Experimental investigation into mechanical properties of Q345 steel after fire

Abstract

The fire hazard has been considered one of the prominent risks for steel structures. After cooling from high temperatures, the mechanical properties of steel can be changed, which has a great influence on structural safety. Therefore, assessing the performance of steel structures after a fire necessitates a profound understanding of the post-fire mechanical properties of steel. Q345 steel is the most commonly used steel grade in China. In this research, an experimental study was undertaken to investigate the post-fire mechanical properties of Q345 steel. Coupon tests of base metal and weld were carried out with the maximum temperatures elevated to 1000 °C. Two cooling methods were adopted, including cooling in air and water. Post-fire performance was obtained and compared, such as the appearance, stress-strain curve, elastic modulus, yield strength, ultimate strength, percent elongation and percent reduction of area. It was found that for base metal, when the temperatures exceeded 600 °C, cooling in water promoted the strength notably but reduced the ductility significantly, while cooling in air increased the ductility. Moreover, the ultimate strength of base metal cooling in water was much higher than that of weld with temperatures above 800 °C, and the deterioration of plastic deformation ability of weld was more severe than in the base metal. Based on the results, a set of predictive formulas is proposed for the variation factors of the post-fire mechanical properties for the Q345 base metal as well as for the weld, helping this way the engineers in evaluating and renovating fire-damaged steel structures.