Study On Fe-based Coating Reinforced By Ti(C_yN_1-y) Particles On The Adamite Roller By Laser Cladding

Adamite steel is being widely used to make rollers in the field of metallurgy because of its better hardness, such as, continuous hot casting and steel rolling. At operating temperature, the working surfaces usually suffer from normal abrasion, corrosion and crack and so on in the procedure of service, which leads to the decrease of production quality and reduction of productivity. At the same time, the user may encounter heavy economic burden if they change new rollers frequently. It is certain that the cost will be improved if you always abandon old mechanical components that are not in good conditions. It becomes necessary for us to study how to maintain and prolong their service life by applying thin hard coating at their surfaces. You know that laser surface cladding process is a promising surface treatment technology for industry applications, due to both an excellent metallurgical bonding and less defects between the layers and substrates. Hence, the technologies of laser cladding composite coating reinforced by ceramic particles have been investigated in the paper. In order to solve above problems and decrease the production cost. The main results involved in the research are as follows.Laser cladding process was studied on the surface of Adamite steel through CO₂ laser modified technology. It was found that laser cladding parameters affected the microstructure and properties of the layers obviously, especially for laser scanning speed (v), laser power (p) and laser beam diameter (d). Under the condition of proper parameters, Fe-based alloy composite coating reinforced by Ti (C_yN_1-y) particles was fabricated on the surface of Adamite steel. It was shown that macroscopic and microscopic qualities are related to the cladding materials and processing parameters of laser cladding. The results showed that width, depth, shape coefficient, dilution and crystal grain of the laser cladding coatings increase with laser power, but thickness of the layer decrease at same parameters. It is also shown that width, thickness, depth, shape coefficient, dilution and crystal grain of the laser cladding coatings decrease with laser scanning speed, but hardness and sensitivity of pores of the layers increase under the same condition. By selecting proper process parameters, such as laser power with 3000w, laser beam diameter with 3.0mm, the prefabricating thickness with 1.0mm, laser scanning speed with 300mm/min. FeCrBSiMo and [TiN+C] powders (the ratio of TiN:C is 1:1) are used to form single laser cladding layer, which possess better appearance and high strength between layer and substrate.Pre-fabricate alloy powder was mixed before laser cladding according to surface properties requirement of Adamite steel. The self-made organic adhesive was used to paste the powders. With optimum parameters, Fe-based alloy composite coating reinforced by Ti (C_yN_1-y) particles was fabricated on the surface of Adamite steel through CO₂ laser cladding technology. The microstructure of laser cladding coating was observed by optical microscopy (OM), x-ray diffractionmetry (XRD), scanning electron microscopy (SEM), electron probe microscopy analyzer (EPMA), and transmission scanning electron microscopy (TEM). The distribution of elements and phases in the coatings were identified using EDS and x-ray diffractionmetry (XRD). Relationship between the microstructure and properties was analyzed in the paper in detail. The results show that Titanium carbonitride Ti(C, N) particles are introduced by an in-situ metallurgical reaction between TiN particles and graphite powders during laser cladding process. Titanium carbonitride particles existed in the layer are fairly fine, ranging from 0.1μm to 40μm, and evenly dispersed in the metal matrix. Most of them take on nearly round shape, and some of them are irregular in shape. The bonding zone is a distinct zone between the layer and substrate, which illustrates that the perfect metallurgical bonding is achieved through this zone The interface between Ti(C, N) particles and the matrix remains clean and is free from deleterious phases by TEM, which insures that the carbide-matrix has a strong interface bond. The in-situ hard carbonitride grains have high bonding strength with the matrix, and form a good carrier capacity system together.According to the basic thermodynamic analysis ofâ–³G_T, it is feasible for the formation of Ti(C, N) that metallurgical reaction may happen between titanium nitride (TiN) and graphite (C) during laser cladding process. First of all, a great deal of energy is absorbed by the raw cladding material during laser cladding process, which causes the complete dissolution of the original titanium nitride (TiN) particles coming into being nitrogen and titanium atoms: TiN→Ti + N. Then, these nitrogen, carbon, and titanium atoms begin to diffuse in the cladding matrix. It is known that diffusing speed of carbon atoms is faster than that of other atoms in the melt pool. There are strong chemical reactions and metallurgy processes between the titanium atoms and graphite powders in the molten metal. At the same time, chemical combination reaction between Ti and N atoms can also reoccur in the cladding coating because of active chemical property of Ti and N atoms. Thus, lots of hard anti-wear phases of carbides TiC and nitrides TiN are respectively formed after the melt pool is rapidly solidified. The formation process can be characterized as follows: [Ti]+[C]→TiC, [Ti]+[N]→TiN. It is known that titanium carbon (TiC) and titanium nitride (TiN) characterize by the same NaCl-type crystal structure and approximately equal lattice constants. They may unlimitedly mix each other and form a solid solution named titanium carbonitride. Finally, Ti(C_yN_1-y) particles are synthesized by a solid-solution metallurgical reaction between TiN and TiC particles in the process of laser cladding: yTiC+(1-y) TiN→Ti(C_yN_1-y). Meanwhile, it is possible to form Ti(C_yN_1-y) that the exchange reaction between TiN and C may happen directly in the cladding coating.Wear behavior and wear-resistant property of laser cladding composite coating reinforced by Ti(C,N) particulates were tested on a tester named M-2000 without lubrication at room temperature. It is shown that dispersive strengthening effect and refining effect of the Ti(C,N) particles evenly distributed in the matrix greatly contribute to increasing the microhardness and wear-resistance of the Fe-based composite coating. On the other hand, solid solution strengthening effect of C, Mo and Cr elements and martensite strengthening effect play an important role in theα-Fe metal matrix.Horizontal and longitudinal cracks occur in the layer on the surface of Adamite steel while laser cladding, especially, it is easy for them to appear in the Fe-based alloy multilayer reinforced by Ti(C_yN_1-y) particles. Inner cracks propagate nearby fusion transition zone in theα-Fe metal matrix. These cracks usually end in the bond zone between layer and substrate. Some of them are inter-granular and others are trans-granular. The cracks which lie in the laser layer are brittle fracture. The susceptibility of occurring crack can be decreased by selecting and optimizing processing parameters. In order to keep from cracks in the Fe-based alloy multilayer reinforced by Ti(C_yN_1-y) particles, the second transition Ni-based layer was built up between the working layer and substrate. The method of transition layers may solve the problem of cracking.