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The latest research progress of gradient structure cemented carbide

cemented carbide products are mainly cemented carbide with uniform microstructure. However, with the continuous development of modern industrial technology, the requirements for cemented carbide products are also increasing, and they often have different performance requirements in different working parts. For example, rock drilling cemented carbide drilling tools are required to be both wear-resistant and impact resistant. If the drilling tools with high surface hardness and good wear resistance can be manufactured, while the drilling tools with good core toughness and impact resistance, their performance is undoubtedly better than the drilling tools with uniform organization. In addition, in order to improve the adhesion between the cemented carbide matrix and the surface coating, and improve the bonding performance and impact resistance of the facing cemented carbide, it is also required that the cemented carbide surface has good strength and toughness

the so-called gradient structure or gradient structure alloy refers to an alloy whose composition and structure show regular differences in different parts of the section. Gradient structure cemented carbide just takes advantage of its special structure or composition gradient change to give different properties to different parts, so that the overall product can obtain excellent comprehensive mechanical properties. The research work of gradient structure cemented carbide began in the late 1970s, and its application in the production of spring testing machine in China was in the late 1980s. The preparation methods of gradient structure cemented carbide at home and abroad are mostly based on traditional processes, and some special process measures are taken in the forming or sintering process to obtain special gradient changes, including: the gradient changes of bonding phase composition or content, the gradient changes of hard phase grain size or adjacency, etc. The most commonly used methods mainly include: composite cemented carbide method, powder layered pressing method, metal melt impregnation method, carbon deficient cemented carbide carburization treatment method, etc

1. Research and application of gradient structure cemented carbide abroad

it is generally believed that gradient structure cemented carbide products with good core toughness and high surface hardness can overcome the shortcomings of traditional cemented carbide that both wear resistance and toughness cannot be considered at the same time. Therefore, foreign countries have done a lot of pioneering work in the research and development of improving the surface hardness of cemented carbide products while maintaining or even improving the toughness of the heart. Among them, Sandvik of Sweden took the lead in developing cobalt rich DP (dual property) alloy in the central area by using low-cost carbon deficient cemented carbide carburizing technology. This technology has applied for a U.S. patent in October 1985 and was formally authorized in March 1988. DP alloy technology mainly includes two aspects. First, WC containing homogeneous and fine decarburized phase with controllable volume fraction is prepared+ λ+η (coxwyc) three-phase abnormal structure alloy, and then carburize the alloy, and effectively control the thickness of each gradient layer in the alloy. Its essence is to produce carbon deficiency with uniform distribution η On the basis of phase cemented carbide, the distribution of bonding phase in the alloy is changed by carburization, giving different properties to different parts of the alloy. After carburization, the phase on the surface of the product is eliminated, and co migrates to the center, so that the content of CO on the surface is low, but there are still phases in the heart, such as ncamp report, and the content of CO is high. This kind of cemented carbide with gradient distribution of cobalt content has high surface hardness, good wear resistance, and good impact toughness in the heart. The wear resistance and toughness of the alloy are well coordinated, and the application effect is significantly improved compared with traditional products

sandvik's mining and engineering tools company has developed three DP ball tooth alloy brand names in the late 1980s, including dp55, dp60 and dp65, for your choice!. Its wear resistance and toughness are significantly higher than that of traditional cemented carbide, and its working life is 3 times higher than that of traditional cemented carbide. For example, in limestone tunnel drilling, a 45mm impact drill with dp55 conical ball teeth is used, with a drilling speed of 1.96m/min and an average life of 3121m; The drilling speed and average life of the original cemented carbide ball tooth bit are 1.48m/min and 1000m respectively. The average service life of dp60 heavy-duty ball gear bit when drilling in quartz ore is 83m, while the service life of the original cemented carbide ball gear bit is only 53m. With its excellent performance, DP products accounted for 30% - 40% of the total output of cemented carbide cylindrical teeth six years after they were put on the market on a small scale in 1986

because Sandvik's DP alloy is a patented technology, there is no detailed report on the basic research work directly related to DP alloy abroad, and its strengthening mechanism is simply explained by the qualitative analysis of the internal stress distribution state of the alloy. Nevertheless, the carburizing technology of carbon deficient cemented carbide adopted by the alloy solves the contradiction that it is difficult to give consideration to both wear resistance and toughness of cemented carbide products at the same time, so that the service life of the alloy is significantly improved, and the process is flexible and simple, which is suitable for industrial production; Using simple sintering and heat treatment methods, a gradient structure alloy with controllable change of bonding phase and gradient distribution is prepared in a single brand of homogeneous cemented carbide, and significantly different wear resistance and toughness are obtained in different parts of the sintered body of cemented carbide

recently, most foreign studies have used sintering and heat treatment processes to gradient cemented carbide products. Among them, the treatment method of controlling the nitrogen content of Ti (CN) - based ceramics is a hot research field in the current research

zackrisson et al. Carried out research on nitriding treatment technology of cemented carbide surface to further improve the wear resistance of cemented carbide surface. They used the composition series of Ti (C, n)/tin/wc/co+ carbon black, sintered it at 1430 ℃ for 90min, and then held it for 20h at 1200 ℃ in a 0.1MPa nitrogen furnace. The composition analysis of the sample by electron probe microanalysis (EPMA) showed that the surface of the nitrided material was rich in Ti and N, and poor in W and C; From about 40 to the surface μ From m to the heart, the W content of the material is higher than that of the material without nitriding treatment; 15 ~ 40 from the surface μ Within the range of M, its CO content is higher than the average CO content of the material. At the same time, fine WC phase and η Phase, grain size about 150nm. The cutting test was carried out on ss2541 steel. The change rate of tool surface wear (VB) of cemented carbide blade after nitriding treatment with time is much lower than that of PVD coated cemented carbide blade (classification code p15). It can be seen that the wear resistance and durability of cemented carbide blades after nitriding treatment are significantly improved compared with PVD coated blades

Hideki Moriguchi et al. First milled TiCN, WC, CO and Ni powders for 24h, and pressed the milled powder to 10mm under 98mpa pressure × 10mm × 5mm test block is sintered in vacuum at 1400 ℃, and then kept in nitrogen for 1h, and then cooled at a suitable speed to make gradient structure cemented carbide with gradient distribution of CO content. The CO content of the treated material increases from the surface to the inside, and tens of microns of TiCN layer is formed on the surface. The outer surface of the material contains almost no Co, and the maximum hardness can reach 2.2GPa; At the same time, the appropriate cooling rate can introduce a residual compressive stress of more than 0.5gpa on the surface, which greatly improves the hot cracking resistance of the alloy tool during cutting. The cutting comparative tests of gears and piston rods with Japanese steel grades of scr420h, S45C, cm420 and SCr420 were carried out respectively. The results showed that the cutting life of the developed gradient structure cemented carbide tool was 2 ~ 4 times that of the ordinary TiCN Cermet Tool

Suzuki et al. Of the University of Tokyo prepared WC Ti (C, n) -co cemented carbide materials by gradient sintering in a nitrogen removal atmosphere. After sintering, Ti (C, n) on the surface of cemented carbide disappears, and part of the bonding phase is transferred from the center to the surface, forming a bonding phase rich layer. The toughness and ductility of the cemented carbide surface can be improved by the denitrification treatment of the cemented carbide surface, forming the enrichment of bonding phase on the surface and eliminating the gradient transition layer of cubic carbon nitride

based on the research results of Suzuki and others, Schwarzkopf of the German Max Planck Society pointed out that the formation of the gradient region on the surface of cemented carbide is the thermodynamic coupling result of the diffusion of N atoms to the outside of the block and the diffusion of W atoms to the center of the block. At the same time, Schwarzkopf et al. Established a computer model to predict and describe the influence of several important variables in the sintering process. They pointed out that the depth of the formation of the surface gradient region is proportional to the square root of the sintering time t and the CO content of the matrix, which conforms to the diffusion law. The thickness of gradient layer x=kt (1), where t is the sintering time and K is a constant, which is directly proportional to the CO content in the matrix

in recent years, frykholm of Chalmers University of technology and Gothenburg University in Sweden have carried out more in-depth research on this surface denitrification gradient modification to more effectively control the formation of surface denitrification gradient layer. They used scanning electron microscope (SEM) and electron probe microanalysis (EMPA) to analyze the micro morphology and composition of the surface gradient region respectively, and carried out computer simulation at the same time. The results showed that the experimental analysis results were quite consistent with the computer simulation results

in addition, drakeeric from reed rock bit, Colin from the Catholic University of Leuven, favrot from the French mining school, Rosso and proto from the University of Turin, Nicolae from the Institute of Metallurgical Research in Bucharest and other researchers used different mixing methods to distribute cemented carbide powders with different grain sizes and CO contents in layers, and sintered them in solid or liquid phase to produce cemented carbide powders with high CO content from the heart The powder with large grain size of hard phase is a gradient structure cemented carbide product whose surface layer is made of powder with low CO content and small grain size of hard phase. The sintered samples have fine surface grains, low CO content, high hardness and good wear resistance, and gradually transition to coarse central grains, high CO content, good toughness and good impact resistance. The bonding between powder layers is good, and the residual compressive stress after sintering effectively improves the fatigue strength of the products. Gasik of Helsinki University of technology has prepared WC Co gradient structure cemented carbide by infiltration method. The sample has dense structure and good gradient structure and hardness distribution

2. The research and application of gradient structure cemented carbide in China

the successful application of DP alloy has prompted many domestic scholars to explore and study the carburization technology of carbon deficient cemented carbide. In recent years, Zhou Jianhua and sun Baoqi of the powder metallurgy plant of Central South University, Yang Weicai of the State Key Laboratory of powder metallurgy and Qin Weijian of Zhuzhou Cemented Carbide plant have successfully prepared gradient structure cemented carbides by using the carburization technology of carbon deficient cemented carbides, and obtained a good gradient of cobalt phase composition change. On the basis of carburizing technology of carbon deficient cemented carbide, sun Xuxin and others used carbon rich potential sintering process to prepare gradient structure cemented carbide at one time. First, WC Co alloy with much lower carbon content is prepared and mixed, and then the carbon rich potential sintering makes the sample form a carbon gradient with high surface carbon potential and low internal carbon potential from the surface to the inside. In this way, when entering the liquid phase sintering stage, the liquid phase appears first on the surface and then inside, and the amount of liquid phase on the surface is also more than the amount of liquid phase inside, forming a liquid phase pressure difference between the surface and inside of the alloy, causing the mass migration of co phase from the surface to the inside, forming a reasonable composition gradient. The experimental results show that the above carbon rich potential sintering process can produce gradient structure cemented carbide with high surface hardness, good wear resistance, high internal strength and good toughness. Kuang Tongchun et al. Found when depositing diamond film on the surface of cemented carbide by DC plasma jet CVD method that the WC particles on the surface of cemented carbide treated by chemical acid etching formed equiaxed fine pure W grains after decarburization by plasma etching, and then formed a fine WC layer on the outer surface of cemented carbide during the deposition of diamond film, and WC particles from the surface to the inside

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