Mechanical properties of ultra-high strength (Grade 1200) steel tubes under cooling phase of a fire: An experimental investigation

There has recently been a growing trend towards using ultra-high strength steel (UHSS) in many engineering applications. However, few researches have focused on the mechanical properties of this kind of steel at elevated temperatures. In this study, the mechanical properties of UHSS at temperatures characteristic of fire and after cooling from fire temperatures, are studied experimentally. The specimens taken from UHSS tubes are subjected to fire temperatures of up to 600 °C and tensile tests are carried out both at elevated temperatures and after the specimens were cooled to room temperature. As expected, the strength of the UHSS specimens decreases significantly when tested under fire temperatures of 450 °C and 600 °C. However, the strength of the UHSS is also considerably reduced after cooling down from high fire temperatures to room temperature. The stress-strain curves, strength and ductility of the UHSS tube specimens are discussed. Furthermore, in order to perform a comparison study, the stress-strain curves for three different grades of steel tubes including UHSS, high strength steel (HSS) and Mild steel (MS) tubes are presented and compared. It is shown that the reduction in the strength of the UHSS after cooling from fire temperatures of up to 600 °C does not occur to the same extent for HSS and MS steels. The effect of the cooling rate after exposure to fire temperatures on the mechanical properties of UHSS tube specimens is also investigated. Micro-structure examination is conducted using optical and scanning electron microscopy (SEM) and the room temperature strength reduction in the UHSS after exposure to the fire temperatures is discussed in terms of the effect on the steel microstructure. A recommendation has been made for separating studies of the effect of simulated fire temperatures on the residual strength of steel into two classes (low and high temperature), depending on whether a critical maximum temperature (which depends on alloy composition) is exceeded and the science underlying this recommendation has been discussed.