【作者】 高志伟；

【导师】 王东坡；

【作者基本信息】 天津大学 ， 材料科学与工程， 2022， 博士

【摘要】 钢制桥梁和海底管道的焊接接头在服役期间不仅承受交变载荷的作用,而且受到腐蚀介质的作用。探究焊接接头在腐蚀环境中的疲劳行为及适用的疲劳延寿技术,对保障腐蚀环境下焊接结构的服役安全和延长焊接结构的疲劳寿命具有重大意义。本文开展了腐蚀环境下焊接接头疲劳行为及延寿机理的研究,探究了在疲劳损伤与氢耦合作用下焊接接头的断裂机理,分析了腐蚀环境下焊接接头的疲劳曲线,并为标准设计曲线的修正提供了一定的疲劳数据支撑。主要结论如下:焊接接头在干燥空气和水雾环境中的疲劳性能研究结果表明,对于含高值焊接残余应力的足尺寸焊接接头,在干燥空气中,IIW和BS7608推荐的设计曲线是保守的;在水雾环境中,标准设计曲线是危险的。对于含低值焊接残余应力的焊接接头,在干燥空气和水雾环境中,标准设计曲线是安全的。对于足尺寸焊接接头,焊后热处理、高频机械冲击和焊后热处理+高频机械冲击复合处理使得焊接接头在水雾环境中的疲劳强度分别提高60%、90%和160%以上。对于小尺寸焊接接头,焊后热处理可使水雾环境中接头的疲劳强度提高约10%,而高频机械冲击处理可提高水雾环境中接头的疲劳强度达50%以上。焊接接头在H₂S环境中的疲劳性能研究结果表明,焊接接头在H₂S环境中的疲劳寿命远低于空气环境,两者的差异超过一个数量级;随着应力水平的降低,H₂S环境和空气环境中焊接接头疲劳寿命的差异逐渐增大。与标准设计曲线对比,H₂S环境中的疲劳曲线远低于标准设计曲线。在H₂S环境中,焊后热处理可使接头的疲劳寿命提高约40%,高频机械冲击处理可提高接头的疲劳寿命达4倍以上,而焊后热处理+高频机械冲击处理复合处理可提高接头的疲劳寿命达2倍以上。高频机械冲击处理的改善效果优于焊后热处理+高频机械冲击处理复合处理,这是因为高频机械冲击处理引入更高的残余压应力抑制了疲劳裂纹的萌生和扩展。焊接接头腐蚀环境下的断裂机理表明,腐蚀介质中的氢与疲劳损伤的耦合作用是造成接头疲劳寿命降低的重要因素。疲劳损伤导致接头的氢含量进一步提高,进而加剧接头的氢脆敏感性。接头微观组织的不均匀性和疲劳损伤会引起位错和晶界的演变,进而影响接头的氢分布及氢脆敏感性。氢致弱键和氢增强局部塑性机制是接头氢脆的主要影响机制。氢渗透结果表明,经疲劳损伤作用后,接头的氢扩散系数降低,氢陷阱捕获的氢浓度增大;焊后热处理使得接头的氢扩散系数增大,而氢浓度和氢陷阱密度均降低。采用数值分析方法研究了影响焊接接头氢分布的关键因素,结果表明,接头氢分布的不均匀性是由氢-应力相互作用导致的。随外加载荷的增大,接头焊趾处的氢浓度逐渐增加;焊接残余拉应力的存在进一步提高了焊趾和母材近表层区域的氢浓度;而高频机械冲击处理能有效地降低焊趾近表面的氢浓度。尽管焊后热处理+高频机械冲击复合处理引入的残余压应力低于高频机械冲击处理,但仍可以抑制氢在焊趾处的聚集和扩散。虽然外加载荷造成高频机械冲击处理态焊接接头残余压应力的释放,从而影响接头氢的分布,但接头焊趾处的氢浓度仍然较低,证明高频机械冲击处理可有效地降低接头焊趾处的氢浓度。更多还原

【Abstract】 The welded joints of steel bridges and submarine pipelines are not only subjected to the effect of alternating load,but also are affected by the corrosive environment.It is of great significance to investigate the fatigue behavior of welded joints and apply appropriate fatigue life improvement technology to ensure the safe service and improve the fatigue performance of welded structures in corrosive environments.In this paper,the fatigue behavior and life extension mechanism of welded joints in corrosive environment are investigated.Meanwhile,the fracture mechanism of welded joints subjected to the coupling effect of fatigue damage and hydrogen is explored.The corresponding fatigue curve of welded joints in corrosive environment has been analyzed to provide a certain amount of fatigue data for the correction of the standard design curve.The main conclusions are as follows:For dry air and water-spray environment,the fatigue performance of welded joints shows for medium-scale welded joints with high welding residual stress,the design curves recommended by IIW and BS7608 are conservative in dry air,while the standard design curve are dangerous in water-spray environment.For welded joints with low welding residual stress,the standard design curve is safe.For medium-scale welded joints,the post-weld heat treatment（PWHT）,high-frequency mechanical impact（HFMI）and PWHT+HFMI increase the fatigue strength of welded joints by more than60%,90%and 160%,respectively.For small-scale welded joints,PWHT and HFMI treatment can improve the fatigue strength of welded joints in water-spray environment by about 10%and more than 50%,respectively.The fatigue performance showed that the fatigue life of welded joints in H₂S environment is much lower than that in air,and the difference between them is more than one order of magnitude.With the decrease of stress range,the difference of fatigue life of welded joints in H₂S and air environment increases gradually.The S-N curve in H₂S environment is far lower than the standard design curve.PWHT can increase the fatigue life of joints by 40%in H₂S solution.HFMI treatment can improve the fatigue life of the joint by more than 4 times,while the PWHT+HFMI treatment can improve the fatigue life of the joint by more than 2 times.The improvement effect of HFMI is better than that of the PWHT+HFMI treatment.This is because HFMI introduces higher compressive residual stress and inhibits the initiation and propagation of cracks.The fracture mechanism of welded joints in corrosive environment is analyzed,which revealed that the coupling effect of fatigue damage and hydrogen from corrosive medium is an important factor in the fatigue life reduction of joints in corrosive environment.Meanwhile,fatigue damage would further enhance the hydrogen concentration in welded joint,and then promotes the hydrogen embrittlement sensitivity of welded joints.Moreover,the evolution of dislocation and grain boundary caused by microstructure heterogeneity and fatigue damage would affect the hydrogen distribution and hydrogen embrittlement sensitivity of the joint.The result proves that hydrogen-enhanced decohesion and hydrogen-enhanced localised plasticity are the main mechanisms affecting the hydrogen embrittlement of welded joints.The result of hydrogen penetration of welded joints shows that after fatigue damage,the hydrogen diffusion coefficient decreased and the hydrogen concentration captured by hydrogen traps increased.The PWHT treatment increases the hydrogen diffusion coefficient of welded joint,and reduces the hydrogen concentration and hydrogen trap density.The critical factors affecting the hydrogen distribution in welded joints are analyzed by numerical analysis.The results show that the interaction of hydrogen and stress would give rise to the non-uniformity of hydrogen distribution in welded joints.The results show that the hydrogen concentration at the weld toe increased with the rise of external load.Moreover,the presence of tensile welding residual stress further increases the hydrogen concentration at the surface area of weld toe and base metal.However,HFMI-treatment can effectively reduce the hydrogen concentration near the surface of the weld toe.Although the compressive residual stress of welded joints introduced by PWHT+HFMI treatment is lower than that of HFMI treatment,it can still inhibit the accumulation and diffusion of hydrogen at the weld toe.Even though the effect of external load would cause the release of an amount of compressive residual stress of welded joints treated with HFMI,resulting in the change of hydrogen distribution of welded joint,the hydrogen concentration at the weld toe is still very low,which demonstrates the effectiveness of HFMI-treatment in reducing the hydrogen concentration at the weld toe.更多还原