TOOL FOR COLDFORMING OPERATIONS

Abstract: An improved hard wearing surface zone is being formed in a tungsten carbide-cobalt cemented carbide. This has been achieved by a post-sintering heat treatment in a boron nitride containing environment of a hard metal of a suitable composition. The effect is most pronounced when the heat treatment is made of a hard metal which has previously been sintered to achieve a high carbon content through a suitable choice of chemical composition and processing conditions.

Description: Tool for coldforming operations The present invention relates to a tool for coldforming operations. Cemented carbide products are used in tools for different coldforming operations of materials like, steels, copper alloys, composite materials etc. Examples of such tools are wire drawing dies, which consist of a cemented carbide nib shrink fit into a steel holder. Such tools should have a hard and wear resistant surface zone which also should have the following additional properties : - good thermal conductivity. - low coefficient of friction (i.e. it may be self lubricating or assist lubrication with a coolant) . - good corrosion resistance. - resistance to micro cracking. - high toughness. When using cemented carbides in tools for the forming of e.g. copper or its alloys, chemical reactions might occur between the binder metal of the hard metal and the copper-rich alloy. In order to minimise the effects of chemical wear of the cobalt binder phase and to improve the wear resistance, a cobalt (binder) content of about 3 % and a grain size <1 μm is used in hard metals for such applications. Often, a low carbon content close to eta phase formation is chosen. In order to maintain the fine grain size, grain growth inhibitors such as VC, Cr3C2 etc are used. In order to further increase the wear resistance, the surface of the tool exposed to wear is often boronised. The boroniεing treatment is generally done by applying a paste of organic or inorganic material containing a boron compound such as boron metal, BN, B4C etc on said surfaces and heat treating in argon atmosphere at 800-1100 °C. During this treatment a thin gradient zone is induced into the surface zone of the hard metal tool. This zone is depleted with cobalt and also contains a boron rich phase which forms during the treatment. This makes the surface zone both harder, tougher and more resistant to thermal cracking. As a result, this treatment offers an improved combination of hardness and toughness and thus increased wear resistance. This effect can also be re-applied as the surface layer is eroded. The worn surface layer of the tool is then repolished, a boron containing paste is applied and heat treated. A tool can typically be retreated several times before it loses its internal bore geometry and the tool becomes unusable. This is the life determining factor of such a tool. It is an object of the present invention to provide a tool for coldforming operations with a further improved combination of high hardness and toughness and, thus, increased wear resistance. Fig 1 shows a drawing die in which A = cemented carbide nib and B = steel casing. Fig 2 shows in xl500 magnification the boronised surface zone of a prior art nib. Fig 3 shows in xl500 magnification the boronised surface zone of a nib according to the invention. It has now surprisingly been found that a tool for coldforming operations with an increased performance of about three times increased life time than prior art tools can be obtained if the tool is made of a cemented carbide comprising WC with a mean grain size of 1.5-2 μm and 5-7, preferably about 6, weight-% Co and with the carbon content close to saturation level with respect to precipitation of graphite, 92-98 % in terms of cobalt magnetic measurements assuming pure cobalt. The tool is boronised using the prior art method. Of course, it can be retreated as before . The invention also relates to the use of a cemented carbide with a boronized surface zone comprising WC with a mean grain size of 1.5-2 μm and 5-7, preferably about 6, weight-% Co and with a carbon content close to saturation level, 92-98 % in terms of cobalt magnetic measurements assuming pure cobalt. The reason for the unexpected great improvement is not completely understood. It is believed that it is the increased hardness of the surface zone in combination with a substrate beneath with high toughness. The surface zone has a gradient created by a carbon-cobalt push in solid state at 800-1100 °C due to the boronizing treatment leaving a surface zone with increased volume of hard phase, lower binder phase and improved surface condition with respect to coefficient of friction, resistance to micro cracking at the working surface. In addition, a tougher zone of increased cobalt content is created beneath this surface zone which adds increased toughness to the tool. Compare Figs 2 and 3. Example Steel wire drawing dies according to Fig 1 were manufactured according to the following: A WC-3%Co, submicron grain size, VC as grain growth inhibitor, prior art. Fig 2 with a cobalt magetic value, CoM, of 2.7 % B WC-6%Co, grain size 1.5-2 μm, low carbon content, CoM=4.7% C WC-6%Co, grain size 1.5-2 μm, medium carbon content, CoM=5.3 % D WC-6%Co, grain size 1.5-2 μm, high carbon content, Fig 3, CoM=5.7 % E WC-6%Co, grain size 2-3.5 μm, high carbon content, CoM=5.8 % F WC-6%Co, sub icron grain size with chromium carbide grain growth inhibitor, CoM=5.2 % G WC-6%Co, grain size 1.5-2 μm, with chromium carbide, CoM=5.4 % The tools were tested in the wire drawing of steel chord with the following results. Performance factor relates to the quantity of product (wire) as length of mass drawn through the different nibs relative to the prior art nib, A. Performance factor A, prior art 1 B, outside invention 0.2 C, outside invention 0.25 D, according to the invent 3 E, outside invention 0.25 F, outside invention 0.20 G, outside invention 0.20 It is obvious from the example that the unexpected properties can only be obtained with the chosen Co- content, WC grain size and carbon level.

Claims: c-l-a-ima 1. Cemented carbide tool with boronised surface zone for coldforming operations c h a r a c h t e r i z e d in that the cemented carbide comprises WC with a mean grain size of 1.5-2 μm and 5-7, preferably about 6, weight-% Co with a carbon content close to saturation level, 92-98 % in terms of cobalt magnetic measurements assuming pure cobalt. 2. Use of a cemented carbide tool with boronised surface zone wherein the cemented carbide consists of WC with a mean grain size of 1.5-2 μm and 5-7, preferably about 6, weight-% Co with a carbon content close to saturation level, 92-98 % in terms of cobalt magnetic measurements assuming pure cobalt for coldforming operations. 3. Use of a cemented carbide tool according to claim 2 for wire drawing nibs .

Inventors: CARPENTER MICHAEL JOHN, SWEETMAN GARY WILLIAM, CARPENTER, MICHAEL, JOHN, SWEETMAN, GARY, WILLIAM
Applicants: SANDVIK AB, SANDVIK AB (PUBL)
Application year: 1997
Main classification: C22C29
IPC codes: B21C3, B21C25, B21J1, B21J5, B21K5, B23B27, B23G5, C22C29, C23C30