Vacuum Sintering of Cemented Carbide
Vacuum sintering is the process of sintering products in negative pressure gas (steam) body medium. Compared with normal hydrogen sintering, vacuum sintering improves the purity of furnace gas, and the negative pressure during sintering improves the wettability of bonding relative hard phase and promotes the densification process. Therefore, sintering temperature or holding time can be reduced to different degrees to prevent the uneven growth of carbide grains and facilitate the preparation of fine grain alloys. The purity of cemented carbide can be improved by using vacuum sintering furnace, the microstructure of cemented carbide can be controlled easily, the density and mechanical properties of cemented carbide can be improved, and the process operation is simple. As the atmosphere effect is basically eliminated, no filler can be used in vacuum sintering, which not only simplifies the operation, but also can avoid the adverse effect of the filler on the surface of the sintered parts, especially the aluminizing effect on the edges and corners of the sintered parts when the filler A1203 is used. The quality control of vacuum sintering is the organic coordination of sintering temperature, vacuum degree and time, so as to make the alloy obtain the structure and performance required by the design and keep the appearance intact.
1. The vacuum degree of a vacuum sintering furnace
The degree of vacuum in vacuum sintering directly affects the degree of deoxidization, carbon content change or drilling loss of the alloy. The press block contains oxygen, and during vacuum sintering, it is reduced by the free carbon in the cemented carbide press block and the carbon in tungsten carbide, so that the press block is both deoxidized and decarbonized. The higher the vacuum, the faster the removal of carbon monoxide, the faster the reaction, the more thorough deoxidation. In addition, with a certain sintering temperature and holding time, the higher the vacuum degree is, the greater the evaporation loss of cobalt will be. When the vacuum degree reaches a certain level, cobalt evaporation remains unchanged, that is, there is a critical vacuum pressure. In order to reduce the loss of cobalt, the vacuum degree in the vacuum sintering furnace can be reduced after the appearance of liquid phase. It can be seen that in the pre-burning deoxygenation stage, in order to completely deoxygenate, the larger the vacuum pump pumping force is better, until the deoxygenation is completed to the highest vacuum degree. After the appearance of the liquid phase, the vacuum degree is required to be as low as possible to avoid the evaporation loss of the drill. But in order to make the product carburization, the higher the vacuum is required, the better.
2. Vacuum sintering, the sintering temperature from room temperature to 200 ℃ is a physical inspiratory, suction desorption stage. After 400 ℃ 300 ℃ "-", into the solid phase, the higher the temperature, the faster the deoxidization reaction, and the higher requirements for vacuum degree. In order to make the deoxidization accelerated from 400 ℃ to liquid phase appeared in front of presintering insulation stage, heating speed can be speed up, quick to attract products inside and outside the degree of crack due to temperature difference is too big. The temperature of the liquid phase varies with the alloy grade, and the first 50 of the liquid phase is generally chosen. C - 100. The temperature of C is used as the pre-firing insulation temperature. The length of holding time depends on the time of deoxygenation. Heat preservation can also eliminate the temperature difference between the center of the vessel and the surrounding area, so that the time of liquid phase of the sintered products is basically the same. The temperature of pre-burning heat preservation exceeds the temperature of the liquid phase, so that the carbon and oxygen reaction occurs in the liquid phase. At this time, the product has a relatively large shrinkage, and the surface of the product forms a liquid cobalt layer, which increases the carbon monoxide escape resistance and leads to the increase of the inner pores of the alloy and the formation of cobalt aggregation and surface "drums"
Bubble ". The sintering temperature has a great influence on the properties of the products. When the alloy is "underburned", due to low temperature, the Co liquid cannot flow evenly, resulting in uneven distribution of Co, poor toughness and low coercivity. When the temperature rises to the optimal sintering temperature, the fluidity of Co liquid increases, and the result shows that the Co distribution is uniform, the alloy toughness is good, and the coercive force is also high. When the temperature continues to rise to the "overburning" temperature, the grain grows, the thickness of Co layer is uneven, the toughness decreases, and the coercivity decreases. In practice, it is difficult to control the optimal temperature. Generally, the temperature range slightly higher than the optimal sintering temperature is adopted as the actual sintering temperature range under control.
The figure above is the typical process of cemented carbide multi-atmosphere vacuum sintering, in which figure (a) is the temperature process and figure (b) is the furnace pressure process. In order to make the press block adsorption of water, gas, and residual wax ruled out as soon as possible, to put the vacuum sintering furnace furnace pressure to A minimum (point A), and then slowly heat up to 400 ℃ (point B)
Wen. As the exhaust (steam) and residual wax volatilize, the pressure in the furnace appears peak (CF) at this time. If the product is not removed before vacuum sintering, and the vacuum furnace needs integrated degumming, hydrogen gas (point D) needs to be injected into the furnace for degumming. After degumming is completed (point E) and hydrogen gas (point F) is removed from the furnace, the insulation stage will end (point G). To speed up the DNA from degumming finish to presintering temperature (1200-1250 ℃) to adopt quick heat (GJ), vacuum pump to maintain maximum pumping, the furnace pressure drop (FH), after the presintering temperature, oxygen carbon reaction quickly, furnace pressure rise gradually appeared peak (I am). When the peak value begins to disappear (L point), it indicates that the oxygen-carbon reaction is completed, and the pre-burning stage ends (K point). The pre-burning holding time depends on the actual vacuum degree, if the vacuum degree can reach 46. Above 66Pa, the heat preservation time can be shortened correspondingly, and vice versa. From pre-sintering temperature to sintering temperature (KM), to post-sintering temperature insulation (MQ), the insulation time depends on the alloy brand and product size. When the atmosphere in the sintering furnace is vacuum (OS), the vacuum degree depends on the specific product and is generally controlled between 10Pa-,40 Pa. If argon atmosphere sintering (PT) is adopted, the pressure in the furnace is generally controlled in the state of micro-positive pressure, usually around 50mbar.
After the vacuum sintering heat preservation, under the process of pumping air into vacuum state, make the oven temperature to 1000 ℃ ~ 1100 ℃ (R). Then argon or nitrogen is injected to make the pressure in the furnace rise to the state of micro positive pressure (ST). The pressure is maintained and the furnace temperature is rapidly cooled.