Study On In-situ TiC Reinforced Fe-based Coating Fabricated By TIG Cladding

Properties of endurance impact, corrosion resistance and wear resistance are the basic requirements for components which are employed in commercial application such as mine exploration, heavy metallurgy and gasoline drilling. The method of development high-hardness and anti-resistant coating on the surface of components is a valuable way to repair it and prolong their service life. In this research, the Fe-based coating reinforced by in-situ TiC was fabricated by tungsten-inert gas (TIG) melting process employing a proper amount of alloy powder prepared on the surface of ordinary carbon steel substrate, and the microstructure, wear properties of the coatings and the growth mechanism of TiC carbides were analyzed systematically. At same time, the factors and rules influencing microstructure and properties were investigated.It is found that the powders constituents are the key factor to synthesize in-situ TiC reinforced coatings by using TIG welding process. Because of the lower energy-flux density of TIG compared to laser and plasma, it is necessary to select powder elements with lower melting point which can quickly form molten pool. And then, metallurgical reaction occurs between titanium and carbon and synthesizes well-shaped overlap with high effective mass of TiC. The reaction equation is as followed: Ti + C→TiC. According to this principle, the powders composed of ferrotitanium and graphite (FeTi+C ) has excellent processing properties, which can produce a good bonding, excellent mechanical and wear resistant properties of the coating. The in-situ TiC in the coating distribute in dendritic mode majority, with small TiC grains in diffusion minority. Although iron-based self-fluxing alloy powder (G314) has a lower melting point and well manufacturability, complex carbide M₂₃C₆ emerges besides the in-situ TiC reinforced phase in the coating, which increases the crack sensitivity of the coatings. In addition, TiC carbides can also be formed via pure Ti element and graphite, however, the active chemical property of pure titanium, it can react with oxygen easily and form TiO₂, which result in the increasing of the components melting point and poor shaping.Under the nonequilibrium condition of TIG fabrication, primary TiC grains in the Fe-based coating reveal dendrite crystal in majority and eutectic form in minority. Because of the directionality of thermal conduction and heat emission in the course of the fabrication of the TiC, component overcooling emerges in the growth front of TiC grains. This results in definite orientation of primary TiC. The primary TiC grows and turns to dendrite distribution. Nevertheless, in the course of eutectic transformation during the solidification of molten pool, the mechanism of TiC nucleation and growth is diffusion. The anisotropic growth of TiC is prohibited by the cladding of iron matrix and the TiC grains become graininess eutectic TiC, the structure and pattern of eutectic TiC_is more close to ideal octahedral shape of face-centered cubic TiC. The growth of in-situ TiC is specific small-plain growth. The exposure plain of TiC grains is the close packing [111] plain which grows slowly. The interface between in-situ TiC carbides and the matrix remains clean and free from deleterious phase. Thus, the carbide-matrix has a strong interface bond.Cladding parameters affected the microstructure and properties of the coatings obviously. By selecting proper cladding parameters, such as thickness of the pre-coated coating with 1.2mm, welding current with 150A, cladding speed with 55-60/min, and tri-layers cladding, TiC reinforced coatings, which possess well appearance, high interface strength with the substrate and superior microstructure and property can be obtained. Multilayer cladding method was employed to reduce the dilution effect of substrate metal on the Ti and C elements, thus increasing the amount of the reinforcements in the cladding. For multilayer cladding, it is also found that the particle distribution can be characterized as combination of primary TiC dendrites and eutectic particles. In addition, as FeTi alloy could combine with carbon to form lots of TiC carbides and decrease the carbon content in matrix, which leads to change the microstructure to low carbon martensite. It is favourable for improving resistance for crack and wear resistance of the coatings. Experimental results showed that cladded coating with high hardness and good wear resistance was obtained. At same time, the problems of poor appearance and cracking were also avoided.The present study shows that the ratio of FeTi and graphite influences directly not only on the formation of carbides but also matrix microstructure and hardness. Optimization of the composition and content of the raw alloy powder can influence the microstructure of the coating matrix and the amount, size and distribution of the TiC reinforcements. While the ratio of Ti:C is 1:1.2, the detrimental phase of Fe₂Ti can be effectively decreased, and the microstructure of the matrix is composed of low carbon martensite and residual austenite. Moreover, with increasing of graphite, the amount of in-situ TiC increased, and the primary dendrites of TiC tended to transform from fine dendrites to coarse dendrites, which improve the wear resistance of the coating.With proper addition of La₂O₃, the nucleus of in-situ TiC was increased and the reinforcement phase of TiC transformed from coarse dendrites to more fine dendrites or diffusive particles. Nevertheless, since the La₂O₃ acted as the heterogeneous nucleus of TiC, the nuclearation and growing mechanism of TiC were changed and the saturation of carbon in TiC was decreased. These brought detrimental effect to the mechanic properties of the TiC crystal, which includes decrease of the hardness of TiC. Consequently, the reinforcing effect was depressed, and the wear resistance of the cladded coating with addition of rear-earth was decreased. When ferrovanadium was added, the TiC/VC grain was formed to produce synergetic reinforcing effect. On the other hand, the grains the coating were thinning and the strength of the substrate were increased.Wear test of the substrate and cladded coating showed that under the same load and wear sliding" distance, the wear volume loss of the in-situ TiC reinforced coating produced using TIG was only one twentieth of the substrate, or even less. Compared with the substrate, the average value of the wear coefficient of the clad coating was lowered 0.05-0.1. TiC grains exposed in the surface coating contact with the grinding wheel dominantly in wearing. As a result, higher drag force is needed to overcome encumbrance and embedment resulting from the reinforced TiC particles and wear loss can be reduced enormously with the same load.According to the results of dry wear test, main abrasive mechanism of the coating is pull-out of the diminished reinforcements and micro-plough of the substrate. In grinding crack, there are a lot of depressions owing to pull-out of the diminished TiC, and short and shallow grooves developed in the substrate. The grinding cracks begin at the pull-out point of diminished TiC and stop at next resistant TiC grain. According to the examination with different layers, three-layer coatings have the best wear resistant properties. The volume fraction of the reinforcements in the coating is enhanced by multi-layer cladding, which result in large TiC grains and embedded on the substrate in the form of accumulated petaliform. It combines with the substrate and bonds tightly. Therefore, the TiC grains can not be pull out easily in wearing and reveals good wear resistant properties.