Post-fire behavior and residual capacity of high-strength grade 8.8 steel bolts

The paper presents results of experimental research involving assessment of the impact of temperature, fire exposure time, and the applied cooling method on the residual load-bearing capacity of high-strength construction steel bolts quenched and tempered (QT) in the production process, and their behavior under loading. The tests consisted in subjecting the bolts to simulated thermal impacts reflecting the environmental conditions of a real fire. During the experiment, a series of static tensile and shear tests were carried out on M20-8.8 construction bolts, exposed to the temperature of 100°C, 150°C, 200°C, 300°C, 400°C, 500°C, 600°C, 700°C, 800°C, 900°C, and 1000°C for the periods of 300, 600, 1200, and 2400, respectively. Moreover, the research took into account different cooling methods and analyzed their impact. After heating, the first batch of bolts was cooled in air, by allowing them to cool freely in ambient temperature conditions. In the case of the second batch, the bolts were cooled down rapidly by immersion in water, thus simulating the effect of a rescue and firefighting operation. In each series – for statistical reasons – 3 samples were tested in order to verify correctness and repeatability of the results obtained. Residual values of the post-fire tensile strength and the post-fire shear strength were determined. Values of reduction coefficients of the residual post-fire load-bearing capacity were determined as the ratio of the current load-bearing capacity of the bolt subjected to the conditions corresponding to a relevant fire situation to its reference load-bearing capacity in the initial condition. In addition, the article discusses changes in the plasticity and behavior of bolts subjected to the described environmental impacts and points out to the observed failure mechanisms. Attention was drawn to causes of the observed phenomena, the sources of which should be sought in microstructural changes of the bolt material that occur in the process of heating and cooling, depending on the temperature reached during the simulated fire exposure.