To know sintering furnace of cemented carbide, we should review the sintering process of cemented carbide at first.

The function of the sintering furnace is to heat  cemented carbide preforms to a certain temperature (sintering temperature), keep them for a certain time (holding time), and then cool them down to obtain the cemented carbide final products with the required performance.

1Four stages in cemented carbide sintering process

Pre burning + removal of forming agent

At the initial stage of sintering, with the increase of temperature, the molding agent gradually vaporizes and is separated out of the sintered body. At the same time, the molding agent more or less carburizes the sintered preform. oxide on the surface of the carbide powder will be reduced by hydrogen. The contact stress between the powder particles is gradually eliminated, and bonding metal powder begins to recrystallize.

Solid phase sintering stage (800 ℃ — eutectic temperature)

At the temperature before the liquid phase appears, in addition to continuing the process in the previous stage, the solid-phase reaction and diffusion are intensified, the plastic flow is enhanced, and obvious shrinkage appears on preforms.

Liquid phase sintering stage (eutectic temperature — sintering temperature)

When the liquid phase appears in preforms, the shrinkage is completed quickly, and then the crystal transformation occurs, forming the basic structure and structure of cemented carbide products.

Cooling stage (sintering temperature – room temperature)

At this stage, the microstructure and phase composition of the cemented carbide will change with different cooling conditions. Cemented carbide products have been formed at this stage with enhanced mechanical and chemical properties.

2the premise of sintering furnaces: VACUUM SINTERING

In order to ensure the smooth dewaxing (lubricant or forming agent), reduction, alloying and microstructure transformation of powder compacts during the sintering process, the sintering furnace needs to be able to accurately control the sintering temperature, protective atmosphere, compacts conveying mode, heating and cooling speed, etc. In the production of cemented carbide products, the vacuum sintering furnace is basically used now. Extract the air in the furnace to vacuum, and then fill the sintering furnace with inert gas to achieve different atmospheric pressure.

3 Widely applied cemented carbide sintering furnaces 1
fig.1 the relationships between pressure in sintering furnace and pore shrinkage of cememted carbide products

3Cemented carbide sintering furnaces widely used in mass production Currently

According to the difference of pressing strength, two kinds of sintering methods are usually used, i.e. pressureless sintering and pressured sintering. Pressureless sintering is mainly applicable to ceramic products. At present, in the field of cemented carbide, pressure sintering is widely used in the following three ways: hot isostatic pressing sintering, hot pressed sintering, and gas pressure sintering.

Hot pressing sintering furnace

Hot pressing sintering refers to a sintering method in which the dry powder is filled into the model, and then heated while being pressed from the uniaxial direction to complete the molding and sintering at the same time.

Because the heating and pressurization are carried out simultaneously, the powder is in a thermoplastic state, which is conducive to the contact diffusion of particles and the flow and mass transfer process. Therefore, the molding pressure is only 1 / 10 of the cold pressing pressure (the cold pressing pressure is about 630mpa); It can also reduce the sintering temperature and shorten the sintering time, so as to resist grain growth and obtain products with fine grain, high density and good mechanical and electrical properties. It can produce ultra-high purity ceramic products without adding sintering aids or molding AIDS. The disadvantages of hot pressing sintering are complex process and equipment, strict production control requirements, high requirements for mold materials, large energy consumption, low production efficiency and high production cost.

3 Widely applied cemented carbide sintering furnaces 2

Hot isostatic pressing integrated sintering furnace

Hot isostatic pressing is a process production technology integrating high temperature and high pressure. The heating temperature is usually 1000 ~ 2000 ℃, and the working pressure can reach 200MPa by using the high-pressure inert gas or nitrogen in the closed container as the pressure transmission medium. Under the joint action of high temperature and high pressure, the workpiece is uniformly compressed in all directions. Therefore, the processed products have high compactness, good uniformity and excellent performance. Because the density is uniform, the ratio of length to diameter is not limited, which is conducive to the production of rod-shaped and tubular thin and long products. Moreover, the hot isostatic pressing generally does not need to add lubricant to the powder, which not only reduces the pollution of the products, but also simplifies the manufacturing process.

The compactness of the sintered product can be significantly improved by treating it in a hot isostatic press under the conditions of pressure of 80-150mpa and temperature of 1320-1400 ~ C for a certain time, and its porosity can be reduced to 1 / 20-1 / 100 or even lower than that before hip treatment, and the bending strength and service life can be significantly improved. However, the design and control of hip equipment are expensive and the maintenance and operation are complicated, so it is not widely used in cemented carbide.

Gas pressure sintering furnace

Air pressure sintering is also called overpressure sintering or low pressure hot isostatic pressing sintering. It is a kind of hot isostatic pressing and sintering process for workpieces at a pressure lower than hot isostatic pressing (about 6Mpa), which is more suitable for mass production.

gas pressure sintering is developed on the basis of vacuum sintering and hot isostatic pressing. The previous concept believed that it requires a large pressure to eliminate the pores in the alloy at the sintering temperature. Later, it was found that at the sintering temperature, a lower pressure can also eliminate the pores in the alloy and avoid the defects of ‘cobalt pool’ in the alloy caused by high pressure. Low pressure sintering can obtain better comprehensive properties than the alloy treated by hot isostatic pressing. Air pressure sintering has advantages in producing large-size cemented carbide products.

Its specific working principle is that after dewaxing with hydrogen at a lower temperature, vacuum sintering is carried out at 1350 ~ 1450 ℃ for a period of time, and then quasi hot isostatic pressing is carried out in the same furnace. Argon is used as the pressure medium, the pressing pressure is about 6Mpa, and then it is kept for a certain time, and then cooled.

compared to HIP sintering

Because the sintering and hot isostatic pressing of the alloy products in the gas pressure sintering furnace are carried out in the same sintering furnace. According to a large number of production data, gas pressure sintering can avoid the “coarse grain” and other structural defects that are easy to appear after conventional hot isostatic pressing treatment,and improve the strength and hardness of the tungsten alloy products to a certain extent.

All processes are carried out in the same furnace body, which saves a lot of equipment investment. The processing cost of gas pressure sintering is twice as low as that of the hot isostatic pressing process after vacuum sintering, which can greatly improve the service life of the product.

6Mpa gas pressure sintering furnace with degreasing function that be put into production in Meetyoucarbide

3 Widely applied cemented carbide sintering furnaces 3
horno de sinterización a presión de gas
Effective space (w * h * l)200*200*400mm300*300*1200mm400*400*1200mm500*500*1200mm500*500*1800mm500*500*2700mm
max load50kg300kg500kg1200kg1500kg2500kg
heating power240KVA320KVA320KVA430KVA560KVA900KVA
heating spacespace Ⅱspace Ⅲspace Ⅲspace Ⅲspace Ⅲspace Ⅵ
cooling time cost≤2h≤4h≤5h≤6h≤7h≤8h
max working pressure58bar
max working temp.1580℃、2100℃
temp. messuring methodW-Re5/26Thermocouple
Extreme vacuum1Pa
Vacuum leakage rate3Pa/h
efficiency of collecting wax≥98%
Type of forming agentParaffin, PEG, rubber, ethyl cellulose, etc
gas allowed to fillNitrogen, argon and hydrogen

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