U.S. patent application number 10/666268 was filed with the patent office on 2005-03-24 for cemented carbide article having binder gradient and process for producing the same.
Invention is credited to Bennett, Stephen L., Jagner, Mark J..
Application Number | 20050061105 10/666268 |
Document ID | / |
Family ID | 34274708 |
Filed Date | 2005-03-24 |
United States Patent
Application |
20050061105 |
Kind Code |
A1 |
Bennett, Stephen L. ; et
al. |
March 24, 2005 |
CEMENTED CARBIDE ARTICLE HAVING BINDER GRADIENT AND PROCESS FOR
PRODUCING THE SAME
Abstract
An eta-phase-free cemented carbide insert with improved surface
hardness and wear resistance containing WC, and possibly cubic
phases of a carbide and/or carbonitride, in a binder phase of Co,
Ni, Fe or a combination thereof, with a binder phase gradient in
the surface and near surface regions, is disclosed. The nominal
binder phase content in the insert is 3-20 weight %. The surface,
and near surface cobalt content is 50-100% of the binder phase
content of the inner portion of the insert. The insert is formed by
standard sintering practices, followed by the chemical removal of
the binder phase from the surface and near surface regions of the
insert. The insert is then heat treated at a temperature of
1300-1350.degree. C. in a carburizing atmosphere, for a time of
5-400 minutes to cause diffusion of the binder phase from the
interior into the binder depleted surface regions.
Inventors: |
Bennett, Stephen L.;
(Antioch, TN) ; Jagner, Mark J.; (Ypsilanti,
MI) |
Correspondence
Address: |
BURNS DOANE SWECKER & MATHIS L L P
POST OFFICE BOX 1404
ALEXANDRIA
VA
22313-1404
US
|
Family ID: |
34274708 |
Appl. No.: |
10/666268 |
Filed: |
September 22, 2003 |
Current U.S.
Class: |
75/236 ; 428/547;
75/242 |
Current CPC
Class: |
B22F 2998/00 20130101;
B22F 2998/00 20130101; C22C 29/08 20130101; Y10T 428/12021
20150115; B22F 2207/03 20130101 |
Class at
Publication: |
075/236 ;
075/242; 428/547 |
International
Class: |
C22C 029/08 |
Claims
1. (Canceled)
2. The article of manufacture of claim 3 wherein said cemented
carbide body further comprises at least one carbide of a metal of
Groups IVA, VA, or VIA.
3. A hard, wear resistant article of manufacture comprising a
cemented carbide substrate, substantially free of eta-phase,
comprising tungsten carbide and 2-30 weight % binder selected from
the group consisting of Co, Ni, Fe, and combinations thereof,
wherein binder concentration increases from approximately zero at a
surface of the substrate to nominal at a selected distance interior
to the surface and does not exceed nominal, wherein the selected
distance interior to the surface ranges from 25 to 500 microns.
4. The article of manufacture of claim 3 wherein said binder is at
least 3 weight % cobalt.
5. The article of manufacture of claim 3 wherein said article has
porosity of less than 0.5 volume %.
6. The article of manufacture of claim 3 wherein binder
concentration increases according to a gradient of substantially
constant slope extending from the surface to the interior of the
substrate.
7. (Canceled)
8. The article of manufacture of claim 9 wherein binder
concentration decreases according to a gradient of substantially
constant slope.
9. An article of manufacture comprising an eta-phase-free cemented
carbide body, composed of well-defined metal carbide grains and a
binder wherein a binder concentration varies within the article
according to a binder concentration gradient decreasing from
nominal concentrations at a selected distance interior of a surface
of the article to less than 1 wt. % at the surface of the article,
wherein the selected distance interior to the surface ranges from
25 to 500 microns.
10-16. (Canceled)
17. The article of manufacture of claim 3 wherein the selected
distance interior to the surface ranges from 100 to 250
microns.
18. The article of manufacture of claim 3 wherein the nominal
binder concentration is 3 to 20 wt. %.
19. The article of manufacture of claim 18 wherein the nominal
binder concentration is 5 to 12 wt. %.
20. The article of manufacture of claim 3 wherein the binder
consists essentially of Co, the nominal binder concentration is 6
wt. %, and the selected distance interior of a surface of the
article ranges from 100 to 250 microns.
21. The article of manufacture of claim 20 wherein the binder
concentration is a minimum at 5 to 10 microns below the surface of
the substrate.
22. The article of manufacture of claim 3 wherein the binder
consists essentially of Co, the nominal binder concentration is 6
wt. %, and the selected distance interior of a surface of the
article is 400 microns.
23. The article of manufacture of claim 22 wherein the binder
concentration is a minimum at 5 to 10 microns below the surface of
the substrate.
24. The article of manufacture of claim 3 wherein the binder
concentration is a minimum in a near surface region of
substrate.
25. The article of manufacture of claim 24 wherein the near surface
region is at 5 to 10 microns below the surface of the
substrate.
26. The article of manufacture of claim 3 wherein the binder
comprises at least 50 wt. % Co.
27. The article of manufacture of claim 9 wherein the metal carbide
grains comprise tungsten carbide.
28. The article of manufacture of claim 9 wherein the metal carbide
grains consist essentially of tungsten carbide
29. The article of manufacture of claim 9 wherein the binder
comprises one or more of Co, Ni, Fe, and combinations thereof.
30. The article of manufacture of claim 29 wherein the binder
comprises at least 50 wt. % Co.
31. The article of manufacture of claim 9 wherein the binder
consists essentially of Co.
32. The article of manufacture of claim 9 wherein the binder
concentration is a minimum in a near surface region of article.
33. The article of manufacture of claim 32 wherein the near surface
region is at 5 to 10 microns below the surface of the article.
34. The article of manufacture of claim 9 wherein the selected
distance interior to the surface ranges from 100 to 250
microns.
35. The article of manufacture of claim 9 wherein the nominal
binder concentration is 3 to 20 wt. %.
36. The article of manufacture of claim 35 wherein the nominal
binder concentration is 5 to 12 wt. %
37. The article of manufacture of claim 9 the binder consists
essentially of Co, the nominal binder concentration is 6 wt. %, and
the selected distance interior of the surface of the article ranges
from 100 to 400 microns.
38. The article of manufacture of claim 37 wherein the binder
concentration is a minimum at 5 to 10 microns below the surface of
the article.
39. The article of manufacture of claim 37 wherein the selected
distance interior of the surface of the article ranges from 100 to
250 microns.
40. The article of manufacture of claim 9 wherein said article has
porosity of less than 0.5 volume %.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a cemented carbide material
which exhibits a gradient in binder concentration. In particular,
the material exhibits a relatively low binder concentration near
surface regions and nominal concentrations at the interior of the
material. The invention also relates to methods of producing the
above.
BACKGROUND OF THE INVENTION
[0002] Various constructions and techniques will be described
below. However, nothing described herein should be construed as an
admission of prior art. To the contrary, Applicants expressly
preserve the right to demonstrate, where appropriate, that anything
described herein does not qualify as prior art under the applicable
statutory provisions.
[0003] Cemented Carbide inserts and articles have been commercially
available for use as cutting tools, wear parts and dies for many
years. Typical cemented carbides are comprised of metal carbides,
normally WC, often with the addition of carbides of other metals
such as Ti, Ta, Nb, V, Zr, etc., and a metallic binder comprising
Co, Ni, Fe or combination thereof. Various combinations of binders
and metal carbides are mixed together in a body to produce the
desired characteristics of hardness, toughness, and chemical and
abrasion resistance. Cemented WC parts incorporating a binder in
nominal concentrations between about 2 and 30 weight %, and cubic
carbides such as TiC, TaC, NbC, VC and ZrC, and combinations
thereof in concentrations up to about 30% by weight of the total
weight have the requisite characteristics for most applications
useful to the automobile and other industries. The parts formed
from such cemented carbides are often coated with one or more
refractory layers to impart desired characteristics which may be
lacking in the substrate material or to otherwise improve
performance of the finished article. Known coatings include
Al.sub.2O.sub.3, ZrO.sub.2, Y.sub.2O.sub.3, AlN, cBN, as well as
nitrides and carbonitrides of Groups IVA and VA, and combinations
thereof.
[0004] Parts combining various amounts of metal carbides and
binders, as well as different carbide phases, have been developed
in attempts to optimize performance. U.S. Pat. Nos. 4,743,515 and
5,856,626 from Fischer et al. attempted to improve the strength of
cobalt cemented carbide by creating a two-layered body utilizing
eta-phase. Eta-phase is understood in the industry to mean
compositions of tungsten, cobalt and carbon, such as M.sub.6C and
M.sub.12C, where M=tungsten and cobalt, for example W.sub.3CoC.
However it is known in the art that eta-phase forms brittle grains
around WC crystals, providing sites for crack initiation and
propagation. The presence of eta-phase results in a marked
reduction in strength of the resulting article. Fischer et al.
disclose parts having an inner layer comprised of WC, Co and
eta-phase, with an outer layer which was eta-phase-free. In the
substrates described by Fischer et al., the cobalt concentration in
the eta-phase-free outer layer varies with depth from about 10-90%
of the nominal value at the surface, to at least 120% of nominal,
and then drops sharply in the inner eta-phase containing layer. The
method for achieving the two layers requires sintering powders
having substochiometric quantities of carbon at high temperature to
generate eta-phase and then transforming the outer layer of
eta-phase via high temperature carburization.
[0005] U.S. Pat. No. 5,453,241 by Akerman et al. seeks to improve
the toughness of products produced per Fischer et al. by
establishing a method of high temperature carburization followed by
rapid cooling. All of the foregoing patents share the drawback of
containing eta-phase, which is brittle and acts as a source for
fracture initiation and propagation. Furthermore, the use of
temperatures greater than 1400.degree. C. for post sintering heat
treatments has the drawback of loss of geometric features, warpage,
and a reduction in hardness, due to excessive grain growth. Another
drawback of this prior art is the presence of porosities in the
substrate which are detrimental to the performance of the finished
article.
[0006] Modifications can also be carried out by increasing the
concentration of the binder phase in the near surface regions of
the part. This binder phase enrichment improves certain properties
of the part, such as toughness, but has the drawback of leaving
residual binder at the surface, which interferes with later coating
of the part. U.S. Pat. Nos. 5,560,839; 5,660,881; 5,618,625; and
5,713,133 detail the removal of the binder from the surface by
etching, grinding, and other means followed by coating of the
article with diamond. A drawback to these methods is increased
porosity in the body and damage to the WC grains at the surface.
U.S. Pat. No. 5,380,408 by Svensson details a method of removing
cobalt from the surface of a part having a cobalt enriched surface
region by chemical etching without removal of cobalt channels
between the hard material grains. Removal of only the surface
cobalt is asserted to improve coating adherence without creating
undesirable porosity in the part.
[0007] Another drawback of the cobalt enriched surface region in
the cemented carbide is a resulting decrease in hardness and
chemical wear resistance, particularly when machining super alloys,
such as titanium and its alloys. This lack of hardness leads to
accelerated tool wear, even when coatings are applied to the part.
Alternative materials such as eta-phase containing cemented
carbides, discussed above, and selected cemented carbides
consisting of WC with very small amounts of cobalt and ceramics,
are thought to lack sufficient toughness to withstand forces
associated with repeated use of tools, wear parts and dies. It is
therefore desirable to produce a cemented carbide part having a
combination of hardness and toughness, which overcomes the
drawbacks of the prior art.
SUMMARY OF THE INVENTION
[0008] It has been demonstrated in the present invention that a
body which is eta-phase-free, and exhibits a binder concentration
which gradually increases from concentrations at or near zero in
the surface regions of the article to higher concentrations, not
exceeding nominal concentration levels, in the interior of the part
provides a desirable combination of hardness and toughness.
[0009] According to a first aspect, the present invention provides
a hard, wear resistant article of manufacture comprising a cemented
carbide substrate, substantially free of eta-phase, comprising
tungsten carbide and 2-30 weight % binder selected from the group
consisting of Co, Ni, Fe, and combinations thereof, wherein binder
concentration increases from approximately zero at the substrate
surface to nominal at a selected distance interior to the surface
and does not exceed nominal.
[0010] According to another aspect, the present invention provides
an article of manufacture comprising an eta-phase-free cemented
carbide body, composed of well defined metal carbide grains and a
binder wherein said binder concentration varies within the part
according to a binder concentration gradient decreasing from
nominal concentrations in the interior of the part to less than 1%
at the part surface.
[0011] According to a further aspect, the present invention
provides a method of making a hard, wear resistant article
comprising the steps of: a) heat treating a green or pre-sintered
article to produce a fully sintered product; b) removing binder
from surface regions of a cemented carbide article to a selected
depth within said article by immersion in a chemical etching
solution; c) heating the etched article in a vacuum between 1225
and 1275.degree. C.; and d) further heating the article in a
carburizing atmosphere between 1300 and 1350.degree. C. for a time
sufficient to diffuse binder from interior regions of the article
into said surface regions.
[0012] According to yet another aspect, the present invention
provides a method of making a hard wear resistant article
comprising the steps of: a) removing binder from surface regions of
a cemented carbide article to a selected depth within said article;
b) heating the article in a vacuum to between 1200 and 1250.degree.
C.; and c) introducing an atmosphere comprising carbon monoxide and
further heating the article to between 1300 and 1350.degree. C. and
holding at that temperature in the carbon monoxide atmosphere for a
period of time sufficient to diffuse binder from interior regions
of the article into said surface regions.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a plot of the cobalt concentration versus the
depth beneath the surface for a 6 weight % Co part according to one
embodiment of the present invention.
[0014] FIG. 2 is a plot of cobalt concentration versus depth
beneath the surface for different embodiments of the present
invention based on different depths of etch and an identical heat
treatment.
[0015] FIG. 3 is a plot depicting the depth at which the cobalt
concentration returns to nominal after heat treatment of an
embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0016] The present invention relates to articles comprising an
eta-phase-free cemented carbide part having well defined metal
carbide grains and a binder concentration between 0 and 1 weight %,
preferably 0 weight %, at the surface of the part. In the inner
regions of the part, binder concentration is equal to the nominal
concentration for the selected cemented carbide grade.
Therebetween, a concentration gradient is present wherein the
concentration of the binder decreases from nominal in the interior
of the part to at or near zero at the substrate surface.
Concentration of the binder in the near surface region is
controlled to form this gradient of decreasing binder concentration
moving from the interior of the part to the outer surface of the
substrate. The decreasing binder concentration of the present
invention extends over a selected distance, which is controlled by
processing parameters, for example duration of each step,
concentration of the etching solutions, temperature, and the
nominal binder concentration. Control of these parameters and their
affects can be readily ascertained based from the methods described
herein by one of ordinary skill in the art. Selected distances over
which the gradient extends range from approximately 25-500 microns,
preferably 100-250 microns.
[0017] Binders may be selected from any of Co, Ni, Fe or
combinations thereof, wherein Co comprises a minimum weight percent
of 50% of the binder. In a preferred embodiment, the binder is
cobalt, with inevitable impurities. Nominal binder concentration
may be selected to achieve various properties in the part, but
generally ranges from 3-20 weight %, preferably 5-12 weight %.
Cemented WC parts of the invention may incorporate cubic carbides
such as TiC, TaC, NbC, VC, HfC, Cr.sub.2C.sub.3 and ZrC, and
combinations thereof in concentrations up to about 30 weight % of
the total weight.
[0018] In a first embodiment of the invention, the substrate is a
cobalt cemented tungsten carbide material having a nominal binder
concentration of 6 weight %. The binder concentration in the
interior of the part is nominal and decreases to less than nominal
along a gradient to the near surface regions of the part,
preferably the near surface regions are from 5-10 microns beneath
the surface. The concentration gradient begins at approximately
100-250 microns below the surface, with the binder concentration
gradually decreasing until it reaches a minimum at the near surface
region, as illustrated in FIGS. 1 and 2.
[0019] In a second preferred embodiment, having a nominal binder
concentration of 6 weight %, the gradient begins at 400 microns
below the surface and reaches a minimum at the near surface region,
as illustrated in FIG. 3.
[0020] The surface has a porosity of less than 1 volume %.
Preferably maximum porosity is less than 0.5 volume %, most
preferably ranging from 0.4 to 0.1 volume %. In the most preferred
embodiment, the porosity is less than 0.2 volume %. The part also
exhibits substantially no distortion, which would contribute to
malformation or warpage of the finished article.
[0021] The present method of producing articles with an
eta-phase-free, binder gradient includes removing binder from the
sintered part surface and near surface regions over a selected
distance into the interior of the part and subsequent heat treating
to cause migration of binder from the interior of the article into
the binder depleted region. The result of this process is a
controlled change in concentration of binder without spikes in
concentration of binder to greater than nominal in the substrate.
While greater than nominal concentrations of binder are considered
beneficial in prior art, it is believed that low concentration
gradually increasing to nominal, without regions of high binder
concentration, provide a more cohesive substrate and better
predictability in performance of the article of the present
invention.
[0022] Optionally, the articles may also be ground, subjected to a
carbon correction or other processing provided that such processing
does not interfere with the effectiveness of the described binder
removal and heat-treating process.
[0023] Binder removal according to the present invention is
generally accomplished through chemical etching. See, for example,
U.S. Pat. No. 5,560,839 which details a variety of chemical systems
for etching cobalt binder, the content of which is incorporated
here by reference in its entirety. While any of the
afore-referenced chemical systems may be used to etch the binder
phase, the preferred embodiment makes use of a ferric chloride
solution to chemically etch the binder from the substrate to a
selected depth. Those skilled in the art will recognize that the
depth of etching of the binder will depend upon the molality of the
ferric chloride solution, the temperature, the reaction time, and
the composition of the part (i.e. -nominal cobalt content, binder
chemistry and carbide grain size) so that some reaction
experimentation is expected prior to industrial application to
define optimum process parameters. Preferred concentrations of
ferric chloride aqueous solutions for use in the present invention
range from 0.005 M to 1.0 M, but other concentrations known in the
art may be used. An approximately 20 micron depth of etch may be
achieved by immersing the selected article in 0.005 M ferric
chloride for a period of approximately 2 hours; doubling the
immersion time yields an approximately 37 micron depth of etch.
[0024] Those skilled in the art will recognize that the preferred
depth of binder removal is dependant upon the end use to which the
cemented carbide part will be put, and the ranges given herein are
not to be construed as limiting, but rather as merely illustrative
of use for a cutting tool insert. Alternately, binder may be
removed via other methods known in the art that are not
incompatible with the remainder of the process.
[0025] The apparatus used in the heat-treating process of the
present invention comprises an enclosed vessel and a retort of
steel or other suitable material. The reaction vessel is provided
with an inlet and an outlet whereby the gaseous atmosphere for
heat-treating enters the vessel through the inlet, flow through a
reaction zone containing the part and exits through an outlet.
Typically the vessel includes a premix area such as a chamber,
where the gases utilized are premixed at a temperature lower than
the heat-treating temperature. This premix area can be internal or
external to the vessel or the reaction zone. In one embodiment
uniformly mixed gases exiting the premix chamber flow into the
inlet and continue into the reaction zone. The apparatus is
equipped with furnace controls for process parameter regulation,
such as monitoring and adjusting processing time, the vessel's
temperature and pressure, the temperature and pressure of the
premix area, flow rate and partial pressures of gasses at selected
points within the apparatus. Preferably, as is typical of
manufacturing level furnaces, the furnace controls can be set at
selected process parameters utilizing a personal computer or other
computer interface with the operator. To maintain repeatability
from batch to batch, in the most preferred embodiment, the process
parameters are computer controlled.
[0026] The articles, cutting tools or inserts to be heated are
positioned in the reaction zone by conventional means, such as
tables, trays or other fixtures known in the art. The reaction
vessel includes heating elements typically in the form of graphite.
The reaction vessel is loaded with articles, cutting tools or
inserts and may be flushed with a suitable inert gas such as
nitrogen, argon, or the like. Typically, the vessel is vacuum
evacuated and the temperature ramped up to within the range of
900-1300.degree. C. and the carburizing gas is introduced. The
temperature may be increased to not more than 1350.degree. C.,
preferably 1300-1330.degree. C., or maintained and the inserts held
at this temperature for a sufficient time to cause diffusion of
binder from the interior of the article into the etched portion,
without causing deleterious warpage of the article or migration of
the binder onto the surface of the article. In a preferred
embodiment of the invention, carbon monoxide comprises the
atmosphere in the reaction vessel during the heating step. The
pressure during the heating step can be atmospheric pressure or
less. Suitable pressures are within the knowledge of one of
ordinary skill in the art based upon the composition and size of
the carbide article and can be readily determined.
[0027] The present invention will become even clearer upon
consideration of the following examples, which are intended to be
illustrative of the present invention, and not limiting.
EXAMPLE 1
[0028] Step 1: Groups of sintered cemented carbide inserts
containing <0.5 weight % cubic carbides and 6 weight % cobalt,
were treated with freshly prepared 0.05 M ferric chloride solutions
for periods of 2 and 4 hours respectively. Upon examination, the
cobalt binder in the inserts was found to have been etched away to
depths of 20.+-.1 microns and 37.+-.2 microns respectively. A
second set of cemented carbide inserts containing <1 weight %
cubic carbides and 12.3 weight % cobalt was top and bottom ground
and then etched to depths of 21.+-.1 micron and 34.+-.2 microns
using freshly prepared 0.05 M ferric chloride solutions for periods
of 4.5 and 9 hours respectively.
[0029] Step 2: Inserts selected from each group prepared in Step 1
were heated in a furnace to approximately 1100.degree. C. for 100
minutes in a vacuum. A second batch of the selected inserts was
heated in a furnace to approximately 1250.degree. C. for 100
minutes in a vacuum. The heat-treated inserts were then
cross-sectioned, polished and examined at a magnification of
1000.times. on an optical microscope. No differences between the
etched and the etched/heat treated inserts were observed.
EXAMPLE 2
[0030] Inserts prepared according to Example 1, Step 1, were heated
in a furnace to approximately 1300.degree. C. for 100 minutes in a
vacuum. The heat-treated inserts were examined as in Example 1,
step 2. Partial filling of voids in the substrate and some
reduction in the cobalt content just below the etched region was
observed.
EXAMPLE 3
[0031] Inserts prepared according to Example 1, Step 1, were heated
in a furnace to approximately 1350.degree. C. for 100 minutes in 1
torr argon. The resulting inserts had cobalt on the periphery and
the edges were distorted.
EXAMPLE 4
[0032] Step 1: Group A sintered cemented carbide inserts containing
<0.5 weight % cubic carbides and 6 weight % cobalt were etched
to depths of 20.+-.1 microns using freshly prepared 0.05 M ferric
chloride solutions for 2 hours, and Group B sintered cemented
carbide inserts containing <1 weight % cubic carbide and 12.3
weight % cobalt, were etched to depths of 21.+-.1 microns using
freshly prepared 0.05 M ferric chloride solutions for 4.5
hours.
[0033] Step 2: Group A and B were heat treated in 10 torr carbon
monoxide. The gas was introduced at 1250.degree. C., the
temperature was ramped up to 1325.degree. C. and held for 100
minutes. The resulting inserts were uniformly light gray in color.
Losses in mass ranged from 0.01% to 0.02% and decreases in magnetic
saturation levels (Ms) ranged from 0.28% to 0.37%. Surfaces of the
inserts were examined optically at 1000.times.; well-defined WC
grains were visualized and a lack of surface Co or distortion was
noted. The heat-treated inserts were also examined as in Example 1,
Step 2.
[0034] Cemented carbide articles which have been made according to
the present invention may also be subjected to coating with
monolayer or multilayer coatings.
[0035] It is intended that the specification and examples be
considered as exemplary only. Other embodiments of the invention,
within the scope and spirit of the aforementioned claims will be
apparent to those of skill in the art from practice of the
invention disclosed herein and consideration of this specification.
All documents referred to herein are hereby incorporated by
reference, in their entirety.
[0036] While the present invention has been described by reference
to the above-mentioned embodiments, certain modifications and
variations will be evident to those of ordinary skill in the art.
Therefore, the present invention is limited only by the scope and
spirit of the appended claims.
* * * * *