690合金表面渗氮纳米晶的腐蚀与微动磨损机理研究

The failure of steam generator heat transfer tube is one of the main reasons for the nuclear power plant shutdown accident, and the failure is mostly caused by stress corrosion cracking and fretting damage. Surface nanocrystallization has been proved to be a promising method to improve the corrosion and wear resistance for metals. In this project, ultrasonic peening is used to produce a nanocrystallization layer on the surface of 690 alloy, then nitrided naocrystals is obtained by plasma nitriding process. Some researches include: Studying the effect of nitride solid solution and precipitation on the grain growth during the heating process to understand the mechanism of nanocrystal stabilization. Based on electrochemical polarization curve and electrochemical impedance test results, the formation and dissolution model for passive film are established according to the point defect model (PDM) theory in order to clarify corrosion mechanism and enrich PDM theory. Surface electron working function is calculated using density functional theory to explain the galvanic corrosion from the electronic level. Analysis the result of fretting curve, friction coefficient with the number of cycles, wear scar characteristic and so on to investigate the influence of nitride solid solution and nitride precipitation on fretting wear mechanism. Through experimental and theoretical researches, the purpose of improving thermal stability, corrosion resistance and fretting wear resistance of nanocrystals is achieved, and a scientific foundation for the application of surface nanocrystallization in 690 alloy is expected to establish.

蒸汽发生器传热管破损是核动力装置停堆事故的主要原因之一，而破损的主要原因是应力腐蚀开裂和微动磨损。金属表面纳米化已为提高金属耐蚀性和耐磨性的重要技术手段，因此本项目以传热管材料690合金作为研究对象，采用超声喷丸表面纳米化和等离子渗氮工艺结合，在690合金表面获得渗氮纳米晶。研究氮化物固溶体及析出相等对晶粒长大的影响规律，了解纳米晶稳定化机理；结合电化学极化曲线与电化学阻抗测试结果，应用点缺陷模型理论建立钝化膜形成和溶解模型，阐明腐蚀机理，丰富点缺陷模型理论；利用密度泛函理论计算合金表面电子功函数，从电子层面解释电偶腐蚀发生的原因；通过对微动运行曲线、摩擦力及摩擦系数与循环次数关系曲线、磨痕分析等测试，探索氮化物固溶体和析出相等对磨损机理的影响规律。通过实验和理论研究，达到提高纳米晶热稳定性、耐蚀性和抗磨损性能的目的，为表面纳米化技术在690合金的应用奠定科学基础。

压水堆核电站蒸汽发生器的传热管是核反应堆一、二回路的压力边界，服役于高温高压强腐蚀性的恶劣环境，要求其材料具备非常优异的热稳定、抗腐蚀和抗微动磨损性能。目前，690镍基合金是应用最广泛的核反应堆蒸汽发生器材料，由于长时间在高温高压强腐蚀性环境中服役易于失效。基于此，本项目采用表面纳米化与等离子渗氮相结合的工艺方法，旨在进一步提高690镍基合金的热稳定、抗腐蚀和抗微动磨损性能。实验结果表明，采用超声滚压技术对690合金表面实施纳米化处理，经过0.2mm压下量的表面纳米化处理后，其硬度值由180HV0.05提高到337 HV0.05，材料抗拉强度增加了31.6%，断裂延伸率降低了53.18 %；渗氮处理最佳条件为400℃+8h，等离子渗氮后690合金的硬度值增加到580 HV0.05。随后对690镍基合金的热稳定性、磨损和抗微动磨损行为和耐蚀性进行分析，结果表明：纳米晶渗氮后有助于提高材料表面纳米晶的热稳定性；磨损行为分析表明，纳米化处理后材料的磨损机理为磨料磨损，而纳米化+渗氮处理后，磨损机制则主要为疲劳磨损，纳米化+渗氮处理可以大幅度提高材料的抗磨损性能，而相较于磨损实验提高的幅度，纳米化+渗氮处理提高微动磨损性能幅度不是很大；通过密度泛函理论研究发现，渗氮后电子功函数呈增大趋势，说明渗氮可以有效提高Ni合金表面的稳定性。渗氮处理可降低氧化性分子与Ni表面之间相互作用的强度，促进H2O分子的分解，并提供了表面与Cr作用的反应位点，增强氧化产物CrxOy在表面的稳定性，水环境下的渗氮纳米晶腐蚀行为实验进一步验证了表面纳米化可以显著提高耐腐蚀性，渗氮处理可进一步提高抗腐蚀性能。项目研究成果为进一步实现690镍基合金表面纳米化及渗氮的实际应用，提高合金的抗腐蚀抗磨损性能以及热稳定性，提供了理论和实践上的依据。