| Hydrogen induced cracking?HIC?and sulfide stress corrosion cracking?SSCC?are two of the major damage phenomena of pipeline steel in H2S environment.In this study,HIC susceptibility of welded MS X70 pipeline steels was evaluated in NACE“A”solution at room temperature.Meanwhile,microstructure and regions near a HIC crack in the MS X70 base steel and its welded joint were analyzed by employing OM,SEM and EBSD.The hydrogen trapping efficiency was also investigated by measuring the permeability?J??and the effective hydrogen diffusivity(Deff).In addition,sulfide stress cracking resistance of welded MS X70 pipeline steels was also evaluated in NACE“A”solution according to NACE TM 0177-2005 at room temperature.The fracture surface of the failed test specimens were observed by FE-SEM.The corrosion rates were measured in the NACE“A”solution by utilizing electrochemical polarization techniques.And the susceptibility to stress corrosion cracking of MS X70 pipeline steel under cathodic protection was investigated by slow-strain-rate tensile test.The main progressions are as follows:?1?Both base metal and welded joint were highly susceptible to HIC and the later steel sample was more vulnerable than the former.The higher HIC susceptibility of the welded joint primarily attributed to the higher hydrogen trapping efficiency of bainitic lath microstructure and low grain boundary.?2?The higher hydrogen trapping efficiency and corrosion rate made the welded joint presented the lower SSCC threshold stress of about 338.42MPa?66.36%YS?compared with the base metal of about362.12MPa?80.47%YS?.?3?The SSCC mechanism of MS X70 pipeline steel mainly depended on hydrogen embrittlement?HE?which caused 88%of the loss of plasticity,whereas anodic dissolution has less impact.These progressions contributed to the development of a science base for improving safety and integrity of pipelines used in H2S environments and give instructive suggestions in the welding process. |
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