Improvement of Structural Design Rule for Efficient Use of Heat Resistant Materials

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The safety factor for determining allowable stress of a material is required to be as small as possible, so that the material can be utilized at as high stress as possible. However, the safety factor is usually remained at a high value because of the uncertainties of material properties at elevated temperatures. This research studied the cause of the uncertainty of long term creep properties of heat resistant steels.1 Long-term Creep Property Prediction : Temperature dependence of creep rupture life often changes due to a change in creep rupture mechanism. Because of this change we often overestimate long term creep rupture life of steels. The contributions in the uncertainty of the long term property from heat-to-heat variation of the material property, error introduced by the change in temperature dependence and the inherent scatter in data are estimated to be 3:3:2 in SU316 stainless steel.2 Premature Failure of High Cr Ferritic Steels : High Cr ferritic steels usually shows a transgranular fracture mode under short term rupture condition, but their fracture mode changes to intergranular fracture under long term rupture condition. One cannot accurately predict the long term property due to the fracture mode change. The intergranular fracture occurs by the growth and coalescence of creep cavities. The cavities are formed at coarse precipitates such as Fe_2W, M_<23>C_6, and growth of the precipitates is a good measure of the fracture mode change.3 Premature Failure in Weldment of High Cr Ferritic Steels : The type IV failure of weldment causes serious premature failure of welded joint of high Cr ferritic steels. Creep cavities are nucleated at coarse MC_6 particles and grow with the assistance of creep strain accumulated in the dual phase HAZ region. Growth of the MC_6 particles is a good measure of the occurrence of type IV failure.