合金结构钢表面高强韧渗氮层的制备、表征及强化机理研究

Gas nitriding is one of the most widely used surface modification techniques to improve surface hardness, wear resistance and corrosion property of alloys and steels through the development of a unique composite structure with an outermost compound layer and subsurface diffusion zone. However, the compound layer suffers from high internal stress that makes it friable and brittle, and it will be crushed under the conditions of impact and/or heavy loads. This will affect the service capability of the nitrided engineering components under heavy load environment. Although the toughness of the surface nitrided layer could be significantly improved by eliminating the compound layer, serious deterioration of the surface hardness and wear resistance will be induced. The project aims to develop a composite method of controlled gas nitriding and surface plastic deformation to prepare a nitrided layer with high strength and toughness on the surface of alloy structural steel. The process involves first preparing a compound-free nitrided layer with excellent toughness by controlled gas nitriding, and then improving surface strength by shot peening without apparent loss of toughness. The gradient microstructure characteristics, hardness, toughness, residual stress state and distribution of the combined treated layer will be fully investigated. Based on these results, the influence of composite treatment on surface wear properties under heavy loads, and the regulations of crack initiation and propagation during thermal fatigue can be determined. Finally, the complex strengthen mechanism is revealed. This research project will break the inverse relationship between hardness and toughness in the traditional gas nitriding process. The expected research results will not only provide technique basis for the preparation of surface modified layer with high strength and toughness for the key mechanical components, but also help broad the application of gas nitriding.

气体渗氮是工业生产中应用最广泛的表面改性技术之一，它可显著提高钢铁材料表面的硬度、耐磨及耐腐蚀性能。然而，常规气体渗氮层因带有化合物层而韧性较差，重载条件下极易剥落，尽管消除化合物层可获得良好的表面韧性，但硬度和耐磨性又会下降。针对上述问题，本项目拟发展一种合金结构钢表面高强韧渗氮层的复合制备方法——可控气体渗氮+表面塑性变形，即首先通过可控气体渗氮获得韧性优良的无化合物渗氮层，而后对其进行表面喷丸处理进一步提高强度，制备综合强韧性能优良的复合改性层。系统表征复合处理渗层的梯度组织特征、硬度、韧性、残余应力状态和分布规律，进而确定复合工艺对渗氮层重载磨损性能的影响规律、热疲劳过程中裂纹的萌生与扩展规律，最终揭示其内在的复合强化机制。本项目的研究将打破单一气体渗氮强化导致的“硬度-韧性”倒置关系，为关键零部件表面高强韧改性层的制备提供技术依据，拓宽气体渗氮工艺的应用范围。

气体渗氮是工业生产中应用最广泛的金属材料表面改性技术之一，可显著提高材料表面的硬度、耐磨性及耐腐蚀性能。然而，常规气体渗氮层因化合物层的形成导致其韧性较差，重载服役时极易破碎甚至剥落。尽管抑制化合物形成或消除化合物层可获得良好的表面韧性，但硬度和耐磨性又会下降。显然，传统气体渗氮表层强化工艺存在“硬度（强度）”和“韧性”之间的矛盾。本项目首先通过可控气体渗氮获得韧性优良的无化合物渗氮层，而后通过喷丸等技术对其进行表面塑性变形进一步提高强度，获得综合强韧性能优良的复合改性层。研究了可控气体渗氮及超声喷丸复合处理42CrMo钢表层的组织特征、硬度、韧性、残余应力状态和分布规律、耐磨性能等。低载荷磨损条件下，化合物层因硬度稍高耐磨性最优，而在重载磨损条件下化合物层因脆性较大磨损严重，可控气体渗氮及超声喷丸复合处理样品凭借优良的硬度-韧性搭配及叠加的残余应力保证了良好的抗磨损性能。通过常规气体渗氮，可控气体渗氮，预喷丸气体渗氮，及可控渗氮后喷丸处理四种工艺，在H13钢表面制备了四种不同的改性层，并从热裂纹的萌生及扩展方面比较了不同改性层的热疲劳性能。与常规气体渗氮化合物层相比，可控气体渗氮表层因韧性优良其裂纹在渗氮表层的扩展较缓慢，但因硬度（分布）稍低，其热裂纹沿深度方向扩展较快。预喷丸渗氮处理显著滞后了裂纹的萌生及其在深度方向的扩展，但表层热裂纹出现后长大迅速，且剥落面积更宽；而可控气体渗氮滞后了裂纹在表层的萌生，又延缓了其在深度方向上的扩展。初步研究了52100高碳铬轴承钢的碳氮共渗行为，依据不同参数搭配下的碳氮共渗热/动力学特性及其对显微组织及残余应力的影响，给出了适用于高碳铬轴承钢碳氮合金化的最佳工艺窗口。本项目的研究优化了常规气体渗氮工艺中存在的“硬度-韧性”倒置关系，为关键零部件表面高强韧改性层的制备提供了技术依据，拓宽了气体渗氮工艺的应用范围。