Role of microstructure on sulfide stress cracking of oil and gas pipeline steels
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I. INTRODUCTION
SULFIDE stress cracking (SSC) is one of the major failure problems that occur in oil and gas pipeline steels when they are exposed to aqueous hydrogen sulfide (H2S) environments. This cracking imposes restrictions on selection of materials for the transportation pipelines. Increasing development of oil and gas industry requires pipeline steels with higher strength and better SSC resistance. Unfortunately, higher strength for the steels is generally associated with lower SSC resistance.[1] In general, pipeline steels used for H2S service have been limited to yield strengths of 690 MPa or hardness of HRC22.[2] Currently, there are numerous efforts aimed at developing the high strength grades of pipeline steels with good SSC resistance in sour gas environments.[3,4] It has been found[4,5] that the API grades X-60 to X-80 of pipeline steels are susceptible to hydrogen embrittlement (HE) and most HE failures have been related to the SSC susceptibility. The chemical composition and microstructure are the main variables that can be optimized during the steel production to improve the SSC resistance.[6,7] These variables are not independent, and for a pipeline steel with one specific chemical composition, different thermomechanical control processes (TMCP) can produce different microstructures, changing its SSC resistance. Pipeline steels currently used and studied can generally be classified as ferritic-pearlitic steels and acicular ferrite steels in view of microstructure. The latter, whose microstructure is dominated by acicular ferrite, i.e., acicular ferrite matrix plus a small amount of polygonal ferrite, is of considerable interest in developing high-performance pipeline steels because of its optimal combination of high strength and good toughness.[8,9] In recent MING-CHUN ZHAO, Ph.D. Student, and BEI TANG, Post Graduate Student, YI-YIN SHAN, Vice-Professor, and KE YANG, Professor, are with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted May 24, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
years, another interest has arisen in the ultrafine ferrite microstructure,[10] since refinement of the grain size is beneficial to both strength and fracture resistance. So far, most work concerning the role of microstructure on SSC resistance has dealt with the microstructures obtained by conventional hot working or heat-treatment process, such as ferritic-pearlitic microstructure, upper/lower bainite, or quenched/tempered martensite.[2,11] However, microstructures obtained by TMCP have been overlooked in previous studies of SSC susceptibility. In particular, both acicular ferritedominated microstructure and ultrafine ferrite microstructure, which are beneficial to the increase of strength and toughness, are far beyond clarity for their SSC resistance. In order to develop high-strength grade pipeline steels with optimum SSC resistance, it is necessary to clarify the role of microstructure obtaine
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