Formation and analysis of stack cracks

Formation and analysis of stack cracks - JOURNAL OF...

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JOURNAL OF MATERIALS SCIENCE 28 (1993) 3322-3328 Formation and analysis of stack cracks in a pipeline steel M. Y. B. ZAKARIA Standard and Industrial Research Institute of Malaysia, PO Box 7035, 40911 Shah Alam, Selangor Darul Ehsan, Malaysia T. J. DAVIES Manchester Materials Science Centre, UMIST, Grosvenor Street, Manchester M1 7HS, UK Extensive cracking of the type known as stack cracking was demonstrated in a cathodically charged X65 microalloyed pipeline steel containing a weldment. It is shown that the formation and propagation of rolling-plane cracks, which constitute the primary stages of the stack cracking, is due to local concentration of hydrogen gas pressure and a lowering of the cohesive strength of a number of interfaces by hydrogen. The characteristic S-shape of individual cracks which occurred during the linking up of cracks was attributed to stress interactions at crack tips and cleavage cracking normal to the rolling plane. An explanation of hydrogen embrittlement fracture is given in terms of electronic state modifications of the steel, including charge polarization. 1. Introduction In an investigation to study the formation of stack cracks and their relation to stress and metallurgical state it was found that in cathodic hydrogen charging tests simulating sour service pipeline conditions, stack cracking occurred in an X65 microalloyed pipeline steel containing a weldment [1, 2]. The steel was silicon and aluminium deoxidized and calcium treated and thus contained very low sulphur (0.001 wt %). Extensive cracking was observed near the weldment and only minor cracking was seen in the parent metal. Longer exposure times were required for cracks to develop in the parent metal than in a region near the weldment [1, 2]. This paper is concerned with the mechanism of initiation and formation of stack cracks. A model is proposed of the fracture process associated with the stack cracking based on experimental observations. Details of the experimental procedure have been pub- lished previously [2]. 2. Initiation and formation of stack cracks It has been found experimentally that the presence of a stress, internal or external, was not an essential pre- requisite to cause stack cracking [1]. It was also found that most of the stack cracks initiated at inclusions; the majority of these inclusions did not contain sul- phur. Thus, it was possible that some of these inclu- sions produced tessellated stresses (tension) in the matrix due to the difference in thermal contractions between inclusions and the matrix, as suggested by Brooksbank and Andrews [3 6]. In the presence of 3322 tessellated stresses, hydrogen will diffuse or be trans- ported by dislocations and accumulate around these inclusions. The subsequent build-up of hydrogen pre- ssure allied with low adhesive forces between the inclusions and the surrounding matrix will promote the formation of cracks.
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