Environmentally assisted cracking behavior of oil and gas line pipe weldments in wet H_2S containing environments

摘要

Hydrogen induced cracking (HIC) or stepwise cracking (SWC) is one of the major problems observed on low carbon and low alloy steels primarily acting as pipeline or pressure vessel in oil and natural gas industries under wet H_2S containing environments. Since atomic H produced by electrochemical corrosion in wet H_2S environment enters into interaction with dislocations and inclusions, when it is diffused into crystalline structure, it may cause detrimental effects on the microstructure and on mechanical properties of steels. Besides resulting embrittlement of microstructure, atomic H converts into molecular form in interstitial locations within iron crystal lattice and forms H_2 and causes deformation of surrounding metal structure. Under more critical conditions, high pressure of gaseous H2 results in blistering or hydrogen induced cracking. This article is focused to determine the environmentally assisted cracking (EAC) behavior of API 5L line pipe (X60, X65 and X70) weldments such as fabrication weldments (i.e. submerged-arc welding & electric welding) and girth weldments (laser welding & shielded metal electric arc welding) in sour service environments including wet H_2S by standard test method NACE TM 0284 in two methods. Standard test method is based on exposure of test samples to an acidic environment for 96 hours under 1 atm. H2S pressure and ambient temperature. In the first method, welded test samples have been exposed to standard test solution under unstressed condition and then HIC behaviors are evaluated. Cross-sections taken from samples in directions specified in the standard are examined by 100 magnification under optical microscope. For determination of resistance against HIC cracking, crack length ratio (CLR) <15%, crack thickness ratio (CTR) < 3% and crack surface ratio (CSR) < 5% as defined in EFC No:16 standard are taken as target values. Secondly, welded test samples taken from the pipes are subjected to mechanical tests specified in API 5L after being exposed to standard test solution. Tensile, bending and hardness tests are performed on welded specimens subjected to HIC experimental solution in the second method. As a result of investigations hydrogen induced cracks have been observed in the base metal of line pipe steels (X60 and X70) independent from the different welding methods. Especially centerline segregations along the rolling direction, pearlite bands and inclusions (MnS) existed in the matrix has been determined favorable sites for HIC cracks than X60 and X70 steels that have conventional ferrite and pearlite microstructure. Keywords: Line Pipe Weldments, Sour Service, Wet H2S, Environmentally Assisted Cracking (EAC), de-cohesion, plasticity.