Microstructural aspects of sulfide stress cracking in an API X-80 pipeline steel
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I.
INTRODUCTION
SULFIDE stress corrosion (SSC) cracking is a degradation process that occurs in steels when they are exposed to wet hydrogen sulfide environments. Under these conditions, a sulfide film, (Fe,X)S, develops at the steel surface, as well as nascent hydrogen. Surface recombination of the atomic hydrogen generated is inhibited by the presence of adsorbed sulfur and results in enhanced hydrogen permeation.[1,2,3] In general, SSC is confined to highly stressed regions where hydrogen accumulation occurs preferentially.[4] The combination of internal or applied stresses and a susceptible microstructure determine the critical hydrogen buildup needed for crack initiation and growth. SSC susceptibility is maximum at room temperature,[5] and it is generally enhanced by (1) the presence of oxide and sulfide inclusions;[6] (2) untempered martenite or bainite;[7,8] and (3) impurities such as C, Mn, P, and others, which give rise to segregation bands.[9,10] Conventionally, the SSC susceptibility of high-strength steels has been estimated using various qualitative tests,[11,12,13] which include the National Association of Corrosion Engineers (NACE) TM-0177-90 tension test. Furthermore, NACE has issued a series of guidelines for safe operation in sour environments (NACE MR-0175-93). Accordingly, the strength of corrosion resistant alloys (CRAs) for sour service has been limited to steels with yield strengths no greater than 690 MPa, or with a maximum hardness of 22 HRC. Nevertheless, in recent years, there has been an increasing demand for CRAs for sour gas and oil transportation systems. New steel refining processes and thermal-mechanical control have been employed to improve ยด PEZ, Associate Professor, and R. RAGHUNATH, Graduate H.F. LO Student, are with the Materials Department, University of WisconsinMilwaukee, Milwaukee, WI 53201. J.L. ALBARRAN, Research Scientist, and L. MARTINEZ, Program Director, are with the Institute of Physics, UNAM, C.P. 621291, Cuernavaca Morelos, Mexico. Manuscript submitted August 23, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
the strength, toughness, and weldability of high-performance pipelines.[10,14] Consequently, the NACE MR-0175-93 standard might no longer reflect the current status of CRAs for sour service. New efforts to improve standardization and to reflect the various sour conditions, temperature effects, and testing procedures are currently underway.[15] Among the various testing methods, fracture mechanics has been employed to measure the effects of stress and the sour environment on SSC crack propagation.[3,15,16,17] By these means, a threshold in the stress intensity factor, KI, below which crack arrest occurs (KISSC) has been determined for a number of CRAs under various microstructural conditions. According to Craig,[10] values of KISSC .33 MPa=m are desirable in the oil industry for satisfactory performance of high-strength steels. Hence, the use of fracture mechanics provides valuable information on the relative susceptibilities of CRAs to crack propagation. R
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