Post-fire mechanical response of ultra-high strength (Grade 1200) steel under high temperatures: Linking thermal stability and microstructure

Recently, ultra-high strength steel (UHSS) tubes with nominal yield strengths of up to 1200 MPa have attracted attention for applications in engineering fields. While many studies have focused on the mechanical behaviour of mild carbon steel at elevated temperatures, there is a scarcity of data for the in-fire and post-fire mechanical response of the UHSS material. In this study, the tensile mechanical properties of the UHSS tube under fire and after cooling from fire temperatures of up to 800 °C to room temperature are studied. The stress-strain curves, strength and ductility of the UHSS material are discussed. It is shown that the in-fire strength of the UHSS tube starts to deteriorate when the specimens are exposed to fire temperatures above 300 °C and is almost disappeared when tested at 800 °C. There is also a major reduction in the strength of the UHSS tube specimens cooled from fire temperatures above 470 °C to room temperature. To investigate the effect of steel grade on the in-fire and post fire mechanical behaviour of steel materials, the stress-strain curves of Grade 800 high strength steel (HSS) tube specimens are presented and compared with those obtained for Grade 1200 UHSS tube. In order to interpret the experimental results, microstructural examination on the UHSS is conducted using optical and scanning electron microscopy (SEM). The plots of the thermodynamic stability of the ferrite and cementite phases in the UHSS and HSS are calculated and the phase changes occurring during each fire temperature exposure are discussed. Based on the results obtained from experimental tests, an empirical constitutive model which takes into account the post-fire behaviour of UHSS material is developed. The constitutive model can be implemented into commercial finite element packages to carry out a rational thermal analysis and perform fire safety design and evaluation.