Ceramic cutting insert of polycrystalline tungsten carbide

Abstract

A polycrystalline tungsten carbide ceramnic cutting insert with chip control is disclosed for high speed machining.

Description

[0001] This invention relates to the field of ceramics and particularly to dense polycrystalline tungsten carbide inserts with chip control.[0002] In the machining process, it is important for the cutting tool to work effectively at high speeds and to have a long tool life. In order for the cutting tool to be effective, it must be made of a material which results in the tool having a high heat hardness and a high transverse rupture strength and fracture toughness, and it must also have a design sufficient to control the flow of chips which are formed in the machining process and to reduce the cutting forces.[0003] Chip control is an important element of the machining process, in order to break up the length of undesirably long chips which may be formed in the machining process. In high speed machining, if the strip taken off from the workpiece by the cutting insert is not broken up, the strip can interfere with the machining process in a variety of ways. For example, an undesirably ...

AI Technical Summary

Benefits of technology

[0017] It is the primary object of the invention to make a molded polycrystalline tungsten carbide ceramic cutting insert having a chip control structure. In drilling operations, chip control is important so that drilling efficiency and tool insert damage does not result. (See FIG. 1) Incorporating chip breaker grooves or lands on the cutting insert blank allows the strip taken off of the workpiece to be broken up into short pieces. These small chips will readily fall away from the machining region into a receiving space or containers, so that the chips are contained and can be removed from the machine tool.

[0020] Tungsten carbide (WC) ceramics of the present invention can be tailored for use in particular applications by an appropriate choice of starting WC powder size and by controlling densification conditions control grain growth.

[0021] Desirable starting powder sizes fall within a range of from greater than 0.001 .mu.m up to 20 .mu.m. The range, depending on application, is preferably from about 0.001 .mu.m to about 10 .mu.m, more preferably from about 0.001 to about 4 .mu.m. In one embodiment, the tungsten carbide powder size is about 1.0 .mu.m. Starting powder sizes of less than 20 .mu.m should provide densified bodies having excellent properties.

[0022] Tungsten carbide powders having an average particle size of less than or equal to 10 .mu.m are commercially available. One such powder, Teledyne type IV, has a nominal average particle size of 8 .mu.m and includes a small amount of vanadium carbide as a grain growth inhibitor. Attriting such a powder simultaneously reduces the average particle size, reduces grain size distribution, and more uniforrnly disperses the grain growth inhibitor. Even in the absence of a grain growth inhibitor, attrition provides the benefits of smaller average particle size and a narrower particle size distribution. As an alternative, the WC powder may have these characteristics as synthesized. As a further alternative, powders with even larger average particle sizes may be used provided they are milled or attrited under conditions sufficient to reduce the average particle size to less than or equal to 0.2 .mu.m. These powders necessarily require longer size reduction procedures and may, as a consequence, pick up additional quantities of impurities from media used to promote size reduction.

[0023] WC powders used in the present invention need not be 100% pure. They may contain very small amounts, e.g., less than 1.5 wt % by volume, of other materials so long as the other materials do not interfere with densification of the powder or adversely affect physical properties of resultant densified bodies. Examples of "other materials" include cobalt, iron, nickel, carbon and silicon. The other materials may, for example, be present as a result of powder synthesis procedures or as residue from milling operations. In some embodiments, cobalt is present from about 0.01% to 1.5% by volume. Preferably cobalt is present at about 0.25%. In addition to the other materials, the WC powders have an oxygen content that varies inversely with particle size. Thus, as particle size decreases, oxygen contents tend to increase. However, the oxygen content should be maintained at a level that does not interfere with densification of the powder or adversely affect physical properties of resultant densified bodies. In some embodiments a binder, e.g., wax is added to the powder to facilitate molding into the die. Preferably, the binder is less than about 5% by volume. More preferably the binder is about 2.25% by volume. Grain size can be controlled by careful control of densification procedures even if the WC powder does not include a grain growth inhibitor. Any conventional densification technique may be used provided it yields the densified ceramic body of the invention. Conventional techniques include pressureless or low pressure sintering, hot pressing, hot isostatic pressing and rapid omnidirectional compaction. Densification is preferably accomplished by hot isostatic pressing.

Problems solved by technology

In high speed machining, if the strip taken off from the workpiece by the cutting insert is not broken up, the strip can interfere with the machining process in a variety of ways.

For example, an undesirably long chip can be re-cut and welded onto a portion of the workpiece, thereby causing poor surface conditions on the workpiece.

An undesirably long chip, if not broken under chip control, can also cause breakage of the machining tool itself.

Additionally, undesirably long chips can feed into the tool holder or other portions of the machine and cause difficulties, e.g., damaging parts of the tool holder or obstructing visibility of the working area.

Further, long ribbons are difficult to handle and can represent a safety hazard to the machine operator.

Early work with WC focused upon densifying WC by heating to a temperature of, for example, 2,000 C.degree.. The densified material was judged unsuitable for use in applications requiring toughness, such as in cutting tools.

The unsuitability stemmed largely from the densified material's excessively brittle character.

However, in formulating these materials, there is a tendency that if wear resistance is heightened, fracture resistance is lowered, and conversely, if fracture resistance is heightened, wear resistance is lowered.

Therefore, in the design of cermet cutting tools, there has been encountered the problem of improving one material property at the expense of another material property by adding cobalt or another iron group that will plastically deform in the heat of high speed machining.

Although cermets and WC have been used extensively in the design of cutting tools, there still has not been a satisfactory resolution to the problem of tailoring the composition of the cermet or WC in order to maximize efficiency of the cutting tool.

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