Study On Microstructure And Properties Of（Nb,Ti）C Reinforced Iron-based Composite Surfacing Layers

Niobium carbide（NbC）reinforced iron matrix composites have excellent wear resistance and are widely used in surface strengthening of key mechanical parts.With the continuous development of advanced manufacturing technology,the service life requirements of iron-based key parts are getting higher and higher.Therefore,higher requirements are put forward for the wear resistance of surfacing coating with surface strengthening.Studies have shown that（Nb,Ti）C composite carbides can be generated with Nb by adding a small amount of Ti alloy elements,thereby improving the poor bonding between Nb C and iron matrix and further improving the wear resistance of surfacing coating.In this thesis,the（Nb,Ti）C reinforced iron-based composite surfacing layer was prepared by the in-situ reaction under the action of arc using the arc surfacing technology of molten electrode gas protection.The effects of different shielding gas,heat input and Nb content on the microstructure evolution,hardness and friction and wear properties of the surfacing layer were studied.In this thesis,the type of shielding gas for arc surfacing is optimized firstly.The research shows that the carbide content precipitated by pure Ar shielding gas is the most,and the carbide precipitation decreases with the increase of CO₂ content in the shielding gas.The microhardness of pure Ar protective gas surfacing layer was the highest,reaching 708.2 HV_0.5,which was about 14.2%higher than that of pure CO₂ protective gas surfacing layer.The wear resistance of surfacing layer with pure Ar shielding gas is the best.The wear form shows a slight adhesive wear,and the wear is the lightest.The wear weight loss is about 9.5%of that under pure CO₂ shielding gas.After determining the protective gas,this thesis continues to optimize the heat input of the surfacing layer.The results show that with the increase of the heat input of the surfacing layer,the weld width,weld depth and weld height of the surfacing layer gradually increase.The number of precipitated（Nb,Ti）C particles gradually decreased,and the size of block（Nb,Ti）C gradually decreased.The microhardness of surfacing layer decreases with the increase of heat input,and the increase of heat input reduces the wear resistance of surfacing layer samples.The microstructure evolution and performance variation of surfacing layer with different Nb content were studied in detail under the optimal surfacing process conditions.The results show that when Nb is not added in the surfacing layer,the surfacing layer consists of onlyα-Fe phase andγ-Fe phase.With the increase of Nb content,the diffraction intensity ofγ-Fe phase in the surfacing layer decreases,while that ofα-Fe phase increases.（Nb,Ti）C phase begins to appear,and the diffraction intensity gradually increases.The microstructure of the surfacing layer without Nb is mainly cellular crystal,and there is no carbide formation.With the increase of Nb content,grain boundaries begin to disappear,carbides precipitate and gradually increase.When Nb content is 6%,（Nb,Ti）C particles precipitate most.The hardness of the surfacing layer decreases slightly and then increases gradually with the increase of Nb content.With the increase of Nb content,the wear resistance of surfacing layer gradually increases,and the wear volume of surfacing layer gradually decreases with the increase of Nb content,and the wear scar becomes shallow.The study shows that the amount,size and distribution of carbide precipitation have a significant impact on the performance of surfacing layer.The higher the content of large size carbide,the more conducive to hinder the wear path and improve the wear resistance of surfacing layer.The change of Nb/Ti ratio will affect the amount of carbide precipitation.With the decrease of Nb/Ti ratio,the proportion of Ti content in the micro zone increases and Ti C tends to grow.At the same time,the nucleation point of Ti C will decrease and the amount of carbides will decrease.In addition,with the increase of heat input,the cooling rate of the molten pool decreases,the precipitation of carbides decreases,and the amount of Nb C gathered in Ti C as the nucleation center decreases,which makes the size of（Nb,Ti）C composite carbides decrease,and the small size carbides are easy to fall off,which cannot well hinder the wear path and reduce the wear performance of the surfacing layer.