激光喷丸奥氏体不锈钢的亚稳点蚀行为研究

Laser shock peening (LSP) is a new surface-hardening treatment which can improve the pitting corrosion resistance of stainless steels. To date, the pitting initiation and metastable growth are not completely understood for austenite stainless steel after LSP. Therefore, in this project, the relationship between the change of the microstructure for 304 stainless steel after LSP and its metastable pitting behaviour will be investigated by using microelectrode technique. The current transients will be recorded by applying various constant potentials. By comparing the transient analysis with the defect characterization using SEM, TEM and XPS et al. statistically, a corresponding relationship between the metastable pitting and microstructure can be established. Moreover, the growth mechanism of metastable pitting will be discussed considering the changing law of transient growing rate with applied potentials and single current transient analysis. In addition, by developing an experimental formula between the main technological parameters of LSP and ipit·rpit, an optimal process of LSP should be proposed expectantly to improve the pitting corrosion resistance.

激光喷丸处理是一种新型的材料表面强化技术，能够提高不锈钢的耐点蚀性能。但目前缺乏激光喷丸后奥氏体不锈钢的点蚀早期行为研究。本项目以304不锈钢为研究对象，拟采用微电极技术研究激光喷丸后表面微观结构的变化对其亚稳点蚀形核及生长的影响。采用FIB-SEM、TEM和XPS等多种实验手段对激光喷丸后奥氏体不锈钢的微观结构和成分进行表征，并与不同极化电位下亚稳点蚀暂态数目及特征参数的统计结果进行对比，揭示亚稳点蚀的萌生机理。对单个暂态生长动力学进行分析，建立亚稳蚀坑的生长速率与极化电位的关系，揭示亚稳点蚀的生长机制；进一步建立峰值暂态电流密度与半球形蚀坑半径的乘积（ipit·rpit）与激光脉冲功率密度/喷丸次数/脉冲宽度参数的关系，定量揭示激光喷丸奥氏体不锈钢亚稳点蚀向稳态点蚀转变的倾向性，为激光喷丸工艺参数的优化提供科学依据。

激光喷丸是一种新型的材料表面强化技术，高速冲击波使材料表层晶粒细化并形成一定深度的残余压应力，能够提高不锈钢的耐点蚀性能，但目前尚不明确激光喷丸提升点蚀抗力的本质因素。本项目以奥氏体不锈钢为研究对象，采用SEM、TEM、EBSD、XRD、AFM、XPS和残余应力等多种测试手段，研究激光喷丸所致的残余压应力、晶粒细化以及形变马氏体对点蚀早期行为的影响。结果表明，存在残余拉应力和形变诱发马氏体时，亚稳点蚀形核率和生长速率都大大提高，点蚀电位显著降低；存在较大残余压应力和形变诱发马氏体时，亚稳点蚀形核率和生长速率略有提高，但此时亚稳点蚀更不容易转化为稳态点蚀。点蚀电位升高，且有残余压应力的点蚀坑不同于原始材料的蕾丝花边形态，而是呈现出开口的浅盘状；调控激光喷丸参数，抑制晶粒细化，仅获得较小表层残余压应力（<200MPa）时，由于变形孪晶和位错密度的增加，导致残余压应力的优势弱化，激光喷丸虽降低了点蚀形核率，却更易出现亚稳点蚀信号的交叠，抑制了亚稳点蚀再钝化，点蚀坑呈多个亚稳点蚀坑聚合形态；较大表层残余压应力和晶粒细化同时存在时，激光喷丸对奥氏体不锈钢耐点蚀能力有明显的提升，表现为亚稳点蚀形核率和生长速率有所降低，亚稳点蚀坑尺寸下降，点蚀电位提高。在激光功率密度6.4GW/cm2时，随着喷丸次数的增加，小角晶界增多。激光冲击5次后小角晶界占比近30%，是未冲击时的十几倍，晶粒细化程度显著。残余应力影响层深度可达400μm左右。鉴于5次冲击后明显的晶粒细化和较大的表层残余压应力（250MPa），其点蚀电位较冲击前提高近100mV。虽然表面粗糙度随着激光喷丸次数的增加而增大，但点蚀形核率仍为下降趋势，体现了激光喷丸引入的晶粒细化和较大残余压应力对耐点蚀性能提升的联合作用。该研究首次从亚稳点蚀角度探究激光喷丸对奥氏体不锈钢点蚀行为影响的本质，为激光喷丸工艺参数的优化提供科学依据。