New Ferritic Steel Beyond Grade 92 and its Creep Degradation Assessment by Hardness Method for Grade 91

Abstract

The alloy development R&D activities for weld construction components, such as boiler piping and headers of high-efficient fossil-fired power plants have been conducted to introduce creep strength enhanced ferritic (CSEF) steels, grades 91, 92, 122, 911, 23 and 24 for last three decades. The grade 92 among these CSEF steels has the highest creep rupture strength to increase the steam temperature up to 620°C however the weld heat affected zone is much weaker in creep than the base metal due to the Type IV failure. Alloy design trials or proposal of the candidate steels in laboratory level has been conducted to improve the creep rupture strength than that of grade 92. Presently it is becoming possible in a very near future to introduce the new high strength ferritic steels beyond grade 92 on the commercial basis, and also the new steels could mitigate the Type IV failure at the welds to be applicable at the maximum use temperature of 625°C and above. But the creep degradation behavior in such new advanced steels is not yet well investigated and it is not verified that the creep degradation/life assessment techniques studied for the existing CSEF steels are applicable. Therefore the creep degradation and softening behavior of new advanced CSEF steels were studied and the hardness creep life assessment technique developed using grade 91 was applied to investigate the similarity of degradation process with the conventional CSEF steels. The present paper introduces the development status of new advanced steels beyond grade 92 in Japan and deals with the creep degradation and softening behavior of the new steels in comparison with grades 91 and 92. Paper published with permission.