Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Reexamination Certificate
2000-04-04
2001-08-14
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
C075S252000, C075S351000, C075S352000
Reexamination Certificate
active
06273930
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a submicron cemented carbide powder with low compacting pressure, particularly useful for cutting tool inserts for turning, milling and drilling steels and stainless steels.
BACKGROUND OF THE INVENTION
Cemented carbide is made by wet milling of powders forming hard constituents and binder phase to a slurry, drying the slurry generally by spray drying, tool pressing the dried powder to bodies of desired shape and finally sintering. During sintering the bodies shrink abour 20% linearly. The shrinkage depends on pressing pressure, WC grain size, WC grain size distribution and Co-content. Pressing tools are expensive to make and are therefore made for a standard shrinkage such as 18%. This shrinkage is obtained by choosing a particular pressing pressure. However, if the grain size is submicron or finer the pressing pressure is relatively high, 250-300 MPa. A high pressing pressure is not desirable because of the risk of pressing cracks in the pressed bodies and abnormal wear and even risk of pressing tool failure. It is therefore desirable to find methods of reducing the pressing pressure when making submicron cemented carbides.
SUMMARY OF THE INVENTION
It has now surprisingly been found that a submicron cemented carbide powder with an apparent reduced compacting pressure at a predetermined weighing in exhibiting no more than 18% shrinkage can be made from powder mixtures having submicron hard constituents with narrow grain size distributions which are conventionally milled at greatly reduced milling times.
According to the present invention, a method of making a powder comprising a plurality of components including hard constituents and a binder phase, the powder having a low compacting pressure, the method comprises the steps of:
(i) premilling a mixture of components which does not include the WC for more than 2 hours;
(ii) adding the WC to the mixture of step (i) and milling for approximately 10 hours; and
(iii) drying the milled mixture to obtain a powder.
DETAILED DESCRIPTION OF THE INVENTION
According to the invention there is now provided a submicron cemented carbide powder with the desired low compacting pressure <200 MPa, preferably <175 MPa at 18% shrinkage and excellent compacting properties for cutting tool inserts comprising WC and 6-15 wt-% Co, preferably 8-12 wt-% Co and less than 1 wt-% Cr, preferably 0.2-0.6 wt-% Cr and/or less than 1 wt-% V, preferably 0.2-0.6 wt-% V.
The WC-grains have an average grain size in the range 0.2-1.0 &mgr;m, preferably 0.4-0.6 &mgr;m with no WC grains greater than 1.5 &mgr;m.
The W-content in the binder phase can he 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 shall preferably be 0.80-1.0, and most preferably 0.80-0.90. 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.
According to the method of the present invention, the milling procedure is started with a >2 hours, preferably about 3 hours, pre-milling step including wet milling in ethanol with cemented carbide milling bodies of all components except of WC such as grain growth inhibitors, carbon black or tungsten powder, binder metal and pressing agent, respectively. The pre-milling step is followed by a further final milling step of about 10 hours with the WC powder included. The amount of milling bodies during the pre-milling and final milling shall be such that the weight ratio milling bodies: WC powder is about 4-7. The cemented carbide powder is then dried preferably by spray drying, pressed to inserts and sintered.
WC-powder with submicron grain size distribution according to the invention with no grains greater than 1.5 &mgr;m is prepared by milling/sieving such as in a jetmill-classifier. According to the present invention, a minimal change in grain size and/or grain size distribution results from the final milling step.
REFERENCES:
patent: 5403541 (1995-04-01), Oskarsson et al.
patent: 5844153 (1998-12-01), Schlatter et al.
patent: 5882376 (1999-03-01), Kim et al.
patent: 5885653 (1999-03-01), Waldenstrom et al.
patent: 5902942 (1999-05-01), Maderud et al.
Burns Doane Swecker & Mathis L.L.P.
Mai Ngoclan
Sandvik AB
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