Experimental investigation of the residual mechanical properties of cast steels after exposure to elevated temperature

Cast steel is extensively used in civil engineering, especially for complex joints in spatial steel structures. Provided that local or global structural collapse of structures that incorporate cast steel structural elements and joints does not occur during or immediately after a fire event, the post-heating residual behavior of the steel castings in steel structures must be accurately characterized to estimate the remaining structural capacity and resulting safety. An experimental investigation was undertaken to explore the post-heating residual mechanical properties of two widely used structural cast steels, namely G20Mn5N and G20Mn5QT. Tensile coupon tests were performed on specimens after exposure to one of 13 preselected maximum temperatures (up to 1000 °C). Both air cooling and water cooling methods were considered, and residual mechanical properties, such as stress-strain curves, elastic modulus, yield strength, ultimate strength, and fracture strains were obtained for both types of cast steel. Additional tests were undertaken to investigate the effects of cyclic heating and cooling. The mechanical properties of both cast steels began to change after exposure to temperatures exceeding approximately 700 °C. With increasing exposure temperatures, up to 1000 °C, G20Mn5N showed maximum variations of approximately 28.6%, 14.8%, and 57% in yield strength, ultimate strength, and fracture strains, respectively; while the corresponding variations for G20Mn5QT were less severe at 16.8%, 7.6%, and 45%, respectively. The influence of different cooling methods was considerable, particularly when the exposure temperature exceeded 700-750 °C, whereas the effects of cyclic heating and cooling appeared to be insignificant. Predictive empirical equations to evaluate the post-heating mechanical properties of the two cast steels studied herein are proposed for both air and water cooling.