Oxidation of WC-16%Co, WC-8%Co-8%Ni and WC-16%Ni hard metals in the temperature range between 500 and 800 °C was investigated by TGA, XRD and SEM methods. The thermodynamics of tungsten carbide oxidation reactions was calculated. Oxidation kinetics of the alloys follows a complex behaviour. The WC-16%Co alloy is oxidized to a lesser degree than the WC-16%Ni alloy in the temperature range 650–800 °C. Oxidation resistance of hard metals is governed by phase composition, formation mechanism of the scales and protective properties of the oxidation products (CoO, NiO, WO3 and complex oxides, based on the formula (Ni, Co)WO4).

Oxidation of WC-Co, WC-Ni and WC-Co-Ni hard metals in the temperature range 500–800 °C

The literature has been reviewed to determine the status of nickel in hardmetals and cermets. It has shown that attractive alternatives to WCCo are possible, both by replacement or partial replacement of the cobalt with nickel and by changing the approach completely by using the alternative hard phase of TiCN bonded with nickel to form cermets, together with altering the machining philosophy.

Nickel in hardmetals

Multi-element alloy “Al0.5CoCrCuFeNi” is a brand new novel metal material. It has high working hardness, high high-temperature strength, and anti-oxidation properties. Multi-element alloy powders can be prepared by blending various pure metal powders using a mechanical alloying method. Different ratios of WC and multi-element alloys can be sequentially mixed and alloyed using a mechanical alloying method. WC particles were dispersed homogeneously in ductile multi-element alloy without forming any second phase. After press forming, the admixed powders were debound at 500 °C and then liquid-phase sintered at 1300–1450 °C. The sintering process of WC/Al0.5CoCrCuFeNi multi-element alloy is similar to the traditional WC/Co. The hardness of the sintering body of WC/Al0.5CoCrCuFeNi alloy is Hv 200–300 higher than the traditional WC/Co sintering body measured at room temperature. Its high-temperature hardness is Hv 200 higher than the traditional WC/Co measured at 900 °C. Substitution of multi-element alloy for Co as the base phase can enormously improve the material properties of WC super-hard composite material.

Novel cermet material of WC/multi-element alloy

A sintered hard metal compact for use in earth-boring bits comprises 80 to 94% by weight tungsten carbide particles, 5.4 to 18% by weight cobalt particles and 0.6 to 2.0% by weight nickel particles wherein the ratio of cobalt to nickel is approximately 9:1 by weight. These materials are formed according to conventional powder-metallurgy techniques to provide a hard, sintered compact for use in earth-boring bits having superior properties for drilling applications.

High-strength tungsten carbide material for use in earth-boring bits

Determination of three-dimensional grain size distribution by linear intercept measurement

The effect of the Ni/Co ratio in WC–10(Co–Ni) and WC–15(Co–Ni) alloys on microstructure, density, hardness, and transverse rupture strength is investigated. The specimens have been prepared by mixing WC powders of 5·7 μm with Co and Ni powders. The compacts have been sintered in vacuum to full density. The density after sintering decreases slightly with increasing Ni content, and the effect is attributed to the increase of C content with Ni, which contains less oxygen than Co in powder form. The hardness decreases with Ni content and appears to depend mainly on the hardness of the matrix alloy phase. The transverse rupture strength does not show any variation with Ni content and appears to depend on the grain size, which also remains unchanged. The results show that Co in WC–Co hardmetal can be replaced by Ni without greatly affecting the transverse rupture strength if the microstructure remains constant. PM/0289

K. Y. Eun

Papers

Variation of mechanical properties with Ni/Co ratio in WC-(Co-Ni) hardmetals