U.S. patent application number 15/398155 was filed with the patent office on 2017-10-05 for carburizing austempering process.
The applicant listed for this patent is GM Global Technology Operations LLC. Invention is credited to Huaxin Li, Shekhar G. Wakade.
Application Number | 20170283899 15/398155 |
Document ID | / |
Family ID | 59960703 |
Filed Date | 2017-10-05 |
United States Patent
Application |
20170283899 |
Kind Code |
A1 |
Li; Huaxin ; et al. |
October 5, 2017 |
CARBURIZING AUSTEMPERING PROCESS
Abstract
A novel combination of heat treatment steps includes the steps
of carburizing a component fabricated of a medium carbon alloy
steel at an elevated temperature for between three and six hours,
subjecting the component to an austempering bath and holding it
there for between fifteen and two hundred forty minutes and finally
cooling the component to room temperature to allow martensitic
transformation. These steps may be followed with cryogenic
treatment to reduce retained austenite if needed. The process
produces components with low distortion, high surface hardness,
from HRC 56 to 62, and high surface compressive residual
stress.
Inventors: |
Li; Huaxin; (Rochester
Hills, MI) ; Wakade; Shekhar G.; (Grand Blanc,
MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GM Global Technology Operations LLC |
Detroit |
MI |
US |
|
|
Family ID: |
59960703 |
Appl. No.: |
15/398155 |
Filed: |
January 4, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62314521 |
Mar 29, 2016 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 6/00 20130101; C21D
9/30 20130101; C23C 8/32 20130101; C23C 8/22 20130101; C23C 8/80
20130101; C21D 9/32 20130101; C21D 1/20 20130101; C21D 1/613
20130101 |
International
Class: |
C21D 9/32 20060101
C21D009/32; C23C 8/32 20060101 C23C008/32; C21D 6/00 20060101
C21D006/00; C21D 9/30 20060101 C21D009/30; C21D 1/20 20060101
C21D001/20; C21D 1/613 20060101 C21D001/613; C23C 8/22 20060101
C23C008/22; C23C 8/80 20060101 C23C008/80 |
Claims
1. A heat treatment process for a metal component, comprising the
steps of: carburizing the metal component at an elevated
temperature for between three and six hours, subjecting the
component to an austempering bath for between fifteen minutes and
four hours, and cooling the component to room temperature thereby
allowing martensitic transformation.
2. The heat treatment process of claim 1, wherein the carburizing
step is achieved by one of gas carburizing, low pressure
carburizing process and carbonitriding.
3. The heat treatment process of claim 1, wherein the austempering
step is achieved in an oil or salt bath at a temperature of between
250.degree. C. and 450.degree. C.
4. The heat treatment process of claim 1 wherein the cooling step
is achieved by exposing the component to atmospheric air.
5. The heat treatment process of claim 1, further including the
step of subjecting the component to a cryogenic bath to cool the
component to a temperature of the cryogenic bath.
6. The heat treatment process of claim 5 wherein the cryogenic bath
is liquid nitrogen.
7. The heat treatment process of claim 1 wherein the metal
component is fabricated of SAE 4340 steel.
8. A heat treatment process for a medium carbon alloy steel
component, comprising the steps of: carburizing the medium carbon
alloy steel component at an elevated temperature for between three
and six hours, subjecting the component to an austempering bath for
between fifteen minutes and four hours, cooling the component to
room temperature thereby allowing martensitic transformation, and
subjecting the component to a cryogenic bath to cool the component
to a temperature of the cryogenic bath.
9. The heat treatment process of claim 8, wherein the carburizing
step is achieved by one of gas carburizing, low pressure
carburizing process and carbonitriding.
10. The heat treatment process of claim 8, wherein the austempering
step is achieved in one of an oil bath and salt bath at a
temperature of between 250.degree. C. and 450.degree. C.
11. The heat treatment process of claim 8 wherein the cooling step
is achieved by exposing the component to atmospheric air.
12. The heat treatment process of claim 8 wherein the medium carbon
alloy steel component is fabricated of SAE 4340 steel.
13. A heat treatment process for a medium carbon alloy steel
component, comprising the steps of: carburizing a medium carbon
alloy steel component at an elevated temperature for between three
and six hours to produce a layer near a surface of the component
having between 0.7% and 1.2% carbon, subjecting the component to an
austempering bath for between fifteen minutes and four hours to
produce bainite and retained austenite on and near the surface of
the component, and cooling the component to room temperature
thereby allowing martensitic transformation.
14. The heat treatment process of claim 13, wherein the carburizing
step is achieved by one of gas carburizing, low pressure
carburizing process and carbonitriding at a temperature of between
850.degree. C. and 1200.degree. C.
15. The heat treatment process of claim 13, wherein the
austempering step is achieved in an oil or salt bath at a
temperature of between about 250.degree. C. and 450.degree. C.
16. The heat treatment process of claim 13 wherein the cooling step
is achieved by exposing the component to atmospheric air.
17. The heat treatment process of claim 13 wherein the medium
carbon alloy steel component is fabricated of SAE 4340 steel.
18. The heat treatment process of claim 13, further including the
step of subjecting the component to a cryogenic bath to cool the
component to the temperature of the cryogenic bath.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This patent application claims the benefit of U.S.
Provisional Patent Application Ser. No. 62/314,521, filed Mar. 29,
2016, which is hereby incorporated in its entirety herein by
reference.
FIELD
[0002] The present disclosure relates to a novel heat treatment
process for medium carbon alloy steels and more particularly to a
novel heat treatment process for medium carbon alloy steels
comprehending carburizing, austempering and cooling steps.
BACKGROUND
[0003] The statements in this section merely provide background
information related to the present disclosure and may or may not
constitute prior art.
[0004] Post fabrication treatment of fabricated metal parts such as
gears, shafts, sprockets, bearings and similar components is
commonplace. The usual reason for such treatment is a desire or
need to increase the strength and durability of the part and most
processes involve heating the part, often in a controlled
atmosphere, followed by controlled cooling. There are many suitable
steel alloys and numerous heat treating processes which may be
combined to achieve improved hardness and durability of metal
parts.
[0005] Unfortunately, there are as well many complications that
arise both from alloy choice and heat treatment selection. For
example, certain heat treatment processes are limited to certain
alloys, that is, medium carbon steels may only be heat treated by
certain processes and these limited processes may not achieve a
desired final condition such as hardness. As another example, the
distortion of a component during heat treating generally
corresponds to the carbon content of the alloy. This consideration
encourages the use of lower carbon steels which, however, may not
be capable of achieving a final desired hardness. A further
consideration relates to quenching speed. Rapid quenching, while
desirable from hardness and microstructure standpoints, may result
in both distortion of the component and the production of residual
internal stress. Finally, if a portion of the heat treatment
process achieves an austenitic to martensitic transformation, it
may result in a change in size of the component.
[0006] Obviously, the foregoing consequences of heat treatment are
undesirable and present an engineering challenge of both maximizing
the desirable metal and process characteristics while minimizing
those undesirable characteristics for a certain component and
application.
SUMMARY
[0007] The present invention comprehends a novel combination of
heat treatment steps including the steps of carburizing a component
fabricated of a medium carbon alloy steel at an elevated
temperature for between three and six hours, subjecting the
component to an austempering bath and holding it there for between
fifteen and two hundred forty minutes (four hours) and finally
cooling the component to room temperature to allow martensitic
transformation. These steps may be followed with a cryogenic
treatment to reduce retained austenite if needed. The process
produces components with low distortion, high surface hardness,
from HRC 56 to 62, and high surface compressive residual
stress.
[0008] Thus it is an aspect of the present invention to provide a
method of heat treating metal components.
[0009] It is a further aspect of the present to provide a method of
heat treating metal components fabricated of a medium carbon alloy
steel.
[0010] It is a still further aspect of the present invention to
provide a method of heat treating metal components including the
step of carburizing the component.
[0011] It is a still further aspect of the present invention to
provide a method of heat treating metal components fabricated of a
medium carbon alloy steel including the step of carburizing the
component.
[0012] It is a still further aspect of the present invention to
provide a method of heat treating metal components including the
step of subjecting the component to an austempering bath.
[0013] It is a still further aspect of the present invention to
provide a method of heat treating metal components fabricated of a
medium carbon alloy steel including the step of subjecting the
component to an austempering bath.
[0014] It is a still further aspect of the present invention to
provide a method of heat treating metal components including the
step of cooling the component to room temperature to allow
martensitic transformation.
[0015] It is a still further aspect of the present invention to
provide a method of heat treating metal components fabricated of a
medium carbon alloy steel including the step of cooling the
component to room temperature to allow martensitic
transformation.
[0016] It is a still further aspect of the present invention to
provide a method of heat treating metal components including the
optional step of cryogenically treating the component.
[0017] It is a still further aspect of the present invention to
provide a method of heat treating metal components fabricated of a
medium carbon alloy steel including the optional step of
cryogenically treating the component.
[0018] Further aspects, advantages and areas of applicability will
become apparent from the description provided herein. It should be
understood that the description and specific examples are intended
for purposes of illustration only and are not intended to limit the
scope of the present disclosure.
DRAWINGS
[0019] The drawings described herein are for illustration purposes
only and are not intended to limit the scope of the present
disclosure in any way.
[0020] FIG. 1 illustrates a plurality of exemplary mechanical
components utilized in motor vehicle powertrain devices that may be
treated by the carburizing and austempering process described
herein; and
[0021] FIG. 2 is a flowchart setting forth in sequence the steps of
the carburizing and austempering process according to the present
invention.
DETAILED DESCRIPTION
[0022] The following description is merely exemplary in nature and
is not intended to limit the present disclosure, application, or
uses.
[0023] Referring first to FIG. 1, several exemplary mechanical
components typically utilized in motor vehicle powertrain devices
that benefit from the carburizing and austempering process
described herein are illustrated. For example, there is illustrated
a sliding camshaft 10, a spur gear 12, a helical gear 14, a helical
face gear 16, planetary gear carrier 18, a splined shaft 20, a
chain sprocket 22 and a blocker ring 24 of a transmission
synchronizer assembly which all may undergo the process described
below and which will thus exhibit high surface hardness, low post
fabrication distortion and surface compressive residual stress. It
will be appreciated that this delineation of treated components is
not, and is not intended to be, inclusive but merely exemplary as
various and numerous other metal components can and will benefit
from the carburizing and austempering process described herein.
[0024] With reference now to FIG. 2, a flowchart illustrating the
plural steps of the present inventive method is generally
designated by the reference number 30. At the outset, it should be
appreciated that a particular steel alloy composition has been
found to provide desirable final strength and hardness
characteristics in components subjected to the carburizing and
austempering process described herein. Such alloy composition has a
carbon content of between about 0.20% to 0.60%, chromium content of
between about 0.50% to 2.0%, manganese content of between 0.0% to
about 3.5%, nickel content of between 0.0% to about 0.8%, vanadium
and/or niobium content of between 0.0% to about 0.2%, molybdenum
content of between 0.0% to about 0.1%, silicon, a maximum of about
0.1%, sulphur, a maximum of about 0.1% and phosphorus, a maximum of
about 0.05%. A commercially available, standard steel alloy
essentially similar to this composition is SAE 4340.
[0025] After the fabrication and final finishing, such as, for
example, machining or grinding, of a component in a step 32, the
component is subjected to a carburizing or carbonitriding process
in an oven at a step 34. Preferably, the component is exposed to a
carburizing temperature of between about 850.degree. C. and
1200.degree. C. for between about three and six hours. The purpose
of this carburizing step 34 is to produce an exterior shell or
layer of the component that is rich in carbon. The carbon potential
near the surface of the component is preferably between about 0.7%
and 1.2%. The carburizing step 34 also depresses the martensitic
start temperature so that the following austempering step 36 can be
carried out at the traditional temperature range. The carburizing
step 34 may be accomplished by either a gas carburizing process, a
low pressure carburizing process or a carbonitriding process.
[0026] Next, in an austempering step 36, the component is cooled in
an oil or salt bath at a temperature of between about 250.degree.
C. and 450.degree. C. for between about fifteen minutes and two
hundred forty minutes (four hours). As noted above, because the
increased carbon content on and near the surface of the component
achieved in the step 32 depresses the martensitic start
temperature, the austempering step 34 produces bainite and retained
austenite on and near the surface of the component. The
austempering step 36 thus has two purposes and objectives: the
first is to reduce distortion of the component and the second is to
provide a final microstructure of bainite, martensite and retained
austenitie on the surface of the component, with the martensitic
interior as described above.
[0027] In a final necessary cooling step 38, the components are
cooled to room (ambient) temperature by exposure to atmospheric air
or other gasses or liquids. The retained austenite is partially
transformed to martensite when cooling from the austempering
temperature to room temperature in the cooling step 38.
[0028] In a final, optional step, that may or may not be necessary
depending upon the final desired hardness and other aspects of the
component such as dimensional requirements, a cryogenic treatment
step 40 may be undertaken. In this step 40, the component is
subjected to a cryogenic bath of, for example, liquid nitrogen at
-185.degree. C., for a time sufficient to fully cool the component.
The cryogenic treatment step 40 enhances surface hardness and
transforms any untransformed, retained austenite into martensite.
The process 30 concludes at an end or stop step 42.
[0029] The forgoing process steps when utilized on a metal
component having an alloy composition substantially as stated above
provide a surface hardness of HRC 58 minimum and a core hardness of
HRC 45 minimum at the thickest wall locations. Typically, the
surface hardness may be HRC 62. In addition to such surface and
core hardness, a component which has undergone the foregoing
process exhibits low post fabrication distortion and high surface
compressive residual stress.
[0030] The description of the invention is merely exemplary in
nature and variations that do not depart from the gist of the
invention are intended to be within the scope of the invention.
Such variations are not to be regarded as a departure from the
spirit and scope of the invention.
* * * * *