in strength, while accompanied by enhanced ductility characterized by ultimate strain at varying low temperatures. Notably, quantitative analyses demonstrated that low temperature exerted a pronounced increase in both strength and ductility of the Q690E butt-welded joints fabricated by different heat inputs. The controlled cooling from ambient temperature to an extreme low-temperature −80 °C, led to significant 18% and 12% enhancements in yield and ultimate strength, respectively. Meanwhile, the ductility, characterized by the ultimate strain, exhibited an enhancement of approximately 48% across the butt joints fabricated with heat-input levels adopted in this study. A heat input of 1.0 kJ/mm enabled an optimal strength-ductility synergy for the Q690E butt-welded joint under low temperature environments. OM observations indicated that increasing heat input led to the coarsening of bainitic microstructures and promoted the development of a ferrite-bainite mixed microstructure in the CGHAZ. ESEM characterization further revealed that increasing heat input shifted the fracture mode from layered micro-void coalescence to a more homogeneous micro-void coalescence mechanism. In contrast, decreasing temperature promoted localized shear deformation characterized by multidirectional tearing ridges, reduced the extent of layered void coalescence, and resulted in a more localized ductile fracture morphology, which inhibited layered void coalescence and microcrack propagation, and enhanced ductility. Based upon the experimental results, the yield and ultimate strengths accounting for the heat-input and low-temperature effects with validated accuracy were formulated for butt-welded joints with nominal yield strength of 690 MPa.