| Plasma cladding technology has the advantages of high precision and mechanization,and was one of the core technologies of surface strengthening technolodges.Fe-Cr-C alloy cladding layer contained a large number of carbide hard phase,and it's hardness and wear resistance were high,but the carbid of hypereutectic Fe-Cr-C alloy cladding layer with high C and Cr was thick and uneven distribution.The internal cutting effect was strong,and the brittle carbid was easily fractured,which caused crack propagation and limited its application in the surface strengthening field.This paper analysed properties of Fe-Cr-C alloy cladding layer with different C and Cr content manufactured by plasma arc welding.To the hypereutectic Fe-Cr-C alloy cladding layer,pure Ti powder was added to modified its properties and explored the influence of Ti content.By the first principle,the elastic constants of the main carbide hard phase of hypereutectic Fe-Cr-C alloy cladding layer were calculated.Based on the Voigt-Reuss-Hill approximation,the elastic modulus and Poisson's ratio were calculated,and the microscopic mechanical properties of carbides could be calculated by the semi empirical formula,which provided theoretical basis for the design of alloy system.Hypoeutectic Fe-Cr-C cladding layer are mainly cellular crystal with the average surface hardness of 42.6HRC.During the wear process,the machanism of hypoeutectic Fe-Cr-C cladding layer was adhesive wear,and its wear resistance was poor.The average surface hardness of near eutectic cladding layer was 52 HRC,and the wear resistance showed a certain improvement with the elongated eutectic carbide.The average surface hardness of hypereutectic Fe-Cr-C cladding layer reached 62 HRC with hexagonal and strip carbides dispersing in the layer's surafce.The friction mechanism of hypereutectic Fe-Cr-C cladding layer was abrasive wear,and its wear resistance was the strongest,but the internal stress cracks caused the need for further research to improve.The mainly hard phase of hypereutectic Fe-Cr-C cladding layer was orthogonal structure of hexagonal prism Cr7C3(PNMA).Through the analysis of the process of phase transition and thermodynamics,the TiC phase was in possession of the priority formation during the cooling of layer with Ti above 1000?.The formation of TiC decreased hypereutectic degree of layer and made the carbid growing orientation,also the preferential formation of TiC became heterogeneous nucleation of Cr7C3 which refined the carbid.The uniform distribution of Cr7C3 reduced the cutting effect and residual stress in the cladding layer,and lead to reduction in the crack tendency of the layer.But excessive Ti addition caused porosity in the cladding layer and the hypereutectic Fe-Cr-C cladding layer with 0.5% to 1.0% Ti content exhibited the best micro mechanical properties.Based on the first principles,the hard phase Cr7C3(PNMA)of the cladding layer presented the characteristics of the metal bond,while the TiC was mainly covalent bond and the valence bond was strong.By Voigt-Reuss-Hill approximate calculation,the orthorhombic PNMA type Cr7C3 and the cubic TiC satisfied the structural stability,and the orthorhombic PNMA type Cr7C3 showed the property of plasticity which consistented with nano probe and micro hardness test results.Combined with the semi empirical formula of theoretical hardness of high hardness materials,the theoretical hardness of PNMA Cr7C3 was 13.13 GPa,while the TiC was 19.73 GPa.The ratio of the theoritical hardness was 0.67 and it was related to the experimental results,which indicated the reliability of the simulation results.This study provided a theoretical basis for the development of high hardness material. |
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