Post-fire properties of bolt steel, quenched and tempered in the production process

The article presents results of a static tensile test of standardized machined test pieces of round cross-section made of 32CrB3 alloy steel, quenched and tempered in the production process, subjected to environmental actions simulating an extraordinary fire situation. The tests were aimed at assessing the impact of simulated fire conditions on the residual mechanical properties of the analysed steel used to produce grade 8.8 steel construction bolts. The tests involved variants of the maximum temperature applied in the soaking process, exposure time to specific thermal conditions, and the cooling method. The specimens were soaked at temperatures of 600oC, 650oC, 700oC, 750oC, 800oC, 900oC, and 1000oC for periods of 300, 600, 1200 and 2400, respectively, and varied cooling methods were applied during the tests. The first batch of specimens, after soaking, was cooled naturally, left to cool down freely at ambient temperature. In the case of the second batch, the specimens were rapidly cooled by immersion in water until they cooled completely, thus simulating the effect of an intensive firefighting operation. The analysis relates to variability of values of the following post-fire parameters obtained in the static tensile test: ultimate tensile strength – fu; θ; post, yield point – fy; θ; post, modulus of elasticity – Ea; θ; post, ultimate strain at maximum force – "u; θ; post, strain at fracture – "f ; θ; post, percentage elongation after fracture – A5; θ; post, and percentage reduction of area – Zθ;post, depending on the temperature and soaking time, as well as the cooling method applied. The results were compared with those reported in the literature. The values of retention factors and recovery factors were determined.