Experimental study of the post-fire mechanical properties of Q690 high strength steel

Abstract

In this paper, the post-fire mechanical properties of Q690 steel obtained experimentally are presented and empirical equations to estimate the post-fire elastic modulus, yield strength, tensile strength and fracture strain are proposed. The variables considered in the present study include the level of temperature exposure (from 300 °C to 900 °C), the cooling methods (natural air and submersion in water) used, the effects of different heating rates, repeated heating and cooling, and different initial loading conditions. The test results show that while the post-fire elastic modulus is not too sensitive to the exposed temperature level and the manner of cooling, it decreases about 10% when a higher initial heating rate, repeated heating and cooling, or a load is applied to the specimen. The post-fire yield strength tends to decrease with the exposed temperature level when the temperature reaches 400 °C if the air cooling method is used and 500 °C if the water quenching method is used. Further reduction in yield strength occurs when the specimen is subjected to a higher initial heating rate, repeated heating and cooling, or an applied load. The post-fire tensile strength does not show significant variations if air cooling is used but for specimens heated to a temperature above 700 °C and rapidly cooled by submersion in water, noticeably higher post-fire tensile strength is observed as a result of the formation of martensite. Martensite formation also reduces the ductility (as measured by the fracture strain) of steel heated above 700 °C and cooled suddenly.