High temperature transient tensile properties of fire resistant steels

Fire resistance of steel for safety and economy has been the subject of extensive investigation in recent years. Preventive and protective measures are needed to minimise loss of life and properties from fire hazard. Fire resistance is governed by steel strength, the member section factor, types of protective measure taken, and load. To improve fire resistance, Nippon Steel of Japan has developed some fire resistant steels based on niobium and molybdenum. This project is to determine the properties of fire resistant steels, at high temperatures using transient tensile test. In such tests, the load on a steel specimen is maintained constant while its temperature is increased at a given rate and the changes in gauge length are constantly recorded. A comparison was drawn between the strength reduction factors, the elongation and the failure temperatures in three Nippon steels. The most heavily alloyed steel clearly outperforms the other two steels in all three of these categories. Overall, these three steels performed in a very consistent manner at high range temperatures and their strength reduction factors have higher values than the factors of conventional structural steels from Eurocodes or British Standard. The factors are higher for experimental steel containing 0.58 percentage niobium, much more than the Nippon fire resistant steels. The strength reduction factors obtained from steady-state tests are slightly higher than the factors obtained from transient tests. Without reference to similar steel from transient tests, the factors from steady-state tests could give rise to misleading answers when used in a fire engineering analysis. From the structural engineers' point of view, the result from the transient tensile testing is useful for steel structural design.