基于EBSD的核电站主管道粗晶奥氏体不锈钢早期疲劳损伤超声评价研究

The ultrasonic evaluation on the early-stage fatigue damage of primary piping has significant implications for the safety operation of nuclear power plant. How to detach the elastic aniosotropic effects of coarse-grain austenite from the ultrasonic features during the damage process are the precondition and foundation of clarifying the ultrasonic response physical mechanism and establishing the ultrasonic evaluation method. So in this project, it is proposed a detaching method based electron backscatter diffraction (EBSD) technique to the coupling effects between the coarse-grain structrue and the fatigue damage. Firstly, the 2-D distribution characteristics of the austenite crystal orientation and the persistent slip bands will be tested, and a intrinstic damage index will be abstracted. Consequently, the fatigue damage will be determined quantitatively and the coupling ultrasonic features will be modified. Then,the barrier effects of the persistent slip bands to the ultrasonic propagation,the crystal orientation distribution and the grain diameter will be considered, a heterogeneous damage evaluation model will be established.In virtue of numerical calculations and experimental examinantions, the relationship between the ultrasonic features, the crystal orientation, and intrinstic damage index will be eastablished and the ultrasonic response physical mechanism of the early-stage fatigue damage could be clarified. The initiation of fatigue crack in the scale of submillimeter could be predicted. The results will provide an important solution to the development of ultrasonic evalution theory for mechanical damage in coarse-grain materials, and surpport for the analysis of early-stage fatigue damage of primary piping material in nuclear power plant.

核电站主管道早期疲劳损伤无损评价对保证管道运行安全十分关键。如何从声学特性测试结果中剥离粗晶奥氏体弹性各向异性的影响，提取反映损伤自身的声学特征，已成为粗晶材料早期疲劳损伤超声评价的瓶颈。本项目以主管道粗晶奥氏体不锈钢为对象，针对粗晶材料、疲劳损伤对声学特性的混叠效应，提出一种基于电子背散射衍射（EBSD）技术的解耦方法。研究不同疲劳阶段驻留滑移带及晶粒晶体取向演变规律，计算与晶体取向角相关的损伤因子，定量评价疲劳损伤程度，修正混叠声学特性数值变化；考虑驻留滑移带对声传播的阻滞效应，借助晶粒形貌、尺寸和取向分析，提出早期疲劳损伤超声评价模型；通过模拟仿真和实验测试，建立声学特性与加载周次、损伤因子的对应关系，揭示粗晶奥氏体不锈钢早期疲劳损伤的声学响应规律，实现亚毫米级疲劳裂纹预报。项目将从材料固有属性出发，丰富粗晶材料力学损伤声学评价理论，为核电站主管道早期疲劳损伤超声评价提供理论支持。

粗晶奥氏体不锈钢广泛应用于核电主管道，其等轴晶直径约1-5 mm，柱状晶长度可达十几mm，是典型的粗晶材料，弹性各向异性显著。由于服役工况复杂，十分容易产生疲劳损伤。超声脉冲回波测试技术灵活方便，其中水浸聚焦方式能实现声束焦斑的有效控制，声学信息能反映材料内部的微观结构演变，在评价材料疲劳损伤方面均具有明显优势。然而，如何从声学特性测试结果中剥离粗晶奥氏体弹性各向异性的影响，提取反映损伤自身的声学特征，已成为粗晶材料早期疲劳损伤超声评价的瓶颈。电子背散射衍射技术（Electron Back-scatter Diffraction，EBSD）能够将微观组织与晶体学结合起来，从晶粒尺度上同时描述晶体取向和损伤程度。因此，本项目以主管道粗晶奥氏体不锈钢Z3CN20-09M早期疲劳损伤为对象，针对粗晶材料、疲劳损伤对声学特性的混叠效应，提出一种基于EBSD技术的解耦方法，以期为核电站主管道早期疲劳损伤无损评价提供理论支持。项目研究了Z3CN20-09M奥氏体不锈钢宏微观组织和晶粒晶体取向分布，分析了与晶体取向角相关的EBSD损伤评价参数的影响因素，并获得了不同损伤阶段评价参数演变规律，定量评价了疲劳损伤，发现不同取向晶粒呈现不同的损伤演变规律；建立了基于EBSD的粗晶奥氏体超声评价模型，基于模拟仿真和水浸聚焦超声脉冲回波实验测试分析了超声波的传播规律，获得了二维声慢度面和声衰减的变化规律，揭示了粗晶奥氏体不锈钢早期疲劳损伤的声学响应规律；建立了EBSD损伤参数变化、声学特性变化之间的关系，剥离粗晶弹性各向异性的影响，实现了粗晶奥氏体疲劳损伤后声学特性修正和粗晶、损伤混叠效应的解耦，揭示了粗晶奥氏体不锈钢早期疲劳损伤的声学响应规律，从材料固有属性出发，发展了粗晶材料早期疲劳损伤的超声评价方法，丰富了粗晶材料力学损伤声学评价理论。