含体积缺陷的高温焊接管道寿命评估理论与方法研究

For energy conservation and greenhouse gas emissions reduction, China is vigorously developing clean and efficient ultra-supercritical thermal power units. However, many volume defects such as weld porosities and slags had often been detected in the regular inspections on high-temperature thick-walled welded pipes in many ultra-supercritical thermal power units. So the integrities of pipes containing defects need be valued accurately for the purpose of safety and saving money. However, there are not specialized evaluation methods for volume defects in creep regime in the current integrity assessment standards. If applying the free assessment condition based on elastic-plastic analysis, the assessment may be dangerous while the result may be too conservative if taking the volume defects as cracks. So, supervision departments of power plants often can't make proper maintenance decisions because of the lack of assessment methods for volume defects. In the present project, the evaluation theories and methods for life of high temperature welded pipes containing volume defects such as weld porosities and slags will be investigated based on creep damage mechanics, creep fracture mechanics, elastic-plastic fracture mechanics and numerical methods, combined with creep crack growth tests. In the study, according to the four stages of volume defect failure evolution (creep damage - creep crack initiation - creep crack growth - failure), the study contains equivalent crack method for volume defects in creep regime, life prediction models of creep crack initiation and creep crack growth considering the combined effects of residual stress and constraint and engineering calculation methods for crack-tip constraint parameters of pipes and so on. At last, the evaluation theories and methods for life of high temperature welded pipes containing volume defects considering combined effects of residual stress and constraint will be established.

为了节能降耗，我国正在大力发展高效洁净的超超临界火电机组。然而，近年来在许多机组的高温厚壁管道中频发焊接气孔、夹渣等体积缺陷。当前完整性评估规范中并没有专门针对蠕变机制下体积缺陷的评估方法，而套用弹塑性条件下的免评条件或直接作为裂纹处理，往往导致评估结果或偏保守或偏危险，严重困扰着电厂运行监督部门对检修方案的制定。本项目拟针对高温厚壁管道焊接接头中常见的气孔、夹渣等体积缺陷，基于蠕变损伤力学、蠕变断裂力学、弹塑性断裂力学及计算科学等理论与方法，结合高温蠕变裂纹扩展试验，根据体积缺陷处蠕变损伤-蠕变裂纹萌生-蠕变裂纹扩展-失效的全寿命周期演变规律，对蠕变机制下体积缺陷的等效裂纹化方法、考虑残余应力和拘束复合效应的蠕变裂纹萌生孕育期预测模型与稳定扩展寿命预测模型、管道拘束参量的工程计算方法等方面开展研究，最终建立考虑多因素的含体积缺陷的高温管道寿命评估理论与方法。

清洁、高效、高参数的超超临界机组的高温厚壁管道中频发焊接气孔、夹渣等体积缺陷，然而当前完整性评估规范中并没有专门针对蠕变机制下体积缺陷的评估方法，而套用弹塑性条件下的免评条件或直接作为裂纹处理，往往导致评估结果或偏保守或偏危险，严重困扰着电厂运行监督部门对检修方案的制定。本项目基于损伤力学理论，通过含埋藏球型缺陷、含长条型、椭圆型缺陷的管道损伤行为研究，基于提出的新的物理参量——平均等效应力，建立了不同类型体积型缺陷的蠕变裂纹萌生寿命预测模型；提出了考虑与载荷无关的拘束参量C*-Q*双参量法蠕变裂纹孕育期预测模型，并考虑基于应力机制的蠕变性能演变规律，发展了改进的C*-Q*双参量法蠕变裂纹孕育期预测模型；提出了复合加载下、耦合拘束效应与残余应力的蠕变孕育期预测模型，建立了复合加载下的蠕变裂纹孕育期的理论计算方法；探明了不同的裂纹深度、不同试样厚度、不同试样形式以及含不同裂纹类型下管道的蠕变裂纹孕育期变化规律并建立工程计算方法，为高温服役构件的寿命评估提供了更加准确的预测模型和计算方法，并结合数值分析和试验验证了预测模型的适用性和准确性；发展了考虑拘束效应的C*-Q（Q*）双参量法蠕变裂纹扩展预测模型，可以准确反映由试样形式、裂纹深度、试样几何尺寸变化引起的裂纹扩展行为的变化；基于载荷无关蠕变拘束参量Q*，表征了含表面裂纹的高温管道的裂纹前沿拘束水平变化规律，建立了含表面裂纹高温管道的拘束参量Q*的工程计算方法；形成了含体积缺陷高温管道的全寿命评估体系。此外，还研究了先进高温耐热钢的蠕变变形与断裂机理、合金高温低周疲劳性能、不同保载时间下钢蠕变-疲劳裂纹扩展行为，为新一代超超临界机组建设、制造、运行和维护中的完整性评估提供数据支持。