Sintering process of cemented carbide

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Cemented carbide is composed of various carbide and iron element, such as WC – Co, WC – TiC – TaC – NbC – Co or TiC – Mo ₂ C – Ni.The typical characteristic of these materials is that almost 100% theoretical density can be achieved through liquid phase sintering. After sintering, low residual porosity is the key to the successful application of cemented carbide in high-stress working conditions such as metal cutting, oil drilling bits or metal forming molds.In addition, the sintering process must be carefully controlled to obtain the desired microstructure and chemical composition.

In many applications, cemented carbide is sintered.The surface of sintered alloy is often subjected to harsh conditions of friction and stress. In most cutting metal applications, the wear depth of the tool head surface exceeds 0.2~0.4mm, and the tool is determined to be scrapped. Therefore, it is very important to improve the surface performance of cemented carbide.

There are two basic methods for sintering cemented carbide: hydrogen sintering, in which the component is controlled by inverse reaction kinetics under normal pressure and hydrogen, and vacuum sintering, in which the component is controlled by slowing down the reaction kinetics in a vacuum or by reducing the ambient gas pressure.Vacuum sintering has more extensive industrial applications.Sometimes, hot isostatic pressure and hot isostatic pressure are also used, which have important influence on the production of cemented carbide.

Hydrogen sintering: hydrogen is a reductive atmosphere, but when hydrogen reacts with the sintering furnace wall or bearing device, other components are changed to provide the appropriate carbonization potential to maintain thermodynamic equilibrium with the carbide.In conventional cemented carbide sintering, the carbon content of the carbide in the mixture should be adjusted to the theoretical value and kept unchanged during the whole hydrogen sintering process.For example, when sintering 94wc-6co cemented carbide, the carbon content is 5.70~5.80% (mass fraction) when entering the furnace, and it should be maintained at 5.76+0.4% when coming out of the furnace
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.Pre-sintering can also be adjusted to increase the strength of the embryo, so that it can be roughed, such as turning and drilling, pre-sintering temperature between 500 and 800 degrees Celsius, which mainly depends on whether the lubricant is removed thoroughly and the required strength of the embryo.

Vacuum sintering: compared with hydrogen sintering, vacuum sintering mainly has several advantages. First of all, vacuum sintering can control the product composition extremely well. Under the pressure of 1.3~133pa, the exchange rate of carbon and oxygen between atmosphere and alloy is very low.The oxygen content in carbide particles, rather than the reaction rate between carbon and rarefied gas in vacuum, is the main factor influencing the change of composition. Therefore, vacuum sintering has an advantage in the industrial production of sintered cemented carbide.

During hydrogen sintering, the oxidation potential of atmosphere gas in the furnace increases due to the infiltration of hydrogen and the reaction between hydrogen and ceramic furnace components.There are no such problems in vacuum sintering, and the oxidation potential in the furnace is lower than that in hydrogen sintering. Therefore, the alloy containing titanium carbide, tantalum carbide and niobium carbide, which are very sensitive to oxidation, is more suitable for vacuum sintering process.

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 an intermittent operation, which can flexibly adjust the sintering system required, while hydrogen sintering is mostly a continuous sintering process, which can achieve accurate control of the temperature of each sintering stage.

If this were possible, the slower rate of warming would give enough time for the carbon-oxygen reaction to be sufficient: the CO gas formed would also come in and escape from the connected pores; if the rate of warming were too fast, the gas would be trapped and form pores.In a vacuum furnace, it is easy to adjust the heating rate of the heating process, while for a mechanical push-boat hydrogen sintering furnace, the temperature in each sintering area and the time to reach the maximum sintering are both limited and difficult 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 referred to as overpressure sintering and pressure sintering.In fact, the sintering furnace is a vacuum sintering furnace that can be filled with pressure. In order to reduce or eliminate residual voids, when closed pores are formed in the part at sintering temperature, inert gas is injected into the furnace to exert equal static pressure on it. The pressure of argon is 1.5~10Mpa, far lower than the normal thermal isostatic pressure.A specific sintering process, including lubricant removal, oxide reduction, and carbide alloy sintering.When the closed hole appears in carbide sintering, the static pressure of low-pressure heat in the furnace rises to a higher level.It has been reported that for the cemented carbide containing co3% ~6% wc-co, the obturator occurs in the range of 1420~1460 ° c at the isostatic pressure of sintering heat of about 0.2Mpa.For cemented carbide containing cubic tungsten carbide, the obturator foramen occurs at 1430~1480 ℃.In terms of activity-based cost, the operating cost of the sintering heat isostatic pressure equipment with the same production capacity is much higher than that of the vacuum sintering furnace.

The thermal isostatic pressure is carried out in a specially designed high-pressure vessel pressurized with argon gas to 100Mpa at roughly the same temperature as conventional sintering.Usually, it is sintered first and then treated with isostatic pressure to eliminate a small amount of residual voids that cannot be eliminated by normal sintering process.Of course, hot isostatic pressure can also be used to solidify the pre-fired embryos.The thermal isostatic press is a major key investment, as a post-processing process for sintering, it increases the operating cost, energy and gas consumption and production cycle.The hard alloy produced by hot isostatic pressing has the characteristics of fine grain and low content, so its strength is higher.

However, no matter whether hot isostatic pressing or post-hot isostatic pressing is adopted, only when an appropriate relationship among time, temperature and pressure is established, can the strength be higher than that of hydrogen sintered and vacuum sintered products.

Learn More : Vacuum Sintering Furnace