U.S. patent application number 14/111693 was filed with the patent office on 2014-02-13 for method of heat treating a steel component.
The applicant listed for this patent is Staffan Larsson, Peter Neuman. Invention is credited to Staffan Larsson, Peter Neuman.
Application Number | 20140041762 14/111693 |
Document ID | / |
Family ID | 47009576 |
Filed Date | 2014-02-13 |
United States Patent
Application |
20140041762 |
Kind Code |
A1 |
Larsson; Staffan ; et
al. |
February 13, 2014 |
METHOD OF HEAT TREATING A STEEL COMPONENT
Abstract
Method for heat treating a steel component (28, 36) to provide
the steel component (28, 36) with a surface having improved wear
resistance. The method comprises the steps of carbonitriding the
steel component (28, 36) at a temperature of 930-970.degree. C.,
cooling the steel component (28, 36), re-heating the steel
component (28, 36) to a temperature of 780-820.degree. C. and
either quenching the steel component (28, 36) to form martensite
and tempering, or quenching the steel component (28, 36) to form
bainite and tempering.
Inventors: |
Larsson; Staffan; (Goteborg,
SE) ; Neuman; Peter; (Goteborg, SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Larsson; Staffan
Neuman; Peter |
Goteborg
Goteborg |
|
SE
SE |
|
|
Family ID: |
47009576 |
Appl. No.: |
14/111693 |
Filed: |
April 4, 2012 |
PCT Filed: |
April 4, 2012 |
PCT NO: |
PCT/SE2012/000050 |
371 Date: |
October 14, 2013 |
Current U.S.
Class: |
148/219 ;
148/318 |
Current CPC
Class: |
F16C 2300/02 20130101;
C23C 8/32 20130101; F16C 33/30 20130101; F16C 2204/66 20130101;
C23C 8/80 20130101 |
Class at
Publication: |
148/219 ;
148/318 |
International
Class: |
C23C 8/80 20060101
C23C008/80 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 13, 2011 |
SE |
1100278-9 |
Claims
1. A method for heat treating a steel component that provides a
surface having improved wear resistance, the method comprising the
steps of: a) carbonitriding the steel component at a temperature of
930-970.degree. C., b) cooling the steel component, c) re-heating
the steel component to a temperature of 780-820.degree. C., and
including one of the following steps; d) quenching the steel
component to form martensite, and tempering, and e) quenching the
steel component to form bainite.
2. The method according to claim 1, wherein step a) further
comprises carbonitriding the steel component at a temperature of
930-970.degree. C. for 5-10 hours.
3. The method according to claim 1, wherein the steel component
(28, 36) comprises steel with a carbon content of 0.6 to 1.20
weight %.
4. The method according to claim 1, wherein step e) further
comprises quenching the steel component to form bainite, and
tempering.
5. The method according to claim 1, wherein the steel component
provides at least one of a rolling element and roller, and a steel
component for an application in which is subjected to alternating
Hertzian stresses.
6. The method according to claim 1, wherein as a result of the
method, the steel component provided with a carbonitrided layer
having a thickness (d) of 0.3-1.5 mm wherein all of the carbides in
the carbonitrided layer have a maximum longitudinal dimension of
0.2-0.3 m.
7. The method according to any of the preceding claims, claim 1,
wherein as a result of the method, the steel component is provided
with a carbonitrided layer having a ratio (d:D) of depth (d) of the
carbonitrided layer measured from the surface of the steel
component to maximum transverse dimension (D) of said steel
component of 1:4000 to 1:17,000 or more.
8. A steel component comprising; a carbonitrided layer having a
depth of the carbonitrided layer measured from the surface of the
steel component of 0.3-1.2 mm, wherein all of the carbides in the
carbonitrided layer have a maximum longitudinal dimension of
0.2-0.3 m.
9. The steel component according to claim 8, further comprising
steel with a carbon content of 0.6 to 1.2 weight %.
10. The steel component according to claim 9, further comprising at
least one of a rolling element and roller, and a steel component
for an application subjected to alternating Hertzian stresses.
11. The steel component according to claim 8, further comprising a
carbonitrided layer having a ratio (d:D) of depth (d) of the
carbonitrided layer of the carbonitrided layer measured from the
surface of the steel component to maximum transverse dimension (D)
of said steel component of 1:4000 to 1:17,000 or more.
12. A steel component for use under contaminated and/or poor
lubricant conditions comprising: a carbonitrided layer having a
depth of the carbonitrided layer measured from the surface of the
steel component of 0.3-1.2 mm, wherein all of the carbides in the
carbonitrided layer have a maximum longitudinal dimension of
0.2-0.3 m.
Description
TECHNICAL FIELD
[0001] The present invention concerns a method for heat treating a
steel component to provide the steel component with a surface
having improved wear resistance. The invention also concerns a
steel component having a carbonitrided layer and the use of such a
steel component under contaminated and/or poor lubricant
conditions.
BACKGROUND OF THE INVENTION
[0002] Carbonitriding is a metallurgical surface modification
technique that is used to increase the surface hardness of a metal
component, thereby reducing the wear of the component during use.
During the carbonitriding process, atoms of carbon and nitrogen
diffuse interstitially into the metal, creating barriers to slip
and increasing the hardness near the surface, typically in a layer
that is 0.1 to 0.3 mm thick. Carbonitriding is usually carried out
a temperature of 850-860.degree. C.
[0003] Carbonitriding is normally used to improve the wear
resistance of steel components comprising low or medium carbon
steel, and not high carbon steel. Although steel components
comprising high carbon steel are stronger, they have been found to
be more susceptible to cracking in certain applications. Components
may for example be used in typically dirty environments where
lubricating oil is easily contaminated, such as in a gear box, and
it is well known that the service life of components can decrease
considerably under such conditions. Particles in the lubricant can
namely get in between the various moving parts of a gear box, for
example, and make indentations in their contact surfaces. Stress is
concentrated around the edges of these indentations and the contact
stress concentrations may eventually lead to fatigue cracking.
Using components damaged in this way may also result in an increase
in the noise generated by the components.
SUMMARY OF THE INVENTION
[0004] An object of the invention is to provide a method for heat
treating a steel component to provide the steel component with a
surface having improved wear resistance.
[0005] This object is achieved by a method that comprises the steps
of: a) carbonitriding the steel component at a temperature of
930-970.degree. C., i.e. a temperature higher than the usual
carbonitriding temperature, in order to dissolve all carbides, b)
cooling the steel component to a temperature below the A.sub.1
transformation temperature, c) re-heating (re-hardening) the steel
component to a temperature of 780-820.degree. C. i.e. a temperature
higher than the A.sub.1 transformation temperature, lower than the
carbonitriding temperature and lower than the temperatures used in
the prior art, and either d) quenching the steel component, in oil
for example, to form martensite and tempering, or d) quenching the
steel component, in a quenching medium bath, such as a salt bath,
polymer solution or oil, to form bainite.
[0006] The surface of steel components subjected to a method
according to the present invention will have a Rockwell hardness
HRC of at least 60 and comprise a considerable quantity of fine
carbides, i.e. carbides having a maximum longitudinal dimension of
0.2-0.3 .mu.m. Changing the microstructure of the surface of the
steel component in this way improves its wear resistance and
enhances its ability to relax stress concentration at the edges of
any indentations in its surface.
[0007] By carrying out the carbonitiriding step at a temperature in
the given temperature range, and the subsequent (re-hardening) heat
treatment, the steel component may be provided with a carbonitrided
layer having a depth measured from the surface of the steel
component of 0.3-1.5 mm, whereby the carbonitrided layer contains
only carbides having a maximum longitudinal dimension of 0.2-0.3
.mu.m and no carbides having a longer maximum longitudinal
dimension.
[0008] According to an embodiment of the invention the method
comprises the step of low temperature tempering the steel component
at a temperature in the range 150-260.degree. C.
[0009] Tempering is carried out to toughen the steel component by
transforming brittle martensite or bainite into a combination of
ferrite and cementite. The brittle material becomes tough and
ductile after it has been tempered.
[0010] According to an embodiment of the invention the method
comprises the step of tempering the steel component at a
temperature of approximately 290 degrees Celsius for 4 hours. This
will lead to an increased hardness of the steel component and also
the steel component can be used, such as a bearing, under higher
operating temperatures with maintained high hardness of the
component. In this embodiment, the operating temperature is up to
250 degrees Celcius. This can be done for both a bainite and
martensite structure. In an embodiment of the steel component, when
the steel component has been subjected to bainite quenching and
followed by tempering at 290 degrees Celcius for 4 hours, the
hardness of the component will be approximately 61.5 HRC. In an
embodiment of the steel component, when the steel component has
been subjected to martensite quenching and followed by tempering at
290 degrees Celcius for 4 hours, the hardness of the component will
be approximately 57.6 HRC.
[0011] According to another embodiment of the invention the method
comprises the step of tempering the steel component at a
temperature of approximately 340 degrees celcius for 4 hours. This
will lead to an increased hardness of the steel component and also
the steel component can be used, such as a bearing, under higher
operating temperatures with maintained high hardness of the
component. In this embodiment, the operating temperature is up to
300 degrees Celcius. This can be done for both a bainite and
martensite structure. In an embodiment of the steel component, when
the steel component has been subjected to bainite quenching and
followed by tempering at 340 degrees Celcius for 4 hours, the
hardness of the component will be approximately 59.5 HRC. In an
embodiment of the steel component, when the steel component has
been subjected to martensite quenching and followed by tempering at
340 degrees Celcius for 4 hours, the hardness of the component will
be approximately 55.5 HRC.
[0012] According to another embodiment of the invention the method
comprises the step of tempering the steel component at a
temperature of approximately 390 degrees celcius for 4 hours. This
will lead to an increased hardness of the steel component and also
the steel component can be used, such as a bearing, under higher
operating temperatures with maintained high hardness of the
component. In this embodiment, the operating temperature is up to
350 degrees Celcius. This can be done for both a bainite and
martensite structure. In an embodiment of the steel component, when
the steel component has been subjected to bainite quenching and
followed by tempering at 390 degrees Celcius for 4 hours, the
hardness of the component will be approximately 58 HRC. In an
embodiment of the steel component, when the steel component has
been subjected to martensite quenching and followed by tempering at
390 degrees Celcius for 4 hours, the hardness of the component will
be approximately 52.5 HRC.
[0013] According to another embodiment of the invention step a)
comprises carbonitriding the steel component at a temperature of
930-970.degree. C. for 5-10 hours. In another embodiment of the
invention, step a) comprises carbonitriding the steel component at
a temperature of 930-970.degree. C. for at least 8 hours. This will
lead to that the carbonitrided layer will go deep into the surface
of the steel component, approximately 1-1.5 mm. This is
advantegeous especially for large steel components, such as large
rolling bearings.
[0014] According to an embodiment of the invention the steel
component comprises or constitutes a rolling element or roller, or
a steel component for an application in which is subjected to
alternating Hertzian stresses, such as rolling contact or combined
rolling and sliding, such as a slewing bearing or a raceway for a
bearing. The component may include or constitute gear teeth, a cam,
shaft, bearing, fastener, pin, automotive clutch plate, tool, or a
die. The steel component may for example constitute at least part
of a roller bearing, a needle bearing, a tapered roller bearing, a
spherical roller bearing, a toroidal roller bearing or a thrust
bearing. The component may be used in automotive, wind, marine,
metal producing or other machine applications which require high
wear resistance and/or increased fatigue and tensile strength.
[0015] According to a further embodiment of the invention the steel
component comprises steel with a carbon content of 0.6 to 1.20
weight %, such as a high carbon bearing steel such as SAE
52100/Gd3. Compared with the prior art, the hardness of both the
carbonitrided layer and the core of a high carbon steel component
is greater than is the case with known components comprising steel
having a low carbon content. The wear resistance and fatigue
strength for rolling contact are improved as a result. Furthermore,
the loading capacity of a component, such as a bearing, will be
increased, whereby the bearing may be of smaller construction for a
particular application. The fatigue resistance on rolling contact
also increases, so that the service life of the bearing can be
extended. Additionally, the disadvantage that through cracking
occurs, described in the prior art, is not found.
[0016] According to a further embodiment of the invention, the
steel component comprises steel with a carbon content of 0.6 to
1.20 weight %, such as a high carbon bearing steel such as SAE
52100 (high carbon chromium steel), wherein the steel before the
heat treating process is spherodized annealed with approximately
15% carbides, which all will be dissolved when carburizing the
steel at 930-970.degree. C. In another embodiment the steel
component comprises steel with a carbon content of 0.7-1.20 weight
%. In another embodiment of the invention, step c) comprises
re-heating (re-hardening) the steel component to a temperature of
780-820.degree. C. which will result in 3-5% residual carbides in
the core of the steel component. It has been found that by using a
high carbon steel of 0.6-1.20 weight % carbon, the reheating step
will create residual carbides in the core. This will increase the
hardness and strength of the core, and the risk that cracking
occurs is reduced significantly.
[0017] According to a further embodiment of the invention, as a
result of said method, the steel component is provided with a
carbonitrided layer having a ratio (d:D) of depth (d) of the
carbonitrided layer measured from the surface of the steel
component to maximum transverse dimension (D) of said steel
component of 1:4000 to 1:17,000 or more. The method according to
the present invention may be used to provide a component of any
size with a carbonitrided layer. The method is however particularly
suitable for providing a large component, having a maximum
transverse dimension of a few metres for example, with a
carbonitrided layer since the higher carbonitriding temperature
provides a carbonitrided layer with greater depth, whereby part of
the carbonitrided layer may be ground away during the manufacture
of the component without substantially affecting the wear
resistance of the component.
[0018] The present invention also concerns a steel component that
comprises a carbonitrided layer having a depth of the carbonitrided
layer measured from the surface of the steel component of 0.3-1.5
mm whereby the carbonitrided layer contains only carbides having a
maximum longitudinal dimension of 0.2-0.3 .mu.m.
[0019] According to an embodiment of the invention the steel
component comprises steel with a carbon content of 0.6 to 1.2
weight %, such as a high carbon bearing steel such as SAE
52100/Gd3.
[0020] According to an embodiment of the invention the steel
component comprises or constitutes a rolling element or roller, or
a steel component for an application in which is subjected to
alternating Hertzian stresses.
[0021] According to another embodiment of the invention the steel
component comprises a carbonitrided layer having a ratio (d:D) of
depth (d) of the carbonitrided layer of the carbonitrided layer
measured from the surface of the steel component to maximum
transverse dimension (D) of said steel component of 1:4000 to
1:17,000 or more.
[0022] The present invention further concerns the use of a steel
component according to any of the embodiments of the invention
under contaminated and/or poor lubricant conditions.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] The present invention will hereinafter be further explained
by means of non-limiting examples with reference to the appended
figures where;
[0024] FIG. 1 shows a heat treatment cycle according to the prior
art,
[0025] FIG. 2 shows a method according to an embodiment of the
invention,
[0026] FIG. 3 shows the carbonitriding layer depth of a component
according to an embodiment of the invention,
[0027] FIG. 4 shows micrographs of carbonitriding layers of
components according to an embodiment of the invention and
schematic representations thereof, and
[0028] FIGS. 5 & 6 show components according to embodiments of
the invention.
[0029] It should be noted that the drawings have not been drawn to
scale and that the dimensions of certain features have been
exaggerated for the sake of clarity.
DETAILED DESCRIPTION OF EMBODIMENTS
[0030] FIG. 1 shows a heat treatment cycle according to the prior
art. A steel component is subjected to a carbonitriding (CN)
process at a temperature of 850.degree. C. The process environment
is provided by the introduction of methane/propane/natural gas (for
carbon) and ammonia (for nitrogen) into a furnace in the presence
of a controlled carrier gas. By maintaining the proper ratios of
the working gases, the component is provided with a thin
carbonitrided layer of carbon- and nitrogen-rich steel. The
component is then re-heated to 820.degree. C. and subsequently
quenched to achieve the full case hardness. Quenching may be
carried out in an oil or salt bath with bath temperatures selected
to achieve the optimum properties with acceptable levels of
dimensional change. Hot oil/salt bath quenching can be used to
minimize distortion of intricate parts. Low temperature tempering
may be carried out to toughen the steel component.
[0031] FIG. 2 shows a method according to the present invention.
The method comprises the steps of a) carbonitriding a steel
component at a temperature of 930-970.degree. C. for 5-10
hours.
[0032] According to an embodiment of the invention the method
includes supplying a higher concentration of ammonia at the
beginning of the carbonitriding step a) to boost the carbonitriding
process. For example, 9.5% ammonia may be used initially; this may
be lowered to 6.5% ammonia and then 0%. 9.5% ammonia may be used
for about 70% of the carbonitriding step a).
[0033] The method then comprises the steps b) cooling the steel
component to a temperature below the A.sub.1 transformation
temperature, c) re-heating the steel component to a temperature of
780-820.degree. C. i.e. a temperature higher than the A.sub.1
transformation temperature, lower than the carbonitriding
temperature and lower than the re-heating temperatures used in the
prior art, and d) quenching the steel component to form martensite,
and low temperature tempering the steel component at a temperature
in the range 150-260.degree. C. After tempering, the component is
cooled to room temperature and may then be used in any application
in which it is likely to be subjected to stress, strain, impact
and/or wear under a normal operational cycle, such as in under
contaminated and/or poor lubricant conditions.
[0034] Alternatively, after step c), in which the steel component
is re-heated to a temperature of 780-820.degree. C., the steel
component may be quenched, in a salt bath for example, to form
bainite. This will induce compressive residual stresses in the
subsurface of the steel component. In an embodiment, the
compressive residual stress in the subsurface of the steel
component is 250-300 MPa. A standard bainitic steel may have a
compressive residual stress of approximately 50-75 MPa. Compressive
residual stress is good for fatigue life, e.g. the fatigue life of
a bearing component which is subjected to alternating Hertzian
stresses. Alternatively, the steel component may subsequently be
followed by a tempering step.
[0035] Components subjected to a method according to an embodiment
of the present invention may be used with or without subsequent
grinding operations.
[0036] Steel components comprising steel with a carbon content of
0.6 to 1.20 weight % may be subjected to a method according to the
present invention.
[0037] FIG. 3 shows the carbonitriding depth in pm from the surface
(x-axis) of a component according to an embodiment of the invention
against hardness (y-axis) of the carbonitriding layer. FIG. 3 shows
measured hardness values for the carbonitriding layers of
components comprising SAE 52100/Gd3 steel which have been subjected
to a method according to the present invention.
[0038] FIG. 3 namely shows the hardness profile of: [0039] a
component subjected to carbonitriding at 930.degree. C. for 8
hours, and re-hardened martensitically and tempered at 160 .degree.
C. for 1.5 hours and 350.degree. C. for 5 hours (profile 10),
[0040] a component subjected to carbonitriding at 970.degree. C.
for 6 hours and re-hardened martensitically and tempered at
160.degree. C. for 1.5 hours and 350.degree. C. for 5 hours
(profile 12), [0041] a component subjected to carbonitriding at
930.degree. C. for 8 hours and re-hardened bainitically at 215
.degree. C. for 4 hours and 240.degree. C. for 4 hours and tempered
at 350.degree. C. for 5 hours (profile 14), and [0042] a component
subjected to carbonitriding at 970.degree. C. for 6 hours and
re-hardened bainitically at 215.degree. C. for 4 hours and
240.degree. C. for 4 hours and tempered at 350.degree. C. for 5
hours (profile 16).
[0043] FIG. 3 also shows the nitriding depth 18 achieved in bainite
and the nitriding depth 20 achieved in martensite. The profiles
10-16 show that the depth of a carbonitriding layer may be tailored
to a specific application by the selection of a suitable
carbonitriding and re-hardening temperatures and times.
[0044] The method according to the present invention may be used to
provide a steel component with a carbonitrided layer having a
thickness of 0.3-1.2 mm whereby all of the carbides in the
carbonitrided layer have a maximum longitudinal dimension of
0.2-0.3 .mu.m.
[0045] FIG. 4 shows a micrograph 21 of a carbonitriding layer of a
component according to the present invention which has been
subjected to carbonitriding at 930.degree. C., cooled to 70.degree.
C. in an oil bath, tempered at 320.degree. C., re-heated to
820.degree. C., quenched in a 215.degree. C. salt bath for four
hours followed directly by transferring to a 240.degree. C. salt
bath for four hours and cooling to room temperature and finally a
350.degree. C. tempering. The white structures in the micrograph 21
and the black structures in the schematic representation are
carbides 24.
[0046] FIG. 4 also shows a micrograph 22 of a carbonitriding layer
of a component according to another embodiment of the present
invention and a schematic representation thereof. The component has
been subjected to carbonitriding at 970.degree. C., cooled to
70.degree. C. in an oil bath, tempered at 320.degree. C., re-heated
to 820.degree. C., quenched in a 215.degree. C. salt bath for four
hours followed by directly transferring to a 240.degree. C. salt
bath for four hours and cooling to room temperature and finally a
350.degree. C. tempering. The white structures in the micrograph 22
and the black structures in the structures in the schematic
representation 22 are carbides 26.
[0047] As can be seen, the carbides 24 in the micrograph 21 are
more coarse than the fine carbides 26 in micrograph 22. The coarse
carbides 24 in micrograph 21 are more remotely spaced than the fine
carbides 26 in micrograph 22, and there are fewer of them per unit
area of the carbonitrided layer. The carbonitrided layer of
components according to the present invention, which contains more
carbides, finer carbides and more closely spaced carbides 24, 26
than the carbonitrided layers of prior art components, has been
found to have superior wear resistance compared to the
carbonitriding layer of prior art components which have fewer,
larger and more remotely spaced carbides. The distribution of
carbides 24 and 26 in a carbonitrided layer of a component may be
tailored to a specific application by selection of suitable
carbonitriding temperature and time.
[0048] FIG. 5 shows an example of a component according to an
embodiment of the invention, namely a rolling element bearing 28
that may range in size from 10 mm diameter to a few metres diameter
and have a load-carrying capacity from a few tens of grams to many
thousands of tonnes. The bearing 28 according to the present
invention may namely be of any size and have any load-carrying
capacity. The bearing 28 has an inner ring 30 and an outer ring 32
and a set of rolling elements 34. The inner ring 30, the outer ring
32 and/or the rolling elements 34 of the rolling element bearing
28, and preferably at least part of the surface of all of the
rolling contact parts of the rolling element bearing 28 may be
subjected to a method according to the present invention.
[0049] FIG. 6 shows a component 36, namely a shaft shown in cross
section, according to an embodiment of the invention. The component
36 has been provided with a carbonitrided layer 38 on its outer
surface using a method according to an embodiment of the invention.
The depth of the carbonitrided layer 38 measured from the surface
of the component 36 is d and the maximum transverse dimension of
the component 36 (the diameter of the shaft in this case) is D. The
ratio (d:D) of the thickness d of the carbonitrided layer 38 to the
maximum transverse dimension D of the component 36 is 1:4000-17,000
or more.
EXAMPLES
[0050] Steel components were subjected to a method according to the
present invention comprising the steps of: carbonitriding the steel
components at a temperature of 930.degree. C. or 970.degree. C.
(step a)), cooling the steel components to 70.degree. C. using an
oil bath (step b)), tempering the components at 320.degree. C.,
re-heating (bainite re-hardening) the steel components to
820.degree. C. (step c)), quenching the steel components in a
215.degree. C. salt bath (step e)) for four hours followed directly
by quenching in a 240.degree. C. salt bath for four hours and air
cooling to room temperature. The measured hardness for the steel
components subjected to this method was 61.5 to 62.0 HRC. [0051]
Steel components were also subjected to a method according to the
present invention comprising the steps of: carbonitriding the steel
components at a temperature of 930.degree. C. or 970.degree. C.
(step a)), cooling the steel components to 70.degree. C. using an
oil bath (step b)), tempering the components at 320.degree. C.,
re-heating (martensite re-hardening) the steel components to
820.degree. C. (step c)), quenching the steel components in a
70.degree. C. oil bath (step d)), air cooling to room temperature,
post-quenching in a 5.degree. C. water bath, tempering at
160.degree. C. for 90 minutes and air cooling to room temperature.
The measured hardness for the steel components subject to this
method was 64.5 to 65.5 HRC. [0052] SAE 52100/Gd 2 steel components
were subjected to carbonitriding at 970.degree. C. for 3 hours;
carbon potential (Cp) 1.4, NH.sub.3 9.5% and CO 20% (both the
carbon potential and the nitrogen potential (Np) was boosted during
the carbonitriding process), SAE 52100/Gd 3 steel components were
subjected to carbonitriding at 970.degree. C. for 13 hours: Cp 1.2,
NH.sub.3 9.5% and CO 20% (the nitrogen potential (Np) was boosted
during the carbonitriding process), SAE 52100/Gd 6 steel components
were subjected to carbonitriding at 970.degree. C. for 1.5 hours,
Cp 1.2, NH.sub.3 6.5% and CO 20%, SAE 52100/Gd 70 steel components
were subjected to carbonitriding at 970.degree. C. for 4.5 hours,
Cp 1.2, NH.sub.3 3.0% and CO 20%. The components were then quenched
in a 320.degree. C. salt bath and re-heated (bainite re-hardening)
at 805.degree. C. for 1 hour, quenched in a 215.degree. C. salt
bath for 18.5 hours followed directly by quenching in a 240.degree.
C. salt bath for six hours and cooled to room temperature. The
measured hardness for the SAE 52100/Gd 2 steel components subjected
to this method was 60.0 HRC. The measured hardness for the SAE
52100/Gd 6 steel components subjected to this method was 61.7 HRC.
The depth of the carbonitrided layer was 2.5-3 mm.
[0053] Further modifications of the invention within the scope of
the claims would be apparent to a skilled person.
* * * * *