Cooling effects on post-fire tensile properties of S235 and S355 steels

This study experimentally investigates the combined effects of elevated temperature and cooling protocol on the residual tensile behavior of S235 and S355 structural steels. A total of 198 tensile coupon tests were conducted on 3.0-mm-thick specimens exposed to ISO 834-based heating at target temperatures ranging from 100 to 800 °C, followed by four cooling protocols: air cooling, foam cooling, water spray cooling, and water immersion cooling. Yield strength, ultimate tensile strength, and ultimate elongation were evaluated together with cooling-sensitive reduction factors. The results showed that both steels retained their tensile properties with only minor fluctuations up to 500 °C, regardless of cooling method. Three-way analysis of variance confirmed significant Temperature × Cooling interactions for yield strength, ultimate tensile strength, and ultimate elongation ( p < 0.001 ), with cooling-related differences becoming systematic from 600 °C onward. Air, foam, and water spray cooling produced progressive strength degradation with increasing temperature, while generally increasing deformation capacity. At 800 °C, water spray cooling caused the most severe average strength reduction, with reduction factors of 0.85 for yield strength and 0.80 for ultimate tensile strength. In contrast, water immersion cooling produced apparent strength increment, reaching average reduction factors of 1.16 and 1.12 for yield and ultimate tensile strength, respectively; however, this improvement was accompanied by a substantial loss of ductility, with the elongation factor decreasing to 0.69. These findings demonstrate that higher residual strength after rapid cooling should not be interpreted as improved post-fire performance without considering deformation capacity. The proposed reduction factors provide an experimental material-level basis for cooling-sensitive post-fire assessment of comparable low-carbon structural steels.