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ASME STP-PT-027-2009 pdf download

The impetus for this activity arises because the new ASME B&PV Code, Section VIII, Division 2 rules permit high strength materials of the type enumerated to be used to temperatures above 700˚F and into their respective creep ranges. A life limiting failure mode is potentially the phenomenon of “creep-fatigue.” We shall define a “creep-fatigue” failure as one in which life is shorter than that expected due to either creep or fatigue acting on a structure independently. This occurs in those regimes of stress, strain-rate, time and temperature where the damage mechanisms due to creep and fatigue can be expected to damage the same microstructure and property characteristics. Creep- fatigue is of concern especially where there may be time-dependent straining and where varying stresses (loads, including start-up and shut down) are among the design conditions. Comprehensive and correct creep-fatigue design rules are needed now for the aforementioned alloys because, under the new Section VIII, Division 2 rules, as the respective creep ranges of the materials are approached, in many cases the allowable stresses are significantly higher than those for which there is applicable service experience that would permit exempting design details from fatigue analysis based on documented “years of relevant experience.” The same must be said for any new alloys and applications for which there is literally no relevant service experience. In summary then, the combination of new materials and applications for advanced energy systems with higher allowable stresses and increased design temperatures requires an understanding of creep- fatigue not now available, analytical models to explain and express damage accumulation and relevant test data in order that new, justifiable and correct rules may be developed.
Relatively high strength alloys such as the very popular 2 ¼ Cr-1Mo-V (22V) and modified 9 Cr- 1Mo-V-Cb-N (91) achieve their superior properties through accelerated cooling of these hardenable alloy steel compositions from high (normalizing) temperatures, transformation of the microstructure to martensite or bainite followed by tempering. For these materials, the specified minimum ambient temperature yield and tensile strengths are 60 and 85 ksi, respectively. Corresponding maximum respective yield and tensile strength values may range up to about 85 and 110 ksi. Typical values of strengths in finished pressure vessels are likely to be about 70 ksi yield and 92 ksi tensile. For the ranges of room temperature strengths usually expected, the time-dependent stress-rupture and creep properties increase directly as shown in Figure 1 for the 100,000 hour stress-rupture strength at 850˚F for the 22V material.
Elevated temperature straining of the alloys under consideration during creep exposure or cyclic stressing will lower the tensile strength and hardness, alter the optimal microstructure from that obtained by proper heat treatment and reduce the creep life. This behavior is well known and has been reported for decades in studies of 1Cr-1Mo-V turbine rotor steels and, more recently, in studies of the modified 9Cr-1Mo-V alloy used in many power piping and similar applications. Figure 2 below contrasts strain softening behavior of a high strength Cr-Mo-V alloy with that of a strain hardening material such as a low tensile strength austenitic stainless steel or a conventional low tensile strength ferritic steel. Data on the latter types of materials are not useful in developing the approach to creep-fatigue design sought in this ASME project for the strain softening materials such as the accelerated cooled and enhanced 1-1/4, 2-1/4 and 9 to 12 Cr alloys.

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