In-fire and post-fire mechanical properties of high nitrogen and low nickel stainless steel S35657

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Stainless steel S35657, a relatively new type of steel material, has been increasingly utilized in construction in recent years. It has the advantages of high strength and low production cost compared to other austenitic stainless steel, due to increasing the nitrogen content (0.235%), reducing the nickel content (3.03%). Fire hazards are normally destructive for steel structures, as both stiffness and strength of steel materials decrease dramatically at elevated temperatures. Investigating their in-fire and post-fire mechanical properties provides evidence for its fire resistance design and post fire repair and reinforcement, which can reduce economic losses caused by fires and improve the sustainability of the building environment. Overall, current research on the in-fire (Rasmussen 2003, Chen et al. 2006, Gardner et al. 2010, Fan et al. 2020 and Suo et al. 2021,) and post-fire (Wang et al. 2014, Huang et al. 2017, 2018, and Tao et al. 2019) mechanical properties of stainless steel is primarily focused on ordinary stainless steels, with limited studies on stainless steel S35657, and there are no refine equations to predict stress-strain relationship of stainless steel S35657 in-fire and post-fire conditions. Hence, there was an eminent need to investigate the deterioration and residual mechanical properties of stainless steel S35657 at high temperatures and after exposure to high temperatures. In this study, in-fire and post-fire mechanical performance tests were conducted on stainless steel S35657. One hundred and nine standard coupons were fabricated, comprising 5 coupons for tensile tests at ambient temperature, 32 coupons for steady-state tests at elevated temperature, and 72 coupons for tensile tests post-fire. In steady-state tests, the effect of exposure temperature (ranging from 20 °C to 1000 °C) and tensile rate (0.5 mm/min, 1.0 mm/min and 3.0 mm/min) on mechanical properties was investigated. In post-fire tests, the exposure temperature (ranging from 100 °C to 1000 °C), heat soak duration (20 min, 60 min and 180 min), and cooling mode (furnace cooling, air cooling and water cooling) were considered. According to steady-state tests, it can be found that as the control temperature increases, the mechanical properties deteriorate continuously. When the temperature exceeds 600 ℃, there is a rapid decline in mechanical properties. When the temperature is controlled below 700 ℃, the curve exhibits distinct ascent stage, strengthening stage, and decreasing stage. However, when the temperature exceeds 700 ℃, the strengthening stage of the curve disappears, and the coupon enters the plastic flow stage. As the tensile rate increases, both the yield strength and ultimate strength of the In-fire and post-fire mechanical properties of high nitrogen and low nickel stainless steel S35657 Xu International Conference on Fire Safety Engineering Research and Practice, 24-27 Nov 2024, Sydney, Australia Page 98 coupon increased gradually. This phenomenon may be attributed to the high-temperature creep effect (Fan et al. 2020). In the post-fire test, the stress-strain curve of the coupon with different cooling methods is similar, manifested as the elastic stage, strengthening stage, horizontal stage (when the strain reaches a certain value, the stress tends to stabilize), and decreasing stage. It can be observed that there is no significant change in the variation curve of each coupon at a temperature of 20-900 ℃. After reaching a temperature of 1000 ℃, the stress-strain curve of the coupon showed a significant change, which was significantly lower than other temperatures. The post-fire stress-strain curves for heating soak duration of 20, 60 and 180 min generally coincided with each other. Previous studies (Huang and Young 2017, 2018) have demonstrated that the soaking time has insignificant effect on post-fire mechanical properties for ferritic stainless steel and lean duplex stainless steel. Analogously, a similar conclusion can also be drawn regarding stainless steel S35657. In addition, the mechanical properties encompassing modulus of elasticity, proof yield strength, ultimate strength of stainless steel S35657 in-fire and post-fire were reported, and corresponding reduction factor were obtained. Based on the test results, it was found that the existing equations cannot accurately predict the mechanical properties of S35657 stainless steel in fire and after exposure to fire. Therefore, new predictive equations were proposed to predict mechanical properties for stainless steel S35657 in-fire and post-fire conditions. The predicted results obtained from the established equations are highly consistent with the test data, which can provide references for the evaluation of residual mechanical property of stainless steel S35657 structures.

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