Creep rupture strength and ductility of Gr. 92 steel

Metallurgical risk factors for creep strength enhanced ferritic steel have been discussed in the ASME Code Committee because of the low ductility of long-term creep rupture regime. However, the creep ductility is not well elucidated, and the influences of the difference such as chemical composition, heat treatment conditions and microstructure are also unknown, especially for Gr. 92. Therefore, in this study, in order to carry out a systematic investigation on the long-term creep rupture strength and creep rupture ductility of the fourteen Gr. 92 heats were compared mainly at 600°C, where this material is used in power plant. Furthermore, the extraction of the influence factor on them was tried.

The difference in creep rupture strength and rupture ductility was large, and the difference in creep rupture strength was observed especially in the long-term region over 10,000 h. Similar to Gr. 91, the higher the Cr content, the lower the creep rupture strength was expected due to the decrease in MX precipitate density due to Z phase formation. However, the effect was not recognized in this test range, and the decrease in creep rupture strength for a long-term region was larger as the initial hardness was higher and the prior austenite grain size was smaller.

Some specimens were creep fractured at the brim part which is for measurement of final gauge length after rupture. This is the origin of the crack due to the stress concentration caused by the brim shape, so called notch weakening. When the elongation at rupture is less than 10%, it is easy to fracture at the brim.

Though the difference in creep rupture ductility between heats is also recognized, the factor which affects creep ductility has not been found at present, and it is necessary to evaluate the effect factor and metallurgical risk continuously.