Material Degradation of Heat-Resistant Steels during Long-Term Creep Exposure

Creep strength and material degradation of heat-resistant steel were discussed, focusing on microstructural changes during long-term creep exposure. The materials and creep rupture data were obtained from Creep Data Sheet Project by National Institute for Materials Science. For determining the allowable stress, the 100,000 h creep rupture strength should be evaluated based on creep rupture data. The selection of TTP and dividing the creep rupture data set are important to evaluate long-term creep strength when creep strength degradation occurs in the long term. In order to establish the allowable stress, minimum creep strength must also be evaluated. The minimum creep strength depends on the standard error of estimate (SEE) for creep rupture data. If the heat-to-heat variation of creep strength is large even for the same material specification, the value of SEE is estimated to be large. Therefore, it is important to clarify the reason for the heat-to-heat variation of creep strength in terms of microstructural changes and chemical compositions. Microstructural degradations were reported for carbon steels, low alloy steels, 9–12%Cr ferritic steels and austenitic heat-resistant steels, focusing on precipitates and dislocation structures. The reason for the heat-to-heat variation of creep rupture strength was discussed in term of the difference of solute atoms, segregation and grain size.