tailieunhanh - Handbook Corrosion (1992) WW Part 10

Tham khảo tài liệu 'handbook corrosion (1992) ww part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Fig. 1 Microstructures of WC-Co a c and e and WC-TaC-TiC-Co b d and f cemented carbides. In a c and e the white areas are cobalt binder phase. In b d and f the darker more rounded grains are the WxTazTizC cubic solid-solution phase. a and b Fine grain structures. c and d Medium grain structures. e and f Coarse grain structures. All 1500x. Source Ref 1 and 2 The first key to the successful development of cemented carbides was that these refractory metal compounds particularly WC are best produced as powders. In fact the only logical way to produce tungsten is the hydrogen reduction of WO3 or ammonium paratungstate powder into tungsten metal powder. The carburization of tungsten to WC also results in a fine powder. The second key was the discovery of the eutectic system WC-Co Fig. 2 . Liquid-phase sintering is possible well below the melting point of the WC and even below the melting point of cobalt. Fig. 2 Quasi-binary phase diagram for the WC-Co system Cemented WC is produced by mixing from 3 wt or less up to as much as 30 wt of cobalt metal powder with a balance of WC powder. The mixed powders are ball milled generally in volatile solvents for times ranging from a few hours to as long as 7 days. Alternatively the powders are milled in an attritor for 1 to 10 h. A suitable transient binder is added to the powder which is then pelletized and pressed to form the shape. Finally the part is sintered at temperatures between 1300 and 1600 C 2370 and 2910 F most often in vacuum. Because a liquid phase is formed during sintering virtually 100 density is achieved. More information on the production of cemented carbides is available in the articles Cermets and Cemented Carbides and Production Sintering Practices in Powder Metal Technologies and Applications Volume 7 of the ASM Handbook. Effect of Composition on Properties The two most common variables in cemented carbides are the cobalt or binder content and the grain size. As shown in Fig. 3 increased grain size decreases .