HYDROGEN INDUCED CRACK GROWTH RATE IN STEEL PLATES UNDER UNIAXIAL STRESS J.L. Gonzalez1, R. Ramirez2, F. Fernandez-Lagos2 and J.M. Hallen1 1Instituto Politecnico Nacional, ESIQIE, Dep. Ing. Metalurgica Labs. de Metalurgia, Apdo. Post. 75-872, México, D.F.07300, MEXICO 2Subgerencia de Mantenimiento a Ductos e Instalaciones, PEMEX Exploración Producción Región Sur Edif. 4 Centro Téc. Adm., Av. Carrizal Esq. Campo Saqmaria, Villahermosa, Tabasco, MEXICO ABSTRACT The purpose of this work was to determine in a quantitative manner the effect of an external uniaxial stress on the kinetics of the hydrogen induced cracking. In this study, the hydrogen induced cracking was done by cathodic charging of API–5L–X52 steel plates. The growth of the induced cracks was recorded at increasing inspection times by ultrasonic inspection. The external uniaxial stress affected the number of initial cracks in the HIC tests, having more initial cracks as the stress increases. This effect is related with the stress fields interaction from the internal pressure produced by the molecular hydrogen and the applied external stress, which weakens the interface between the matrix and the non-metallic inclusions, thus facilitating the nucleation of cracks by the pressure mechanism. A greater number of individual cracks causes that the number of cracks interconnections to increase, thus affecting the crack shape and growth. The external uniaxial stress, however did not show a significant effect on the hydrogen induced crack growth rate, at least in the interval of the applied stresses used in this work. KEYWORDS: API-5L-X52 steel, hydrogen induced cracking, crack growth, pipelines 1. INTRODUCTION There have been a number of failures of line-pipe steels due to the presence of delaminations and blisters which are formed when atomic hydrogen resulting from the corrosion reaction of the steel pipe in the presence of sour gas ( H2S). Although the actual trend in industry is to continue developing and evaluating new steels resistant to hydrogen induced cracking (HIC), there are thousands of kilometers of installed sour gas pipelines that still experience HIC. This facts justifies a need to be able to predict the life span of these pipelines in terms of the extension of HIC. The HIC is due to the absorption of the atomic hydrogen generated by the corrosion reaction between the sour gas and steel, which diffuses towards some internal discontinuities, such as non-metallic inclusions and pre-existing cracks, where it recombines to form molecular hydrogen. The internal pressure in the cavity increases until that the stress intensity in the edge overcomes the toughness of the material and a crack propagates (1). The cracks usually are formed parallel to the surface of the pipe; when they are located approximately at the half thickness, are called delaminations, and when they are located near a free surface, deforming the pipe wall, they are called blisters (1-4). The HIC is possible even in the absence of an external stress, because the stress concentration at the crack tip provides the driving force for the growth (1-4). Iino (5) found that the manner of extension of the hydrogen induced cracks is considerably influenced by the presence of external stress (pipe hoop stress): in absence of the external stress, cracks extension develops by stepwise linking the hydrogen induced cracks, founding considerable plastic deformation accompanies the stepwise linking. In the presence of an external stress, cracks extension develops by linking the cracks formed in stacked arrays out of a plane approximately perpendicular to the stress axis. The linking between cracks, in this case, is characterized by the absence of plastic deformation. However, recently Zakaria (6) mentions that external stress did not significantly affect the manner of formation and extension of the hydrogen induced cracks. In his investigation, Zakaria found that in the absence of external stress the cracks still form in a stacked array. Although the actual trend in industry is to continue developing and evaluating new steels resistant to hydrogen induced cracking (HIC), there are thousands of kilometers of installed sour gas pipelines that still experience HIC. This facts justifies a need to be able to predict the life span of these pipelines in terms of the extension of HIC.
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