Effects of gaseous hydrogen on fatigue crack growth in pipeline steel
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INTRODUCTION
HYDROGEN has been identified as a potential alternative energy medium for a number of applications. One such application is as a direct substitute for natural gas. As such, hydrogen must be transported safely and economically from production areas to distribution facilities. One possible method of accomplishing this goal is to use the existing natural gas transmission pipeline system. Because these pipelines transport large volumes of gas at high pressure over hundreds of miles, there is considerable concern over the potential for hydrogen degradation of the mechanical properties of pipe steels. One area of particular concern is accelerated growth of fatigue cracks from preexisting flaws that are cyclically loaded by pressure variations in the hydrogen gas being transported. Existing natural gas pipelines are constructed largely from carbon-manganese steels of relatively low strength [yield strengths in the range 173 to 414 MPa (25 to 60 ksi)]. However, many newer pipelines are being constructed with somewhat stronger microalloyed steels [yield strengths in the range 414 to 483 MPa (60 to 70 ksi)] that have reduced carbon contents, special processing (controlled rolling), and contain small amounts of elements such as columbium, molybdenum, vanadium, or titanium for strengthening. Such steels, by virtue of their relatively low strength levels, are generally considered to be fairly immune to embrittlement by gaseous hydrogen under conditions of sustained loading or monotonically increasing load. However, under cyclic loading conditions hydrogen acceleration of crack growth can occur in steels with virtually any strength level. In pipelines, cyclic loading arises from two principal sources: (1) approximately daily pressure fluctuations during normal operation that are of the order of - 1 0 pct of the nominal operating pressure, and (2) shutdowns and startups for regular service or as a result of an upset condition, in which the gas pressure decreases to zero or practically zero (i.e., complete unloading) and then is increased to the nominal operating pressure. The influence of hydrogen on fatigue crack growth in ferritic steels was first reported by Walter and Chandler. 1 In that study, precracked specimens of ASTM A517 and H. J. CIALONE, Principal Research Scientist, and J. H. HOLBROOK, Manager, Physical Metallurgy Section, are with Battelle, Columbus Laboratories, 505 King Avenue, Columbus, OH 43201. Manuscript submitted March 22, 1984. METALLURGICAL TRANSACTIONS A
ASTM A533B pressure-vessel steels that were fatigue loaded in 6.9 MPa (1000 psi) hydrogen experienced reduction in low-cycle fatigue life of as much as 90 pct. Since that time, numerous investigations on fatigue crack growth of moderate-strength carbon and low-alloy steels have demonstrated that hydrogen dramatically increases crack growth rates. Steels with yield strengths ranging from 207 to 1697 MPa (30 to 246 ksi) which were cyclically loaded in hydrogen environments have been reported to undergo approximately a tenfold or greater i
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