Decisive effect of SSPT on post-fire mechanical properties of structural steel

The heating and cooling processes during a fire essentially constitute a simple heat treatment for structural steels, in which solid-state phase transformation (SSPT) often plays a decisive role in developing their mechanical properties. However, the post-fire mechanical properties of structural steels have rarely been studied following the general principles of heat treatment. The mechanical properties of Q355D steel after simulated fire exposure were investigated in this study. The steel was heated to target temperatures of 600°C, 700°C, 800°C, and 900°C in a furnace. Two cooling methods, cooled in air and cooled in water, were adopted respectively to investigate the influence of the cooling rate. Metallographic examination showed that the SSPT occurring in Q355D steel during the simulated fire exposure follows the general principles of heat treatment. Tension coupon tests were carried out for post-fire Q355D steels. It was found that the SSPT during the simulated fire exposure has a decisive effect on post-fire mechanical properties of Q355D steel. Post-fire Q355D steels significantly hardened only when the target temperature reached Ac1 and the steels were cooled rapidly in water. To verify the generality of these findings, results of fire tests on various steels in the literature were collected and analyzed. Variations in yield strength and tensile strength of all these steels after simulated fire exposure can be well explained by general principles of heat treatment of hypoeutectoid steels. When no SSPT occurs during simulated fire exposure, the post-fire tensile strength of these steels varies within 15 %. For mild and cast steels, the SSPT induces significant hardening of the post-fire steels, typically increasing the post-fire tensile strength by over 20 %. For high-strength and ultra-high-strength steels, the SSPT may lead to varying degrees of post-fire hardening or softening, depending on the peak temperature, cooling rate, and the fractions of different phases in both the original and post-fire steels. The decisive effect of SSPT caused by the heating and cooling processes during simulated fire exposure on post-fire mechanical properties of structural steels was confirmed.