Preparation of WC Based Cemented Carbide by Using Ammonium Paratungstate as Binder

Non-magnetic tungsten carbide (WC) is a kind of hard alloy material with no magnetism or weak magnetism.  It has excellent properties of high hardness, high T.R.S. and high corrosion resistance.  Its applications include the production of standard parts, cold heading, cold punch, and cold mold. It can also be used in non-magnetic wear parts.

The magnetism of hard metal usually comes from the composition of Cobalt (Co), Nickel (Ni) or Iron (Fe), which is usually used as binder material. As to realize no magnetization of the hard tool materials, Ni based nonmagnetic solid solution powder is worthy of serious thought for binder of hard tool materials.  

Ni powder and ammonium paratungstate (APT) as the precursor material of the mechanical alloying method was used. APT is the the raw material, and the synthesis the WC based cemented carbide using this nonmagnetic Ni(W) solid solution as the binder had been conducted as following steps:

The Ni(W) solid solutions were prepared by using APT and pure tungsten powder as the tungsten source, respectively. Nickel (99.9%, 36 μm) and APT (99% purity, 0.8 μm) powders were mixed to constitute powder blends with APT contents of 30 wt.% (convert to 20 wt.% W). Blended mixtures were mechanically alloyed for 48 with a speed of 360 rpm with WC-Co balls and the ball-to-powder weight ratio was 7:1. Diffusion of Ni-APT powders were conducted at 550 °C for 2 h and then 800 °C for 2 h under H2 atmosphere. For the sample without using APT, Ni and W powders were mixed to powder blends with the composition of 80 wt.% Ni and 20 wt.% W. The condition of mechanical alloying of the Ni-W powder blend was the same as that of the Ni-APT powders. Diffusion of the powder was conducted at 800 °C for 2 h under H₂ atmosphere.

In conclusion, the preparation process of nonmagnetic WC based cemented carbide by APT as the binder has been successfully conducted. It was found that powder particles using APT as the W source had larger contact area between W and Ni and higher activity, which was conducive to the mutual diffusion between Ni and W, leading to a faster solid phase reaction and higher content of W diffused into Ni. And the mechanical and nonmagnetic properties of WC based cemented carbide derived from nonmagnetic Ni(W) solid solution as the binder are much better than those derived from pure Ni as the binder.

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