Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Metal and nonmetal in final product
Reexamination Certificate
2000-04-06
2001-04-10
Jenkins, Daniel J. (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Metal and nonmetal in final product
C419S035000, C419S038000, C075S240000
Reexamination Certificate
active
06214287
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a cemented carbide cutting tool insert, particularly useful for turning, milling and drilling in steels and stainless steels.
Conventional cemented carbide inserts are produced by powder metallurgical methods including milling of a powder mixture forming the hard constituents and the binder phase, pressing and sintering. The milling operation is an intensive milling in mills of different sizes and with the aid of milling bodies. The milling time is of the order of several hours up to several days. Such processing is believed to be necessary in order to obtain a uniform distribution of the binder phase in the milled mixture. It is further believed that the intensive milling causes reactivity of the mixture which further promotes the formation of a dense structure. However, milling has its disadvantages. During the long milling time the milling bodies are worn and contaminate the milled mixture. Furthermore even after an extended milling a random rather than an ideal homogeneous mixture may be obtained. Thus, the properties of the sintered cemented carbide containing two or more components depend heavily on how the starting materials are mixed.
There exist alternative technologies to intensive milling for production of cemented carbide. For example, particles can be coated with binder phase metal. The coating methods include fluidized bed methods, solgel techniques, electrolytic coating, PVD coating or other methods such as disclosed in e.g. GB 346,473, U.S. Pat. Nos. 5,529,804 or 5,505,902. Coated carbide particles can be mixed with additional amounts of cobalt and other carbide powders to obtain the desired final material composition, pressed and sintered to form a dense structure. U.S. Pat. No. 5,993,730 discloses a method of coating carbide particles with V, Cr, Ti, Ta or Nb.
During metal cutting operations like turning, milling and drilling the general properties of the material such as hardness, resistance against plastic deformation, and resistance against formation of thermal fatigue cracks are to a great extent related to the volume fraction of the hard phases and the binder phase in the sintered cemented carbide body. It is well known that increasing the amount of the binder phase reduces the resistance to plastic deformation. Different cutting conditions require different properties of the cutting insert. When cutting in steels with raw surface zones (e.g. rolled, forged or cast) a coated cemented carbide insert must consist of tough cemented carbide and have a very good coating adhesion as well. When turning, milling or drilling in low alloyed steels or stainless steels the adhesive wear is generally the dominating wear type.
Measures can be taken to improve the cutting performance with respect to a specific wear type. However, such action will often have a negative effect on other wear properties.
SUMMARY OF THE INVENTION
It has now surprisingly been found that cemented carbide inserts made from powder mixtures with Cr-coated submicron hard constituents and manufactured without conventional milling have excellent toughness performance for machining of steels and stainless steels.
The present invention provides a method of manufacturing a cemented carbide powder, comprising the steps of: coating a hard constituent powder with a coating selected from the group of Cr and Cr+Co to form a coated hard constituent powder, wet-mixing without milling the coated hard constituent powder and with binder metal and pressing agent, to form a wet-mixed powder, and drying said wet-mixed powder to form a dried cemented carbide powder.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
According to the invention there is now provided cemented carbide inserts with excellent toughness properties for machining of steels and stainless steels made from a dried powder of WC and 6-12 wt. % Co, preferably 8-11 wt. % Co, most preferably 9.5-10.5 wt. % Co and 0.1-0.7 wt. % Cr, preferably 0.2-0.5 wt. % Cr. The WC-grains preferably have an average grain size in the range 0.2-1.0 &mgr;m, more preferably 0.6-0.9 &mgr;m.
The microstructure of cemented carbide according to the invention is preferably further characterized by a grain size distribution of WC in the range 0-1.5 &mgr;m.
The amount of W dissolved in binder phase is controlled by adjustment of the carbon content by small additions of carbon black or pure tungsten powder. The W-content in the binder phase can be expressed as the “CW-ratio” defined as
CW-ratio=M
s
/(wt. % Co * 0.0161)
where M
s
is the measured saturation magnetization of the sintered cemented carbide body in kA/m and wt. % Co is the weight percentage of Co in the cemented carbide. The CW-ratio in inserts according to the invention should preferably be 0.80-1.0, more preferably 0.8-0.90.
The sintered inserts according to the invention are used coated or uncoated, preferably coated with conventional PVD (TiCN+TiN) or PVD (TiN).
According to the method of the present invention coated WC-powder with submicron grain size distribution is wet mixed without milling with binder metal and pressing agent, dried preferably by spray drying, pressed to inserts and sintered.
WC-powder with grain size distributions according to the invention with coarse grains tails greater than 1.5 &mgr;m having been eliminated can be prepared by milling and sieving such as in a jetmill-classifier. It is an important feature of the invention that the mixing takes place without milling i.e. there should be no change in grain size or grain size distribution as a result of the mixing.
According to the method of the present invention the submicron hard constituents, after careful deagglomeration are coated with a grain growth inhibitor metal such as Cr, V, Mo, W, preferably Cr using methods disclosed in U.S. Pat. No. 5,993,730 and, optionally, an iron group binder metal, preferably Co, using methods disclosed in patent U.S. Pat. No. 5,529,804. In such case the cemented carbide powder obtained from the above method includes Cr-coated, or optionally Cr+Co coated, WC, possibly with further additions of Co-powder in order to obtain the desired final composition.
The following examples are given to illustrate various aspects of the invention.
REFERENCES:
patent: 5505902 (1996-04-01), Fischer et al.
patent: 5529804 (1996-06-01), Bonneau et al.
patent: 5993730 (1999-11-01), Waldenström et al.
patent: 819490 (1998-01-01), None
patent: 346473 (1931-04-01), None
patent: 1438728 (1976-06-01), None
Patent Abstracts of Japan, vol. 018, No. 487 (M-1671), Sep. 12, 1994 & JP 06 158114 A (Mitsubishi Materials Corp), Jun. 7, 1994.
Patent Abstracts of Japan, vol. 017, No. 442 (C-1097), Aug. 16, 1993 & JP 05 098385 A (Sumitomo Electric Ind Ltd), Apr. 20, 1993.
Burns Doane Swecker & Mathis L.L.P.
Jenkins Daniel J.
Sandvik AB
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