U.S. patent application number 11/339033 was filed with the patent office on 2006-06-08 for method and device for heat treatment of metal workpieces as well as a heat-treated workpiece.
Invention is credited to Bernd Edenhofer.
Application Number | 20060118209 11/339033 |
Document ID | / |
Family ID | 8185420 |
Filed Date | 2006-06-08 |
United States Patent
Application |
20060118209 |
Kind Code |
A1 |
Edenhofer; Bernd |
June 8, 2006 |
Method and device for heat treatment of metal workpieces as well as
a heat-treated workpiece
Abstract
A process for heat treating metal workpieces contains with
respect to an efficient process control the following successive
operations following directly one after the other: a heating phase;
an enrichment phase; a first cooling phase; a boriding phase; a
second cooling phase; and a concluding quenching phase. Workpieces
processed by a method of this type are distinguished by a
comparatively great fatigue limit and fatigue strength with
simultaneous high resistance to wear and tear.
Inventors: |
Edenhofer; Bernd; (Kleve,
DE) |
Correspondence
Address: |
CANTOR COLBURN, LLP
55 GRIFFIN ROAD SOUTH
BLOOMFIELD
CT
06002
US
|
Family ID: |
8185420 |
Appl. No.: |
11/339033 |
Filed: |
January 25, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10328555 |
Dec 23, 2002 |
|
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11339033 |
Jan 25, 2006 |
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Current U.S.
Class: |
148/330 |
Current CPC
Class: |
C23C 8/34 20130101; F27B
5/04 20130101; C21D 9/0062 20130101; F27B 5/13 20130101; F27B 5/12
20130101 |
Class at
Publication: |
148/330 |
International
Class: |
C22C 38/32 20060101
C22C038/32 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 4, 2002 |
EP |
02 002 530.0 |
Claims
1. Workpiece that is made of a metal material and is heat treated,
the workpiece comprising: an outer iron boride layer from 10 .mu.m
to 100 .mu.m thick and a case hardening layer beneath the iron
boride layer with a Vickers hardness between 600 and 900 and a case
hardening depth between 0.2 mm and 2.0 mm.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application is a divisional application of U.S.
patent application Ser. No. 10/328,555, filed on Dec. 23, 2002, the
entire contents of which are incorporated herein by reference. The
10/328,555 application claimed the benefit of the date of the
earlier filed European Patent Application No. 02002530.0 filed Feb.
4, 2002 priority to which is also claimed herein.
FIELD OF THE INVENTION
[0002] The invention concerns a method for the heat treatment of
metal workpieces, especially for the combined carburization,
boriding and hardening of ferrous products. It furthermore relates
to a device by means of which such a method can be implemented, and
a workpiece heat-treated using the method.
DESCRIPTION OF THE RELATED ART
[0003] For generating defined workpiece properties, such as perhaps
a great hardness or resistance to wear and tear, metal workpieces
are usually subjected to thermochemical heat treatment. The goal of
this heat treatment is, for example, with case-hardening, first of
all to carburize the edge layer of the workpiece, i.e. to
strengthen it with carbon, in order to bestow a comparatively high
degree of hardness upon the workpieces on the basis of the altered
material composition resulting therefrom through subsequent
hardening. Furthermore, heat treatments where the surface of the
workpieces is coated with a layer creating the desired mechanical
characteristics are known. Thus, in boriding with the diffusion of
boron, a hard boride layer is created on the surface of the
workpiece, which leads to a high resistance to wear and tear and
resistance to corrosion of the workpieces.
[0004] A compilation of the most varied types of heat treatment is
found, for example, in DIN 8580. Moreover, methods are known in the
state of the art which combine individual types of heat treatment
with one another. These so-called combination, hybrid or duplex
methods make use of synergy effects, which arise with a combination
of various types of heat treatment (cf. O. H. Kessler et al.:
"Combinations of coating and heat treating processes: Establishing
a system for combined processes and examples," Surface and Coatings
Technology 108-109 (1988), pages 211 to 216; T. Bell et al.,
"Realizing the potential of duplex surface engineering," Tribology
International, Volume 31, Number 1-3 (1998), pages 127 to 137). In
this way, it is possible to bestow characteristics upon workpieces
that could not be attained by the individual types of heat
treatment. The workpieces can consequently meet complex standards
which, for example, require a great fatigue strength as well as a
high resistance to wear and tear as well as to corrosion.
[0005] But not every arbitrary combination of various types of heat
treatment gives rise to a synergistic result, as Bell et al. point
out (op. cit., page 128). In contrast, for example, the combination
of CVD (chemical vapor deposition) and quench hardening has a
positive action with regard to workpieces provided with a hard
surface. For, as Kessler et al. (op. cit.) explain, the surface
layer generated with such a duplex process through the
plasma-activated vapor deposition process has a great hardness.
SUMMARY OF THE INVENTION
[0006] The invention is based upon the objective of creating a
process and a device for heat-treating metal workpieces, by means
of which a comparatively great hardness, especially fatigue limit
and fatigue strength can be attained, with simultaneous high
resistance against wear and tear of the workpiece.
[0007] For accomplishing this objective, a process for the heat
treatment of metal workpieces, especially for combined the
carburizing, boriding and hardening of ferrous products, contains
the following operations: [0008] a) Heating the workpieces to a
first temperature under a vacuum or a neutral or reducing gas
atmosphere during a heating phase; [0009] b) Carburizing the
workpieces at the initial temperature and an initial pressure
reached at the end of the heating up phase and for a first period
of time in a gas atmosphere containing a hydrocarbon during an
enrichment phase following directly upon the heating phase; [0010]
c) Cooling the workpieces from the first temperature to a second
temperature under a vacuum or a gas atmosphere containing chiefly
nitrogen (N2) during a first cooling phase following immediately
upon the enrichment phase; [0011] d) Boriding the workpieces at the
second temperature reached at the end of the first cooling phase
and at a second pressure for a second period of time in a gas
atmosphere containing boron (B) during a boriding phase following
directly upon the first cooling phase; [0012] e) Cooling the
workpieces from the second temperature to a third temperature under
a vacuum or in a gas atmosphere containing mainly nitrogen (N2)
during a second cooling phase following directly upon the boriding
phase and [0013] f) Quenching the workpieces from the third
temperature to a temperature below 150.degree. C. during a
quenching phase following upon the second cooling phase.
[0014] Such a process is based upon the knowledge that the boriding
phase can be used in order to allow the carbon which accumulated
upon the edge layer of the workpiece during the enrichment phase to
diffuse into the interior of the workpieces. An independent
diffusion phase for generating the desired carbon content in the
edge layer, as is customary with conventional carburizing,
consequently becomes dispensable. A carbonitriding process, if in
addition nitrogen is also added in the gas atmosphere, can also be
understood as carburization in the sense mentioned above.
[0015] The fact that the temperature difference to be bridged
during the first cooling phase immediately following upon the
enrichment phase is generally small moreover contributes to an
efficient process control. For the second temperature necessary for
boriding is probably not smaller, or only slightly smaller than the
first temperature necessary for the enrichment phase for most
carbon-poor ferrous products, such as, for example, case hardening
steel C 15. Depending on the application, the second temperature
can also be greater than the first temperature so that the
workpieces in this case are not to be cooled, but to be heated.
[0016] The carbon profile in the edge layer of the workpieces
generated during the enrichment phase and the boriding phase
serving as a diffusion phase for carbon leads, together with the
subsequent quenching, to residual compressive stresses in the edge
layer of the workpieces and therewith to a fatigue limit and
fatigue strength, which withstands comparatively high dynamic
stresses. In addition to this, the wear and tear-resistant boride
layer formed during the boriding phase on the surface of the
workpieces by the subsequent quenching of the workpieces has a
higher load carrying capacity. For the configuration of the
carburized and hardened workpieces existing beneath the boride
layer possesses a sufficiently high hardness of typically ca. 800
HV which in this way forms a load-carrying sub-structure for the
boride layer having as a rule a hardness according to Vickers of
ca. 2000. Contrary to a CVD process or a PVD process (physical
vapor deposition), the danger of a splitting off of the hard boride
layer in connection with dynamic stress is consequently ruled
out.
[0017] The first temperature to which the workpieces are heated
during the heating up phase and at which the workpieces are
carburized or carbonitrided during the enrichment phase, the second
temperature to which the workpieces are exposed during the boriding
phase, the third temperature from which the workpieces are
quenched, the length of the first period of time, the length of the
second period of time and the amounts of carbon and
boron-dispensing mediums introduced during the enrichment phase and
the boriding phase are chiefly oriented toward the material of the
workpiece that are supposed to be treated, the specific composition
of the gas atmosphere necessary for attaining the desired carbon
content in the edge layer of the workpieces and the sought
treatment success, possibly the desired carburization depth and
thickness of the boride layer. The process parameters, which depend
upon the material properties of the workpieces to be processed, can
be gathered for a certain material from generally accessible data
bases such as perhaps Calphad (Calculation of Phase Diagrams).
Depending on each application, it can be necessary after this to
heat the workpieces during the first and/or second cooling phase to
the second or third temperature. Cooling in the aforementioned
sense can therefore also represent a warming process.
[0018] The objects of the dependent claims represent advantageous
embodiments of the method of the invention.
[0019] Hence it is advantageous to heat the workpieces to a first
temperature between 800.degree. C. and 1100.degree. C. suited for
carburizing or carbonitriding commercially available ferrous
products during the heating up phase. It is furthermore
advantageous to cool the workpieces to a second temperature between
800.degree. C. and 950.degree. C. during the first cooling phase in
order to maintain a temperature usable for boriding the workpieces.
It is moreover advantageous to cool the workpieces to a third
temperature between 800.degree. C. and 900.degree. C. during the
second cooling phase in order to attain a hardening temperature
corresponding to the respective material. Preferably the materials
are cooled to room temperature during the quenching phase so that
they can subsequently processed further without delay.
[0020] An especially advantageous type of process moreover results
when the first period of time amounts to between 60 min. and 360
min. and the second period to between 30 min. and 360 min. The
first and second periods of time are appropriately selected as a
function of the temperatures prevailing at any given time such that
a boride layer with a thickness from 10 .mu.m to 100 .mu.m arises
and the edge carbon content directly beneath the boride layer is
between 0.6% by weight and 0.9% by weight of a hardening depth of
between 0.2 mm and 2.0 mm.
[0021] In accordance with an advantageous embodiment of the process
of the invention, a support by a plasma, i.e. a strong current glow
discharge, takes place during the enrichment phase and/or during
the boriding phase. Such a plasma-activated process is described in
connection with boriding, for example, by H.-J. Hunger et al. in
the article "Plasma-activated gas boriding with boron trifluoride,"
HTM 52 (1997) 1. Supporting by a plasma as a rule takes place at
low pressure and offers in comparison to a purely thermal
activation the advantage of a low consumption of carbon or
boron-dispensing mediums. Appropriately the gas atmosphere contains
boron trichloride (BCl.sub.3) and/or boron trifluoride (BF.sub.3)
and/or diborane (B.sub.2H.sub.6) during the boriding phase. Above
all, the use of boron trifluoride as a boron-dispensing medium has
proven advantageous for plasma-activated boriding. For first of
all, a thermal activation is omitted during boriding with boron
trifluoride so that the boriding process is restricted to the
workpieces situated in the region of the cathode fall, and a
boriding on the internal walls of a boriding chamber is avoided.
Second, boron trifluoride exists in the form of a gas already at
room temperature so that a vaporizer can be economically
foregone.
[0022] Furthermore, it is appropriate if the workpieces are
quenched during the quenching phase at a third pressure, preferably
a high pressure of more than 1,013.25 mbar in a reducing or neutral
gas atmosphere or in a liquid quenching medium in order to assure a
sufficient rate of cooling. The workpieces hardened in this manner
can subsequently (as known from case hardening) be tempered at a
temperature between 150.degree. C. and 200.degree. C. in a separate
furnace.
[0023] An especially advantageous embodiment furthermore exists
when the workpieces are made of a carbon-poor ferrous product,
preferably a case hardening steel according to DIN 17,210. Contrary
to the state of the art, the method of the invention is not
restricted to ferrous products, which initially already possess a
relatively high carbon content, such as, for example, customary
heat treatable steels Ck 45, Ck 60 or 42 CrMo 4. It is rather
possible with the method of the invention to boride carbon-poor
ferrous products, such as, for example, common case hardened steels
Ck 10, Ck 15 or 20 MoCr 4. The reason for this is that the
enrichment phase performed before the boriding phase makes an
enrichment of the edge layer of the workpieces with carbon
possible, which allows a carbon content that is sufficient with
respect to the required carburization to remain in the edge layer
after completion of the boriding phase and therewith of the
diffusion phase.
[0024] In a preferred embodiment of the method of the invention,
the initial pressure as well as the second pressure are between 0.1
mbar and 30 mbar. The pressure here primarily depends on the
temperature prevailing at any given time and the respective
composition of the gas atmosphere. Thus, for example, the initial
pressure should be set such during the enrichment phase that, on
the one hand, a comparatively rapid carburizing of the edge layer
of the workpieces is attained and on the other, the generally
undesired carbide or soot formation on the surface of the
workpieces is avoided. The initial pressure and the second pressure
need not be equal during the enrichment phase and the boriding
phase and also need not necessarily be constant. They can rather be
selectively varied, for example pulsed, in accordance with the
desired treatment result.
[0025] Moreover, in accordance with claim 14, a device for
implementing the method previously described is proposed for
accomplishing the above-mentioned objective, containing at least
one treatment chamber in which the heating up phase, the enrichment
phase, the first cooling phase, the boriding phase, the second
cooling phase and the quenching phase can be conducted one after
the other.
[0026] Such a device can be a one-chamber vacuum furnace in the
simplest case, in which the operations described above can be
conducted successively and without transport of the charge.
[0027] A preferred configuration of such a device provides two
treatment chambers, whereby in the first treatment chamber the
heating phase, the enrichment phase, the first cooling phase, the
boriding phase and the second cooling phase are conducted and
whereby the quenching phase is conducted in the second treatment
chamber. Since a separate treatment chamber is available for the
quenching phase, a high pressure gas quenching process can be
conducted in a simple manner, by means of which comparatively high
quenching rates are obtained.
[0028] With respect to a higher throughput, a second preferred
configuration of the device of the invention provides three
treatment chambers, whereby the heating phase and the enrichment
phase are conducted in the first treatment chamber, whereby the
first cooling phase, the boriding phase and the second cooling
phase are conducted in the second treatment chamber, and whereby
the quenching phase is conducted in the third treatment
chamber.
[0029] A third preferred configuration of the device of the
invention provides treatment chambers arranged in series or
parallel, whereby the heating phase is conducted in the first
treatment chamber, whereby the enrichment phase or the enrichment
phase and the first cooling phase are conducted in the second
chamber, whereby the first cooling phase, the boriding phase and
the second cooling phase or the boriding phase and the second
cooling phase are conducted in the third treatment chamber and
whereby the quenching phase is conducted in the fourth treatment
chamber.
[0030] A fourth preferred configuration of the device of the
invention provides six treatment chambers which are arranged in
series or parallel, whereby the first treatment chamber is
constructed as a heating chamber for conducting the heating phase,
the second treatment chamber is constructed as an enrichment
chamber for conducting the enrichment phase, the third treatment
chamber is constructed as a cooling chamber for conducting the
first cooling phase, the fourth treatment chamber is conducted as a
boriding chamber for conducting the boriding phase, the fifth
treatment chamber is constructed as a cooling chamber for
conducting the second cooling phase and the sixth treatment chamber
is constructed as a quenching chamber for conducting the quenching
phase. Since an independent treatment chamber is available for each
operation, such a heat-treatment facility is distinguished by a
comparatively easy to control process with an especially high
throughput.
[0031] Finally, a workpiece is proposed in agreement with claim 19,
which is made of a metal material and is heat treated by the method
of the invention, whereby the workpiece is provided with an outer
iron boride layer from 10 .mu.m to 100 .mu.m thick and a case
hardening layer under the iron boride layer with a hardness
according to Vickers between 600 and 900 and a case hardening depth
between 0.2 mm and 2.0 mm.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] Details and further advantages of the method of the
invention and the corresponding device result from the following
description of preferred embodiments. Depicted in particular in the
associated drawings are:
[0033] FIG. 1 A diagram illustrating the temperature and pressure
curves of the method of the invention over time;
[0034] FIG. 2 A schematic representation of a one-chamber vacuum
furnace with gas quenching;
[0035] FIG. 3 A schematic representation of a two-chamber vacuum
furnace with gas quenching;
[0036] FIG. 4 A schematic representation of a two-chamber vacuum
furnace with oil quenching;
[0037] FIG. 5 A schematic representation of a heat treatment
facility with six treatment chambers and
[0038] FIG. 6 A schematic representation of a three-chamber vacuum
furnace with gas quenching and flushing sluices.
DETAILED DESCRIPTION
[0039] With the diagram represented in FIG. 1, time t is
represented on the abscissa and temperature o and pressure p are
represented on the ordinate. The heat treatment method illustrated
on the basis of FIG. 1 is a duplex process in the sense mentioned
at the beginning and serves the combined carburizing, boriding and
hardening of workpieces, which are made of a carbon-poor iron
material, for example case hardened steel C 15 (material 1.0401).
The entire process sequence can be subdivided into six phases A to
F.
[0040] During the first phase, the heating phase A, the workpieces
to be processed are heated to a first temperature .phi..sub.1 of
about 1000.degree. C. The device used for this purpose, possibly a
heat treating system in accordance with FIG. 5, is first evacuated
after introducing the workpieces and subsequently heated to
temperature .phi..sub.1. Alternatively, the workpieces can be
heated in an inert or a reducing gas atmosphere, possibly of
nitrogen (N.sub.2), up to temperature .phi..sub.1.
[0041] After heating them up to the temperature .phi..sub.1, the
workpieces are transported into a second treatment chamber where
they are exposed to a gas atmosphere containing a hydrocarbon
during a second phase directly following upon the first phase, the
enrichment phase B, for a first period of time .DELTA.t.sub.1,
which amounts to between 60 min and 360 min according to the
required carburizing depth. The amount of the pressure p.sub.1
prevailing during the enrichment phase is basically directed
according to the desired treatment result as well as the type of
hydrocarbon used and amounts to ca. 10 mbar in the present case.
The enrichment phase B can be plasma-activated if need be.
[0042] Subsequent to the enrichment phase B, the workpieces are
conveyed to a third treatment chamber where they are cooled from
temperature .phi..sub.1 to a second temperature .phi..sub.2 of ca.
900.degree. C. under a vacuum during a first cooling phase C
directly following upon the enrichment phase B. Alternatively the
workpieces can be cooled in a primarily nitrogen-containing and
therewith inert gas atmosphere to temperature .phi..sub.2.
[0043] At the end of cooling phase C, the workpieces are
transported to a fourth treatment chamber and borided at
temperature .phi..sub.2 and a second pressure p.sub.2 of ca. 0.1
mbar for a second period of time .DELTA.t.sub.2 in a
boron-containing gas atmosphere. During boriding, the carbon
deposited during enrichment phase B on the edge layer of the
workpieces diffuses into the interior of the workpieces so that the
boriding phase D at the same time represents a diffusion phase for
the carburizing process. The period of time .DELTA.t.sub.2 for this
boriding phase D immediately following upon cooling phase C is
between 30 min and 360 min according to the desired treatment
result. The gas atmosphere contains boron trichloride, boron
trifluoride, diborane or several of the previously named substances
as boron-dispensing mediums during boriding phase D. If need be,
the boriding phase D can be plasma-activated. The use of boron
trifluoride as a boron-dispensing medium is especially suited for
this case.
[0044] A second cooling phase E follows directly upon the boriding
phase D during which the workpieces are cooled from temperature
.phi..sub.2 to a third temperature .phi..sub.3 of ca. 800.degree.
C. under a vacuum or alternatively in an inert gas atmosphere in a
fifth treatment chamber of the heat-treating system. For the
purpose of balancing the temperature within the batch, the
workpieces are maintained at a third temperature .phi..sub.3 for
ca. 15 min to 30 min, as can be recognized in FIG. 1.
[0045] Finally, the workpieces are quenched during a quenching
phase F directly following upon the second cooling phase E from
quenching temperature .phi..sub.3 to a temperature of less than
150.degree..quadrature. C., for example room temperature. For this,
the workpieces are transported into a sixth treatment chamber and
cooled at a high pressure p.sub.3 from more than 1,013.25 mbar in a
reducing or neutral gas atmosphere. Alternatively, the workpieces
can also be quenched in a liquid quenching medium.
[0046] Various embodiments of a device are shown in FIG. 2 through
6 in which the previously described method can be implemented. The
device according to FIG. 2 is a one-chamber vacuum furnace 10 in
which all operations A to F are conducted in one and the same
treatment chamber 11. The workpieces assembled into a batch 12 are
here quenched by gas during quenching phase F.
[0047] In contrast, the device represented in FIG. 3 is a
two-chamber vacuum furnace 20, which has a first treatment chamber
21 and a second treatment chamber 22. Processing steps A to E are
conducted in the first treatment chamber 21, while the second
treatment chamber constructed as a gas quenching chamber serves for
quenching the batch 12 during the quenching phase F. A two-chamber
vacuum furnace 30 shown in FIG. 4 differs from the device in
accordance with FIG. 3 mainly in that an oil bath 34 is present in
a second treatment chamber 32, which is separated from the first
treatment chamber 31 by a door 33, in which the batch 12 is
quenched during quenching phase F. Operations A to E are conducted
analogously to the device in accordance with FIG. 3 in the first
treatment chamber 31.
[0048] A heat treating system 40 is depicted in FIG. 5 which is
provided with six treatment chambers 41 to 46 arranged parallel.
Treatment chamber 41 serves as a flushing sluice when the batch 12
enters into the heat treatment system 40 and as a high pressure
quenching chamber during quenching phase F at the end of the
treatment cycle. Treatment chamber 42 is constructed as a heating
chamber in which the batch 12 is heated during the heating phase A
to the first temperature .phi..sub.1. The batch 12 is carburized
during enrichment phase B in the treatment chamber 43. Cooling of
the batch 12 to the second temperature .phi..sub.2 takes place in
the treatment chamber 44 during the first cooling phase C. Boriding
of the batch 12 takes place during boriding phase D in the
treatment chamber 45, while the treatment chamber 46 is provided
for cooling and equalizing the batch 12 to the third temperature
.phi..sub.3 during the second cooling phase E. A three-chamber
vacuum furnace 50 can be recognized in FIG. 6. Apart from three
treatment chambers 51 through 53 arranged behind one another, the
vacuum furnce 50 contains a flushing sluice 54, through which the
batch 12 is introduced into the vacuum furnace 50. The treatment
chamber 51 serves the purpose of warming the workpieces to the
first temperature .phi.1 during the heating phase A and for
carburizing the batch 12 during the enrichment phase B. In the
treatment chamber 52, the cooling to the second temperature .phi.2
occurs during the first cooling phase C, the boriding of the batch
12 during the boriding phase D and the cooling and equalizing of
the batch 12 to the third temperature .phi.3 during the second
cooling phase E. Treatment chamber 53 is provided for a concluding
gas quenching during quenching phase F.
[0049] The workpieces treated using the method described above have
an outer iron boride layer from 10 .mu.m to 100 .mu.m thick and a
case hardening layer lying under the iron boride layer with a
Vickers hardness between 600 and 900 and a case hardening depth
between 0.2 mm and 2.0 mm. They are distinguished by a
comparatively great fatigue limit and fatigue strength with
simultaneous wear and tear resistance. The reason for this is the
combination of carburizing, boriding and hardening obtained through
operations A to F. Synergy effects thus arise through operations A
to F, which directly follow upon one another and allow for an
efficient process control. This is true because the process can be
conduced in a single cycle and in a single heat treatment system
without interruption owing to which significant economic advantages
can be obtained in comparison with the previously usually separate
carburizing, cooling, boriding and hardening processes.
* * * * *