Cemented carbide

Details Cemented carbide Cemented carbide

1984-03-13

1985-02-05

Bell, Mark

Specialized metallurgical processes, compositions for use therei

Compositions

Consolidated metal powder compositions

75236, 75242, C22C 2900

Patent

active

044976607

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to a new type of hard metal with excellent properties especially when used for construction parts and wear parts but also as cutting tools and in rock drilling. More exactly the invention relates to a sintered hard metal alloy, in which the hard material principally is tungsten carbide (WC), and the binder phase is based on Ni with optimized additives of above all the elements Cr and Mo.
Since a long time hard metal, in which the hard material consists mainly of WC and the binder phase of Co, dominates when used in said applications. Principally in applications for high temperatures it has been favourable substituting WC for one or more of the carbides where Ti, V, Cr, Nb, Hf, Mo and Ta are the metal components. When high oxidation resistance or high corrosion resistance is demanded, hard metal with TiC or Cr.sub.3 C.sub.2 as main component of the hard material and with a binder phase having Ni as main component, has been commercially used in rare applications. This in spite of the fact that the latter types of hard metal from a general point of view have got unfavourable properties of toughness.
Hard metal of the type in which WC is the hard material but the binder phase consists of Ni has hitherto had only a limited use. Principally it is used in certain applications in the nuclear power industry where WC-Co cannot be used because of Co-isotopes of long half-lives.
However, metallic Ni has several advantages, with respect to properties, over metallic Co. Thus, both the oxidation resistance and the corrosion resistance are better because of the higher electropotential of Ni than of Co in most reagences. Furthermore, Co is around 10 times more expensive than Ni (Nov. 78) and the mean occurrence of Ni in the earth crust is around 4 times larger than the occurrence of Co.
Ni is used as an alloying material in Co-alloys because of the higher corrosion resistance and oxidation resistance of Ni. This indicates especially favourable properties of Ni-bound hard metal. This is especially valid in applications in critical working environments under reducing or oxidation conditions. Furthermore, a long life, often for years, is a necessary demand for an economically favourable use of an expensive hard metal part compared with for example a steel part, which is much cheaper.
The physical and mechanical properties of hard metal where WC is the main component of the hard materials, are characterized mainly by the mean grain size of WC, by the concentration of binder phase and the composition of the binder phase. For hard metals the highest E-module, the lowest coefficient of thermal expansion and the highest thermal conductivity have hitherto been obtained when WC is the hard material. Furthermore, the highest toughness and a very favourable strength have been obtained for pure WC-Co hard metal. Generally the elasticity module of hard metal is influenced mainly by the composition and amount of the hard material and for comparable elasticity modules the transverse rupture strength is a good measure of the general properties of toughness of the hard metal. The hardness, the resistance of the material to plastic deformation, is a measure of strength.
It is earlier known that for additives of Cr and Ni, respectively, to the binder phase of Wc-Co hard metal, which gives improved oxidation and corrosion resistances, a decrease of especially the toughness is obtained for Cr additives, whereas additives of Ni result in decrease of both toughness and strength. Additives of Cr in greater concentrations can furthermore lead to difficulties in controlling the carbon balance in sintered hard metal and to the formation of brittle double carbides in which the binder phase metals are components, which will result in drasticly decreased toughness. Additives of Fe cause still lower toughness than additives of Ni.
Thus, it is well known that for hard metal with WC as main component of the hard material the alternative with Co in the binder phase is favourable principally from a toughness point of view. Espec

REFERENCES:
patent: 2147329 (1939-02-01), Willey
patent: 3215510 (1965-11-01), Kelly et al.
patent: 3322513 (1967-05-01), Corbett
patent: 3677722 (1972-07-01), Rymas
patent: 3746519 (1973-07-01), Hara et al.
patent: 3816081 (1974-06-01), Hale
patent: 3993446 (1976-11-01), Okawa

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