| The iron and steel industry played an important role in national economy. Because of its short process and high efficiency, the continuous casting process was widely used in steel metallurgy. Quality and output of the iron and steel metallurgy depended on the efficiency and reliability of continuous casting. Crystallizer was the key component of continuous caster. Copper alloy was used in crystallizer’s inner sleeve, its quality and service life directly affected the quality, yield and production efficiency of continuous casting. Due to the crystallizer’s inner sleeve directly contacting liquid steel for a long time and tough serving conditions, Crystallizer became a key consumable in the process line of horizontal continuous casting. With the increasing demand of iron and steel metallurgy, amount of scrap crystallizer inner sleeve also increasing year by year, the domestic annual consumption of copper crystallizer was more than2billion Yuan in metallurgical industry. So the surface treatment technology and the modified material of crystallizer’s inner sleeve copper alloy were important. Improving inner sleeve’s copper alloy performance of erosion resistance on high temperature liquid steel, friction and wear resistance, it not only could improve the service life of crystallizer that ensure the production reliability, also could reduce the cost of production, save resources that meet the needs of national economy sustainable development.CuCo2Be alloy was chose as the base material of crystallizer inner sleeve in this paper, because it had good thermal conductivity, corrosion resistance and fatigue resistance. Then it was set heat treatment. The multilayer structure of coating was designed reasonable. Cr3C2-NiCr was used as working layer because of its good performance of high temperature wear resistance. NiAl was used as bonding layer, it not only can improve the interface bonding strength, but also reduce the thermal expansion difference about transition layer and the substrate material; Plasma spraying technology was used to prepare Cr3C2-NiCr coating, NiAl bonding coating and Cr3C2-NiCr/NiAl multilayer coating on the sample surface respectively, the thickness of those coating was more than0.3mm.Through a series of spray test, three kinds spraying parameters of coatings was optimized. Using SEM, EDS, XRD and other means, we studied three kinds of coating’s microstructure, morphology, interfacial behavior, the element and the forming mechanism. And then, the three kinds of coating prepared by optimum parameters was tes under high temperature thermal shock, the differences of cold and hot fatigue life and failure mechanism of the coating were studied and compared between Cr3C2-NiCr/NiAl composite coating and Cr3d-NiCr coating at high temperature. And through friction and wear test, the change rule between coating wear performance and spraying process parameters were studied, and the wear failure mechanism of coating was also studied under different temperature. The relationship between spraying process and organization, performance, forming mechanism of the coating, was studied and established, in order to determine the best preparation of high performance and long working life spraying coating, the test results will provide a scientific basis for improving the crystallization used copper surface and subsequent production use.The following conclusions were drawn through studing:the optimized spraying process parameters of three coating:the spraying process parameters and influence law of Cr3C2-NiCr coating:Power (kw)20> feeding rate (18g·min-1)> main gas flow (58L·min-1)> spraying distance (95mm), bond strength of Cr3C2-NiCr coatings under preparation for this condition:43Mpa; The spraying process parameters and influence law of NiAl coating: Power (20kw)> spraying distance (90mm)> feeding rate (25g·min-1)>main gas flow (61L·min-1), bond strength of NiAl coatings under preparation for this condition:48MPa; The spraying process parameters and influence law of Cr3C2-NiCr/NiAl composite coating: Power (22kw)>spraying distance (100mm)> main gas flow (64L·min-1)> feeding rate (30g·min-1). bond strength of the composite coatings under preparation for this condition:47MPa; Two exothermic reaction of NiAl composite powder were verified by means of DTA in600℃to671.8℃, it was proved that coating generated AlNi3, Al3N12, AlNi phase in this temperature range by means of XRD. Through SEM it was analysed three coating interface combined closely, coating present continuous layer, there were undeformed Cr23C6, Cr7C3, Cr3C2hard phase in working layer, there was angular nickel aluminum compound in bonding layer; through XRD analysis of the phase of three different kinds of coating layer in the depth, it was showed that:Cr3C2-NiCr coating was mainly composed of Cr23C6, Cr7C3, Cr3C2, NiCr amorphous phase composition; External surface of the NiAl coating was mainly composed of nickel, aluminum single phase, Cu3Al2, Cu0.81Ni0.19, Al1.1Ni0.9compounds such as phase were detected at close to interface. The interface of Cr3C2-NiCr/NiAl working layer and bottom layer generated AlCr2, Cr3Ni2, there were Al2Cu, Cu0.81Ni0.19, Al1.1Ni0.9phase near the interface about0.05mm, EDS analysis showed the substrate bonding three kinds of coating and matrix was the machinery anchor-based bonding, between bottom layer and base, coating and bottom layer, metallurgical bonding was formed because of element diffusion.Cr3C2-NiCr/NiAl composite coating, Cr3C2-NiCr coating prepared by optimum process parameters was chose to be carried out the thermal shock experiment in500℃,550℃,600℃,650℃.When500℃, the composite coating complete failure times was235, single working layer times was average119. It can be concluded that through the reasonable structure design of the coating, the difference of volume expansion between working layer and matrix was effectively buffered, the stress distribution was more litter, the thermal shock resistance of the coating was improved. Thermal shock test was carried out in550-650℃complete failure times of composite coating suddenly from120times to1-2times, the complete failure number of single working layer suddenly from83times to1-2times, it showed that as the temperature up to600℃, thermal shock resistance of the coating declined rapidly. Through SEM, XRD, EDS analysis it was concluded that the main failure mechanism was coating and interface had gap and shrinkage, they were easy inhalation of much oxygen and oxidizing, and the existence of thermal stress between bottom layer and working layer, all of these factors together caused coating peeling. NiAl bottom coating prepared by optimum process parameters was choosed to be carried out the thermal shock experiment in500℃, it was found that Ni/Al coating was not cracking after hot-cold cycle350times, we can conclude NiAl coating had very good thermal shock resistance. To establish the relationship between the friction and wear performance and coating spraying process, Cr3C2-NiCr/NiAl composite coating was carried out friction and wear test. Results showed:when room temperature, the friction and wear mechanism was mainly abrasive wear, adhesive friction and scratches friction were complementary, and the high temperature, wear mechanism was adhesive wear. Because the friction and wear mechanism was different at the different temperature, the coefficient of friction of changes would occur. At the room temperature, friction pair had not yet entered the friction pair in area, the friction coefficient was relatively high and unstable, After entering in pair area, coefficient was reduced and relatively stable.When high temperature friction and wear test was going on, the friction pair entered the friction pair in area soon and fast entered into the sticky friction area, and then friction coefficient was up to a certain degree of stability, so it was showed friction and wear resistance of matrix was lower than the coating. |
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