Effect of Temperature on Mechanical Properties of Steel Bolts

AbstractThe strength characteristics of bolts under ambient and fire conditions are among the major factors affecting the performance of bolted connections in steel structures. At present, the evaluation of fire resistance of steel bolts is done under the assumption that the mechanical properties of steel bolts are identical in both the heating and cooling phases of fire. The reason is the scarcity of data regarding the high-temperature properties of steel bolts in the cooling phase of fire. This paper is an attempt to experimentally study the high-temperature mechanical properties of Grade 10.9 high-strength steel bolts both in the heating and cooling phases of fire. The researchers carried out steady-state tension tests on bolt specimens in a temperature range of 20°C–800°C. The test results were used to assess the stress-strain response and the ultimate and yield strength as functions of temperature for Grade 10.9 bolts. Furthermore, the influence of the heating-cooling process on the microstructure and failure mode of bolts was investigated. The obtained data show that Grade 10.9 bolts experience rapid decrease in strength at temperatures exceeding 400°C and respectively reach 40% and 5% of their original strength when heated to 600°C and 800°C. Test data also indicate that Grade 10.9 bolts act differently in the cooling phase compared with the heating phase with respect to strength degradation. This is due to the irreversible transformations that occur in the microstructure of bolts when they are exposed to a heating-cooling cycle.