X70 MS管线钢焊接接头硫化物应力腐蚀敏感性及氢捕获效率

Sulfide Stress Corrosion Susceptibility and Hydrogen Trapping Efficiency of the Welded Joint of X70 MS Pipeline Steel

目的分析探讨氢在X70级抗酸管线钢(X70 MS)母材及焊接接头中的氢捕获效率及其对硫化氢应力腐蚀开裂(SSCC)敏感性的影响机理。方法依据NACE TM 0177标准,采用应力环拉伸试验分别获得X70 MS管线钢母材及焊接接头SSCC的临界应力门槛值(σscc)。采用场发射扫描电子显微镜(FE-SEM)观察拉伸断口形貌、氢显试验后表面形貌以及电化学充氢后裂纹区域EBSD观察。利用改进的D-S双电解池氢渗透技术,联用Letry应力腐蚀试验机,测量母材及焊接接头的氢渗透动力学参数。结果X70 MS管线钢母材和焊接接头发生SSCC的临界应力门槛值σscc分别为362.1 MPa和338.4 MPa,拉伸断口均呈脆性断裂特征。与母材相比,焊接接头的氢渗透通量J∞和表观氢浓度Capp较大,氢有效扩散系数Deff较小,被捕获的晶格氢、可逆氢和不可逆氢浓度均较高。结论X70 MS管线钢焊接接头具有比母材较高的SSCC敏感性,一方面是由于焊接接头中的板条贝氏体组织晶界及其亚晶界均是氢扩散通道及捕获陷阱,具有较高的氢捕获效率;另一方面,焊缝区中有相对较多的易于裂纹穿晶扩展的{101}//ND取向晶粒,在应力和可逆氢的共同作用下,裂纹易穿过这些取向晶粒快速扩展。

The work aims to analyze and explore the hydrogen trapping efficiency in base metal and welded joint of X70 MS pipeline steel and its influence mechanism on sulfide stress corrosion cracking(SSCC)susceptibility.According to NACE TM 0177 standard,the threshold stress(σscc)of SSCC in the base metal and welded joint of X70 MS pipeline steel was obtained by uniaxial tensile testing,and the fracture morphology,the surface morphology after hydrogen microprint and the EBSD of the cracks area after electrochemical hydrogen charging were observed by field emission scanning electron microscope(FE-SEM).The dynamic parameters of hydrogen permeation in the base metal and welded joint were measured by the improved D-S double cell hydrogen permeation technology and Letry stress corrosion tester.Theσscc of SSCC in base metal and welded joint of X70 MS pipeline steel was 362.1 MPa and 338.4 MPa respectively,and the fracture showed brittle fracture characteristics.Compared with the base metal,the hydrogen permeation flux J∞and apparent hydrogen concentration Capp of welded joint were larger,the effective hydrogen diffusion coefficient Deff was smaller,and the trapped lattice hydrogen,reversible hydrogen and irreversible hydrogen concentration were higher.The welded joint of X70 MS pipeline steel has higher SSCC susceptibility than the base metal.On the one hand,the grain boundary and sub-boundary of lath bainite in the welded joint are hydrogen diffusion channels and traps,which have higher hydrogen trapping efficiency;on the other hand,there are more{101}//ND orientation grains in the welded zone,which are easy for transgranular extension of cracks.Under the combined action of force and reversible hydrogen,the cracks can easily propagate through these grains.