Revision of Long-Term Creep Strength of Base Metal of Asme Grade 91 Type Steel

In a preliminary study on the creep rupture life scatter estimation of base metals of ASME Grade 91 type steel under actual service conditions, it was found that the creep rupture life curves and allowable stress of Grade 91 type steel pipes which were determined in Japan in 2015 may not be appropriate. Therefore, those were re-evaluated in this work by the Assessment Committee on Creep Data of High Chromium Steels, which consists of the Federation of Electric Power Companies of Japan, material producers, power plant manufacturers, and research institutes in Japan, which had evaluated the material in 2015. Creep rupture data were again provided for the re-evaluation by each organization of the committee, and creep rupture strength was assessed using the latest dataset of Grade 91 type steel by the region splitting analysis method in the same manner as in the previous study. It was suggested that the assessment should be performed at once simultaneously regardless of the product form such as pipes, plates, and tubes. As a result, the creep rupture life curves and allowable stresses of steel pipes were revised to the lower strength side than those in 2015. Furthermore, on the basis of the data and knowledge regarding the effect of chemical composition on the creep life of Grade 91 type steel, revisions to Japanese material codes regarding the chemical composition of Grade 91 type steel were proposed to prevent the production of low-strength Grade 91 type steel.