改进奥氏体不锈钢加工表面抗应力腐蚀性能的原理与方法研究

Stress corrosion cracking （SCC）is an major failure mode for the service materials exposed to pressurized water reactor (PWR) primary environment. Conventional surface cutting inherently affects the microstructure and mechanical properties of the subsurface material，and generally results in reduction of the stress corrosion resistance of austenitic stainless steels. This project aims at the parts that need to be machined to ensure geometric accuracy, and studies the principle of new machining-based surface treating processes for controlling surface microstructure transformation to improve the resistance to intergranular stress corrosion. At first, quantitative analysis will be conducted to study the relationship between the machining-induced plastic deformation and the microstructure evolution of the material by relating grain and sub-grain sizes to the deformation parameters. Based on the surface machining process proposed in this project, the relationship between the thermal-mechanical loading and the microstructure and grain boundary of the subsurface material will be established. Then, by means of molecular dynamics simulation, we will simulate the response of the special grain boundaries and dislocation channels to the environment and stress, and reveal the inherent relationship between SCC and the microstructure and mechanical properties of the material caused by the surface treatment. At last, under the simulated high temperature water environment, the process parameters of surface treatment based on machining is optimized through the SCC susceptibility tests of austenitic stainless steels, and the improvement of surface treatment on stress corrosion resistance is evaluated. The research results can provide the basic theoretical support and economic method for improving the stress corrosion resistance of machined parts servicing under high temperature water environment of nuclear power stations.

应力腐蚀是核电站服役材料重要失效形式，采用传统工艺参数的车、铣等表面切削加工会影响材料表层的微观结构和物理特性，通常会降低奥氏体不锈钢应力腐蚀性能。本项目针对需要采用切削加工保证几何精度的零件，研究利用加工导致塑性变形效应调控材料表层微观结构、提高加工表面抗沿晶应力腐蚀的原理。首先定量分析研究不同切削载荷分布产生的加工塑性变形对材料微结构的影响，建立热、力载荷与表层材料微结构和晶界变化关系模型；然后，通过分子动力学模型，模拟特殊晶界、位错等对环境因素和应力的响应，揭示材料表面处理导致的材料微观结构和机械性能与应力腐蚀开裂之间的内在联系。最后，在模拟核电压水堆高温水环境下，通过应力腐蚀实验研究，优化基于切削的表面处理方法的工艺过程，评价处理表面对应力腐蚀性能的改进效果。研究成果可为改进核电站高温水环境下零部件加工表面抗应力腐蚀性能提供基础理论支持和经济方法。

传统的车、铣等切削加工在表面成形的同时改变了材料表层的微观结构和物理特性、产生表面残余拉应力，降低了奥氏体不锈钢耐腐蚀和抗应力腐蚀性能。本项目研究了表面非均质微结构和表面晶界特征分布优化增强切削加工表面耐晶间腐蚀和抗应力腐蚀性能的原理，提出用机械加工过程的热力作用于材料表面形成表面纳米梯度结构和基于加工预变形处理的表面晶界工程方法。.（1）研究了304、316L奥氏体不锈钢材料车削、铣削和表面机械碾磨加工对表面微结构和机械性能的影响，建立了加工表层微结构和机械性质的预测模型，揭示了加工工艺参数对加工表面机械特性和微观结构的影响规律。.（2）基于反应力场分子动力学模拟，分析了高温水环境下FeCr合金表面初始氧化成膜形成与微结构演变过程。开展了不同晶界类型在应力作用下响应的分子动力学模拟，分析了不同晶界与位错在应力作用下裂纹形核与扩展过程。.（3）提出了以表面碾压碾磨形成纳米梯度结构提高抗应力腐蚀性能的机械加工方法，开展了恒应变应力腐蚀和慢应变实验，揭示了表层微观结构、应力分布与应力腐蚀微裂纹萌生与发展的联系。发展了表层梯度纳米结构在抗应力腐蚀结构的应用，将不锈钢加工表面应力腐蚀裂纹萌生应力水平提高了4倍以上。.（4）实现了一种基于简单机械加工预变形的表面晶界工程方法，其利用加工导致的表面梯度塑性变形驱动退火过程中表层微结构演化。提出的表面晶界工程方法在304奥氏体不锈钢表面产生75%以上的低∑重位点阵(CSL)晶界，改善材料的晶间腐蚀性能。 .本项目所发展的加工方法可在复杂曲面零件完成，突破了传统晶界工程和表面纳米化加工只能在平面零件上实现的限制，扩展了纳米梯度结构和表面晶界工程在零件抗应力腐蚀开裂领域的应用。.