Mechanical response of ultra-high strength (Grade 1200) steel under extreme cooling conditions

Ultra-high strength steels (UHSS) with nominal yield strength of up to 1200 MPa have many potential applications in engineering fields. However, due to inadequate knowledge on the mechanical behaviour of these materials under extreme loading conditions (e.g. severe temperature changes), their extent of application in civil construction has remained restricted. This paper characterises the mechanical properties of Grade 1200 UHSS under extreme cooling conditions, which is defined as cooling from fire temperatures with an extreme cooling rate or cooling from ambient state to sub-zero temperatures. In order to simulate the extreme cooling rate, UHSS standard dog-bone specimens are heated up to different fire temperatures and cooled to room temperature using water-quenching (WQ) technique. To evaluate the effect of extreme cooling rate on the post-fire mechanical response of the tested materials, the residual mechanical properties of water-quenched specimens are compared to the air-cooled (AC) ones for different fire temperature exposures. For simulation of extreme cooling temperatures, the test specimens are cooled to sub-zero temperatures using Liquid Nitrogen (LN) injection. The changes in the mechanical properties of the tested specimens are then quantified and discussed. In both set of experiments, the mechanical behaviour of UHSS under extreme cooling conditions is compared to the common structural steels. Microstructural evaluation of UHSS tested specimens are also presented using optical and scanning electron microscopy (SEM) techniques.