The susceptibility to sulphide stress cracking of low alloy steels

Abstract

The susceptibility to sulphide stress cracking (SSC) of low alloy Mn‐V and Cr‐Mo‐Nb steels in standard NACE solution was determined. The critical stress (Sc) and threshold stress (σth) of steels were determined in both as‐received and heat treated condition. In the as‐received state the steel shows a highly susceptibility to SSC dominantly due to its microstucture. By quenching and tempering of steels at high temperature the SSC resistance was increased. It was found that the susceptibility to SSC for Cr‐Mo‐Nb steel is lower than that of Mn‐V steel at a comparable yield strength. It was explained by the presence of different second phases in the ferrite matrix and spheroidized carbide particles resulting from the higher tempering temperature. In addition to Fe3C, the presence of Cr3C2, Mo2C and NbC in the ferrite matrix of Cr‐Mo‐Nb steel play an important role in their resistance to SSC. In all cases tested was Sc>σth which suggests of the importance of plastic deformation during the test. The microfractographic analysis of specimens before and after SSC testing have been done. Occurrence of cracks in the near outer surface of non‐failed tensile specimens during 720 hours test suggested that SSC could be explained as a combination of hydrogen‐induced cracking (HIC) and stress corrosion cracking (SCC).

Experimental Characterization of Hydrogen Embrittlement in API 5L X60 and API 5L X80 Steels

“…Additions of elements that are carbides formers, which are used for grain refinement, precipitation hardening and improves the toughness, also provide irreversible trapping for hydrogen. For example, steels containing Nb have shown good resistance due to formation of particles of NbC in the ferrite matrix [9]. In addition, precipitates of Mo 2 C, VC and TiC also increase resistance to hydrogen embrittlement [8].…”

Fragilización por Hidrógeno de los Aceros API 5L X60 y API 5L X80

“…Adicionalmente, aceros con contenido de Nb presentan buena resistencia a la FH debido a que forman partículas de NbC (Gojic et al, 1998); precipitados de Mo 2 C, VC y TiC también aumentan esa resistencia. Por lo tanto, como la cantidad de hidrógeno que puede ser liberada por la micro-estructura es la que se torna más importante durante los procesos de FH, el hidrógeno atrapado reversiblemente es más perjudicial que el atrapado de forma irreversible (Luppo y Garcia, 1991).…”

Influence of external tensile stress on corrosion and trench formation of low alloy steel in a low H2S content sour corrosion environment

“…SSC is a type of environmentally assisted cracking phenomenon and proceeds perpendicularly to the loading axis under external load from a steel surface exposed to a sour environment. [1] From previous studies, [2][3][4][5][6][7][8][9][10][11][12][13][14][15] SSC in low alloy steels is regarded as a type of hydrogen embrittlement (HE), and pipeline steels with hardness higher than HRC 22 (equivalent to 550 MPa yield strength) are generally considered to have high SSC susceptibility. [16] Currently, TMCP (thermomechanical-controlled process) low alloy steel that satisfies this hardness constraint is used for the transportation of LP applications.…”

“…Many previous studies on SSC have been based on a NACE TM-01-77 compliant 1-bar H 2 S saturated environment or a high-pressure, high-concentration H 2 S environment. [2][3][4][5][6][7][8][9][10][11][12][13][14][15] As H 2 S is well known to act as a catalyst that promotes penetration of hydrogen into steel, [17][18][19][20][21][22] it is reasonable to conclude that SSC in a saturated H 2 S solution or a high-pressure, high-concentration H 2 S environment is a kind of HE phenomenon. However, actual sour environments include many low-concentration H 2 S partial pressure environments.…”