涡轮增压器转轴渗氮钢的环境－缺陷致裂机理及超长寿命评估

Rotor shaft is one of the core components of turbocharger with high failure rate, the reliable assessment of service performance, failure mechanism and safe life for its structural material has always been the key link restricting its development with high performance and long life. The current design method based on the conventional fatigue limit and disregarding the effect of service environment is far from meeting the requirement of the rotor shaft for long term and safety use. In this proposal, starting from the characterization of mesomechanical properties of microstructure and defect, and considering the effect of typical machining feature and service environment of the rotor shaft, the fundamental research on ultra-long life fatigue properties of nitriding Cr-Mo steel for the rotor shaft will be performed. Some scientific problems, such as failure laws, failure mechanisms and multiscale life assessment in the ultra-long life regime under the effect of environment, will be investigated. The reciprocal effect of some factors such as nitriding, notch and moderate temperature on ultra-long life fatigue S-N characteristics and crack growth behavior of this steel will be clarified. Some evaluation models of the probabilistic competing failure mode, the fatigue strength related to the threshold value of controlling crack propagation and the multi-scale crack growth, will be established, and finally an ultra-long life assessment method of this steel based on the failure mechanism and the combination of the safe life design and the damage tolerance design will be proposed. This not only can provide a fundamental reference for material selection and design of new rotor shaft, but also can provide a safe and reliable guidance of residual life assessment and failure prevention for ageing rotor shaft.

转轴是涡轮增压器中发生故障率较高的核心部件之一，其结构材料服役性能、失效机理及安全寿命的可靠性评估一直是制约其高性能且长寿命研发的关键环节，无视服役环境影响的传统疲劳极限设计方法已远不能满足其长期安全使用的要求。本项目从微结构/缺陷的细观力学性能表征入手，虑及转轴典型加工及服役环境特征，开展渗氮转轴Cr-Mo钢超长寿命疲劳性能的基础研究，揭示材料在环境－微结构/缺陷影响下的超长寿命疲劳失效规律、机理以及多尺度超长寿命评估等科学问题，阐明渗氮、缺口、中温等多因素交互作用对材料超长寿命疲劳S-N特性及裂纹扩展行为的影响，建立材料概率竞争失效模式、疲劳强度-裂纹扩展门槛值以及多尺度裂纹扩展等评估模型，形成一套基于失效机制及安全寿命设计和损伤容限设计相结合的多尺度超长寿命评估方法。这不仅能为新转轴的选材、设计提供基础参考依据，又能为高龄化转轴的剩余寿命评估、失效预防做出安全可靠性指导。

涡轮增压器由于其高效、节能、减排等优点已逐步在汽车发动机行业得到广泛应用。转轴是涡轮增压器中发生故障率较高的核心转动部件之一，其结构材料服役性能、失效机理及安全寿命的可靠性评估一直是制约其高性能且长寿命研发的关键环节，无视服役环境影响的传统疲劳极限设计方法已远不能满足其长期安全使用的要求。本项目从微结构/缺陷的细观力学性能表征入手，虑及转轴典型加工及服役环境特征，开展了（1）渗氮层微结构及内部缺陷的力学性能表征和评估、（2）常规超长寿命疲劳S-N 特性、失效机理及缺口效应、（3）中温-载荷交互作用对超长寿命疲劳性能及失效机理的影响以及（4）基于失效机制的多尺度超长寿命评估方法四个方面的研究。重点揭示了表面渗氮强化钢在轴向拉压加载下的多元化疲劳失效模式（短寿命表面失效、长寿命内部失效（夹杂或不均匀微结构-鱼眼）及超长寿命内部失效（夹杂-细晶粒区-鱼眼））及机理；明确了缺口效应对表面失效疲劳寿命的影响较大，而对内部失效疲劳寿命影响很小的特征；阐明了高温会降低表面或内部疲劳强度且促进材料发生内部失效的可能性；以多元化疲劳失效模式为基础，结合混合威布尔分布，构建了一种多元化失效模式的概率S-N曲线模型；研究了夹杂尺寸、鱼眼尺寸、疲劳寿命和应力幅值之间的关系，同时虑及夹杂尺寸分布特性，构建了一种基于夹杂和鱼眼失效特征的疲劳强度评估模型和疲劳寿命设计方法；基于“薄弱环节”的概念，通过特征参数求解修正，建立了超高周疲劳竞争失效的局部失效概率预测模型；考虑平均应力、应力比、温度的影响，提出了虑及裂纹萌生及扩展的超高周疲劳内部失效全寿命预测方法；虑及应力比、表面残余应力、缺口效应、温度的影响，提出了一种虑及裂纹萌生及扩展的超高周疲劳表面失效全寿命预测方法。该项目成果在丰富了超高周疲劳研究的基础上，不仅能为新转轴的选材、设计提供基础参考依据，又能为高龄化转轴的剩余寿命评估、失效预防做出安全可靠性指导。