Experimental Study on the Mechanical Degradation of Q420C Steel Welded Joints after Simulated Fire Exposure

Abstract

This study investigates the evolution of toughness in Q420C low-alloy steel after simulated fire exposure. Specimens were heated to 200, 400, 600, and 800°C for 30 minutes, followed by either natural cooling or water quenching. Charpy V-notch tests were conducted over a temperature range of −60 to 20°C. Toughness exhibited a three-stage trend: a significant decline at 400°C, substantial improvement at 600°C (39–53% higher than at 400°C), and sharp deterioration at 800°C The heat-affected zone consistently outperformed the weld zone in toughness, while cruciform butt-welded joints showed inferior low-temperature performance, with the weld zone being most sensitive. Notably, water quenching at 400°C reduced embrittlement, whereas air cooling yielded better toughness at 600°C and 800°C, indicating a temperature-dependent effect of cooling methods on the post-fire fracture resistance of welded joints. Scanning electron microscope analysis revealed ductile dimples in the 600°C group, while the 400°C and 800°C groups were dominated by cleavage features. The cruciform butt weld zone exhibited the most severe cleavage characteristics at low temperatures.