高温压力管道焊接接头的非均匀蠕变损伤机理及寿命预测方法研究

Formulating life extension strategies based on precise life prediction for high temperature pressure piping after long-term service has been one of the major issues for energy industry. The aim of this project is to conduct research on nonhomogeneous creep damage of weldment of high temperature pressure piping. The following issues will be included in the present project: (1) Characterizing the nonhomogeneous creep damage along the thickness of the pipe and at the local regions in the weldment by microstructural analysis and small angle neutron diffraction (SANS) investigations. Studying the degraded mechanical properties in the nonhomogeneous damage situation in terms of micro-hardness test, tensile test and creep test. (2) Comparing the distribution of creep voids in the columnar grain zone (CGZ), the equiaxed grain zone (EGZ) and the recrystallized grain zone (RGZ). Establishing the relationship between creep damage and microstructural morphology, i.e., grain shape, carbides types and so on. Figuring out the creep damage mechanism of multi-pass weld metal with nonhomogeneous microstructures. (3) Building a detailed geometrical model of multi-pass weld metal based on the microstructure observation. Establishing a revised creep damage constitutive model by coupling the microstructure evolution mechanism with the creep rate equation. Developing a creep life prediction method for weldment with fully considering the effect of nonhomogeneous creep damage. The project is expected to provide new insights into creep residual life assessment of weldment of high temperature pressure piping and to give scientific support to the state inspection and life extension analysis.

针对超期服役的高温压力管道焊接接头，进行精确的寿命预测，制定合理的延寿决策，是能源行业亟需解决的关键问题之一。本项目围绕高温压力管道焊接接头的非均匀蠕变损伤问题进行研究，主要包括：（1）采用微观组织分析与小角度中子衍射测试相结合的方法，定量表征管道壁厚方向和接头局部微区的蠕变损伤分布情况，通过显微硬度、拉伸和蠕变试验,研究非均匀损伤条件下的力学性能退化规律；（2）对比柱状晶区、等轴晶区和重结晶区的蠕变孔洞分布规律，反演蠕变损伤与晶粒形状、析出物类型等的映射关系，理清多道焊缝非均匀组织在长时蠕变条件下的损伤机理；（3）参照微观组织排布情况建立细化的多道焊缝金属几何模型，构建耦合组织演化机制的蠕变损伤本构方程，制定充分考虑焊接接头非均匀蠕变损伤特性的寿命预测方法。本项目有望为超期服役高温压力管道焊接接头的寿命评估提供新的思路，为状态检测及延寿分析提供科学支持。

针对超期服役高温压力管道，从蠕变损伤表征、机理分析、寿命预测及修补等角度进行了延寿评估方法研究。对比CoMoV原始钢及服役钢的微观组织、拉伸性能、蠕变性能发现：长时服役后的蠕变损伤主要为晶粒明显长大，珠光体严重球墨化，碳化物密度、尺寸均显著增大，且晶界发生了贫化；长时服役后拉伸性能的退化规律与温度有关，室温条件测试获取的拉伸性能老化规律不能代表真实服役条件下拉伸性能老化规律。服役钢接头持久强度略大于服役钢母材强度，在寿命评估中为保守和简化起见，可以近视采用服役钢母材的蠕变持久强度预测高温压力管道的剩余寿命。服役钢的蠕变延性不受应力水平影响，采用唯象的延性耗竭蠕变本构方程可以有效描述长时服役后的蠕变变形行为；修正后的Monkman-Grant关系在有效预测长时蠕变寿命方面具有一定潜力。垂直受力方向等轴晶、柱状晶晶界长度的差异是引起非均匀蠕变损伤的重要因素。考虑到管道工作过程中，蠕变裂纹长度在壁厚1/3之前扩展速率缓慢，而后快速扩展，当裂纹长度达到壁厚的1/3时需进行更换或维修。