ASME STP-PT-027:2009 pdf download

ASME STP-PT-027:2009 pdf download EXTENDED LOW CHROME STEEL FATIGUE RULES
2 MATERIALS 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.
3 CREEP-FATIGUE DATA Creep-fatigue data have been developed in tests utilizing many combinations of strain range, hold time, temperature and load measurement. For the most part, creep-fatigue tests are run with loads that cycle between compressive and tensile and with tensile hold periods ranging from seconds to times exceeding a few minutes, but rarely more than an hour. Plastic strain amplitudes typically do not exceed 1-2 percent and are usually only a fraction of 1 percent.
The total number of cycles applied before failure or a specific load reduction is reached may extend into the thousands, but because of the high cyclic frequency, the total time of exposure may be only tens or, at most, a few hundreds of hours. For the purpose of this study, creep-fatigue data on several of the strain softening alloys were gathered from many sources. The data from which the plastic strain range may be estimated are shown in Figure 3. Included in the plot are some data from tests that show the effects of tensile hold times. Most of the data are from relatively high frequency tests where the accumulated time at creep temperature is very short. It appears that longer hold time tests result in fewer numbers of cycles, i.e. there is a creep-fatigue interaction.
A line provided by a producer of 2 ¼ Cr-1Mo-V alloy, shown in Figure 3, did not include significant hold time effects. The most widely scattered points in the figure are for hold time tests of one brittle heat of an alloy for which the “no hold time” tests were also mainly outside the scatter band. It is not expected that pressure vessel alloys of interest in this project will behave in a creep brittle manner when tested in uniaxial tension.