Cemented carbide body with increased wear resistance

Details Cemented carbide body with increased wear resistance Cemented carbide body with increased wear resistance

1999-04-01

2001-04-03

Jenkins, Daniel (Department: 1742)

Powder metallurgy processes

Powder metallurgy processes with heating or sintering

Metal and nonmetal in final product

C419S035000, C419S038000

Reexamination Certificate

active

06210632

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to coated cemented carbide bodies particularly useful in tools for turning, milling and drilling of steels and stainless steels.
Cemented carbide bodies are manufactured according to powder metallurgical methods including milling, pressing and sintering. The milling operation is an intensive mechanical 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 days. Such processing is believed to be necessary in order to obtain a uniform distribution of the binder phase in the milled mixture, but it results in a wide WC grain size distribution.
In U.S. Pat. Nos. 5,505,902 and 5,529,804 methods of making cemented carbide are disclosed according to which the milling is essentially excluded. Instead, in order to obtain a uniform distribution of the binder phase in the powder mixture, the hard constituent grains are precoated with the binder phase, the mixture is further wet mixed with pressing agent dried, pressed and sintered. In the first mentioned patent the coating is made by a SOL-GEL method and in the second, a polyol is used.
EP-A-665 308 discloses a coated cutting insert with a bimodal distribution of WC grain size with WC grains in two groups 0.1-1 &mgr;m and 3-10 &mgr;m. The insert according to this application is produced with conventional milling technique resulting in a broadening of the WC grain size distribution.
OBJECT AND SUMMARY OF THE INVENTION
It is an aspect of this invention to provide a method of making a cemented carbide body comprising wet mixing without milling of at least two different WC-powders with deagglomerated powders of other carbides, binder metal and pressing agent such that the WC-powders are coated with the binder phase, said WC-grains being carefully deagglomerated before and after being coated with binder metal, the grains of the WC-powder being classified in at least two groups in which a group of smaller grains has a maximum grain sixe a
max
and a group of a larger grains has a minimum grain size b
min
, each group containing at least 10% of the total amount of WC grains wherein b
min
−a
max
>0.5 mm, the variation in grain size within each group being >1 &mgr;m, drying said mixture, pressing to a desired shape and sintering said pressed bodies.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION
It has now surprisingly been found that a further improvement of the properties of a cemented carbide according to EP-A-665 308 can be obtained it such a material is made using the technique disclosed in the above mentioned U.S. Pat. No. 5,505,902 or 5,529,804.
The present invention relates generally to a cemented carbide body comprising WC with an average grain size of <10 &mgr;m in a binder phase. The WC grains are classified in at least two groups in which a group of smaller grains has a maximum grain size a
max
and a group of larger grains has a minimum grain size b
min
. Each group contains at least 10% of the total amount of WC grains. The cemented carbide body according to the invention is characterized in that b
min
−a
max
>0.5 &mgr;m and that the variation in grain size within each group is >1 &mgr;m.
More particularly, the invention relates to a coated cutting insert with a bimodal distribution of the WC grains particularly useful for machining of steels and stainless steels comprising WC and 4-20 wt-% Co, preferably 5-12.5 wt-% Co and 0-30 wt-% cubic carbide, preferably 0-15 wt-% cubic carbide, most preferably 0-10 wt-% cubic carbide such as TiC, TaC, NbC or mixtures thereof. The WC grains have a narrow bimodal grain size distribution with grain sizes in the ranges 0-1.5 &mgr;m and 2.5-6.0 &mgr;m respectively, and with a weight ratio of fine WC particles (0-1.5 &mgr;m) to coarse WC particles (2.5-6.0 &mgr;m) in the range of 0.25-4.0, preferably 0.5-2.0.
The amount of W dissolved in the 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-value in inserts according to the invention shall be 0.82-1.0, preferably 0.86-0.96.
The sintered inserts according to the invention are used coated or uncoated, preferably coated by MTCVD, conventional CVD or PVD, with or without Al
2
O
3
. In particular, multilayer coatings comprising TiC
x
N
v
O
z
with columnar grains followed by a layer of &agr;-Al
2
O
3
, &kgr;-Al
2
O
3
or a mixture of &agr;- and &kgr;-Al
2
O
3
, have shown good results. In another preferred embodiment, the coating described above is completed with a TiN-layer which can be brushed or used without brushing.
According to the method of the present invention, a cemented carbide body is made comprising wet mixing without milling of at least two different WC-powders with deagglomerated powders of other carbides, generally TiC, TaC and/or NbC, binder metal and pressing agent, dried preferably by spray drying, pressed to inserts and sintered. The grains of the WC-powder are classified in at least two groups in which a group of smaller grains has a maximum grain size a
max
and a group of larger grains has a minimum grain size b
min
each group containing at least 10% of the total amount of WC grains wherein b
min
−a
max
>0.5 &mgr;m and the variation in grain size within each group is >1 &mgr;m. Preferably, prior to mixing, the WC grains are carefully deagglomerated before and after being coated with binder metal.
Particularly according to the method of the present invention, WC-powders with two narrow grain size distributions of 0-1.5 &mgr;m and 2.5-6.0 &mgr;m respectively and a weight ratio of fine WC particles (0-1.5 &mgr;m) to coarse WC particles (2.5-6.0 &mgr;m) in the range of 0.25-4.0, preferably 0.5-2.0 are wet mixed without milling with other carbides generally TiC, TaC and/or NbC, binder metal and pressing agent, dried preferably by spray drying, pressed to inserts and sintered.
It is essential according to 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.
In a preferred embodiment, the hard constituents, at least those with narrow grain size distributions, are after careful deagglomeration coated with binder metal using methods disclosed in U.S. Pat. No. 5,505,902 or 5,529,804. In such case, the cemented carbide powder according to the invention is preferably of Co-coated WC+Co-binder, with or without additions of the cubic carbides such as TiC, TaC, NbC, (Ti,W)C, (Ta,Nb)C, (Ti,Ta,Nb)C, (W,Ta,Nb)C, and (W,Ti,Ta,Nb)C coated or uncoated, preferably uncoated, possibly with further additions of Co-powder in order to obtain the desired final composition.
The invention is additionally illustrated in connection with the following Examples which are to be considered as illustrative of the present invention. It should be understood, however, that the invention is not limited to the specific details of the Examples.


REFERENCES:
patent: 4966627 (1990-10-01), Keshaven et al.
patent: 5505751 (1996-04-01), Mehrotra et al.
patent: 5505902 (1996-04-01), Fischer et al.
patent: 5529804 (1996-06-01), Bonneau et al.
patent: 5593474 (1997-01-01), Keshavan et al.
patent: 5624766 (1997-04-01), Moriguchi et al.
patent: 5674564 (1997-10-01), Ljungberg et al.
patent: 5773735 (1998-06-01), Dubensky et al.
ASM Handbook, vol. 7, Powder Metallurgy, 9th ed., pp. 798801, 1984.*
Derwent WPI, Dialog Accession No. 008183518, WPI Accession No. 90-070519/199010, (Mitsubishi Metal Corp), “Tungsten carbide base hard alloy end mills—cont. cobalt, nickel, chromium, vanadium, titanium carbide, niobium carbide”, & JP,A,2022438, 19900125, 199010 B.
Derwent WPI, Dialog Accession No. 007185293, WPI Accession No. 87-182302/

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