Post-fire hardness and impact resistance tests of high-strength grade 8.8 steel bolts

The article presents results of hardness and impact resistance tests of grade M20-8.8 bolts, previously subjected to simulated fire conditions. The tests were aimed at assessing the effect of temperature, fire exposure time, and cooling method on the hardness and impact resistance of bolts subjected to a fire in the context of fastener post-fire suitability for further use. Knowledge of these two parameters may be crucial from the point of view of expert assessment of the safety of structures that survived the fire. This may be of particular importance in the case of structures located in seismic and paraseismic areas, and subject to dynamic loads. Test specimens were received from bolts previously subjected to simulated thermal actions, which were supposed to reflect environmental conditions of a real fire. They were heated at temperatures 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 periods of 30’, 60′, 120′ and 240′, respectively, and then cooled at various rates, which resulted in the differentiation of the material microstructure. After heating, some of the bolts were cooled naturally, left to cool down freely at ambient temperature, whereas some other were rapidly cooled by immersion in water until they cooled completely, thus simulating the effect of an intensive firefighting operation. The obtained results were elaborated on to assess their usefulness in the analyzes of structures that survived the fire and, due to the nature and extent of damage, are the subject of considerations regarding the possibility of their further use.