CEMENTED CARBIDE WITH A BINDER PHASE GRADIENT AND METHOD OF MAKING THE SAME

Abstract: The present invention relates to a cemented carbide body, preferably for rock drilling, mineral cutting and wear parts, in which the content of binder phase in the surface is lower than the nominal one and in the centre higher than the nominal one. In the centre there is a zone having a uniform content of binder phase. The WC grain size is uniform throughout the body.

Description: The present invention relates to a method of making a sintered body of cemented carbide with varying contents of binder phase. The invention is more precisely defined in the appended claims. In order to obtain good properties in cemented carbide it is often desirable to have a tough core (with a high content of binder phase) surrounded by a more wear resistant cover (having a low content of binder phase). One method of attaining this effect is to make a sintered body with a tough and less wear resistant grade in the centre surrounded by a more wear resistant and less tough grade. During the sintering usually a levelling of the binder phase content takes place, however, which in many cases leads to a body having an almost uniform binder phase content as final result. A varying content of binder phase in a sintered body of cemented carbide can be obtained, however, by means of so called compound hard metal technique. By using cemented carbide powder with different grain sizes (for example according to European patent EP 111 600) or by having the cemented carbide body divided in zones with different grain sizes (for example according to GB-A 806 406) it has generally been possible to obtain a certain difference of binder phase content between different parts of the cemented carbide body. In this case, however, there is not obtained any difference in wear resistance between the different parts because the fine grained part will obtain a greater binder phase content than the more coarse grained part. EP-A-0182759 discloses cemented carbide bodies comprising a core of cemented carbide containing eta-phase surrounded by a surface zone of cemented carbide free of eta-phase and having a low content of binder phase in the surface and a higher content of binder phase next to the eta-phase zone. It has now surprisingly been found that a body having varying binder phase contents can also be obtained by, starting from a essentially homgeneous powder, first making a body with a lowered content of carbon, usually 0.05-0.5 %, preferably 0.1-0.4 % lower than the stoichiometric content, and so that the body obtains a fine-grained, uniformly distributed eta phase i.e. a phase of carbides of the metals of the alpha-(WC)- and beta-(binder)-phases often written Co₃W₃C. The body is then carburized during a time being chosen so long that all eta phase will disappear. The supply of carbon is performed in a carburizing atmosphere of for example methane, carbon monoxide etc at a temperature of 1200-1550°C. The time is determined by experiments because it depends upon the size of the sintered body, temperature etc. As a result of the carburizing treatment a body is obtained with low contents of binder phase in a surface zone (possibly with small amounts of free graphite) and having a high content of binder phase in the centre. The explanation for the obtaining of a varying content of binder phase in a cemented carbide body by carburizing an eta phase containing structure can be given by several hypotheses of theoretic nature. These hypotheses are essentially assumptions, however, and therefore the result must be considered very surprising for a person skilled in the art. The binder phase content in the surface is 0.1-0.9, preferably 0.4-0.7, of the nominal content. The binder phase content in the centre is at least 1.2, preferably 1.4-2.5 of the nominal binder phase content and it is present preferably is the form of a zone having a uniform binder phase content and an extension of 0.05-0.5, preferably 0.1-0.3 of the diameter. A nominal binder phase content is obtained within 0.1-0.8, preferably 0.2-0.6, of the radius. The WC grain size is uniform throughout the body. Compared with the prior art, in particular with compound cemented carbide bodies having different grain sizes and different binder metal contents, it has thus been found possible according to the invention to use principally only one or a single cemented carbide grade for reaching the desired effect concerning a binder phase gradient with a controlled variation of the binder phase content. According to the invention it has thus been possible to reach a considerable difference in wear resistance and toughness between the different parts of the body. The positive effect on wear resistance and toughness depends upon the fact that the lower binder phase content in the outer part of the body in relation to the inner part leads to that compressive stresses are formed in the outer part during the cooling after the sintering. The outer binder phase depleted part has a smaller heat expansion than the binder phase rich inner part. The great amount of hard constituents in the outer part also leads to an increased wear resistance. The invention is directed to all kinds of cemented carbide for rock drilling and wear parts based upon WC having a binder phase based upon the metals of the iron group preferably cobalt and with a WC grain size between 0.5 and 8 µm, preferably 1-6 µm. An alternative but less suitable way is to decarburize a cemented carbide with normal structure and then carburize the same. The invention has been described above with reference to circular cylindrical bodies but it is naturally applicable to bodies with other cross sections such as square, rectangular, triangular etc. Example 1 From a WC 6 % Co powder with 0.3 % substoichiometric carbon content (5.5 % C instead of 5.8 % C) and WC grain size 2.5 µm buttons were pressed having a height of 16 mm and diameter of 10 mm. The buttons were pre-sintered in N₂-gas for 1 h at 900°C and standard sintered at 1450°C. After that the buttons were sparsely packed in fine Al₂O₃ powder in graphite boxes and thermally treated in a carburizing atmosphere for 2 h at 1400°C in a pusher type furnace. At the sintering a structure of alpha + beta phase and uniformly distributed, fine grained eta phase was formed. At the thermal treatment there was formed in the surface of the buttons a very narrow zone of merely alpha + beta structure because carbon begins to diffuse into the buttons and transform the eta phase to alpha + beta phase. After 4 hour's sintering time a sufficient amount of carbon had diffused and transformed all the eta phase. The content of cobalt at the surface was determined to 3.5 % and in the centre to 10.0 % in the form of a zone with about 3.5 mm diameter. The width of the part having a low content of cobalt was about 3.5 mm. See Fig 1. Example 2 Tests with ⊘45 mm rock drill bits, underground mining. Rock: Hard abrasive granite with small amounts of leptite. Compressive strength 2800-3100 bar. Machine: Atlas Copco COP 1038HD. Hydraulic drilling machine for heavy drifter equipment. Feeding pressure 85 bar, rotating pressure 45 bar, number of revolutions 200 rpm. Bits: ⊘45 mm button bits. Two wings with ⊘10 mm buttons with height 16 mm. Ten bits per variant. Cemented carbide: Variant 1 - Standard 6 % Co, 94 % WC, WC grain size 2.5 µm. Variant 2 - According to the invention, 3 % Co in the surface zone, 10 % Co in the centre, Nominal content of Co 3 mm from the surface. The zone of Co had a diameter of 3 mm. Drilling procedure: The bits were drilled for 5 m holes according to "the rotation method". After every 35th drilled meter the wear was determined. The bits were removed from the drilling at the first button damage and the number of drilled meters was noted. Example 3 In drawing of automatic welding wire (grade 3RS17) drawing dies were used with the dimensions 1.75, 1.57 and 1.47 mm, respectively, hole diameter. The drawing speed was 6 m/s. As cooling liquid water was used (counter flow cooling). The drawing dies, standard, were made of a cemented carbide grade with 6.0 % Co rest WC, grains size 1 µm, hardness 1750 HV. In the drawing section there were tested alternately drawing dies of standard type and dies made according to the invention. (Starting material 6 % Co, rest WC and W). In the zone close to the drawing channel the hardness was 1980 HV3 and in the inner zone 1340 HV3. The following result was obtained: Mean value, standard drawing die: 2,1 tons Mean value, drawing die according to the invention: 3.9 tons The drawing dies according to the invention showed a mean increase of life of 86 %.

Claims: 1. Method of making a cemented carbide body preferably for rock drilling, mineral cutting and wear parts, said body containing WC (alpha-phase) and a binder phase (beta-phase) based upon at least one of Co, Fe and Ni, the content of binder phase in the surface being 0.1 - 0.9 and in the centre at least 1.2 of the nominal content of binder phase and the grain size of the alpha phase being uniform throughout the body, wherein a sintered body with a uniformly distributed phase of carbides of metals of the alpha- and beta-phases, so-called eta-phase, is prepared starting from an essentially homogeneous powder and that said body thereafter is carburized characterised in that all eta-phase is transformed to alpha- and beta-phase. 2. Method according to claim 1, characterized in that in the centre of the cemented carbide body there is a zone of alpha- + beta-phase having a uniform content of binder phase and an extension of 0.05 - 0.5 of the diameter.

Inventors: AKERMAN, JAN, FISCHER, UDO, HARTZELL, TORBJOERN, Hartzell, Torbjörn
Applicants: SANTRADE LTD.
Application year: 1987
Main classification: C22C29
IPC codes: C22C1, B22F3, B22F5, C23C8, C22C29, C23C30