核电不锈钢管道插套焊完整性评定与延寿方法研究

The stainless steel small pipe socket welds have been widely used in the nuclear power plants. It was reported that more than 80% nuclear piping fatigue failures were due to the high-cycle and low-cycle fatigue failures of socket welded joints. Thus, the safety operation of nuclear power plants is seriously threatened. In the present project, based on fracture mechanics, fatigue science, plasticity mechanics, metallography and welding metallurgy, the effects of parameters such as axial gap, load condition on the fatigue behavior of small pipe socket welded joints using experimental and simulation technology are investigated. Then, the welding procedures will be modified to improve the high-cycle fatigue life of root weld. Meanwhile, the Low Transformation Temperature(LTT) welding wire will be used in weld toe dressing to improve the high cycle and low cycle fatigue life of weld toe. In the life extension section, the effect of welding residual stress and complex stress state on the fatigue resistance will be discussed. Based on the welding metallurgy principle, LTT welding wire with high impact toughness which can form compressive residual stress will be developed. The interaction mechanism of ratcheting effect, residual stress and fatigue behavior will be studied. Finally, the fatigue life extension and integrity assessment approach will be proposed.

不锈钢小支管插套焊在核电机组中得到广泛使用，其焊接接头的高周和低周疲劳失效占到核电机组疲劳失效的80%以上，严重威胁核电机组的安全运行。本项目拟针对核电不锈钢小支管插套焊焊接接头，基于断裂力学、疲劳学、塑性力学、金属学和焊接冶金学方法，借助试验技术和数值模拟技术，在弄清轴向间隙、载荷条件等因素对插套焊接头疲劳失效影响机理的基础上，一方面改进焊接方案，增加焊接坡口来提高焊根的高周疲劳寿命，另一方面使用低相变点（LTT）焊丝进行焊趾熔修来提高插套焊焊趾的高、低周疲劳寿命。在延寿方法研究中，本项目将探明焊根处焊接残余应力与复杂应力状态对其疲劳抗力的影响规律；基于焊接冶金学原理开发能形成压缩残余应力并具有高冲击韧性的焊趾熔修用LTT焊丝；弄清插套焊焊趾处棘轮效应、残余应力、疲劳等三种因素的交互作用机理，最终形成核电不锈钢小支管插套焊疲劳延寿及完整性评定方法。

本项目针对核电不锈钢小支管插套焊接头出现的疲劳失效问题，开展了插套焊接头的疲劳失效机理、疲劳延寿方法等方面的研究，首先，借助试验技术和数值模拟技术，研究原始设计的插套焊接头径向间隙、焊根熔深、焊脚尺寸和焊趾处夹角等因素对插套焊接头疲劳开裂的影响机制，在此基础上，一方面通过研究坡口尺寸、焊接层数对插套焊接头焊根处复杂应力的影响规律，进而分析其对接头疲劳性能的影响，根据此结果制定合理的坡口的参数和焊接工艺方案，通过改进插套焊装配工艺，增加焊接坡口来提高焊根的高周疲劳寿命；另一方面通过开发LTT焊丝，基于焊接冶金学原理，优化LTT焊丝成分，使其在室温环境下，发生马氏体相变，产生压缩残余应力，同时提高其焊缝金属的冲击韧性，使用LTT焊丝对插套焊焊趾进行熔修，提高焊趾的低周疲劳寿命。研究预制坡口和LTT熔修这两种改进措施对插套焊残余应力分布及疲劳寿命的影响，还对插套焊残余应力随循环加载的释放再分布进行研究。最后通过对材料棘轮行为的研究，分析残余应力、棘轮效应和疲劳三者之间的交互作用，并从微观层面上对材料的循环行为及棘轮行为进行解释，这对研究整个插套焊接头完整性评定具有重要意义。