Mechanical properties of heat-treated high strength steel under fire/post-fire conditions

• Mechanical properties of high strength steel at elevated temperatures are reported.
• Post-fire residual strength of heat-treatment hardened high strength steel is studied.
• Heat-treated hardened steel return to its parent material after fire exposure.
• A new reduction factor for post-fire residual strength assessment is proposed.

Heat treated high strength steel (HSS) has attracted more and more applications in both on-shore and off-shore structures. According to the classical theories of metallurgy, the hardening effects in the microstructures that grant HSS its high strength may be weakened or eliminated by heat. Structures built using such materials may behave differently after they are heated and cooled down (e.g. during and after a fire situation). This experimental study investigated the mechanical properties of reheated, quenched and tempered (RQT) HSS plate manufactured to grade S690 at elevated temperatures as well as its residual strength after heating and cooling down. It is found out that while RQT-S690 HSS showed good resistance to low temperature (below 400 °C) heating, serious deterioration in strength would occur at higher temperatures and its post-fire mechanical properties are different from those of traditional hot-rolled normal strength steels.