X60、X70管线钢及其焊接接头氢脆敏感性的对比研究

Comparative Study on Hydrogen Embrittlement Susceptibility of X60 and X70 Pipeline Steels and Their Welded Joints

目的开展电解渗氢管线钢及其焊接接头显微组织和力学性能研究,为掺氢天然气管线服役安全可靠性评估提供支持。方法X60、X70管线钢及其焊接接头试样在100mA/cm^(2)下进行24h电解渗氢后,开展有、无渗氢情况下相关试样的显微组织结构、氢热脱附、拉伸力学性能与断口形貌测试分析,揭示渗氢对X60、X702种管线钢及其焊接接头氢脆敏感性的影响规律。结果X60及X70管线钢以细小多边形铁素体为主,其焊缝和热影响区以粗大的粒状或板条贝氏体为主。相对于X70管线钢,其焊接接头具有更高的氢渗透性和稳定性。渗氢导致X60、X70管线钢及其焊接接头试样的强度与塑性降低,渗氢X70焊接接头力学性能衰减最为明显,拉伸断口呈准解理断裂特征。结论X70管线钢的氢脆敏感性高于X60管线钢,管线钢焊接接头的氢脆敏感性高于母材。

Surface damages,e.g.,hydrogen embrittlement or hydrogen-induced crack,of the pipeline steel usually take place during long-term service in the hydrogen-contained environment.In this paper,microstructures and mechanical properties of X60 and X70 pipeline steels and their welded joints with/without hydrogenation were studied to provide a support for the hydrogen damage and safety reliability estimation of the hydrogen-contained natural gas pipeline.All samples of the X60 and X70 pipeline steels and their welded joints were electrolytically hydrogenated at a current of 100 mA/cm^(2) for 24 h.Microstructures,thermal desorption spectrum(TDS)characteristics,tensile mechanical properties and fractographies of the samples with/without hydrogenation were comparatively studied to reveal different effects of hydrogen permeation on their microstructures and mechanical properties.Further,the hydrogen embrittlement susceptibility of the X60 and X70 pipeline steels and their welded joints was evaluated.The X60 and X70 pipeline steels were mainly composed of fine polygonal ferrites,and their welding seams and heat affected zones mainly consisted of coarse granular and lath bainites with more prior austenite grain boundaries and the carbide/matrix interfaces.Therefore,the base metals of the pipeline steel welded joints may have higher mechanical properties than the welding seams and heat affected zones of them.Taking the X70 pipeline steel and its welded joint as examples,the compositions and microstructures of the welded joint were more complicated than those of the X70 base metal.As a result,when the hydrogen thermal desorption curve of the welded joint sample arrived at its peak,the peak still sustained for a long time to form a hydrogen thermal desorption platform ranged from 125℃to 200℃.It indicated that more hydrogen atoms were absorbed in the welded joint samples,and thus resulted in a higher hydrogen embrittlement susceptibility of them.Electrolytic hydrogenation resulted in simultaneous reduction of strength and plasticity of the X60 and X70 pipeline steels and their welded joints.The strength reduction ratios of all samples were roughly the same,but the specific elongation reduction ratios of them were very different.The elongations of the X60 pipeline steels and their welded joints were almost unchanged,on the contrary,those of the X70 pipeline steels and their welded joints were decreased by more than 21%,suggesting a very high hydrogen embrittlement susceptibility of the X70 pipeline steels and their welded joints.Electrolytic hydrogenation resulted in decrement of the dimple aggregation plastic fracture characteristics of the samples during tensile test,and that was more apparent for the X60 and X70 welded joints.Above all,the degradation of the X70 welded joints was the most remarkable,and the hydrogenated area of the X70 welded joints was in a quasi-cleavage fracture mode,rather than a ductile fracture one.The hydrogen embrittlement susceptibility of the X60 and X70 welded joints is higher than that of their base metals.The hydrogen embrittlement susceptibility of the X70 pipeline steel and its welded joint is higher than that of the X60 pipeline steel and its welded joint.The damage more likely takes place in the surface hydrogen permeation layer at the heat affected zone of the welded joints.