Cooling-Regime Effects on the Residual Mechanical Properties, Hardness and Fracture Characteristics of Fire-Exposed Hardox Abrasion-Resistant Steels

This study examines how cooling history affects the residual properties of 4-mm-thick Hardox 450 and Hardox 500 Tuf abrasion-resistant steels after thermal exposure. The programme includes an unheated reference at 23 °C and exposures to 300, 400, 500, 600, 800 and 1000 °C, followed by air cooling (AC), water spray cooling (WSC) or blanket cooling (BC). Room-temperature tensile tests and Vickers microhardness measurements characterize the residual response; X-ray diffraction (XRD) and scanning electron microscopy (SEM) examine the reference and 800 °C conditions. Each hardness value represents the mean of five indentations. Both grades retain most of their reference ultimate tensile strength at 300 °C, whereas strength loss becomes pronounced at 500–600 °C. After 1000 °C exposure, WSC specimens retain approximately 52.5% and 79% of the reference ultimate tensile strength for Hardox 450 and Hardox 500 Tuf, respectively, with strains at maximum stress of 3.50% and 3.79%. Local post-tensile hardness remains substantially below the reference level. The results therefore show that recovery of tensile strength does not establish restoration of the original property balance. XRD and fractography support qualitative interpretation at 800 °C but do not verify the transformation mechanism at 1000 °C. Continuous piecewise linear equations describe the measured trends within the investigated conditions; their predictive accuracy and between-specimen variability remain unestablished.