LAUSR

Abstract Q460 and Q690 are two most typical high strength steels applied extensively in industrial construction, mechanical engineering and other engineering structures due to their cost efficiency, high strength and toughness. Their residual mechanical performance after fire accidents is a crucial indicator for an appropriate post-fire safety evaluation. In addition, water has long been a universal agent for fire suppression, however it is typically ineffective on chemical or oil fires, and can be dangerous. Therefore, the post-fire mechanical properties of both Q460 and Q690 high strength steels cooled by fire-fighting foam from seven different high temperatures are investigated in this study. Test specimens are initially subjected to an elevated temperature ranging from 200 to 900 °C to simulate various fire conditions, and then these heated specimens are cooled to ambient temperature by fire-fighting foam. Subsequent tensile tests are performed on the fire-affected specimens to observe the failure mode, and obtain the stress-strain curve and other associated mechanical performance parameters. The visual appearances of Q460 and Q690 high strength steel tensile coupons are clearly changed after being heated and cooled by fire-fighting foam. In the case of fire-fighting foam cooling, the post-fire mechanical properties (except for the elastic modulus) of Q460 and Q690 high strength steels change drastically after exposure to temperatures above 600 °C (up to 900 °C). When exposed to the temperature of 900 °C, Q460 and Q690 high strength steels are able to regain 67% and 64% of their initial ultimate strength in the case of fire-fighting foam cooling, respectively. In contrast, the ductility of Q460 and Q690 high strength steels cooled from 900 °C by fire-fighting foam can gain 139% and 152% of their initial ductility, respectively. Meanwhile, the micro-fracture behavior is also observed to understand the effects of elevated temperatures and fire-fighting foam cooling on the post-fire mechanical properties. By comparing the results of previous studies on other two cooling methods, significant differences in the post-fire mechanical properties of Q460 and Q690 high strength steels cooled by different methods are discussed. At last, the predictive equations are developed to determine the post-fire mechanical properties of fire-affected Q460 and Q690 high strength steels when the fire-fighting foam cooling method is applied in fires.