Note: Descriptions are shown in the official language in which they were submitted.
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Cemented carbide tool for woodworking
The present invention relates to a cemented carbide
with excellent properties for use as tool for
woodworking. More particularly, the invention relates to
a sintered, very fine-grained cemented carbide material
in which a corrosion and oxidation resistant phase has
been distributed in a monophase binder based on nickel
and cobalt and of one or more of the following metals:
chromium, molybdenum, iron and vanadium. Most
particularly, the binder is based on nickel and/or
cobalt powders made by the polyol process.
Cemented carbide is usually manufactured by
blending the WC, Ni, Co and other powderous sources and
an organic binder (typically wax-based) and mixing these
in a ball mill before spray-drying the slurry into a
flowable ready-to-press powder. But most of the powder
sources of Ni and Co are made from calcinated oxalates
or hydroxides subsequently reduced in a hydrogen
atmosphere. These powders usually have a broad particle
size distribution and strongly agglomerated particles.
The particles are difficult to deagglomerate, even by
ball milling. This may lead to Co-lakes and a
heterogeneous cemented carbide microstructure resulting
in varying physical and chemical properties.
Polyol cobalt and nickel powders made according to
US patent no.4,539,041 contain predominately near-
spherical grains with little agglomeration and a
submicron average particle size. The use of polyol
powders as binders in cemented carbides is described in
US patent 5,441,693. By using polyol powder sources the
microstructure becomes more homogeneous through better
dispersion of the binder phase particles. Thereby fewer
binder phase-lakes are present after sintering and
further the sintering temperature may be decreased.
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Cemented carbide is one of the most common
materials currently being used in the wood industry for
woodworking tools. In the quest for extended lifetime of
the woodworking tools and for cutting quality it is
important to optimise specific properties of the cutting
tool material. Two of the most important properties of
the woodworking tool are the resistance to corrosion and
oxidation and the hardness.
Since these tools are used for working solid wood,
such as logs etc. but also man-made products based on
wood particles, fibres or chips, such as chipboard,
particle board and medium and high density fibre boards
(MDF/HDF), the different types of corrosion are
important:
~ Solid wood contains organic acids and other
compounds that degrade the binder in the tool material
and cause outbreak of the hard WC-particle from the
matrix and thus wear the tool material.
~ Man-made wood products contain organic binder
material, based on compounds such as urea,
formaldehyde, wax, glue fillers etc. and possibly
laminated with plastic layers, such as melamine. The
temperature may get very high (>500°C) when working
these wood products and then the content of the
product will degrade into corrosive compounds
chemically attacking the cemented carbide binder.
Furthermore the high temperature may result in
oxidation of the cemented carbide binder, also
degrading and wearing the cutting edge.
The hardness usually should be as high as possible
in order to retain a sharp and wear resistant cutting
edge. The factors determining the hardness are
predominately grain size and binder content. The lower
the grain size and/or the binder content the higher
hardness usually will be. Usually, a compromise has to
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be reached between grain size and binder content in
order to get optimal sinterability, e.g. low porosity of
the sintered compact and low sintering temperature. A
very fine grain -size usually necessitates a higher
binder content than slightly higher grain size in order
to have the WC grains wet properly and homogeneously by
the binder constituents. The influence of grain size in
a Ni- based corrosion resistant cemented carbide for
woodworking is discussed in EP0568584.
The problem is to produce a cemented carbide with
very fine grain size and low binder phase content
avoiding a heterogeneous microstructure, sintered
porosity and property differences within the material
when using the highly agglomerated standard Ni- and Co-
powder sources.
In order for the cemented carbide material to work
optimal it is important that the microstructure be as
homogeneous as possible. Polyol binder phase elements
will promote the optimal distribution of the binder
phase around the ultra-fine WC grains and make it
possible to combine very fine WC grain size and a low
binder.
The invention is primarily concerned with the use
of near-spherical, submicron nickel and/or cobalt
powders manufactured by the polyol process in cemented
carbide grades particularly for woodworking having a
corrosion and oxidation resistant monophase binder
phase. The said polyol Ni- and Co-powders shall be
produced separately by the polyol process, preferably as
a mixture of Ni and Co by the polyol process. The total
content of said binder phase varies between 0.5 and 30
wt~, preferably between 1 and 10 wt~, most preferably
between 1.5 and 5 wt~, the remainder being tungsten
carbide. The composition of said binder phase shall be
30 to 80 wt~ Co, max 5 wt~ Mo, max l5 wt~ Cr, max 10 wt~
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V, max 20 wt~ Fe, balance Ni. The average sintered WC
grain size shall be less than 0.7 ~..im, preferably less
than 0.5 um, most preferably less than 0.3 ~zm.
In a preferred embodiment particularly useful for
machining of chip board, particle board and medium and
high density fibreboard (MDF/HDF), the cemented carbide
consists of 2.3 wt~ Co, 0.9 wt~ Ni and 0.3 wt~ Cr, the
rest being tungsten carbide. The average sintered WC
grain size is <0.5 ~.Lm, preferably <0.3 ~.m.
The invention also relates to the use of a cemented
carbide with very fine sintered grain size and with a
binder phase containing Ni and Co produced by the polyol
process as a tool, such as saw tips or indexable
inserts, for cutting and machining of wood and wood-
based products, particularly chipboard, particle boards
and medium or high density fibre boards (MDF/HDF).
The present invention further relates to a method
of making a cemented carbide body based on tungsten
carbide and a binder phase based on Ni and Co and any of
the following constituents: Mo, Cr, Fe and V, by powder
metallurgical methods by milling, pressing and sintering
of powders forming hard constituents and binder phase
whereby said binder phase contains Ni and/or Co made by
the polyol process. Preferably sintering is performed
gas pressure sintering also referred to a sinter-HIP.
The advantages offered by the use of polyol Ni
or/and Co are as mentioned a more homogeneous
microstructure and hence improved physical and chemical
properties.
Example
An alloy (A) consisting of 2.3 wt~ Co, 0.9 wt~ Ni
and 0.3 wt~ Cr, the rest being tungsten carbide with an
average sintered grain size of 0.2um was tested against
an alloy (B) containing 1.9 wt~ Co, 0.7 wt~ Ni, 0.3 wt~
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Cr and 0.1 wt~ Mo, the remainder being WC with an
average sintered grain size of 0.8 dun. The determination
of the average sintered WC grain size was made on
scanning electron micrographs made in an instrument
equipped with a field emission gun. The evaluation was
made using a semiautomatic equipment and taking geometry
effects into consideration.
Both grades were manufactured by conventional
cemented carbide techniques. In the prior art powder Co
and Ni-powders were made by reduction of Co-and Ni-
oxalate; in the invention powder Co and Ni-powders were
made from Co-and Ni-hydroxide thermally reduced in
polyol thus obtaining powders with a submicron average
particle size.
The test comprised machining particleboard plates
with a top cutter containing three identical indexable
inserts. The cutting speed was 18000 rpm, feed rate
1m/min and cutting depth 2 mm.
Cutting Wear A, Wear B,
Distance invention prior art
(m) (1~) (dun)
1300 36 45
4000 58 72
It is clear from the test results that the wear of
the alloy made according to the invention decreases by
up to 20~ compared to prior art.
