Sintering and Machining
Because the melting point of tungsten is as high as 3410℃, tungsten crucible is widely used in the core container of sapphire single crystal growth furnace, quartz glass melting furnace, rare earth smelting furnace and other industrial furnaces, and its working temperature environment is generally above 2000℃. Especially for sapphire single crystal growth furnace, tungsten crucible with high purity, high density, no internal crack, precise size, bright and clean inside and outside wall, etc. plays a key role in the success rate of seed crystal, quality control of crystal drawing, decrystallizing and sticking pot and service life of sapphire crystal growth process.
1. Material preparation
Tungsten crucible crucible materials mainly for the tungsten powder. Because the tungsten products hardness at room temperature is higher, therefore, no longer have commonly after sintering mechanical processing. So, it is not special requirements for tungsten powder. But in order to improve the high temperature strength of tungsten crucible and corrosion resistance, often joined by about 8% molybdenum powder for solid solution strengthening. Will the sifted tungsten, molybdenum pretend after mixed in proportion.
2. The tungsten crucible molding
(1) mold design design tungstenk crucible mold, mainly consider the following parameters: the sintering rear axle to the shrinkage rate is approximately 12%, radial shrinkage rate is approximately 13%. External mold using soft mode, internal model USES the steel mould. Steel wall thickness is about 5 mm, the taper is approximately 0.4%, the mold wall should be clean, facilitate ejection, the rough side should be welded on the cross beam, facilitate hoisting.Latex ring cork sealed on both ends.
(2) molding and loading after ejection mould ready. When charging should be uniform thickness, to ensure consistent wall thickness after sintering, some measures should be taken when lifting, prevent the decline in press freedom.
Forming technology in cold isostatic pressing machine, pressure control between 200 ~ 220 mpa, keep 3 minutes, two levels of pressure relief and prevent the compact cracking phenomenon caused by unloading too fast.
After forming of tungsten crucible hoisted out compact, can install furnace sintering. But because of without sintered crucible compact size is larger, so will have to remove a layer of insulating material and on the second floor of induction coil, can put in tungsten crucible compact heating sintering
3. The tungsten of the crucible sintering hook of the crucible sintering general use three sintering process. In the induction furnace sintering is the first time in the induction coil on the bottom of refractory brick in a compact and crucible radius of about the same size, thickness of 30 mm graphite circular plate, put compact graphite round plate, in which the piled up again some tungsten or molybdenum scrap material, for temperature measurement for temperature test points, insulation at 1500 ℃ temperature 5 hours. The second sintering is to through the first sintering has certain mechanical strength of the hook crucible hoisted out, to the top after fall down into the induction furnace, at 1700 ~ 1800 ℃ temperature insulation 5 hours, the cooling after 7 hours. The third sintering is after the second sintering hook crucible according to the above method, induction coil, add remove the original back into the furnace refractory, in above the highest use temperature about 100 ℃ temperature holding 3 hours, to facilitate the uniformity of the alloy composition is increased. After cooling, hoisted out after exchange of upper and lower end, can be put into use.
The highest application temperature environment: 2400℃.