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Will contain briquetting forming agent in a graphite boat, filling a carbon content of alumina filler or graphite particle packing, then, is usually a load of continuous pusher molybdenum wire furnace, sintering under the hydrogen protection, this process is hydrogen sintering process.
A, gas phase reaction sintered body principle:
1, deoxidization reaction:
MeO + H2 = Me + H2O
2, tungsten carbide decarburization reaction (in 800-950 ℃ temperature above) :
WC + H2O = W + H2 + CO
WC + 2 h2 = W + CH4 (methane)
3, the carburization reaction:
CH4 = C + 2 h2 (CH4 from forming agent decomposition of cracking in 400-500 ℃)
2 co = CO2 + C
4, the generation of water gas reaction:
H2O + C = CO + H2 (carbon from the graphite boat, packing and free carbon)
Second, actual common chemical reaction sintering process
1, the cobalt oxide reduction: some of the fine particles of cobalt powder mixture in the form of the oxide, cobalt powder surface coarser will also form oxide film; For cobalt powder stored in the air, wet grinding mixture, drying and briquetting prolonged exposure to air, briquetting further drying; Usually fine particle mixture oxygen levels even more than 1.0%, but the cobalt oxide can completely be hydrogen and carbon reduction:
2 + 2 = 2 c the coo co + CO2
2 coo + WC co + CO2 + W = 2
3 + 3 wc = W3CO3C coo + CO + CO2
2, wet hydrogen decarburization: due to the moisture content of hydrogen is as high as 1000 mg/M3, the temperature is above 950 ℃, the tungsten carbide sharp decarburization and form eta phase; In the solid phase sintering, due to the serious decarburization form eta make press block size increases. Line size can cause increase of 6-7%, briquetting higher porosity shrinkage difficulties; Uneven decarburization cause deformation. In order to avoid the decarburization, caused by high moisture content in the hydrogen should pack or carbon graphite particles of alumina for protection.
3, gas carburizing: because of existence of carbon monoxide and methane carburizing furnace gas carburizing.
: (1) solid carburizing stage in front of the sintering process of two stages, and the sintered body as a liquid, press block or a porous body, when the furnace gas carburizing is through carbon gases in pores on the surface of the separating carbon sintered body, and then further to the activated carbon particles internal diffusion, thus completing the process of carburizing, use high carbon filler sintering, for lack of carbon increase carbon sintered body, prevent alloy appear eta phase in the organization.
(2) liquid carburizing stage: after sintered body into the liquid phase sintering stage, rapid contraction achieve dense state. The carburizing process of sintered body is through liquid cobalt carbon absorption process. Due to liquid phase sintered body, carbon solubility in the liquid phase increases rapidly, the liquid carburizing, obviously improve the temperature, increase the heat preservation time, increase the carburizing gas concentration can improve the carburizing speed, high cobalt content of alloy, fine carbide particles, is conducive to liquid carburizing.
Three, forming agent role of carbon:
In the low temperature sintering stage, the decomposition of forming agent increased carbon sintered body.
1, forming agent carburization process and features:
: (1) rubber heat fission, happens in the low temperature sintering stage is decomposed into hydrogen, methane and other hydrocarbons and free carbon. Carbon in the sintered body, give a sintered body carburization, hydrocarbons into the furnace, furnace gas by gas carburization reaction to secondary thickening of carbon sintered body.
(2) the paraffin wax, paraffin in the process of evaporation condensation will occur some changes. Paraffin wax cracking at high temperatures can produce hydrocarbon gas, make the sinter carburizing, carburizing, however, can only produce more carbon high melting point of paraffin.
2, the factors influencing forming agent increased carbon:
To join (1) forming agent quantity is proportional to the increased amount of carbon;
(2) the heating speed, the faster the temperature, the more carbon. On the contrary, the less.
(3) hydrogen flow: the increase of hydrogen flow rate, can reduce the forming agent increases the amount of carbon.
Four, the choice of the hydrogen sintering specification:
1, sintering temperature, sintering temperature of the carbide shall be higher than that of its main carbide and binder usually eutectic temperature 40-100 ℃; Such as WC - CO eutectic point is 1340 ℃, the sintering temperature is 1370-1440 ℃
2, sintering time, sintering process must ensure enough time to complete the organization transformation. Although in a certain range, sintering temperature and time can complement each other. Such as high temperature fast or slow at low temperature. However, the scope is limited. If the temperature is too low, longer sintering time can't make the density of alloy should be. To the sintering temperature can reach equilibrium state, and have good organizational transformation time, insulation is usually 1 to 2 hours. However, sintering time also affected by other factors. Such as product size is an important factor.
The effect of 3, the choice of packing: packing is hydrogen sintering is very important, it can cause a kind of suitable for sintering in local space body sintering atmosphere, so as to ensure the carbon content of alloy in the allowed range, alloy tissue to normal.
Packing: (1) carburizing graphite powder 1-3 mm size;
(2) the decarburization sex packing: magnesium oxide; W powder;
(3) the alumina packing: add a small amount of 07% (0.1 -) carbon, carbon carrier effect, low temperature carbon absorption, decarburization at high temperature;
4, heating speed, intermittent production furnace per unit time to a rise in temperature degrees; Continuous furnace tends to push the boat speed (mm/min). Heating up too fast, increased carbon sintered body increased, but also can make the furnace hydrocarbon gas concentration is far higher than normal, cause peeling waste; Heating up slowly, increase carbon reduction in the sintered body, still can make sintered body increases oxygen decarburization.
1, hydrogen purity: usually use dew point minus 40 DHS. The lower the water content, the better. Such as negative 70 DHS.
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