Research on mechanical properties of duplex stainless steel S22053 at elevated temperature and after fire

Currently, research on the mechanical properties of stainless steel is primarily focused on austenitic stainless steel, with limited research on duplex stainless steel. To address this research gap, an in-depth investigation was conducted on the mechanical properties of duplex stainless steel S22053 at elevated temperatures and after fire. The findings of this investigation can provide practical guidance for the fire resistance design and post-fire performance evaluation of stainless steel structures. In this research, a total of 58 standard coupons were fabricated, with 4 coupons for tensile tests at ambient temperature, 18 coupons for steady-state tests at elevated temperatures, and 36 coupons for tensile tests after fire. The coupons used for steady-state tests were heated to the target temperatures and held for 15 minutes before tensile loading. The heating rate was set at 20 ℃/min. The target temperatures ranged from 100 ℃ to 900 ℃, with 9 levels in total. The strains of coupons were measured by a high-temperature extensometer. Considering the measuring range limitation, the extensometer was removed when the deformation of the coupons exceeded 5 mm. For coupons under post-fire tests, the heating and loading process were the same as the steady-state test. However, the coupons were allowed to cool down before tensile loading. During the cooling stage, the coupons were divided into two batches and subjected to two different cooling methods: water cooling and air cooling, to investigate the influence of cooling methods. Based on the tensile tests, the major mechanical parameters and stress-strain curves of duplex stainless steel S22053 are obtained. At ambient temperature, S22053 exhibits an elasticity modulus of 203.9 GPa, nominal yield strength and ultimate strength of 558.3 MPa and 784.7 MPa, and an elongation of 36.4%, indicating high strength and good plasticity. At elevated temperatures, the mechanical properties gradually deteriorate as the control temperature increases, with a significant decrease observed between 500 ℃ and 900 ℃. The coupons at 800 ℃ and 900 ℃ enter a plastic flow stage instead of a strengthening stage after reaching nominal yield strength. On the other hand, the post-fire mechanical properties of S22053 at different heating temperatures are relatively similar to each other. Only at 800 ℃, the coupons exhibit a significantly higher strength and brittleness due to the phase transformation (Tao et al., 2019),. To accurately describe the stress-strain relationships of duplex stainless steel S22053, an approach (Fan et al., 2020) based on theoretical derivation and data fitting was adopted, and the expressions for stress-strain relationships of duplex stainless steel S22053 at elevated temperatures and after fire were proposed respectively. The proposed models (Gardner et al., 2010; Rasmussen, 2003) were validated by compared with existing models. It was found that the proposed models performed good prediction for the mechanical properties of S22053 at elevated temperatures and after fire. The reduction formulas for six major mechanical parameters (E0, σ0.05, σ0.2, σ1.0, σ2.0 and σu) of duplex stainless steel S22053 under two fire conditions were obtained by recalibrating the fitting parameters of the prediction equation proposed by Chen & Young (Chen and Young, 2006). The existing models and Eurocode 3 were also evaluated, and there were certain deviations from the test results. It was found that the strength values of S22053 at elevated temperatures changed insignificantly between 200 ℃ and 500 ℃, which was attributed to the “475 ℃ embrittlement” (Alhegagi, 2015) phenomenon. Regarding the post-fire condition, the reductions in initial elasticity modulus and ultimate strength are negligible, while the nominal strength tends to slightly increase from 200 ℃ to 500 ℃, with a maximum increase of 10%, followed by a rapid decrease with a maximum decrease of 15% when reaching 800 ℃. In addition, it can be seen that different cooling methods (water cooling and air cooling) have little affected on the mechanical properties when temperature does not exceed 600 ℃. Finally, reliability analysis was conducted by the Kolmogorov-Smirnov (K-S) test. The results showed that all the six major mechanical parameters satisfy the assumption of a normal distribution at a significance level of 0.05. The six corresponding standard values are proposed, in which E0 is 203.9 GPa, σ0.2 is 527.4 MPa and σu is 757.6 MPa.