Alloys or metallic compositions – Tungsten base – Carbon – boron or nitrogen containing
Reexamination Certificate
2000-10-06
2003-02-04
Sheehan, John (Department: 1742)
Alloys or metallic compositions
Tungsten base
Carbon, boron or nitrogen containing
C420S432000, C148S423000
Reexamination Certificate
active
06514456
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention concerns the use of a tungsten-cobalt cutting metal alloys as components shaped from cutting metal blanks by means of electrical discharge machining methods. These alloys can be used for cutting and punching tools having a complex configuration, which are shaped by wire eroding or cavity sinking.
A large number of different cutting metal types, used, e.g. as dies in machining tools, are being employed to an increasing extent as materials for wear-resistant components or structural parts, for example, in the manufacture of tools.
The cutting or shaping of workpieces by electrical discharge machining was developed in the
50
s
for the processing of steel materials and is used today, more and more, for the shaping of workpieces made of cutting metal. The production of geometrically complex molded articles by electrical discharge machining methods is clearly more economical, for comparable quality, than the usual compression molding and the subsequent metal cutting.
The “electrical discharge machining of a workpiece” is understood to mean its shaping under the explosive evaporation of material parts from the surface of the workpiece, caused by local heating as a result of electrical current pulses with a plasma formation between the workpiece and the electrical discharge machining tool in a liquid dielectric—usually water or organic substances, such as petroleum or kerosene. To this end, an electric capacitor is charged and is discharged upon reaching the disruptive voltage between the workpiece and the tool as a short-term, high current pulse, with the formation of a plasma (T>10,000 K) in the dielectric. In actual practice, cavity sinking and wire eroding or erosive cutting have proven particularly effective. With cavity sinking, the tool or the eroding electrode can have the negative shape of the workpiece configuration. In accordance with the cutting process, the electrode is lowered onto the workpiece. With wire eroding, a wire electrode, usually having a diameter of 0.05 mm to 0.3 mm, is conducted past a path on the tool, which corresponds to the workpiece profile.
Whereas with the process of cavity sinking, erosion performance during roughing is between 20 mm
3
/min and 50 mm
3
/min and with planing, up to 1 mm
3
/min, it is possible to attain roughing performances with wire eroding of 200 mm
3
/min to 500 mm
3
/min and with planing, approximately 10 mm
3
/min. The surface roughness R
a
has a value of ca. 1.4 m with planing by means of cavity sinking; with wire eroding, it is ca. 0.2 &mgr;m in the most favorable case. Thus, eroded workpiece surfaces, as a rule, do not require mechanical surface processing.
Cutting metals are generally parts having a hard material phase embedded in a metal binder phase. The large number of known, different cutting metal alloys have very different, frequently mutually exclusive characteristics. Cutting metal types with a nickel binder, sometimes with a chromium additive, are more corrosion-resistant than those with cobalt binders. However, the former, as a rule, have a lower degree of hardness and as a result, lower abrasive wear and toughness characteristics. The actual characteristic values differ, in turn, with the particle or grain size in the cutting metal, which, in turn, encounter grain growth during the cutting metal sintering process. Hard materials, with different compositions and based on carbide and/or carbonitride, in connection with a large number of additives to the binding phases on a cobalt and/or nickel basis, are known for influencing or control of these characteristics.
Of the previously described cutting metal types, based on WC—Co or with predominantly Co binders, the following are mentioned in considering this invention.
DE 27 19 532 (Auslegeschrift) mentions a WC—Co cutting metal type with 20 to 95 wt % tungsten carbide and the added carbides TiC, TaC, NbC, HfC, VC, and/or MO
2
C and 2 to 30 wt % of a cobalt-rhenium binder alloy with a 5 to 80 wt % rhenium fraction. The cobalt can be replaced, in part, by iron or nickel. Whereas hitherto known additives to the cobalt binder reacted with the carbide phase, with the formation of brittle, complex carbides and thus compensated negatively for any quality improvement of the binder phase, this limitation does not apply to the cobalt-rhenium binder. In contrast to the pure cobalt binder, the binder has a higher heat resistance, which, for example, is advantageous to a cutting insert during metal cutting encountering working temperatures up to 800° at the cutting tip.
The technical essay “Sintering of WC—10Co Hard Metals containing Vanadium, Carbonitride and Rhenium;” authors: R. Hulyal et al., which appeared in “Int. J. Refr. Metals and Hard Materials,” (1991), Vol. 10(1), pages 9 to 13, mentions the addition of 0.2 wt % rhenium into WC—10Co and WC/V(C,N)—10Co and the “Re effect” is described with the attaining of higher hardness values in a WC-rich cutting metal.
The SU patent, Application No. 91/4952086, describes a WC—Co cutting metal alloy for advantageous utilization as an electrode in ultrasound-microwelding tools because of the advantage of a great work stability. The alloy consists of 0.1 to 3 wt % rhenium, 0.15 to 3.85 wt % cobalt, 0.05 to 2 wt % chromium carbide, with the remainder, tungsten carbide.
To now, cutting metal components or tool inserts were typically manufactured by means of electrical discharge processing methods for applications in which it was important to have a high impact resistance and a resistance to abrasive wear. For example, for tool inserts for cutting and punching tools, tungsten carbide-cobalt cutting metal types, such as the ISO types K30 and K40, having a standard grain size of 1-2 &mgr;m, were usually used. In the carbide phase, the type K40 also contains, in addition to WC, <1 wt % TiC+Ta(Nb)C and 12 wt % cobalt as the binding phase, with reference to the total material. Occasionally cutting metal having nickel fractions in the cobalt binder are also used.
Cutting metals of WC—Co, used to now for shaping by means of electrical discharge processing, exhibit, as a serious disadvantage, a phenomenon, which is frequently designated in technical circles as “pitting”—that is, holes with a depth of up to 20 &mgr;m appear in the workpiece surface in components manufactured by means of erosion, with a probability of approximately 1 to 5% in statistically irregular individual cases. The reason for this is the dissolution of binder material (cobalt) from the workpiece surface during the erosion process in the area of these holes. The reason is apparently an electrochemical corrosion process between the individual phase components of a cutting metal alloy with the dielectric of the eroding unit. This corrosive dissolution process can be the direct consequence of an insufficient rinsing of the cutting gap between the workpiece and the eroding electrode with the liquid dielectric, which brings about an increase in the electrical conductance there, or an excessively high conductance of the dielectric. Frequently, the cobalt dissolution cannot be seen without a microscope, since the surface appears to be in an optically satisfactory condition prior to removal of the carbide phase. Remarkably, pitting occasionally appears on a part of a workpiece surface which is not directly eroded, apparently as a result of the increase in the conductance in the entire dielectric.
A local increase in conductance at the cutting point is counteracted, in actual practice, by a continuous, local replacement of dielectric by means of active rinsing of the cutting gap. With complex-dimensioned cutting profiles and with demanding manufacturing parameters, however, a sufficient rinsing of the cutting gap cannot be ensured up to now and “pitting” can, therefore, not be ruled out. Rather, the known countermeasures are limited to the publication of “Points of departure for solving the problem of pitting,” such as: “Note, constantly, the conductance of the dielectric and maintain smaller than 5 &mgr;m S/cm” or “Inter
Ferstl Werner
Kn{overscore (u)}nz Gerhard
Lackner Andreas
Martinz Hans-Peter
Prandini Klaus
Morgan & Finnegan L.L.P.
Oltmans Andrew L.
Plansee Tizit Aktiengesellschaft
Sheehan John
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