Allowable stress design of rupture strength based on confidence level of heat resistant steel

A statistical analysis of creep rupture data for 9Cr1Mo and HP steels is carried out in this paper. The scattering of creep rupture data is represented by Z-parameter method based on the time-temperature parameter method. With the application of Z-parameter, reliability design of allowable stress is carried out according to design life. The higher the value of confidence level, the lower the allowable stress. In comparison with allowable stress designed by safety factor method, it can be seen that confidence level is different with the same safety factor for different steels. The dispersion of material rupture property can not be taken into account by safety factor method. Reliability design provides more precise results by considering the real distribution of creep rupture property. And reliability design provides more flexible choice for design due to the need of safety and economy.