等离子熔覆Fe-Cr-C系熔覆层改性研究

发布时间：2018-03-29 16:33

  本文选题：等离子熔覆　切入点：Fe-Cr-C系合金　出处：《中国矿业大学》2017年硕士论文

【摘要】：等离子熔覆技术具有高精度、机械化等优点,是机械类零部件表面强化的核心技术之一。Fe-Cr-C系合金熔覆层内含有大量的碳化物强化相,硬度及耐磨性均较高,但高C与Cr含量的过共晶Fe-Cr-C系合金熔覆层内碳化物粗大且分布不均匀,内部切削作用强,脆性较大,在使用过程中易发生裂纹扩展及脆性断裂造成熔覆层失效,限制着该合金在表面强化领域的使用。本文通过等离子熔覆工艺制备不同C与Cr含量的Fe-Cr-C系合金熔覆层,对比分析其性能的差异,通过加入纯Ti粉原位生成TiC对过共晶Fe-Cr-C系合金熔覆层进行改性研究,探索不同Ti含量对于该合金体系性能的影响,结合第一性原理对熔覆层内碳化物硬质相进行弹性常数理论计算,根据Voigt-Reuss-Hill近似计算碳化物弹性模量及泊松比,参考半经验公式对碳化物的微观力学性能进行理论计算分析,为设计合金体系提供理论基础。亚共晶Fe-Cr-C系熔覆层组织以胞状晶为主,表面平均硬度值为42.6HRC,磨损过程中以黏着磨损为主,耐磨性较差;近共晶熔覆层表面平均硬度值为52HRC,熔覆层开始出现长条状亚共晶碳化物,耐磨性有一定的提升;过共晶Fe-Cr-C熔覆层内弥散的分布着六边形及粗条状碳化物硬质相,表面平均硬度值达到62HRC,以磨粒磨损为主,耐磨性最强,但内部存在应力裂纹,需要进一步研究改良。过共晶Fe-Cr-C系熔覆层硬质相主要为正交结构六棱柱形Cr7C3(PNMA),通过对熔覆层冷却过程相变及热力学分析,在1000℃以上时含Ti熔覆层冷却过程中优先生成TiC,降低熔覆层过共晶程度的同时在熔覆层内形成局部成分过冷,碳化物生长具有一定的方向性,且优先形成的TiC作为Cr7C3的异质形核核心,起到细晶强化作用。Cr7C3的均匀分布降低熔覆层内部的微观切削作用及残余应力,减小熔覆层的裂纹产生倾向。过量的Ti导致熔覆层内部出现气孔,Ti含量为0.5%-1.0%时的过共晶Fe-Cr-C系熔覆层呈现出最佳的微观力学性能。结合第一性原理对熔覆层硬质相微观力学性能研究,熔覆层硬质相Cr7C3(PNMA)呈现出金属键特性,而TiC则以共价键为主,价键较强。通过Voigt-Reuss-Hill近似计算,正交结构PNMA型Cr7C3及立方结构TiC晶体均满足结构稳定性,且正交结构PNMA型Cr7C3呈现出一定的塑性,与纳米探针及显微硬度实验结果相一致。结合高硬度材料理论微观硬度值半经验公式计算,PNMA型Cr7C3理论硬度值为13.13GPa,TiC的理论硬度值则为19.73GPa,两种硬质相理论硬度的比值为0.67,与实验中测得两种强化相的比例相对应,说明模拟计算结果的可靠性,此计算方法可以为高硬度材料的开发提供一定的理论参考基础。
[Abstract]:Plasma cladding technology has the advantages of high precision and mechanization. It is one of the core technologies for surface strengthening of mechanical parts. Fe-Cr-C alloy cladding contains a large number of carbide strengthening phases, and its hardness and wear resistance are high. However, Hypereutectic Fe-Cr-C alloy with high C and Cr content is characterized by large and uneven distribution of carbides in the cladding layer, strong internal cutting and high brittleness. Crack propagation and brittle fracture are easy to occur in the process of application, which results in the failure of the cladding layer. The application of the alloy in the field of surface strengthening is limited. In this paper, Fe-Cr-C alloy cladding with different content of C and Cr was prepared by plasma cladding, and the difference of its properties was compared and analyzed. The effect of Ti content on the properties of hypereutectic Fe-Cr-C alloy cladding was studied by adding pure Ti powder to form TiC in situ. In combination with the first principle, the elastic constants of the hard phase in the cladding are calculated, the elastic modulus and Poisson's ratio of the carbide are calculated according to the Voigt-Reuss-Hill approximation, and the microscopic mechanical properties of the carbides are calculated and analyzed with reference to the semi-empirical formula. In order to provide the theoretical basis for the design of alloy system, the microstructure of hypoeutectic Fe-Cr-C cladding layer is mainly cellular crystal, the average surface hardness is 42.6 HRC, the adhesion wear is dominant in the wear process, and the wear resistance is poor. The average surface hardness of near-eutectic cladding is 52HRC, and long stripe hypoeutectic carbides begin to appear in the cladding layer, the wear resistance is improved to some extent, and the hard phases of hexagonal and coarse carbides are distributed in the hypereutectic Fe-Cr-C cladding layer. The average hardness value of the surface is 62 HRC, the abrasive wear is the main, the wear resistance is the strongest, but there are stress cracks in the surface. The hypereutectic Fe-Cr-C cladding is mainly composed of hexagonal hexagonal Cr7C3PNMAs. The phase transition and thermodynamics of the cladding layer are analyzed. During cooling process of the cladding layer containing Ti at above 1000 鈩，

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