Stress-assisted hydrogen attack cracking in 2.25Cr-1Mo steels at elevated temperatures
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INTRODUCTION
H Y D R O G E N attack (HA) is a degradation process that can occur in steels when they are exposed to high-pressure hydrogen at elevated temperatures. The microprocess causing the degradation is the nucleation, growth, and linkup of methane bubbles to form fissures along grain boundariesY 1 Typical operating conditions for such vessels in petroleum refinery and petrochemical plants are 20.7 MPa (3000 psi) hydrogen and 400 ~ to 480 ~ Carbon steels cannot be used because the carbon activity is so high that HA brings about premature failure. Addition of carbide-forming elements, such as Cr, Mo, and V, can generate stable carbides and inhibit HA. 2.25Cr-lMo steel is widely used in such vessels, because it is the leanest alloy that still gives good HA resistance. Safe operating limits of hydrogen pressure and temperature are suggested for various steels by the America Petroleum Institute, r31 the so-called Nelson curves. The curve for 2.25Cr-lMo steel shows that this steel can be used in hydrogen with pressure higher than 13.8 MPa (2000 psi) as long as the temperature is kept below 454 ~ However, this curve is defined by only two parameters, i.e., hydrogen pressure and temperature. No information about the effects of applied stress, degree of tempering, or structure change due to welding is included. The primary motivation for this work was the question of crack growth in vessels in service.
LONG-CHING CHEN, Formerly Graduate Student, Materials Science and Engineering Department, The Ohio State University, is an engineer with Walsin-Cartech Steel, Yenshue, Tianan, Taiwan, Republic of China. PAUL SHEWMON, Professor Emeritus, is with the Materials Science and Engineering Department, The Ohio State University, Columbus, OH 43220. Manuscript submitted May 13, 1994. METALLURGICALAND MATERIALSTRANSACTIONS A
That is, does the stress and strain at the root of a crack significantly increase the local rate of bubble formation and thus enhance crack growth? Evidence for such enhancement has been found in carbon steels. E4] Recently, several groups have reported the premature failure of 2.25Cr-lMo steel loaded in creep at temperatures near or above the Nelson curve, i.e., 454 ~ and exposed to hydrogen pressures higher than 13.8 MPa (2000 psi). Ciuffreda et al. conducted stress-rupture tests on 2.25Cr1Mo steels in high-pressure, hot hydrogen and showed that the creep ductility (especially reduction of area) was substantially reduced if tests were carried out under conditions of hydrogen pressure and temperature such that HA would not be expected in the absence of creep loads, tSj Woods and Scott showed that the critical hydrogen pressure for methane bubble formation would be lowered by the existence of creep stresses (125 to 172 MPa) during hydrogen exposure at 510 ~ or 524 ~ Similarly, scanning electron microscopy (SEM) observation conducted by Wanagel et al. on hydrogen-treated Cr-Mo steel showed that the methane bubble formation was greatly enhanced by creep load. ~71 Woods and Scott also investigated the eff
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