| The local interface of traditional high manganese steel parts and components is easy to wear failure as low impact stress fail to obtain the ability of work hardening. The technology of plasma cladding Fe-Cr-Ni alloy coating has been presented in high manganese steel surface to improve the wear resistance of high manganese steel. The cracks initiated and propagated in coating with residual stress and widmanstatten structure transformed in heat affected zone(HAZ) duo to the high cyclic heating and cooling. The process parameters of cladding have optimized from orthogonal experiment based on six factor that affect the quality of cladding layers. The different heat treatments has applied for coating for improving the stability of microstructure, relieving residual stress and transforming widmanstatten structure to austenite structure to obtain excellent cladding layer with wear resistance and without cracks, It’s focused on the evolution mechanism and expansion mode of cracks in coating, the evolution of microstructure and residual stress in coating and matrix after heat treatment, the friction and wear property of cladding layer and matrix after heat treatment.The results indicated that the microstructure of coating consist of dendrite austenite that contain residual cementite and eutectic skeleton. There are made up of the equiaxed crystal zone, dendrite zone, cellular crystal zone and planar crystal perpendicular to cladding path. The matrix maintained original austenite, while heat affected zone transformed to widmanstatten structure. With the increasing of heat treatment temperature, the austenite structure of coating is homogenized, the volume of skeleton eutectic decrease, the content of Cr, Ni in the interior of the grains increase, the particle phase (Cr,Fe)mCn were precipitated from austenite grain and eutectic area. The carbide phase (Cr,Fe)mCn are precipitated completely and transformed from metastable phase (Cr,Fe)7C to stable phase (Cr,Fe)23C6 with heat treatment of 900 ℃. At the meantime, the widmanstatten structure recrystallized to fine pearlite. Multi-channel coating is better than single-channel coating on quantity and stability of carbide phase. The test experiment of residual stress shows that residual stress of coating is thermal stress mainly. The value of residual stress in multi-channel coating is accumulative effect upon single-channel coating and it’s 40 percent higher than single-channel coating. It has decreased by 81% and 67% respectively in coating of single-channel and multi-channel after heat treatment of 900℃ because of the combining effect of thermal stress release, carbide phase(Cr,Fe)mCn precipitation entirely and carbide phase steady state transition.Effect of different sbstrate temperature distribution on evolution mechanism and expansion mode of coating cracks have been investigated from the point of microcosmic view. The cracks of the transverse were generated after the end of cladding, which caused transcrystalline fracture without preheating. As the preheating temperature of substrate was 250℃, tiny transgranular cracks and tiny intergranular cracks have formed and crack density was significantly decreased in the coating. With the preheating temperature increase to 350℃, there are no cracks found in the cladding coating. All cracks originated at the interface of coating and substrate and perpendicular to the scanning orientation. The driving force of cracks is provided by thermal stress caused by temperature gradient and the difference of thermal expansion coefficient in matrix and cladding coatings. Substrate preheating could eliminate the cracks by reducing the residual stress which duo to the combined effect of diminishing temperature spread and benefiting carbide precipitation from supersaturated solid solution.The results of mechanical tests showed that the abrasion resistance of cladding layer was improved 7.4 times than matrix and cladding layer with heat treatment at 900℃ higher 3.3 times than matrix. It means that heat treatment could improved the quality of cladding layer at the same time maintain good abrasion resistance. Under dry friction condition, wear of matrix was dominated by typical abrasive wear. To the Fe-based coating, the wear mechanism are abrasive wear and adhesive wear compared with adhesive wear to the coating with heat treatment at 900 ℃. The micro-hardness of coating with heat treatment at 900℃ was improved 2.7 times than matrix and the strength was equal to matrix, significantly higher 70.8% and 59.7% than coating without heat treatment. All phenomenons are the combined effect of the distribution of various element become uniform, carbide phase precipitation entirely from supersaturated solid solution and carbide phase steady state transition. |
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