Evaluating Post-fire Mechanical Behavior of Ultra-high Strength (Grade1200) Steel Tubes

Abstract

This paper presents an experimental study on the post-fire mechanical behavior of ultra-high strength steel (UHSS) tubes with nominal yield strength of 1200MPa. In order to understand the changes occurring during cooling phase of a fire, the in-fire mechanical behavior of UHSS is also studied experimentally. The temperatures characteristic of fire simulated in this study are within the range of 300ºC to 800ºC. In the experimental tests, standard dog-bone UHSS specimens are subjected to quasistatic tensile tests at fire temperatures and after being cooled to room temperature. The stress-strain curves, strength and ductility of the tested specimens are discussed. To investigate the effect of steel grade on the post-fire mechanical response, the strength deterioration of different grades of steels available in literature including grades of 460, 690, 800 and 960, are compared with those of UHSS (Grade 1200). Finally, the obtained results are explained based on the phase changes occurring in the steel material during fire by calculating the thermodynamic stability of the ferrite and cementite phases in the tested materials.