Mon, 09 Jul 2018 02:57:19 +0000
Cemented carbide sintering furnace – sintering process
Hard alloys are made up of a variety of carbide and iron elements, such as wc-co, wc-tic – tac-nbc-co or TiC-Mo yard-ni. The typical characteristic of the material is, through liquid phase sintering can reach almost 100% theoretical density, sintering, low residual porosity is successful application of cemented carbide in metal cutting, drilling bit or metal forming mould contour is the key to stress the operating conditions. In addition, the sintering process must be carefully controlled to obtain the desired microstructure and chemical composition.
In many applications, cemented carbide is used in sintering state. State sintered alloy surface under harsh conditions of friction and stress, often in most of the metal cutting applications, the depth of abrasion on the surface of the cutting head as long as more than 0.2 ~ 0.4 mm, tools were judged scrap, so, improve the surface performance of cemented carbide is quite important.
Cemented carbide of two basic methods: one is the hydrogen sintering in hydrogen and pressure through composition, instead we should learn to control the moving parts is another kind of vacuum sintering, adopts vacuum environment or reduce the environment pressure, by reducing the reaction kinetics to control hard alloy composition. Vacuum sintering has more extensive industrial applications. Sometimes, static pressure such as sintering heat and static pressure such as heat are used.
Hydrogen sintering: hydrogen is a reducing atmosphere, but when the hydrogen react with sintering furnace wall or bearing device will change other components, provide appropriate carbon potential in order to maintain and carbide thermodynamic equilibrium. In traditional hard alloy sintering, the carbon content of the carbide in the mixture should be adjusted to the theoretical value, and this value should be maintained during the whole hydrogen sintering process. For example, when sintering 94wc-6co hard alloy, the carbon content is 5.70~5.80% (mass fraction) when entering the furnace and 5.76+0.4% when coming out
Hydrogen atmosphere of sintering process control ability for class cobalt tungsten carbide is enough, but for steel cutting tools with titanium carbide carbide tantalum and niobium carbide alloy, high oxidation potential of the atmosphere, causes the alloy composition change, usually to reduce these with vacuum sintering, the content of metal oxide, hydrogen sintering with mechanical push boat commonly done through continuous sintering, can use a separate presintering furnace after removal of lubricant to prevent pollution of volatile matter and high sintering process. Presintering can also adjust product embryo strength, enabling to rough machining, for example, turning and drilling, presintering temperature between 500 ~ 800 degrees Celsius, which depending on whether the lubricant removed thoroughly and required strength of embryo.
Vacuum sintering: compared with hydrogen sintering, vacuum sintering mainly to several advantages, first of all, vacuum sintering can very good control product ingredients, in 1.3 ~ 133 pa pressure, carbon and oxygen in the atmosphere and alloy between the exchange rate is very low. The main factors influencing the composition change is the oxygen content in carbide particles, rather than carbon and thin gas in the vacuum of the reaction rate, thus in the industrial production of cemented carbide, vacuum sintering the advantage.
When hydrogen is sintered, the oxidation potential of the atmosphere gas in the furnace is increased due to the infiltration of hydrogen and the reaction between hydrogen and ceramic furnace components. That vacuum sintering is not lower than when hydrogen sintering furnace oxidation potential, therefore, contains is sensitive to the oxidation of titanium carbide, tantalum and niobium carbide carbide alloy, vacuum sintering process, the more appropriate.
Second, vacuum sintering flexible control sintering system, especially the heating rate of heating temperature stage, in order to meet the needs of production, for example, when sintering containing titanium, tantalum, niobium carbide carbide carbide alloy, must slow heating, also have a middle temperature in the heat preservation stage to get high quality products. Vacuum sintering is intermittent operation, flexible adjustment needed for sintering system, and the hydrogen sintering is mostly continuous sintering process, can realize the different sintering stages for accurate temperature control.
If can achieve, slower rate of warming will have enough time to make carbon – oxygen sufficient reaction: the formation of CO gas and escape from the open pore also, if the heating rate is too fast, the gas will stay down, form a pore. In vacuum furnace, it is easy to adjust the heating rate of heating process, and for mechanical push boat type hydrogen sintering furnace, reached the highest in the sintering temperature and sintering time, are restricted to adjust.
Industrial vacuum sintering operation cost is low, now before the vacuum sintering equipment, is a small induction furnace, the furnace large energy consumption, cooling time, and requires a separate clear lubricant before sintering, now application of vacuum sintering furnace at least than hydrogen sintering low cost in the following aspects: large amount of furnace charging and resistance heating, with forced air cooling, and can remove lubricant in the oven.
Sintering hot isostatic pressure: sintering hot isostatic pressure is sometimes called overpressure sintering and compression sintering. Sintering furnace is actually a pressurized vacuum sintering furnace, in order to reduce or eliminate residual gap under the sintering temperature after the parts inside closed pore space, filled with inert gases exert to furnace, such as static pressure, argon gas pressure in 1.5 ~ 10 mpa, hot isostatic pressing pressure is far less than usual. A specific sintering process, including lubricant removal, oxide reduction and carbide alloy sintering. When a closed hole occurs in carbide sintering, the static pressure of low pressure heat in the furnace is raised to a higher level. It has been reported that for WC-Co cemented carbide with Co3%~6%, the static pressure of sintering heat is about 0.2Mpa. For hard alloys containing cubic tungsten carbide, a closed hole appears between 1430 and 1480 degrees Celsius. In terms of operating cost, the operating cost of sintering equipment with equivalent production capacity is much higher than that of vacuum sintering furnace.
Thermal isostatic pressure is carried out in a specially designed high pressure vessel using argon gas to pressurize to 100Mpa at roughly the same temperature as the traditional sintering temperature. Usually, it is sintered first, followed by isostatic pressing to eliminate a small amount of residual space that cannot be eliminated by normal sintering process. Of course, thermal static pressure can also be used to solidify only pre-fired embryos. Thermal isostatic press is the main key investment. As a post-treatment process of sintering, it increases the operation cost, energy and gas consumption and production cycle. Hot isostatic pressing production of cemented carbide has the tiny grains, the characteristics of low content, higher strength but whether using sintering after hot isostatic pressing or hot isostatic pressing, only by establishing the appropriate relationship between the time, temperature and pressure, can obtain the than hydrogen sintering and the strength of the high vacuum sintering product.