Two significant improvements were achieved in 2019 and 2021 Editions of ASME Boiler and Pressure Vessel Code, Section III, Division 5. One is an update of allowable stress values in Gr.91 steel as 9Cr-1Mo-V steel. The time for which the values of Sr, expected minimum stress-to-rupture, is defined were extended from 300,000 to 500,000 hours and the values in relatively high temperature and long-term region were reduced compared with those in the region less than 300,000 hours. The other is that the equations for monotonic stress-strain curves and creep strain curves were provided. These improvements affect the creep damage calculation in the evaluation of creep-fatigue damage. The reduction of Sr values increases creep damage and providing the equations allows calculating the creep damage more precisely. In this study, the impact of these improvements on the creep damage calculation is assessed by some sample problem calculations. The stress dividing coefficient of K′ for Gr.91 was improved from 0.9 in 2017 Edition to 1.0 in 2019 Edition. The effect of the improvement on creep damage was also assessed. The total creep damage almost tripled by using the updated Sr table, although and decreased by about 20% by using the provided equations rather than graphic calculation in a sample problem case. The change of the value of K′ halved the creep damage in the sample problem case. The creep damage calculations during the stress relaxation were based on the Division 5 procedures of HBB T-1433(a) and (b), and these results were compared to a calculated result using creep damage calculation procedure using the elastic follow-up concept adopted in the Japanese elevated temperate design code. Material test results of notched bar specimens are also used for comparison. The calculated result indicated that the procedure using the elastic follow-up concept was comparable to those by HBB T-1433(a) and (b), while calculation using the elastic follow-up concept is easier than that of HBB T-1433(a) and (b).

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