SE518013C2 - Sintering cemented carbide body with initial cooling in hydrogen and preferably argon - Google Patents

Abstract

Cemented carbide bodies are sintered by a method including heating the bodies to the sintering temperature in a suitable atmosphere and then cooling, to at least below 1200 deg C, in an atmosphere comprising 0.4-0.9 bar hydrogen and the balance >0.1 bar of a noble gas, preferably argon, with a total pressure of 0.5-100 bar and preferably 0.5-10 bar.

Description

25 30 o c n ø OI 518 013 precision, which leads to an increased spread in the properties of the final product - ie. the coated insert. It also leads to damage to the grains of the hard component in the surface. In Swedish patent application 9202142-7, however, it is shown that blasting with fine particles gives an even removal of the binder phase layer without damaging the grains of the hard component.

Chemical or electrolytic methods can be used as alternatives to mechanical methods. U.S. Patent 4,282,289 discloses a method of gas phase etching using HCl in an initial stage of the coating process. EP-A-337 696 proposes a wet chemical method for etching in nitric acid, hydrochloric acid, hydrofluoric acid, sulfuric acid and similar or electrochemical methods. From JP 88-060279 it is known to use an alkaline solution, NaOH, and from JP 88-060280 to use an acidic solution. JP 88-053269 describes etching in nitric acid before diamond coating. There is a disadvantage of these methods, that they are, in fact, incapable of removing only the cobalt layer. They also result in deep etching, especially in surfaces near the edge. The etching medium not only removes cobalt from the surface but also penetrates areas between the hard component grains and as a result an undesirable porosity is obtained between layer and substrate while the cobalt layer may partially remain on other surfaces of the insert. US 5,380,408 discloses an etching method according to which electrolytic etching is performed in a mixture of sulfuric acid and phosphoric acid. This method provides a smooth and complete removal of the binder phase layer without depth effect, i.e. zero Co ~ content is achieved on the surface. On the other hand, in some cases it is not desirable to reach zero Co content on the surface from an adhesion point of view, but rather a Co content on the surface of near nominal content.

The above methods require additional production steps and are therefore less attractive for large-scale production. It would be desirable that the sintering could be performed in such a way that no binder phase layer is formed or alternatively can be removed during cooling.

It is therefore an object of the present invention to provide a method for sintering cemented carbide in such a way that no binder phase layers are present on the surface after sintering but a well-defined Co content.

Fig. 1 shows in 4000x magnification a top view of the surface of a cemented carbide insert partially covered with a binder phase layer.

Fig. 2 shows in 4000x magnification a top view of the surface of a cemented carbide insert sintered according to the invention. In these figures, the dark gray surfaces are the Co layer, the light gray angular grains are WC and the gray rounded grains are the so-called gamma phase which is a (Ti, Ta, Nb, W) C.

According to the present invention, the heating and high temperature steps of the sintering are performed in a conventional manner. However, the cooling from sintering temperature down to at least below 1200 ° C is carried out in a hydrogen and argon atmosphere of 0.4 to 0.9 bar, preferably 0.5 to 0.8 bar hydrogen pressure and the remainder argon. The total pressure should be 0.5 to 100 bar, preferably 0.5 to 10 bar, preferably 0.5 to 1 bar, the argon pressure should always be> 0.1 bar. The optimum conditions depend on the composition of the cemented carbide, on the sintering conditions and to some extent on the construction of the equipment used. Those skilled in the art can experimentally determine optimal hydrogen pressure to avoid binder phase layers and unwanted carburizing of the cemented carbide. The sintering results in a Co content on the surface of nominal content +6 to -4%, preferably +4 to -2%. The Co content can be determined, for example, using an SEM (Scanning Electron Microscope) equipped with an EDS (Energy Dispersive Spectrometer) and compare the intensities of Co from an unknown surface and a reference, such as a polished section of a sample of the same nominal composition.

The method according to the invention can be applied to all types of cemented carbide, preferably cemented carbide with a composition of 4 to 15% by weight of Co, up to 20% by weight of cubic carbides such as TiC, TaC, NbC etc. and the rest WC.

Preferably, the cemented carbide has a composition of 5 to 12% by weight of Co, less than 12% by weight of cubic carbides such as TiC, TaC, NbC, etc. and residual WC. The average grain size for WC should be <8 sqm, preferably 0.5-5 sqm.

Inserts according to the invention are provided after sintering with a thin durable coating comprising at least one layer by means of CVD, MTCVD or PVD technology, known in the art.

The invention has been described with reference to argon but it is obvious that the same result can also be obtained when using other noble gases.

Example 1 CNMG 120408 cemented carbide inserts with 5.5 wt% Co, 8.5 wt% cubic carbides and 86 wt% WC of Zlum average WC grain size were sintered in a conventional manner at 1450 ° C and cooled to room temperature in argon.

The surface was up to 50% covered with a Co layer, Fig. 1. from 1200 ° C in a pure argon atmosphere. The surface was about 6% covered with Co, which corresponds to the nominal Co content, Fig. 2.

Claims (5)

10 15 20 o n o n o n c a n u n a u s. Co n 51 8 Û '| 3 xšf. ššêl Method of sintering cemented carbide bodies comprising heating the bodies to sintering temperature in a suitable atmosphere and cooling, characterized in that the cooling is carried out at least below 1200 ° C in a hydrogen atmosphere of pressure 0.4-0.9 bar and the residue noble gas, preferably argon, with a pressure of> 0.1 bar, the total pressure being 0.5 to 100 bar, preferably 0.5 to 10 bar. 2. A method according to claim 1, characterized in that the hydrogen pressure is 0.5-0.8 bar. 3. A method according to any one of the preceding claims, characterized in that the cemented carbide has the composition 4 to 15% by weight of Co, up to 20% by weight of cubic carbides such as TiC, TaC, NbC etc. and the rest WC. 4. A method according to any one of the preceding claims, characterized in that the cemented carbide has a composition of 5 to 12% by weight of Co, less than 12% by weight of cubic carbides such as TiC, TaC, NbC, etc. and the remainder WC. 5. A method according to any one of the preceding claims, characterized in that said bodies are provided with a thin durable coating comprising at least one layer applied by means of CVD, MTCVD or PVD technology.