The roller is made of WC and CO by powder metallurgy. There are two types of WC roller: integral type and combined type (see cemented carbide roller sleeve). It was born with the development of metal processing industry after the advent of powder metallurgy technology in 1909.
Since the introduction of carbide drawing die in Germany in 1918, the research of cemented carbide in various countries has been stimulated, and rollers for various purposes have appeared one after another. However, the wide application of WC roller was after 1960. In 1964, the first high-speed non twist wire rod mill of Morgen company was born, which increased the finishing speed of wire rod four times at that time.
Because the finishing mill works under high speed and high stress, the wear resistance of cast iron roll and tool steel roll is poor, the service life of rolling groove is short, the repair and loading and unloading of roll are very frequent, which affects the efficiency of the mill and can not meet the requirements of finishing production. Therefore, it is replaced by a combined tungsten carbide roll. There are more than 200 sets of Morgan mills in the world, which consume hundreds of tons of WC rolls annually.
In the 1980s, tungsten carbide rolls developed in China have been successfully applied to high-speed wire rod mills. Ch series rolls of General Iron and Steel Research Institute of Ministry of metallurgy and YGH series rolls of Zhuzhou 601 plant have been mass-produced.
The composition, microstructure and properties of WC rolls vary with the working conditions of the rolls. Even for the same finishing mill, the composition of the rolls in different stands is different. The high-speed wire rod mill roll is mainly made of tungsten carbide with cobalt as a binder. Due to the poor water quality, a few continuous casting and rolling mills also use tungsten carbide rollers with Co-Ni Cr as binders, so the cost is low.
T.C roller chemical composition & properties of grade
The morphology of the WC roll is shown in figure 1. The properties of the rolls with two common components are shown in the table. The metallographic structure is composed of matrix phase WC and binder phase Co, as shown in Figure 2: white phase is WC, black phase is Co. It can be seen from the table that the hardness of carbide roll increases with the increase of WC content; The toughness increases with the increase of binder content.
The application characteristics are shown in the table. The hardness of the WC roller is high, and its hardness value changes little with temperature. The hardness value at 700℃ is four times that of high-speed steel; The elastic modulus, compressive strength, bending strength and thermal conductivity of the tool steel are more than one time higher than those of the tool steel. All these properties ensure that the wear resistance and deformation resistance of WC roll are much better than that of tool steel roll in the working process, so as to ensure the smoothness of roll groove and the constancy of size, which is very important to ensure the surface quality and dimensional accuracy of wire rod.
Due to the high thermal conductivity of WC roll, the heat dissipation effect is good, the time of high temperature on the roll surface is short, so the high-temperature reaction time of the roll with harmful impurities in the cooling water is short. Therefore, WC roller is more resistant to corrosion and thermal fatigue than tool steel roller.
Production method tungsten carbide roll is produced by powder metallurgy method with tungsten carbide powder and cobalt powder (a small amount of other hard phases or alloy elements can also be added) as raw materials. The above two kinds of powders were wet ground and mixed by ball mill. After mixing, drying and sieving, the mixture become the intermediate product cemented carbide mixture. The mixture is pressed and sintered into roll blank. After grinding, polishing or re-grooving, the roll blank can be used for production.
The quality standards and influencing factors of WC roller are: material porosity, WC grain size, total carbon and free carbon content, density, hardness, magnetic saturation strength, coercive force, bending strength, impurity content, etc. In addition, there are the machining accuracy and surface roughness of the roll. Each index reflects the quality of the roller and indicates its performance. The influencing factors of roller quality are: dispersion, particle size and particle size distribution of WC powder and Co powder in mixture, the content of total carbon, free carbon, oxygen and iron, etc; The type and quantity of binder, the temperature and time of degumming, the temperature, time and atmosphere of sintering, etc. In addition, the precision of the grinding machine and the quality of the diamond grinding wheel affect the quality of roll.
Tungsten carbide roller has been used in various fields of metal plastic processing. From non-ferrous metals to steel, from hot-rolled wire rod, pipe to cold-rolled foil, rebar and so on, tungsten carbide roller is used. China has been able to produce from φ 8～ φ There are dozens of varieties and specifications of 300 mm tungsten carbide rolls. However, with the high speed of wire rods finishing rolling (up to 130m / s), people are exploring ways to improve the hardness and strength of WC roller. This is mainly due to the addition of a small amount of TAC, NBC, Ni, Cr, Mo and other elements into the WC roll; By means of hot isostatic pressing, vacuum sintering low pressure hot isostatic pressing, heat treatment and so on, the roller with better quality can be made.
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