The ASME allowable stress is evaluated for 2.25Cr-1Mo, Mod.9Cr-1Mo and type 304 (18Cr-8Ni) steels. The change in coefficient for tensile strength for the allowable stress from 1/4 to 1/3.5 causes a shift of boundary temperature between the tensile and creep regions to lower temperature only by 20–30°C, although it increases the allowable stress by 15% in the tensile region for 2.25Cr-1Mo steel. Whether the allowable stress is determined by 80% of the minimum stress or 67% of the average stress to cause rupture at the end of 105 h depends on the width of scatter band of creep rupture. Type 304 steel exhibits large heat-to-heat variation in time to rupture at long times and the 105 h creep rupture stress distinctly depends on impurity contents such as Al. The stress to produce a creep rate of 0.01% /1000h is about 50 MPa lower than the stress to produce a minimum creep rate of 0.01%/1000h, corresponding to 10−5%/h, for 1Cr-0.5Mo steel. This results from the strain accumulation due to easy deformation in the transient or primary creep.

This content is only available via PDF.
You do not currently have access to this content.