表面车削加工工艺对国产核级不锈钢材料后期腐蚀行为的影响

Different machining processes of mechanical parts can cause surface damage layers with different levels. The surface deformed layer can affect the corrosion behavior and service life of these mechanical parts a lot during the following service process. As a result, it is a key issue for the fabrication of the mechanical parts with long life that the selection of proper machining parameters and the removal of surface damage. The purpose of this study is to study the influence of different turning parameters on the corrosion behavior of domestically made austenitic stainless steel with nuclear grade widely used in the advanced pressured water reactors (PWRs) of our country. The microstructures of the near surface deformed layers of samples with different surface states caused by different turning processes will be characterized by series advanced analysis methods,such as synchrotron radiation, focused-ion-beam system and high resolution transmission electron microscope et al. The chemical composition and microstructures of the surface oxide scales formed on stainless steel with different surface state in the simulated high temperature and high pressure environments of nuclear power plants will also be analyzed.The effect of near surface microstructure of samples on the growth of surface oxide films in the simulated primary water of PWRs will be studied. The effect of surface turning process on the stress corrosion cracking (SCC) sensitivity of samples with different surface state will also be discussed. The finial goal of this study is to find the proper parameters of surface lathe turning operations for the austenitic stainless steel. The experimental results will be useful for the mechanical machining and the later safe service of the new generation nuclear power plants of our country.

机械零部件加工工艺的不同选择可能会造成不同程度的表面损伤层。该表面损伤层在后续服役过程中会对材料的腐蚀行为和服役寿命产生很大影响。因而在保证光洁度的前提下，合理选择加工工艺参数和降低表面损伤是制备长寿命构件的关键。本项目拟对在我国先进压水堆核电站中具有广泛应用的国产核级奥氏体不锈钢进行不同加工工艺的表面车削处理，综合利用如同步辐射,聚焦离子束和高分辨透射电镜等先进分析手段,在微纳米尺度研究车削加工工艺对材料近表面变形层微观结构的影响；详细表征由于加工工艺不同导致的不同表面状态样品在模拟核电高温高压水中表面生长氧化膜的化学组成与微观结构，揭示不同表面状态样品在模拟核电环境中生长的表面氧化膜的保护性与其微观结构之间的联系；阐明不同表面状态样品在模拟核电环境中应力腐蚀开裂敏感性差异及微观机制。最终确定该材料适宜的表面车削加工参数的范围，为我国正在发展的新一代核电站加工制造及安全服役提供基础数据。

大型核电装备表面车削加工工艺的选择会显著影响工件近表面变形层的微观结构，进而会影响装备在模拟核电高温高压水中的长期服役行为。本项目通过与国内大型核电装备制造企业进行合作，采用不同车削加工参数，制备了6种具有工况现场代表性的不同表面状态的样品。建立了车削加工参数与样品的表面粗糙度与近表面变形层微观结构之间的关系。发现相同切削深度和近似切削速度，进给量越大，样品表面越粗糙。表面车削加工能够在不锈钢样品表面形貌明显的变形层，促使近表面由原始的奥氏体组织转变为腐蚀溶解速度更大的马氏体组织。样品近表面变形层截面均由三部分组成：最表面为剧烈变形导致的纳米晶区，之后是微米晶区，而靠近正常基体位置则是滑移带区；因而由样品最表面向基体方向，材料的变形程度逐渐降低。当进给量相同和切削深度也相同时，切削速度的降低会导致近表面变形层中亚晶粒尺寸和微观应变均增加；高的切削速度主要导致晶粒细化，而低的切削速度主要导致微观应变增加。相同切削深度和近似切削速度，进给量的降低会导致样品近表面变形层中的亚晶粒尺寸和微观应变增加；高的进给量主要导致晶粒细化，而低的进给量主要导致微观应变增加。表征了不同样品表面腐蚀行为的差异。不同样品表面生长的氧化膜的厚度、致密度及局部化学组成具有明显差异，主要由不同样品近表面的微观结构差异引起的。近表面严重的变形与组织变化能够显著提高不锈钢材料的应力腐蚀敏感性。在实际机械加工过程中，应该合理选择机械加工参数，严格控制样品近表面变形程度。因而推荐的车削加工工艺是降低走刀量，配合中等或较快的切削速度。当走刀量低于0.15 mm/r时，可以适当提高切削速度至400 r/min，切削深度也可以增加至2 mm；当走刀量大于等于0.2 mm/r，切削速度必须低于400 r/min；同时应尽量避免过大的走刀量，如0.5 mm/r。工况现场的车削机加工参数的具体选择，不但要考虑表面粗糙度标准，还必须要考虑机加工过程中工件近表面的变形程度。