表面细晶化对高碳奥氏体耐热钢晶间腐蚀敏感性和脱敏特性的影响

The intergranular corrosion of a novel Super304H heat-resistant austenitic stainless steel, which is being widely used in ultra supercritical fossil-fired boilers, is to be investigated. The fine grained surface microstructure via shot peening, which has been used for improving the high temperature oxidation resistance of Super304H steel, is to be utilized to solve the problem of its high intergranular corrosion susceptibility (IGCS) caused by the high carbon content. Firstly, severe plastic deformation layer with fine/ultrafine/nano grains on Super304H steel will be obtained by surface shot peening and the effect and mechanism of grain refinement on the IGCS of austenitic stainless steel will be analyzed and revealed, respectively. Then the diffusion kinetics of Cr in the coarse and fine grained austenitic stainless steels, including diffusion rate and diffusion coeficiency, will be investigated to understand the effect of grain refinement on Cr diffusion kinetics under the condition that the surface fine grain structure is stable. Finally, the rechromization possibility of eliminating the chromium depletion zone and sensitization of Super304H steel will be probed into by the non-traditional diffusion heat-treatment in austenitic stainless steels. This study aims at clarifying the general rules and mechanism of the influence of grain refinement on the IGCS of austenitic stainless steels on the one hand and exploring a novel approach to surppress or eliminate the high IGCS of Super304H steel on the other hand. The research results may realise the co-existance of good oxidation resistance, high temperature strength and intergranular corrosion resistance in the Super304H steel and have significance in both theoretical and applicable aspects.

本课题以超超临界机组中广泛使用的高碳Super304H耐热奥氏体不锈钢为研究材料，拟利用该耐热钢中改善高温抗氧化性的表面喷丸细晶结构来解决其高晶间腐蚀敏感性问题。首先利用表面喷丸在Super304H钢表面获得不同晶粒尺寸的大塑性变形层，分析和认识晶粒细化对奥氏体不锈钢晶间腐蚀敏感性影响的规律和作用机理，然后测定晶粒细化对Cr扩散速度和扩散系数的影响，掌握在细晶结构热稳定的前提下晶粒细化对奥氏体不锈钢中Cr扩散动力学的影响规律；最后探讨通过奥氏体不锈钢中的非传统扩散热处理消除Super304H钢中贫铬区和晶间腐蚀敏感性的脱敏工艺。本课题一方面澄清晶粒细化对奥氏体不锈钢晶间腐蚀敏感性影响的基本规律和机理，另一方面探索解决高碳Super304H耐热钢高晶间腐蚀敏感性的新途径，研究结果可以实现Super304H耐热钢在高温抗氧化性能、高温强度和抗晶间腐蚀性能上的共赢，具有重要的理论和现实意义。

本课题以超超临界机组中广泛使用的高碳Super304H耐热奥氏体不锈钢为研究材料，采用喷丸工艺获得表面梯度纳米细晶结构，利用此结构改善该材料高晶间腐蚀敏感性问题。首先利用表面喷丸在Super304H钢表面获得不同晶粒尺寸的大塑性变形层，发现喷丸参数0.5MPa-8min获得的细化层结构和厚度均匀，为较优的喷丸工艺。关于细晶结构对奥氏体不锈钢晶间腐蚀敏感性影响的规律和作用机理的研究发现，Super304H变形结构中的晶粒细化以及形变马氏体的增加一方面引起时效过程中M23C6快速析出，导致敏化过程加速；另一方面会促进Cr的扩散，使得M23C6析出带来的贫铬区能快速消失，材料快速到达脱敏状态。在650℃下时效脱敏能兼顾较高的脱敏效率并保存细晶结构，同时使应力诱发马氏体逆转变回奥氏体，消除其对均匀腐蚀性能的影响，因此是推荐的脱敏处理温度。通过继续延长喷丸时间增大塑性变形程度虽然能加快脱敏效率，但研究发现大塑性变形存在一个临界变形值，超过临界变形能量时，会带来过饱和变形的不均匀结构，这种变形不均匀性带来的局部应力集中高能位点在时效过程中会诱发σ相的异常早期快速析出，带来新的贫铬区，严重恶化了脱敏效果。目前研究结果的推荐脱敏工艺为0.5MPa-8min喷丸+650℃-24h时效。