U.S. patent application number 12/428059 was filed with the patent office on 2009-10-29 for method of carburizing and quenching a steel member.
This patent application is currently assigned to AISIN AW CO., LTD.. Invention is credited to Kouji OHBAYASHI, Kazuaki Okada.
Application Number | 20090266449 12/428059 |
Document ID | / |
Family ID | 41213811 |
Filed Date | 2009-10-29 |
United States Patent
Application |
20090266449 |
Kind Code |
A1 |
OHBAYASHI; Kouji ; et
al. |
October 29, 2009 |
METHOD OF CARBURIZING AND QUENCHING A STEEL MEMBER
Abstract
A method of carburizing and quenching a steel member includes: a
reduced pressure carburization step in which a steel member is
contacted with carburization gas under reduced pressure, a slow
cooling step in which the steel member is then slowly cooled in a
cooling gas, and a quenching step of heating a selected portion of
the cooled steel member using high-density energy and subsequently
subjecting the selected portion to rapid cooling. The steel member
subjected to the low-pressure carburization step includes a first
portion in which a diffusion rate of carbon taken therein during
carburization is high because of its shape and a second portion in
which the diffusion rate of carbon is lower than that of the first
portion. The reduced-pressure carburization step is controlled to
give a carbon concentration at the surface of the first portion in
a range of 0.65.+-.0.1 weight % after diffusion.
Inventors: |
OHBAYASHI; Kouji; (Anjo-shi,
JP) ; Okada; Kazuaki; (Anjo-shi, JP) |
Correspondence
Address: |
BACON & THOMAS, PLLC
625 SLATERS LANE, FOURTH FLOOR
ALEXANDRIA
VA
22314-1176
US
|
Assignee: |
AISIN AW CO., LTD.
Anjo-shi
JP
|
Family ID: |
41213811 |
Appl. No.: |
12/428059 |
Filed: |
April 22, 2009 |
Current U.S.
Class: |
148/222 ;
148/223 |
Current CPC
Class: |
C21D 3/04 20130101; C23C
8/80 20130101; C23C 8/22 20130101; C21D 1/18 20130101 |
Class at
Publication: |
148/222 ;
148/223 |
International
Class: |
C23C 8/22 20060101
C23C008/22; C23C 8/00 20060101 C23C008/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 25, 2008 |
JP |
2008-115024 |
Claims
1. A method of carburizing-quenching a steel member, comprising:
(a) a reduced-pressure carburization step of carburizing a steel
member in a carburization gas at a pressure below atmospheric
pressure giving a first portion of the steel member a carbon
concentration at 0.65.+-.0.1 weight % after diffusion at the
exterior surface of the first portion; (b) a slow cooling step of
subjecting the steel member, treated in step (a) to have a carbon
concentration of 0.65.+-.0.1 weight % after diffusion at the
exterior surface, to a first rate of cooling in a cooling gas; and
(c) subsequent to step (b), a quenching step of selectively heating
a selected portion of the cooled steel member using high-density
energy and subsequently subjecting the selected portion to a second
rate of cooling, more rapid than the first rate of cooling, wherein
the first portion of the steel member subjected to the
reduced-pressure carburization step (a) has an exterior surface
contoured in a manner facilitating the introduction of carbon by
diffusion during the reduced-pressure carburization step and
wherein the steel member subjected to the reduced-pressure
carburization step (a) also has a second portion with an exterior
surface shaped to have a carbon diffusion rate less than the carbon
diffusion rate of the first portion.
2. The method according to claim 1, wherein: the pressure in step
(a) is 1 hPa to 100 hPa; step (a) includes heating the steel member
to at least its austenitizing temperature while the steel member is
in contact with the carburization gas; the slow rate of cooling in
step (b) is from 0.1 degrees C./second to 3.0 degrees C./second and
at a pressure of 100 hPa to 650 hPa; the heating of the selected
portion in step (c) by impinging an electron beam on the selected
portion; the rapid cooling in step (c) is 200 degrees C./second to
2000 degrees C./second; the first portion has an external surface
contour with surface portions meeting at an angle of 130 degrees to
180 degrees.
3. The method according to claim 2 wherein the steel member is a
gear having a circumferential surface with gear teeth arranged
spaced thereon, each gear tooth having a flat top surface and side
surfaces meeting the top surface to form corners and wherein the
exterior surface of the first portion includes a portion of the top
surface between the corners and the exterior surface of the second
portion includes the corner.
4. The method according to claim 1, further comprising, subjecting
other steel members of the same material to step (a) wherein at
least one process parameter is varied between different tests to
determine a value for the at least one process parameter resulting
in a carbon concentration of 0.65.+-.0.01 weight % after diffusion
at the surface of the first portion and subsequently using the at
least one process parameter having the determined value in step
(a).
5. The method according to claim 4 wherein the at least one process
parameter is time of contact of the steel member with the
carburization gas in step (a).
6. The method according to claim 4 wherein the at least one process
parameter is selected from temperature, type of carburization gas,
pressure, processing time and combinations thereof.
7. The method according to claim 1, wherein the reduced-pressure
carburization step (a) is performed under a condition which gives a
carbon concentration at the surface of the first portion in a range
of 0.65.+-.0.05 weight % after diffusion.
8. The method according to claim 1, wherein the reduced-pressure
carburization step (a) is performed under a condition which gives a
carbon concentration at the surface of the second portion of 0.85
weight % after diffusion or lower.
9. The method according to claim 1, wherein the reduced-pressure
carburization step (a) has a carburization period during which the
steel member is in contact with a hydrocarbon carburization gas and
carbon is introduced at a surface of the steel member, and a
diffusion period during which carbon is diffused inside the steel
member no longer in contact with a carburization gas.
10. The method according to claim 1, wherein the first portion of
the steel member is any portion having a surface with an angle in
cross-section of 130 degrees to 180 degrees.
11. The method according to claim 1, wherein the steel member is a
gear having a tooth section, and the first portion is at least one
of a tooth face and a tooth bottom of the tooth section.
12. The method according to claim 1, wherein the reduced-pressure
carburization step (a) is at a temperature at least as high as the
austenitizing temperature of the steel member and at a pressure of
1 to 100 hPa.
13. The method according to claim 1, wherein the slow cooling step
is performed at a cooling rate at which the steel member does not
transform to martensite during cooling.
14. The method according to claim 1, wherein the slow cooling step
is performed at a cooling rate from 0.1 degree C./second to 3.0
degrees C./second while the temperature of the steel member is
equal to or higher than an A1 transformation point temperature.
15. The method according to claim 1, wherein the cooling gas used
in the slow cooling step (b) is selected from the group consisting
of nitrogen, helium, argon, and combinations thereof.
16. The method of manufacturing a steel member according to claim
1, wherein the slow cooling step is performed by contacting the
steel member with cooling gas having a reduced pressure lower than
atmospheric pressure.
17. The method according to claim 16, wherein the reduced pressure
of the cooling gas used in the slow cooling step is in a range from
100 hPa to 650 hPa.
18. The method according to claim 16, wherein the reduced pressure
of the cooling gas used in the slow cooling step is in a range from
100 hPa to 300 hPa.
19. The method according to claim 16, wherein the reduced pressure
of the cooling gas used in the slow cooling step is raised after
the temperature of the steel member becomes equal to or lower than
the A1 transformation point.
20. The method according to claim 1, wherein the quenching step is
performed by heating a selected portion of the steel member, using
high-density energy, to a temperature equal to or higher than the
austenitizing temperature of the steel member and subsequently
rapidly cooling the steel member at a cooling rate equal to or
higher than a critical cooling rate, for rapid cooling at which
martensitic transformation occurs in a carburization layer.
21. The method according claim 1, wherein the quenching step is
performed by induction heating by high frequency a selected portion
of the steel member to heat the selected portion and then rapidly
cooling the steel member by water quenching.
22. The method according to claim 21, wherein the high-density
energy is applied to a succession of steel members, one by one, and
subsequently cooling the heated steel members by spraying cooling
water onto each steel member while rotating the steel member.
23. The method according to claim 1, wherein the quenching step is
performed by emitting a high-density energy beam onto a selected
portion of the steel member to heat the selected portion and then
rapidly cooling the steel member by self-cooling.
24. The method according to claim 23, wherein the high-density
energy beam is an electron beam.
25. The method according to claim 13, wherein the second rate of
cooling is performed at a cooling rate at which the steel member
transforms to martensite during cooling.
26. The method according to claim 1, wherein the second rate of
cooling is performed at a cooling rate at which the steel member
transforms to martensite during cooling.
Description
INCORPORATION BY REFERENCE
[0001] The disclosure of Japanese Patent Application No.
2008-115024 filed on Apr. 25, 2008 including the specification,
drawings and abstract is incorporated herein by reference in its
entirety.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a method of carburizing and
quenching ("carburizing-quenching") a steel member, for example a
gear, wherein the steel member has a first portion in which the
diffusion rate of the carbon introduced during carburization is
high because of the shape of the member and a second portion in
which the diffusion rate of the introduced carbon is lower than in
the first portion.
DESCRIPTION OF THE RELATED ART
[0003] A steel member such as a gear is typically subjected to a
carburizing-quenching process in order to improve its surface
hardness while maintaining its toughness. The carburizing-quenching
process is a process in which a steel member is subjected to
carburization to increase the carbon concentration in a surface
region of the member while heating the steel member at a
temperature equal to or higher than its austenitizing temperature
and subsequently quenching to secure toughness of a core region of
the member and the surface hardness.
[0004] One gas carburizing method is a continuous gas carburizing
method conducted in a large thermal processing furnace with an oil
quenching bath at its outlet. The carburization is conducted over a
long period, immediately followed by oil quenching. In gas
carburizing, carbon diffuses into a steel member by an
equilibration reaction, thus requiring the steel member to be
exposed to a carburizing gas atmosphere at a high temperature for a
long period of time. Oil is used as a coolant for quenching to
suppress distortion by relatively slower cooling as compared to
water cooling. However, in oil quenching, where a large number of
steel members are simultaneously immersed in an oil bath,
distortion is caused within each single steel member because of a
quenching time difference as between that part of the steel member
which first enters oil and the remaining portion which follows into
the oil. Thus, in spite of the use of oil for quenching, it is
difficult to solve the problem of distortion in steel members
obtained by the above-described carburizing-quenching process.
Furthermore, a variation in the quenching quality also occurs among
steel members depending on the positions in which the steel members
are placed.
[0005] Furthermore, because the related-art carburizing-quenching
process requires prolonged carburization in a large furnace as
described above, it consumes a large amount of energy. Accordingly,
a need exists to shorten the time required for the
carburizing-quenching process, to reduce the consumption of energy,
and to downsize the carburizing-quenching equipment.
[0006] The use of reduced-pressure carburization (vacuum
carburization) is considered to be effective in reducing the
consumption of energy in the carburizing-quenching process.
[0007] With regard to quenching after carburization, a
high-frequency quenching method has been suggested in which a
component is subjected to localized quenching instead of
simultaneously quenching the whole body of the component (see
Japanese Patent Application Publication No. JP-A-11-131133).
SUMMARY OF THE INVENTION
[0008] The inventors of the present invention have now identified
disadvantages in using the reduced-pressure carburization method.
In the case of the conventional gas carburizing method, the
carburizing is through an equilibration reaction and thus a carbon
potential can be calculated in advance to set conditions. In
reduced-pressure carburization, however, setting of such conditions
is difficult because a non-equilibrium reaction is used. The
present inventors also found that, when a steel member having a
toothed surface, such as a gear, is subjected to reduced-pressure
carburization, diffusion rates of the introduced carbon differ for
different portions of the gear. Thus, the resultant surfaces have
different carbon concentrations depending on shape and thus a
desired effect may not be provided at a portion that should be
surface modified by carburization.
[0009] Accordingly, it is an objective of the present invention to
provide a method for treating a steel member by which a steel
member, having first and second carbon diffusion rates in different
portions, can be subjected to a reduced-pressure carburization
under optimal conditions.
[0010] The present invention provides a method of manufacturing a
steel member that includes: a reduced-pressure carburization step
of carburizing a steel member in carburization gas under a reduced
pressure, a slow cooling step in which the steel member obtained
through the reduced-pressure carburization step is slowly cooled in
cooling gas, and a quenching step in which a desired portion of the
cooled steel member is heated using high-density energy and
subsequently subjected to rapid cooling. The steel member subjected
to the reduced-pressure carburization has a shape including a first
portion in which the diffusion rate of carbon introduced during
carburization is high, and a second portion in which the diffusion
rate of the introduced carbon is lower than that in the first
portion. The reduced-pressure carburization step is performed under
conditions regulated to give a carbon concentration at the surface
of the first portion in a range of 0.65.+-.0.1 weight % after
diffusion.
[0011] The method according to the present invention includes a
reduced-pressure carburization step and a quenching step in which
the steel member is heated by high-density energy and subsequently
rapidly cooled, and also an intermediate slow cooling step. By this
method, the steel member can be carburized-quenched equal to or
better than in related art, while distortion is significantly
suppressed. The process time is also significantly shorter than in
the related art. Thus, the amount of energy used and the cost are
significantly reduced.
[0012] As noted above, the reduced-pressure carburization of the
present invention is performed under conditions giving a carbon
concentration at the surface of the first ("easy carbon diffusion")
portion in a range of 0.65.+-.0.1 weight % after diffusion.
[0013] Further, the reduced-pressure carburization of the present
invention, under the conditions described above, results in a
second ("difficult carbon diffusion") portion, which has a carbon
diffusion rate lower than that of the first portion, and which has
a surface with a carbon concentration higher than that of the first
portion and equal to or lower than 0.85 weight after diffusion.
Accordingly, the overall carbon concentration of substantially the
entire surface of the steel member is within a range from 0.55 to
0.85 weight % after diffusion. By limiting the carbon concentration
at the surface within this range, the steel member can be subjected
to the subsequent special quenching step in which the member is
selectively heated with high-frequency energy and is subsequently
subjected to rapid cooling. As a result, even in a portion whose
surface carbon concentration is close to the lower limit (first
portion), the quenching effect will be sufficient. At the same
time, in the second portion having a carbon concentration at its
surface close to the upper limit ("difficult carbon diffusion
portion"), formation of cementite due to excessive carbon is
reduced. Thus, a superior modified surface can be obtained by
quenching.
[0014] The carburization conditions as described above must be
determined by performing a plurality of preliminary experiments
with different temperatures, types of carburization gas, pressure
and/or processing times, for example, in the reduced-pressure
carburization step to find conditions in which the carbon
concentration at the surface of the first portion is within the
specified range. When each steel member to be processed has the
same shape, the number of preliminary experiments can be reduced
through the use of accumulated data. The first portion and the
second portion of the steel member may be determined by actually
measuring carbon concentrations at a plurality of positions in the
preliminary experiments. However, the first portion and the second
portion are relatively easily judged based on their shapes, and
thus can be determined through observation of their shapes.
[0015] The quenching step is performed as described above by
heating a selected portion of the steel member using high-density
energy and subsequently subjecting that heated portion to rapid
cooling. In this quenching step, not the entire steel member, but
only the desired (selected) portion of the steel member (i.e., that
portion whose strength is to be improved by quenching) is rapidly
heated using the high-density energy and then rapidly cooled. As a
result, compared with a case where the entire steel member is
subjected to quenching as in the related art, distortion during
quenching can be significantly reduced, and the shape before the
quenching step of the present invention can be substantially
maintained even after quenching.
[0016] In this quenching step, high-density energy is used and thus
the effect of increasing the strength by quenching can be
increased. Furthermore, because improved quenching is obtained,
even if the level of carburization in the reduced-pressure
carburization step, i.e., the depth of carburization is reduced,
the reduced level can be compensated for by the improved quenching
capability. The present invention actively uses this superior
characteristic so that the carbon concentration of the first
portion, resulting from the reduced-pressure carburization step,
can be set to 0.65.+-.0.1 weight % after diffusion, which is lower
than conventional. More specifically, by combining the quenching
step using high-density energy with the above-described
reduced-pressure carburization step, the carburization time in the
reduced-pressure carburization step can be shortened to provide
higher efficiency, and formation of excessive cementite can be
reduced to provide improved quality.
[0017] The high-density energy may be, for example, a high-density
energy beam such as an electron beam or a laser beam, or non-beam
high-density energy such as induction heating by high-frequency
heating.
[0018] On the other hand, even with a quenching step using
high-density energy to suppress distortion, if the steel member is
distorted before this step, of course, it is difficult to obtain a
precisely manufactured steel member. In order to solve such a
problem, a slow cooling step that suppresses the distortion of the
steel member is interposed between the reduced-pressure
carburization step and the quenching step.
[0019] As described above, the method of manufacturing a steel
member of the present invention includes reduced-pressure
carburization in which a relatively small amount of carburization
gas is used while maintaining the interior of a carburization
furnace at a high temperature and a reduced pressure. Thus, the
steel member can be processed with higher efficiency and lower
energy than in the related art. Further, as described above, as the
carburization condition, the carbon concentration of the first
portion is set to 0.65.+-.0.1 weight % after diffusion, which is
lower than in the conventional process, and the above-described
quenching method is used. Thus, a high quality surface modification
is obtained. Furthermore, the use of the slow cooling step provides
a precisely manufactured steel member having little distortion.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1A is a graph of temperature versus time illustrating a
heating pattern in an embodiment of the method of the present
invention (Example 1);
[0021] FIG. 1B illustrates a heat pattern of another method, used
for comparison, in Example 1;
[0022] FIG. 2A is a schematic diagram of thermal processing
equipment used for performing the method of the present invention
in Example 1;
[0023] FIG. 2B is a schematic diagram of carburizing-quenching
equipment used in the comparison method of Example 1;
[0024] FIG. 3A is a plan view of the steel member treated in
Example 1;
[0025] FIG. 3B is a cross-sectional view of the steel member taken
along the arrow A-A in FIG. 3A;
[0026] FIG. 4 is a graph of hardness versus distance from the
surface, illustrating hardness distribution after carburization and
quenching in Example 1;
[0027] FIG. 5 is a table of values for distortion for the treated
articles in Example 1;
[0028] FIG. 6 is a graph of residual stress versus distance from
the surface for the samples treated in Example 1;
[0029] FIG. 7 is a graph of hardness versus carbon content,
illustrating the relationship between the carbon concentration at
the surface and the surface hardness after quenching in Example
1;
[0030] FIG. 8 is a perspective view of another example of a gear
treated in Example 1;
[0031] FIG. 9 is a perspective view of yet another example of a
gear treated in Example 1;
[0032] FIG. 10A is a partial perspective view of the details of a
tooth section of a gear treated in Example 1;
[0033] FIG. 10B is a partial expanded cross-sectional view of a
tooth shown in FIG. 10A;
[0034] FIG. 10C is a partial expanded view showing the exterior
surfaces of the first and second portions of a tooth shown in FIG.
10A.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0035] The reduced-pressure carburization step of the present
invention requires that, as a carburization condition, the carbon
concentration of a first portion be set to 0.65.+-.0.1 weight %
after diffision. When the carbon concentration of the first
portion, resulting from the reduced-pressure carbonization step, is
lower than 0.55 weight % after diffusion, even when a quenching
step is used in which heating with high-density energy is followed
by rapid cooling, sufficient hardening may not be obtained in the
first portion. On the other hand, when the carbon concentration in
the first portion exceeds 0.75 weight % after diffusion, the carbon
concentration of the second portion will probably exceed 0.85
weight % after diffusion, causing formation of cementite and
reducing the surface modification effect.
[0036] Thus, the reduced-pressure carburization step is preferably
performed under conditions which give a carbon concentration at the
surface of the first portion within a range of 0.65.+-.0.05 weight
% after diffusion.
[0037] More preferably, the reduced-pressure carburization step is
performed under conditions that are obtained in advance through
experiment and that give a carbon concentration at the surface of
the second portion equal to or lower than 0.85 weight % after
diffusion.
[0038] In the reduced-pressure carburization step the steel member
is exposed to contact with a hydrocarbon-based carburization gas
and carbon is diffused inside the steel member at a reduced
pressure. In this case, conditions can be set in a relatively easy
manner. In particular, the present invention controls a target
value for the carbon concentration at the surface of first portion
as a process condition. The carbon concentration can be changed by
adjusting the carburization period, i.e., the time of contact with
the hydrocarbon gas while heating. The specific time must be
confirmed through preliminary experiment, which may be easily
performed if data for similar types of shapes is available.
[0039] The first portion of the steel member is a portion where the
angle of the surface, in cross-section of the first portion, is at
least 130 degrees. The first portion and the second portion are
preferably subjected to reduced-pressure carburization as a
preliminary experiment to measure and compare the carbon
concentrations of the respective portions to determine optimum
conditions. They may be empirically and accurately identified based
on their shapes. In particular, as described above, if the
comparison of the cross-sectional shapes of the respective parts
shows that a certain portion has angle .theta. at the surface of
the cross-section of 130 degrees or more (i.e., the portion is not
empirically judged as a protrusion), that portion may be taken as
the first ("easy carbon diffusion") portion. While the side
surfaces 811 and the top surface 812 are slightly curved, they
approximate planar surfaces.
[0040] Further, as shown in FIG. 10B, if the comparison of the
cross-sectional shapes of the respective parts shows that a certain
portion has an angle .theta. at the surface of the cross-section of
130 degrees or less, that portion may be taken as the second
("difficult carbon diffusion") portion.
[0041] The angle .theta. of the second portion, of gears in an
exemplary, commercially available automatic transmission have been
measured as: 118.15.degree., 125.7.degree., 112.7.degree.,
111.5.degree., 124.8.degree., 119.degree., and 113.7.degree.,
respectively. It will be recognized that these are exemplary values
only, and that the second portion can include any suitable angle
.theta. of 130 degrees or less.
[0042] As is illustrated in FIG. 10C, an exterior surface 816 of
the first portion, shown by lines and arrows, includes a portion of
the top surface between corner sections 813 of the gear teeth.
Exterior surfaces of the first portions can also include portions
of the tooth bottom 815 and the tooth face 811. As is also shown,
an exterior surface 817 of the second portion, shown by lines and
arrows, includes the corner section 813.
[0043] The first and second portions in the present invention are
both portions that are subjected to carburization and the
subsequent quenching for surface modification. Therefore, any
portion not subjected to quenching should not be regarded as either
of first and second portions.
[0044] When the steel member is a gear having teeth, the first
portion is preferably a tooth face (or tooth faces) or a groove
bottom (or bottoms) between teeth. The tooth face and the groove
bottom have a gently curved plane relatively close to a flat plane
and thus are portions which are treated as first portions in the
present invention, i.e. portions into which carbon is more easily
diffused, as compared to the second portions. On the other hand, a
tooth tip corner section between a tooth tip face and a tooth face
has a protruding shape and thus is treated in the present invention
as a second portion whose surface is treated to have a higher
carbon concentration than the surfaces of the first portions.
Experimental results obtained for the present invention show that,
when the steel member is a gear, even for steel members of slightly
different shapes, by subjecting the steel member to
reduced-pressure carburization in such a manner that the tooth
faces or the groove bottoms between the teeth have a carbon
concentration of 0.65.+-.0.1 weight % after diffusion, the carbon
concentration at the corners 813, where the tooth tip faces 812 and
the tooth side faces 811 intersect, will be higher than that of the
first portions and equal to or lower than 0.85 weight % after
diffusion. Thus, a superior surface modification effect is
obtained.
[0045] The reduced-pressure carburization step is preferably
performed while heating the steel member at a temperature equal to
or higher than its austenitizing temperature and under a reduced
pressure of 1 to 100 hPa. The disadvantage of a reduced pressure
during carburization lower than 1 hPa, is that high-cost equipment
for maintaining a high degree of vacuum is required. When the
reduced pressure during carburization exceeds 100 hPa on the other
hand, soot is generated during carburization, thus causing a
problem of uneven carbon concentration. The carburization gas may
be, for example, a hydrocarbon gas such as acetylene, propane,
butane, methane, ethylene, or ethane.
[0046] The slow cooling step is preferably performed at a cooling
rate less than that at which the steel member transforms to
martensite during cooling. Accordingly, distortion can be
reduced.
[0047] The reduced-pressure carburization step can use high
concentration carburization in which the surface carbon
concentration is increased to a level higher than in conventional
carburization, to precipitate a compound of iron and carbon in the
top layer, or carburization nitriding in which carburization and
nitriding are simultaneously performed.
[0048] The slow cooling step is preferably performed at a cooling
rate from 0.1 degree C./second to 3.0 degrees C./second while the
temperature of the steel member is equal to or higher than an A1
transformation point temperature. When the cooling rate of the slow
cooling step exceeds 3.0 degrees C./second, while the temperature
of the steel member is equal to or higher than the A1
transformation point temperature, distortion during cooling may not
be sufficiently reduced. On the other hand, when the cooling rate
of the slow cooling step is lower than 0.1 degree C./second, while
the temperature of the steel member is equal to or higher than the
A1 transformation point temperature, a long time is required for
reaching the A1 transformation point temperature where carburized
carbon is increasingly diffused into the steel member. The progress
of diffusion during slow cooling varies depending on the
temperatures of the first and second portions, thereby causing
variation in diffusion rates and a variation in carbon content.
[0049] The cooling gas used in the slow cooling step is preferably
nitrogen, helium, argon or a combination thereof. These gases are
so-called inert gases and can prevent a steel member from becoming
oxidized during slow cooling.
[0050] The slow cooling step is preferably performed with the
cooling gas at a pressure lower than atmospheric pressure to
further suppress distortion during cooling.
[0051] When the cooling gas is agitated during cooling, the
low-pressure cooling gas having a difference in the cooling rates
between the upwind side and the downwind side of the circulating
cooling gas is reduced as compared with the case where the cooling
gas is at atmospheric pressure. Specifically, when slow cooling is
performed at atmospheric pressure, heat exchange is promoted by
mere contact between the steel member and the cooling gas. In this
latter case, gas convection induced by forced gas agitation or heat
creates an upwind side and a downwind side with different cooling
rates. The difference in the cooling rates causes a temperature
difference in the member undergoing treatment, thus causing
distortion due to thermal stress. On the other hand, the cooling
gas at the reduced pressure can provide, at both the upwind side
and the downwind side, a slow heat exchange rate and thus a
difference in the cooling rate is prevented. Thus, the use of the
reduced-pressure slow cooling using cooling gas at a reduced
pressure (below atmospheric pressure) promotes cooling in a
relatively uniform manner and thus it is less likely to cause
distortion due to thermal stress. Even without agitation, cooling
gas at a reduced pressure can reduce, compared to use of cooling
gas at atmospheric pressure, the difference in the cooling rates
due to residual cooling gases having different temperatures.
[0052] By using a cooling gas at a reduced pressure as described
above, the steel member treated in the reduced-pressure slow
cooling step undergoes little, if any, distortion and thus can be
subjected to the quenching step while maintaining high dimensional
accuracy.
[0053] By performing the reduced-pressure carburization step at a
reduced pressure (vacuum) and the slow cooling step at a reduced
pressure continuously, the equipment can be structured with a
reduced-pressure carburization chamber directly connected to a slow
cooling chamber. Therefore, a preliminary chamber for adjusting the
degree of depressurization is not required between the chambers.
Specifically, the reduced-pressure carburization step and the slow
cooling step are both performed at a reduced pressure and thus a
pressure difference therebetween can be kept small. Thus, a product
obtained through the reduced-pressure carburization can be
subjected to the slow cooling at a low pressure without being
subjected to normal pressure and thus processed efficiently while
suppressing distortion.
[0054] The pressure cooling gas at a reduced pressure used in the
slow cooling step is preferably in a range from 100 hPa to 650 hPa.
When the cooling gas has a pressure higher than 650 hPa, the effect
of the reduced pressure may not be sufficiently obtained. When the
cooling gas has a pressure lower than 100 hPa, on the other hand,
the slow cooling step may become difficult due to the configuration
of equipment. Therefore, the cooling gas used in the slow cooling
step more preferably has a reduced pressure in a range from 100 hPa
to 300 hPa.
[0055] The cooling gas at a reduced pressure used in the slow
cooling step is preferably made higher than in its previous state
after the temperature of the steel member becomes equal to or lower
than the A1 transformation point. In the slow cooling at a reduced
pressure, the higher the degree of depressurization, i.e. degree of
vacuum, the more distortion is suppressed, but the cooling
efficiency is also lowered. However, distortion is avoided when the
temperature of the steel member is equal to or lower than the A1
transformation point. Thus, the effect of reducing distortion can
be maintained even when the pressure of the cooling gas is
increased to increase the cooling efficiency.
[0056] Next, in the quenching step a desired (selected) portion of
the cooled steel member is heated using high-density energy and
subsequently rapidly cooled. By heating the steel member using
high-density energy to a temperature equal to or higher than its
austenitizing temperature as described above, localized heating can
be easily realized and thus the effect of reducing distortion can
be significantly improved as compared with heating of the entire
steel member.
[0057] The cooling rate for the rapid cooling is desirably in a
range from 200 degrees C./second to 2000 degrees C./second. When
the cooling rate is lower than 200 degrees C./second, a quenching
effect may not be sufficiently obtained. On the other hand, a
cooling rate exceeding 2000 degrees C./second makes it difficult to
realize rapid cooling. Pressure is not particularly important in
the rapid cooling.
[0058] It is important that the quenching step be performed by
heating the selected portion of the steel member using high-density
energy at a temperature equal to or higher than its austenitizing
temperature and subsequently rapidly cooling the steel member at a
cooling rate equal to or higher than a critical cooling rate for
rapid cooling at which martensitic transformation occurs in a
carburization layer. Accordingly, a sufficient hardening in the
carburization layer can be obtained.
[0059] The heating using high-density energy in the quenching step
is preferably high-frequency heating and the rapid cooling is
preferably by water quenching. The high-frequency heating can be
induction heating in a non-contact manner with a higher efficiency
and accuracy.
[0060] The rapid cooling when induction heating by high-frequency
is used is preferably by water quenching. The use of the induction
heating by high-frequency allows a portion of a steel member to be
selectively heated. In other words, the steel member is partially,
not entirely, heated. Thus, even when water quenching is
subsequently performed using water providing a very high cooling
rate, quenching distortion can be minimized. The superior rapid
cooling effect with water quenching can improve quenching
characteristics, and thus a quenched part can be imparted with yet
higher strength. Thus, a required strength can be attained even
when carburization is simplified (i.e., the processing time is
shortened), which means a carburization layer of reduced thickness.
In this case, the time required for the entire thermal processing
step can be further shortened.
[0061] Preferably, steel members are subjected one by one to
induction heating by high-frequency and the heated members
subsequently cooled by spraying cooling water directly toward the
steel members while rotating the steel members. In this case, the
steel members can be uniformly cooled and thus distortion can be
further suppressed.
[0062] The quenching step can also be performed by focusing a
high-density energy beam onto the selected portion of the steel
member to selectively heat that portion and then rapidly cooling
the steel member, e.g. by water quenching or self cooling.
Specifically, a high-density energy beam, e.g. an electron beam or
a laser beam, can be used to extremely rapidly heat a selected
surface area on which the beam is focused. By limiting the heated
portion to the outer (exterior) surface, a sufficient rapid cooling
effect can be obtained by self cooling when the heating with the
high-density energy beam is stopped or the steel member is removed
from the heating chamber.
[0063] The high-density energy beam is preferably an electron beam.
Electron beams can be easily changed to have a different output, a
different beam diameters and/or a different emission region, for
example. Thus, the desired region can be selectively heated with
high accuracy.
[0064] When an electron beam is used, to the selected portion on
which the beam impinges can also be melted rapidly. Thus, the
quenching step is preferably performed by emitting an electron beam
onto a desired portion of the steel member to heat only an outer
layer to a temperature equal to or higher than the melting point to
form a melt, and by subsequently rapidly cooling the melted portion
until reaching a martensitic transformation region, to obtain a
martensite structure and to form a hardened layer.
[0065] The hardened (outer) layer preferably has a thickness of 0.2
mm or less. A hardened layer having a thickness exceeding 0.2 mm
may be an obstacle to self cooling after melting. A hardened layer
that is too thin on the other hand may result in reduced
durability. Thus, the hardened layer more preferably has a
thickness in a range from 0.1 mm to 0.2 mm.
[0066] The steel member to which the method of the present
invention is applied may be a component of an automobile drive
system (drive train). Components for an automobile drive system
include, for example, a gear, a ring-shaped member, and other
components of an automatic transmission. They are components that
require both high strength and high dimensional accuracy. Thus, by
using the above-described superior thermal processing method, the
manufacture can be streamlined with low cost and the resultant
products have high quality.
EXAMPLES
Example 1
[0067] An example of the method of manufacturing a steel member
according the present invention will now be described with
reference to the accompanying drawings. In this example, a
manufacturing method in accordance with the present invention and a
related-art manufacturing method for comparison (comparative
example) were applied to a ring-shaped steel member 8 (ring gear),
used as a component of an automatic transmission, to evaluate the
surface hardness and the distortion. In this example, the steel
member 8 to be treated included, as shown in FIG. 8 (FIG. 3 is a
schematic view of FIG. 8), a tooth section 81 provided at the inner
circumferential surface of a ring-like body 80. The steel member 8
is a component in which the tooth section 81 has high hardness, and
for which roundness is very important.
[0068] FIG. 1 shows a heat pattern A of the method of the present
invention and, for purposes of comparison, a heat pattern B
(comparative example). In FIG. 1, the horizontal axis shows time
and the vertical axis shows temperature, and the temperatures of
the steel members during thermal processing are shown as the heat
patterns A and B.
[0069] As can be seen from the heat pattern A of FIG. 1, the method
of the present invention starts with a reduced-pressure
carburization step a1 for heating a steel member to a carburization
temperature of 950 degrees C. and retains the steel member at that
temperature for 49 minutes. Thereafter, the steel member is
subjected to a reduced-pressure slow cooling step a2 by which the
steel member is cooled down to 150 degrees C. or less for 40
minutes. Thereafter, the steel member is subjected, in a
high-frequency quenching step a3, to induction heating by
high-frequency to rapidly heat the member up to a quenching
temperature of 950 degrees C. and is subsequently subjected to
water quenching.
[0070] On the other hand, as is seen in the heat pattern B, the
comparative method starts with a conventional carburization step b1
to heat a steel member to a carburization temperature of 950
degrees C. and retains the member at that temperature for 220
minutes. Thereafter, the member is subjected to a quenching step b2
in which the member is maintained at a quenching temperature of 850
degrees C. and is subsequently subjected to oil quenching. In the
comparative example, a post-washing step b3 for washing off coolant
(oil) which the steel member retained from oil quenching and a
tempering step b4 for securing the toughness of the hardened layer
are performed, during which the temperature of the member is
increased to some degree. Distortion evaluation, strength
evaluation, and residual stress evaluation, which will be described
later, were performed after this tempering step b4.
[0071] Before describing details of these processes, the thermal
processing equipment 5 for carrying out the method of the present
invention and carburizing-quenching equipment 9 for carrying out
the comparative example will be briefly described.
[0072] As shown in FIG. 2A, the thermal processing equipment 5 for
carrying out the method of the present invention includes: a
reduced-pressure carburization slow cooling apparatus 52 that
includes a pre-washing bath 51 for washing a steel member before
carburization and quenching, a heating chamber 521, a
reduced-pressure carburization chamber 522, a reduced-pressure slow
cooling chamber 523; a high-frequency quenching machine 53 and a
magnaflux apparatus 54 for inspecting for defects.
[0073] As shown in FIG. 2B, the carburizing-quenching apparatus 9
for carrying out the comparative example included: a pre-washing
bath 91 for washing the steel member before carburization and
quenching; a large elongated carburization furnace 92 that includes
a carburization furnace 921 for performing heating, carburization,
and diffusion, and a quenching oil bath 922; a post-washing bath 93
for washing the steel member after carburization and quenching; and
a tempering furnace 94 for tempering.
[0074] Next, steel members 8 were subjected to carburization and
quenching in the apparatus of FIG. 1A and FIG. 1B, respectively.
Then, the steel members 8 were compared with regard to strength,
distortion, and residual stress.
[0075] In the method of the present invention, as shown in the heat
pattern A of FIG. 1, the steel member was subjected to
reduced-pressure carburization in step a1 to carburize the steel
member placed in carburization gas under a reduced pressure, to
reduced-pressure slow cooling in step a2 to cool the steel member
in cooling gas at a pressure lower than atmospheric pressure, and
to the high-frequency quenching step a3 in which a desired portion
of the cooled steel member was subjected to induction heating by
high-frequency and then to water quenching.
[0076] The reduced-pressure carburization step a1 has a
carburization period during which the steel member is maintained in
the hydrocarbon carburization gas and carbon enters (diffuses) into
the surface of the steel member, and a diffusion period during
which carbon diffuses into the interior of the steel member. In
this example, both of these processes (the carburization and
diffusion processes) were conducted by retaining the steel member
for 49 minutes at 950 degrees C. which is a temperature equal to or
higher than its austenitizing temperature. These processes were
performed under conditions with the carburization chamber drawn
down to a pressure of 1 hPa and wherein the carburization gas was
acetylene. In other words, both of the carburization period and the
diffusion period were at a reduced pressure as described above, and
acetylene was introduced to the carburization chamber during the
carburization period, while the introduction of acetylene was
stopped and only a reduced pressure was applied during the
diffusion period. The temperature was fixed as described above
during the carburization period and the diffusion period. Note that
the steel member 8 is a ring gear. Due to the shape of the ring
gear, the steel member 8 has: a first ("easy carbon diffusion")
portion in which, during carburization, the carbon has a high
diffusion rate, which first portion is composed of a tooth bottom
815 and a tooth face 811; and a second ("difficult carbon
diffusion") portion in which, during carburization, the carbon
diffusion rate is lower than that in the first portion and which is
composed of a tooth tip corner section 813 (a corner section
between the tooth face 811 and a tooth tip 812). The
reduced-pressure carburization step is performed under conditions
which give the tooth bottom 815 a carbon concentration at the
surface within a range of 0.65.+-.0.05 weight % after diffusion. As
is shown in FIG. 10A, the tooth bottom 815 and the tooth face 811
have surface angles in the cross-sectional shapes that are close to
180 degrees and are thus identified as portions having an angle
equal to or greater than 130 degrees.
[0077] In the reduced-pressure slow cooling step a2 the cooling gas
was nitrogen N.sub.2), the reduced pressure was 200 hPa, the
cooling gas was agitated, the reduced-pressure slow cooling step
was performed during a period extending from a temperature just
after carburization equal to or higher than its austenitizing
temperature to a temperature of 150 degrees C. lower than the A1
transformation point, and the cooling rate was within a range from
0.1 to 3.0 degrees C./second (specifically, 10 degrees C./minute
(0.17 degrees C./second)).
[0078] In the high-frequency quenching step a3 the tooth section 81
on the inner circumferential surface of the steel member 8 was
subjected to induction heating by high-frequency to a temperature
of 950 degrees C., i.e. to a temperature equal to or higher than
its austenitizing temperature, and subsequently subjected to water
quenching where water was sprayed onto the tooth section 81 to
provide a cooling rate equal to or higher than the critical cooling
rate for rapid cooling, resulting in martensitic transformation in
a carburized layer. This cooling rate by water quenching was 268
degrees C./second. The induction heating by high-frequency was by a
method in which the steel members 8 were transported one by one to
heat the individual steel members, and were subjected to cooling
after the heating by cooling water sprayed directly onto the steel
members 8 while the steel members 8 were rotated to cool them, one
by one to minimize distortion.
[0079] In the comparative example, as is seen in the heat pattern B
of FIG. 1, a steel member was heated to the carburization
temperature of 950 degrees C. and retained at that temperature for
220 minutes, and subsequently subjected to the conventional
carburization step b1. Thereafter, the member was retained at the
quenching temperature of 850 degrees C., and subsequently subjected
to the quenching step b2 with oil quenching. The carburization in
the comparative example was performed under conditions in which
carbon potential was adjusted to give substantially the entire
surface of the steel member 8 a carbon concentration of 0.8 weight
% after diffusion. In the comparative example, the quenching step
b2 was followed by the post-washing step and the post-washing step
b3 was followed by the tempering step b4.
[0080] Both the comparative example and the example representative
of the method of the present invention used SCM420 (JIS) steel
which is suitable for carburization.
[0081] The steel member obtained through carburization and
quenching was tested to determine Vickers hardness (Hv) with
distance from the surface at the tooth bottom 815 of the gear (FIG.
3) and the measured Vickers hardness was used to evaluate strength.
The results are shown in FIG. 4. In FIG. 4, the horizontal axis
represents the distance from the surface (mm) and the vertical axis
represents the measured Vickers hardness (Hv). The results for the
steel member treated by the method of the present invention are
shown by reference numeral E1 while the results for the steel
member obtained by the comparative example are shown by a reference
numeral C1.
[0082] As seen in FIG. 4, the method of the present invention (E1),
results in a hardness which is slightly reduced at a given distance
from the surface as compared to the hardness for the comparative
example (C1). However, the hardness at the outer surface is higher
for the method of the present invention as compared to that
obtained with the comparative example. As can be seen from these
results, the method of the present invention provides superior
thermal processing equal to or higher than the related art. In
particular, by the method of the present invention, a tooth bottom
section 815 had a carbon concentration (0.65.+-.0.05 weight % after
diffusion) that was lower than the carbon concentration there in
the comparative example (0.8 weight % after diffusion); but
sufficient quenching was obtained.
[0083] Furthermore, the measurement of the carbon concentration at
the surface of the tooth tip corner section 813 of the steel member
8, obtained in the method of the present invention, was 0.8 weight
% after diffusion, and the hardness thereof was equal to that of
the tooth bottom section 815. These results demonstrate the
effectiveness of the reduced-pressure carburization step in the
method of the present invention.
[0084] In FIG. 7, the horizontal axis represents the carbon
concentration (content) at the surface, and the vertical axis
represents the surface hardness after quenching. In FIG. 7, curve A
shows the relationship between the surface carbon concentration
obtained in the reduced-pressure carburization step of the method
of the present invention and hardness after water quenching, which
relationship was experimentally determined, and curve B shows the
relationship between the carbon concentration and the hardness
after oil quenching in the carburization step of the related-art
method that was also experimentally obtained. A region S (of a
carbon concentration of 0.85% or more after diffusion) is also
shown where an abnormal structure tends to be generated due to
excessive carburization.
[0085] In FIG. 7, with regard to the actual carbon concentration
finally reached when carburization conditions for a carbon
concentration of 0.8 weight % after diffusion are selected for the
related-art method, the first ("easy carbon diffusion") portion
(the tooth bottom 815) is shown by the position of the tip end of
an arrow b1 and the second ("difficult carbon diffusion") portion
(the tooth tip corner section 813) is shown by the position of the
tip end of an arrow b2.
[0086] With regard to the carburization concentration finally
reached when carbon conditions for a carbon concentration of 0.8
weight % after diffusion of the carbon easy diffusion portion (the
tooth bottom 815) are selected for the method of the present
invention, the first ("easy carbon diffusion") portion (the tooth
bottom 815) is shown by the position of the tip end of an arrow cl
and the second ("difficult carbon diffusion") portion (the tooth
tip corner section 813) is shown by the position of the tip end of
the arrow c2.
[0087] With regard to the carbon concentration finally reached when
conditions for a carbon concentration of 0.60 weight % after
diffusion at the surface of the first ("easy carbon diffusion")
portion (the tooth bottom 815) are selected for the method of the
present invention, the first ("easy carbon diffusion") portion (the
tooth bottom 815) is shown by the position of a tip end of an arrow
a1 and the second ("difficult carbon diffusion") portion (the tooth
tip corner section 813) is shown by the position of a tip end of
the arrow a2.
[0088] As is shown in FIG. 7, by the related-art method, the
hardness after quenching becomes higher as the carbon concentration
increases. On the other hand, excessive carburization is
problematic and thus the carbon concentrations of all portions are
desirably set to be 0.8 weight % after diffusion. With regard to
this point, the related-art method can provide the first and second
portions both having substantially the same carbon concentration.
Thus, the carbon concentration conditions can be set to 0.8 weight
% after diffusion to provide high hardness for the entire
article.
[0089] By the method of the present invention, the hardness after
quenching is substantially the same in a range of carbon
concentration from 0.6 to 0.8 weight % after diffusion. This is a
result of using the above-described superior quenching step.
[0090] When the reduced-pressure carburization was performed so
that the first portion (the tooth bottom 815) had a carbon
concentration of 0.8 weight % after diffusion (c1), a carbon
concentration (c2) of the second portion (the tooth tip corner
section 813) entered the excessive carbon region S.
[0091] On the other hand, when the reduced-pressure carburization
was performed so that the first portion (the tooth bottom 815) had
a carbon concentration of 0.6 weight % after diffusion (a1), the
carbon concentration (a2) of the second portion (the tooth tip
corner section 813) was reduced to a range equal to or lower than
0.8 weight % after diffusion. Even when there was a difference in
the carbon concentration as described above, the resultant hardness
can be maintained at substantially the same level, as can be seen
from FIG. 7.
[0092] When the steel material suitable for carburization as used
in the related-art method was used in the method of the present
invention (E1), it might be expected that a significantly-reduced
carburization time would result in a proportionally-reduced
carburization depth and reduced strength. However, these potential
strength-related disadvantages were overcome by changing the
material used and by using water quenching. There is also the
possibility that the interior strength can be improved to a level
equal to that of the related-art product by selection of a material
of appropriate composition.
[0093] Next, the sizes of the steel members obtained by the
carburization and quenching treatment were measured to compare the
degrees of distortion. The sizes of the steel members were measured
based on two types of shapes, i.e. "BBD" and "BBD ellipse". As
shown in FIG. 3, the "BBD" indicates a size in which steel balls 88
having a predetermined diameter are placed so as to abut valleys in
the tooth face 81 and an inner diameter between the opposing hard
balls 88 is measured. This measurement was performed at three
positions in the axial direction (positions a, b, and c in FIG. 3B)
for the entire circumference, and an average value (Ave), the
maximum value (Max), and the minimum value (Min) of the
measurements were obtained.
[0094] Next, the difference between the maximum value and the
minimum value of the "BBD" at the respective measured positions in
the axial direction was calculated as "BBD ellipse (.mu.m)". Then,
as above, an average value (Ave), the maximum value (Max), and the
minimum value (Min) of the measurements were obtained.
[0095] FIG. 5 illustrates the results with the "BBD" and "BBD
ellipse". In FIG. 5, the left column shows the results of the
method of the present invention at three different points in time,
i.e. (1) "before reduced-pressure carburization", (2) "after
low-pressure carburization and reduced-pressure slow cooling", and
(3) "after high-frequency quenching". In FIG. 5, the right column
shows the results for the comparative example "before carburization
and quenching" and "after carburization and quenching". The lines
shown in the respective columns are obtained by plotting the
maximum values, the minimum values, and the average values at the
positions a, b, and c in FIG. 3B, respectively, and connecting each
of the maximum values to each of the minimum values by a thick
line. Average values at the three positions are connected by a thin
line.
[0096] As seen in FIG. 5, the method of the present invention
suppresses the distortion even after quenching. Further, it can be
seen that suppression of distortion has already been obtained by
the reduced-pressure slow cooling after the reduced-pressure
carburization. In contrast, it can be seen that significant
distortion is caused by carburization and quenching in the
comparative example.
[0097] Next, residual stresses of the steel members after
carburization and quenching were measured and were compared. The
measurements are shown in FIG. 6. In FIG. 6, the horizontal axis
represents distance from the surface of the tooth bottom 815, and
the vertical axis represents residual stress by showing a tensile
direction by the mark + and a compression direction by the mark
-.
[0098] By the method of the present invention (El), compressive
residual stress is found at least at the outermost surface. In the
comparative example (C1) on the other hand, tensile residual stress
is found at the outermost surface. When the residual stress at the
outermost surface is tensile stress, various problems may arise.
Thus, thermal processing or surface modification processing is
required to reduce the tensile residual stress. Thus, the method of
the present invention provides another advantage in that a process
for improving such residual stress is not particularly
required.
[0099] The advantages of the present invention are not limited to a
ring gear but can be obtained when the method is applied to various
other gears with teeth. As examples of such gears, the
above-described ring gear 8 is shown in FIG. 8, and a worm wheel
802 with external teeth is shown in FIG. 9. FIG. 10A is an exploded
view illustrating the tooth section 81 in the external tooth gear
and shows the tooth section 81 as having a tooth bottom 815, a
tooth face 811, a tooth tip 812, and a tooth tip corner section
813. When the treatment method of the present invention is applied
to a gear as described above, conditions of a reduced-pressure
carburization step may be set so that the tooth face 811 and/or the
tooth bottom 815 is given a carbon concentration of 0.65.+-.0.1
weight % after diffusion.
Example 2
[0100] In this example, the method of the present invention as in
Example 1 was applied to a steel member 8 (ring gear) having the
same shape and material as in Example 1 but with a different target
value for the carbon concentration at the surface of the tooth face
811. The tooth tip corner section 813 and the tooth face 811 of the
resultant tooth section 81 were measured for carbon concentration
and surface hardness, and the surface structures of the both were
observed. The target value of the carbon concentration of the
surface of the tooth face 811 in the reduced-pressure carbon step
was set, as shown in Table 2, to be 0.65 weight % after diffusion
in Example 1 of the present invention, 0.57 weight % after
diffusion in Example 2 of the present invention, and 0.75 weight %
after diffusion in Example 3 of the present invention.
[0101] For comparison, steel members were also treated by two
comparative methods.
[0102] In comparative Example 1 the target value of the carbon
concentration at the surface of the tooth face 811 in the
reduced-pressure carburization step was increased to 0.78 weight %
after diffusion as shown in Table 2.
[0103] Comparative Example 2 is a method in which oil quenching is
performed just after related-art gas carburization as in
comparative Example 1. In this case, the target value for the
carbon concentration at the surface of the tooth face 811 and the
tooth tip corner section 813 in the gas carburization step was set
to 0.75 weight % after diffusion as shown in Table 2.
[0104] Table 1 and Table 2 show the conditions and results for
Examples 1 to 3 of the present invention and Comparative Examples 1
and 2.
TABLE-US-00001 TABLE 1 Carburization/ Cooling After Cooling Rate
Diffusion Carburization/ After Test No. Method Component Material
Carburization Temperature Diffusion Carburization/Diffusion Case
Present Ring Gear SCM42H Vacuum 950.degree. C. Gas cooling
0.16.degree. C./sec. to Example 1 Invention carburization
1.8.degree. C./sec. (at applied 600.degree. C. or higher) Case
Present Ring Gear SCM42H Vacuum 950.degree. C. Gas cooling
0.16.degree. C./sec. to Example 2 Invention carburization
1.8.degree. C./sec. (at applied 600.degree. C. or higher) Case
Present Ring Gear SCM42H Vacuum 950.degree. C. Gas cooling
0.16.degree. C./sec. to Example 3 Invention carburization
1.8.degree. C./sec. (at applied 600.degree. C. or higher)
Comparison No use of Ring Gear SCM42H Vacuum 950.degree. C. Gas
cooling 0.16.degree. C./sec. to Example 1 target values
carburization 1.8.degree. C./sec. (at in low 600.degree. C. or
pressure higher) carburization Comparison Comparison Ring Gear
SCM42H Gas 950.degree. C. Oil cooling Rapid Cooling Example 2
method carburization (110.degree. C./sec.) (related-art gas
carburization)
TABLE-US-00002 TABLE 2 Carburization Target Concentration
carburization After Diffusion concentration (mass %) Surface
Hardness Surface structure (presence value after Tooth tip Tooth
tip or non-presence of Reheating/ diffusion (tooth corner Tooth
corner Tooth abnormal structure) Test No. Quenching face, mass %)
section face section face Tooth tip Tooth face Case High 0.65 0.74
0.65 Hv820 Hv810 Martensite Martensite Example 1 frequency heating
+ Water quenching Case High 0.57 0.71 0.57 Hv818 Hv787 Martensite
Martensite Example 2 frequency heating + Water quenching Case High
0.75 0.84 0.74 Hv825 Hv812 Martensite Martensite Example 3
frequency heating + Water quenching Comparison High 0.78 0.91 0.78
Hv823 Hv805 Cement Martensite Example 1 frequency precipitation
heating + (abnormality) Water quenching Comparison Not applied 0.75
0.76 0.73 Hv780 Hv765 Martensite Martensite Example 2
[0105] As can be seen in Table 1 and Table 2, steel members (ring
gears) obtained by the method of the present invention in Examples
1 to 3 show that both the tooth tip corner section 813 and the
tooth face 811 had carbon concentrations within a range from 0.65
to 0.85 weight % after diffusion and extremely superior hardness,
and that the structure was a sound martensite structure with no
cementite precipitation.
[0106] In Comparative Example 1, on the other hand, the target
value for the carbon concentration at the surface of the tooth face
811 of the first ("easy carbon diffusion") portion in the
reduced-pressure carbon step was set to a value exceeding 0.75
weight % (0.78) after diffusion. Thus, the actual value of the
carbon concentration at the surface of the tooth face corner
section 813, which served as the second portion, exceeded 0.85
weight % and reached 0.91 weight % after diffusion, resulting in an
abnormal structure with cementite precipitation.
[0107] In Comparative Example 2, the surface hardness was slightly
lower than in examples 1-3 of the present invention. This clearly
shows that, so long as the carbon concentration is in a range from
0.55 to 0.85 weight % after diffusion, the present invention
provides superior results in terms of strength characteristics and
hardness.
[0108] The invention may be embodied in other specific forms
without departing from the spirit or essential characteristics
thereof. The present embodiments are therefore to be considered in
all respects as illustrative and not restrictive, the scope of the
invention being indicated by the appended claims rather than by the
foregoing description, and all changes which come within the
meaning and range of equivalency of the claims are therefore
intended to be embraced therein.
* * * * *