Preparation Of Ultrafine W, WC, Spherical Co Powder And Related Application In Cemented Carbides

Fine, ultrafine and nanophase WC-Co cemented carbides have anoptimal combination of hardness and toughness, which made them widelyused in the production of micro-drill, pinhead, cutting tool, precise die andwear part, etc. In china, only a few manufacturers can produce ultrafinecemented carbides, but the raw materials powders depend on import,besides, we have to import lots of ultrafine cemented carbides annually. So,it is necessary for us to produce raw materials for ultrafine cementedcarbides. This paper studied the preparation of homogeneous fine/nanotungsten powder, ultrafine tungsten carbide powder, ultrafine sphericalcobalt powder, and the related application in ultrafine cemented carbides.The main results have been obtained:1. Prepare fine and nano tungsten powderThe homogeneity of the particle size of fine, ultrafine or nanotungsten powder has important effect on the properties of cementedcarbides. Presently, the main technology for producing tungsten powder isreducing tungsten oxides by hydrogen using four-tube furnace ormulti-tube furnace. In this technology, it is necessary to load tungstenoxide powders in boat. In the process, larger tungsten powders have oftenbeen obtained in the front and four comers of the boat, which debases thehomogeneity of tungsten powder. Also, the temperatures in different tubesfor the same furnace have some difference that also influence the sizedistribution of tungsten powder. Belt continuous boat-less reducingfurnace can avoid above disadvantages. Also, it can produce the powdercontinuously, which not only reduces the effect of human factors, but alsoimproves working condition and alleviates hardness. In this paper, a beltfurnace was utilized to produce tungsten powder. The raw materials weretungsten blue oxide and ultrafine tungsten yellow oxide. The resultsshowed that the properties of tungsten powder were mainly controlled bythe temperature, gas feed rate, belt speed and the thickness of rawmaterials. The fine tungsten powder made from tungsten blue oxide usingbelt furnace had a more uniform size than that using four-tube furnace. The homogeneous nano tungsten powder with low-oxygen content couldbe produced by reducing ultrafine tungsten yellow oxide. The nanotungsten powders had 3.45 m²/g in BET and around 90 nm in size.2. Prepare ultrafine tungsten carbide powderIn this paper, a novel direct carbonization process was used toproduce ultrafine tungsten carbide powder. The raw materials weretungsten trioxide and carbon black. The process comprised four steps: （a）wet milling of the tungsten trioxide and carbon black; （b） drying; （c）carburizing; （d） adjusting carbon content. The characteristic was that themole ratio of C and WO₃ in reactive materials was greater than thestoichiometric proportion to prepare WC at thermodynamics reactiontemperature. Using excess carbon had two advantages: one was to makereaction more rapidly and entirely, the other was the excessive carbonblack could be used as dispersant to prevent powder reunite.So, the homogeneous WC powder with the size of around 0.3μmcould be produced at 1000～1100℃carbonization temperature. Theultrafine WC powder had 6.09% combined carbon, 0.05% free carbon, and0.3% oxygen content. It was found that the quantity of consumed carbonin process and the size of tungsten carbide powder increased with theincreasing of carbonization temperature.3. Prepare ultrafine spherical cobalt powderIn this paper, ultrafine spherical cobalt powder had been producedusing carbonate as raw materials in decomposed ammonia atomospherewith belt furnace. The results showed that the morphology of cobaltpowder is controlled mainly by the morphology of raw materials, sospherical carbonate powder can easierly produce spherical cobalt powder,which make it easily to get homogeneous WC-Co mixture in mixxingprocess compared to the tree-like cobalt powder from oxalate.spherical cobalt powder with the average size of 0.9μm could beproduced by reducing carbonate at 420℃～460℃for 2.5～3.0 h, withthickness of 50 mm and gas feed rate of 1.75 m³/h. the cobalt powderswere mainly composed by fcc phase. The oxygen content can be decreasedfrom 0.45% to 0.28% by CO₂ gas passivation treatment.4. Prepare ultrafine cemented carbides WC-10Co Cemented carbides had been prepared by vacuum/pressuresintering using the ultrafine WC powder and spherical cobalt prepared byabove technology, with adding grain growth inhibitors. The results showedthat the cemented carbides had the homogeneous crystal size of about0.4μm, hardness of 93 HRA, bend strength of 3700 MPa, and the coerciveforce of 38.0 KA/M.