节镍型双相不锈钢三类典型晶间腐蚀规律及机制与评价方法研究

Lean duplex stainless steel containing low nickel is one kind of the important developing stainless steels in the world. Three types of grain boundaries (Ferrite-Austenite, Ferrite-Ferrite and Austenite-Austenite)in duplex stainless steels exist side by side and differ from each other in the dynamics of heat sensitivity, leading to the requirement of new technology to evaluate intergranular corrosion resistance quantitatively; and the technology must be based on the in-deep knowledge of the evolution of grain boundary precipitates and chromium depleted zone and the relevant corrosion mechanism. This project aimed at systematically studing the thermal evolution process of three kinds of grain boundaries and the corresponding intergranular corrosion behavior and mechanism for low nickel duplex stainless steel, giving corresponding relationship between grain boundary microstructure evolution and corrosion resistance; establishing new technology to evaluate intergranular corrosion resistance quantitatively. The characteristics and advantages lie in that: in the grain boundary region of less than 500nm, revealing the evolution of different grain boundary precipitates and chromium depleted region scale with the thermal process, and established a new electrochemical technology revealing three kinds of intergranular corrosion sensitivity at the same time, and gaving corresponding corrosion mechanism clearly, and establishing the model of grain boundaries solid mass transfer and the theory of three intergranular corrosion of duplex stainless steels. The academic and practical significance lies in: the establishment and application of the method to evaluate complex phase boundaries ,providing the principle of relevant alloy design and microstructure control proposed from corrosion resistance, serving for the development and application of the relevant steels in our country.

节镍型双相不锈钢是国际上不锈钢重要发展方向之一，钢中铁素体-奥氏体、铁素体-铁素体、奥氏体-奥氏体三类晶界的同时存在以及它们在热敏化行为动力学上的差异，导致对钢种整体晶间腐蚀定量评价新技术的需求；而这种技术必须建立在对各类晶界析出相与贫铬区演变和对相关腐蚀机制的深入了解之上。本项目针对节镍型系列双相不锈钢的三类晶界热演变过程与对应的晶间腐蚀行为与机制进行系统研究，给出晶界组织演变与腐蚀性能的对应关系；在此基础上建立晶间腐蚀定量评价新技术。研究特色与先进性在于：在小于500nm近晶界区中，揭示不同晶界析出相种类与贫铬区程度与尺度随热过程的演变情况，建立同时显现三类晶间腐蚀敏感性的电化学评价新技术，清晰给出对应腐蚀机制，建立晶界固体传质与双相钢三类晶间腐蚀理论模型。学术与实际意义在于：复相晶界评价方法的建立与应用；从抗蚀性角度提出相关合金设计与组织控制原则，直接服务于我国相关钢种的开发与应用。

为满足镍资源节约、成本降低与综合性能提升的多重需求，节镍型双相不锈钢是国际上不锈钢重要发展方向之一，其微观组织中铁素体-奥氏体、铁素体-铁素体、奥氏体-奥氏体三类晶界同时存在且在敏化行为上存在差异，为解决其晶间腐蚀相关问题，必须在对各类晶界析出相及组织演变深入了解的基础上，结合相关腐蚀机制，建立新的整体晶间腐蚀定量评价技术。本课题针对节镍型系列双相不锈钢的三类晶界组织演变过程与对应的晶间腐蚀行为和机制进行系统研究，给出晶界组织演变与腐蚀性能的对应关系,建立晶间腐蚀定量评价新技术。通过对四种节镍型双相不锈钢（2002,2101,2204,2304）晶间腐蚀敏感性的研究，分别建立了针对每个钢种的DL-EPR法测试最优条件:2002 DL-EPR测试最优化条件为：1 M H2SO4 + 0.5 M NaCl，溶液温度为30℃，扫描速率为2.5mV/s；2101 DL-EPR测试最优化条件为：33% H2SO4+0.1% HCl，溶液温度为20℃，扫描速率2.5mV/s；2204 DL-EPR测试最优化条件为：1.5 M H2SO4 + 0.5 M NaCl +0.5 M HCl，溶液温度为30℃，扫描速率为1.667mV/s；2304 DL-EPR测试最优化条件为：1 M H2SO4 + 0.2 M NaCl +1 M HCl，溶液温度为30℃，扫描速率为1.667mV/s。形貌分析材料随敏化处理过程的微观组织演变情况导致了不同种类、不同敏化程度晶界晶间腐蚀抗力差异。2101鼻尖温度为700℃，敏化处理6分钟，开始有少量析出物在奥氏体/铁素体相界处析出，主要析出相为Cr2N，2002鼻尖温度为650℃，析出相为Cr2N和M23C6。不同热处理条件下（450℃-900℃不同温度-时间敏化），每种晶界的碳氮化合物析出行为不同。在600-700℃敏化较短时间可以明显观察到碳氮化合物（主要是Cr2N和M23C6）铁素体/奥氏体相界优先析出，随后在铁素体晶界析出，敏化时间进一步延长，奥氏体晶界也开始出现析出物。对2204的研究发现，敏化时间达到8h时，σ相开始在奥氏体/铁素体相界析出，128h时，铁素体晶粒内部也有大量带状σ相析出。最后，本课题探讨了双极性电化学技术在双相不锈钢晶间腐蚀表征中的应用，证实了该方法在单一测试条件下实现双相不锈钢复杂晶界的晶间腐蚀抗力表征的可行。