不锈钢表面“膨胀”α相层形成与强韧化机理

Martensitic stainless steel is an important raw material used for critical components of high-end equipment in aerospace, nuclear power and shipping ocean engineering and other fields. Low temperature nitriding/carburizing of martensitic stainless steel can form an “expanded” α phase on its surface, i.e. metastable supersaturated solid solution phase of nitrogen/carbon atoms solid soluble in the α-Fe interstitial site, and then the stainless steel possessed an excellent comprehensive performance, but the formation mechanism of the phase is still not clear, and it is also difficult to obtain a single “expanded” α phase quickly, which are the bottleneck of restricting its development and application to be resolved. This application will be introduced the rare earths (RE) to low temperature plasma nitriding/carburizing/nitrocarburizing process, after precisely regulation, a single “expanded” α phase layer could be formed on martensitic stainless steel surface. Based on the analysis of microstructure and the characterization of strengthening and toughening of the layer, as well as the electronic structure, energy, and properties of the “expanded” α phase calculated by first-principles, the microstructure evolution law of “expanded” α phase layer could be studied; the layer growth dynamics equation of “expanded” α phase layer could be established; the diffusion behavior of the penetration elements during low temperature penetration with RE and the effects of RE on the structure stabilities of the “expanded” α phase were clarified; then the layer formation and the strengthening and toughening mechanisms of “expanded” α phase were revealed, and then the regulation technology of the single “expanded” α phase layer on martensitic stainless steel was obtained, which would expand the application of existing stainless steel in critical components of high-end equipment. Therefore, the present study possessed not only important theoretical significance but also application value.

马氏体不锈钢是航空航天、核电和船舶海洋工程等领域高端装备关键件制造的重要原料。低温渗氮/碳处理可在马氏体不锈钢表层形成“膨胀”α相，即氮、碳间隙原子在α-Fe中的亚稳过饱和固溶体，综合性能优良，但其形成和强韧化机理尚不清楚，且难以快速获得单一相层，已成为制约其发展和应用的瓶颈。本申请将稀土引入到低温等离子体渗氮/渗碳/氮碳共渗过程中，经调控，在马氏体不锈钢表面快速获得单一“膨胀”α相层。基于渗层显微组织结构分析、强韧化性能表征以及“膨胀”α相电子结构、能量和性质的第一性原理计算，研究“膨胀”α相层组织结构演变规律，建立“膨胀”α相层生长动力学方程，阐明低温稀土共渗过程中渗入元素的扩散行为和稀土对“膨胀”α相结构稳定性的影响，揭示“膨胀”α相层形成与强韧化机理；形成马氏体不锈钢表面单一“膨胀”α相层调控技术，拓展现有不锈钢在高端装备关键件中的应用。因此，本研究具有重要理论研究意义和应用价值。

马氏体不锈钢是航空航天和船舶海洋工程等领域高端装备关键件制造的重要材料。低温渗氮/碳处理可在其表面形成“膨胀”α相层，综合性能优良，但其形成和强韧化机理尚不清楚，成为制约其发展和应用的瓶颈。本研究对典型不锈钢进行了低温等离子体稀土多元共渗处理，获得了不锈钢表面“膨胀”α相层调控技术，揭示了“膨胀”α相层组织结构演变规律，计算了“膨胀”α相的电子结构、性质以及氮碳的扩散激活能，采用实验表征和第一性原理计算相结合的方法，揭示了“膨胀”α相层的形成与强韧化机理。主要结论如下：（1）设计优化了有无稀土含氮碳“膨胀”α相的超晶胞模型，对有无稀土“膨胀”α相的电子结构和性质进行了第一性原理计算，证明了无稀土含氮碳“膨胀”α 相形成的可能性。在“膨胀”α相模型在氮含量＜25 at.%，碳含量＜20 at.%时，均可形成，且满足能量、结构和力学稳定性条件。（2）设计优化了低温等离子体多元共渗工艺，进行了动力学试验研究，建立了渗层增长动力学方程，分析了氮、碳原子的扩散行为。经低温多元共渗处理，可在不锈钢表面形成单一“膨胀”α相层，其中含碳“膨胀”α相层厚度与时间呈近似抛物线关系，渗碳动力学方程分别为y=-3x2+25x-32。计算得到碳原子和氮原子的扩散激活能分别为262.13 kJ/mol和78.11 kJ/mol。（3）对“膨胀”α相层的显微组织结构进行了分析，揭示了“膨胀”α相层组织结构演变规律，结合第一性原理计算结果，揭示了其形成机理。渗层主要由氮碳“膨胀”α相组成且晶粒尺寸非常细小，稀土添加有利于“膨胀”α相的形成和晶粒的细化；随着温度的提高和时间的延长，OM显示组织变暗，XRD和TEM也表明有氮、碳化合物形成。（4）对渗层的硬度、耐磨性和耐蚀性能进行了测试，结合第一性原理计算，阐明了“膨胀”α相层强韧化机理。含“膨胀”α相的渗层硬度显著提高，稀土添加可提高100HV左右，耐磨性明显改善；膨胀α层均为韧性的；“膨胀”α相的渗层耐蚀性不降低，且部分条件下有所提高。综上，本研究具有重要的理论研究意义和应用价值。