Study On The Preparation And Performance Of Gradient Coating On The Surface Of Shield Machine Hob

Shield hob is the main cutting component in the process of tunneling.Its working environment is harsh,the stress conditions are complex,and the service life is short.Frequent replacement of the hob will affect the construction efficiency and increase the cost of tunneling.During the working process of the hob,it mainly relies on the squeeze and friction of the cutter ring to break the rock.Therefore,the cutter ring needs to have high hardness and wear resistance.In view of the problems of rapid wear and short life of the cutter ring during excavation,this paper takes the material of the shield machine hob cutter ring H13 steel as the research object,and uses laser cladding technology to prepare gradient cladding layers on the surface of the H13 steel substrate to improve its hardness and wear resistance.In this paper,nickel-based and cobalt-based alloy powders are used as cladding materials,and TiC and WC ceramic particles are added to the alloy powders.By controlling the content of ceramic particles in the composite powders,TiC particle-reinforced Ni-based alloy gradient coatings,WC particle-reinforced Ni-based alloy gradient coatings,TiC particle-reinforced Co-based alloy gradient coatings,WC particle-reinforced Co-based alloy gradient coatings are prepared by multilayer laser cladding.The residual stress distribution,microstructure,microhardness distribution,friction and wear resistance,impact resistance of the four gradient coatings were tested and analyzed by corresponding testing methods and equipment.The research results are as follows.The finite element analysis software ABAQUS is used to simulate the residual thermal stress between the coating and the substrate,and the magnitude and distribution of the residual stress of different gradient coatings are obtained: the residual stress is mainly concentrated at the bonding position of the coating and the substrate,and the value of the stress shows a decreasing trend from the center to the edge,and from the substrate to the coating surface;The higher the content of TiC or WC,the smaller the maximum residual stress,the good thermal stability and low thermal expansion coefficient of ceramic particles can significantly reduce thedifference between the thermal expansion coefficient of the composite coating and the substrate;The residual stress distribution of the Co-based gradient coating is more uniform,and the residual stress value is significantly lower than that of the Ni-based gradient coating.Good metallurgical bonding is achieved between the gradient coating and the substrate.The microstructure of the cross-section of the gradient coating is gradiently distributed from the bottom to the top layer,the regular plane crystal is on the joint surface,and the coarse columnar crystal and cellular crystal are on the bottom of the cladding layer.With the increase of undercooling,the microstructure is gradually refined and transited to fine dendrite.The entire cladding layer structure shows a trend of change from coarse and loose to fine and dense from bottom to top.The preparation of the gradient coating on the surface of H13 steel not only improves the surface hardness of the substrate,but also shows a gentle gradient decrease from the surface of the cladding layer to the substrate.The ceramic particles in the cladding layer have the effects of solid solution strengthening,fine grain strengthening and dispersion strengthening.The microhardness of various cladding layers from high to low is as follows: WC-reinforced Ni-based gradient coating,WC-reinforced Co-based gradient coating,TiC-reinforced Ni-based gradient coating,TiC-reinforced Co-based gradient coating.The friction and wear tests were carried out on the surface of the substrate and different gradient coatings.The test results show that the wear loss of the cladding samples is much lower than that of the substrate material,and the wear loss of the cladding layers decreases first and then increases with the increase of the content of hard particles,and the wear resistance of Ni-based gradient coatings are better.Observation of the surface wear morphology reveals that there are obvious scratches on the surface of the H13 steel substrate,the wear mechanisms are mainly abrasive wear and adhesive wear,the surface of Ni-based and Co-based cladding layers have a large number of exfoliation pits,the wear mechanism is mainly brittle exfoliation.The substrate and cladding samples were cut into standard impact parts for impact testing.The test result shows that the impact resistance of Ni-based andCo-based cladding layers are significantly lower than that of the substrate.The fracture morphology show that the fracture form of H13 steel is ductile fracture,and the fracture form of the cladding layers is brittle fracture.