Cermet alloys and composite mechanical parts made by employing t

Details Cermet alloys and composite mechanical parts made by employing t Cermet alloys and composite mechanical parts made by employing t

1989-06-23

1991-01-08

Lechert, Jr., Stephen J.

Specialized metallurgical processes, compositions for use therei

Compositions

Consolidated metal powder compositions

75236, 75244, 75242, 428552, C22C 2904

Patent

active

049832123

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

This invention relates to cermet alloys having high degrees of wear resistance and strength at elevated temperatures, and various kinds of composite mechanical parts having a strong layer of a cermet alloy formed on the outer or inner surface of a base material.


BACKGROUND ART

The materials which are used for making rolls for rolling mills, dies, punches, etc. are required to having high degrees of toughness, impact resistance, high-temperature strength, etc. Therefore, it has been usual to employ cast steels, tool steels, etc. for those purposes. These materials have, however, the drawback of being low in wear resistance and being, therefore, capable of making only parts having a short life.
Attempts have, therefore, been made to improve the drawback of those materials by employing, for example, an ultrahard WC--Co alloy composed of a hard phase of WC and a binding phase of Co, or a cermet alloy composed of a hard phase of titanium compound, such as TiCN, and a binding phase of Ni.
The ultrahard WC-Co alloy consists mainly of WC and contains 15 to 25% by weight of Co which binds WC. As WC is essentially of high wear resistance, the alloy can make e.g. rolls of high wear resistance, as compared with the rolls formed from any conventional materials, such as cast or tool steels.
The ultrahard WC--Co alloy has, however, a number of drawbacks including (1) resulting in a part having a large weight, as WC, of which it mainly consists, has a high specific gravity (about 15), (2) being difficult to employ for making any part to be exposed to heat, as WC is liable to oxidation, particularly at high temperatures, and (3) being low in breaking or chipping resistance, as it contains only a small amount of binding phase and is substantially a lump of carbide particles.
The attempt to improve the chipping resistance of the ultrahard WC--Co alloy by increasing the amount of its binding phase (i.e. the amount of Co which it contains) results not only in a sharp reduction of wear resistance, but also in a reduction of hardness and toughness, if its Co extent exceeds 25% by weight, as is well known in the art. Therefore, a practically acceptable ultrahard WC--Co alloy is one containing 15 to 25% by weight of a binding phase.
The first cermet alloy appeared in the market in 1971. A great many attempts have since been made to obtain improved cermet alloys by employing carbides of Group IVa, Va and VIa metals, such as WC, TaC and NbC, for replacing a part of a titanium compound such as TiCN. The cermet alloys play an important role in the manufacture of cutting tools.
It is, however, known that if a cermet alloy contains over 40% by weight of a metal binding phase consisting basically of Ni, the binding phase between hard particles has an average thickness (m.f.p.) exceeding an adequate range for the alloy and resulting therefore in such a great reduction in strength of the alloy as to make it unsuitable for practical use, and that the amount of the binding phase in any such alloy should, therefore, be limited to a maximum of 40% by weight (see e.g. Hisashi Suzuki: "Ultrahard Alloys and Sintered Hard Materials" (Published by Maruzen), pages 307 to 372).
The conventional cermet alloys are, therefore, low in toughness and impact strength and unsatisfactory in breaking resistance, too, though they are light in weight and high in oxidation resistance, as compared with the ultrahard WC--Co alloys. Therefore, they have found only a limited scope of application to hot and cold working rolls, extrusion dies, or wire drawing dies.
It is usual to add a component, such as Mo.sub.2 C or WC, to a cermet alloy to improve the wetting property of its hard phase composed of particles of a titanium compound (e.g. TiCN) with its binding metal phase. The component undergoes dissolution in the binding metal phase and precipitation in the hard phase during a sintering process and forms a structure surrounding the particles of the titanium compound and thereby improving their wetting property with the binding metal phase. Theref

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