Type 304H and 316H stainless steels are codified in the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code for Section III, Division 5, Class A construction for up to 1500°F (816°C) and 300,000 hours. The extension of design lifetimes to 500,000 hours has been undertaken by various research projects and by the ASME Section III, Division 5 Code Committees. Due to a long-standing issue related to non-classical creep behavior on the use of the time to the onset of tertiary creep as one of the time-dependent allowable stress criteria, little progress was made in the extension of the time-dependent allowable stresses for Type 304H and 316H stainless steels. A recent effort by Dabrow and Nestell (Impact of Tertiary Creep on Time Dependent Allowable Stresses for Type 304H and 316H Stainless Steels, Report 0300-0003-RPT-001, MPR Associates, Alexandria, VA, 2020) has provided a rational method for the treatment of the highly variable data for the time to onset of tertiary creep. This paper presents the background formulas for determining the time-dependent allowable stress values of Type 304H and 316H stainless steels, as assembled from relevant project work. These updates and extensions are being recommended to the ASME Section III Code committees for approval and are subject to change upon feedback from those committees.

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