U.S. patent application number 11/154919 was filed with the patent office on 2005-11-10 for rolling elements.
This patent application is currently assigned to KOMATSU LTD.. Invention is credited to Hamasaka, Naoji, Morioka, Noriko, Takayama, Takemori.
Application Number | 20050247377 11/154919 |
Document ID | / |
Family ID | 31884535 |
Filed Date | 2005-11-10 |
United States Patent
Application |
20050247377 |
Kind Code |
A1 |
Takayama, Takemori ; et
al. |
November 10, 2005 |
Rolling elements
Abstract
The pitting resistance of a gear is increased by hardening its
tooth flanks through application of carburizing/quenching, bright
hardening and induction hardening to a steel material capable of
providing significantly improved softening resistance in tempering
at a low temperature of 300 to 350.degree. C. To this end, the
steel material prepared so as to satisfy the relationship described
by: 5.ltoreq.4.3.times.Si (wt %)+7.3.times.Al (wt %)+3.1.times.V
(wt %)+1.5.times.Mo (wt %)+1.2.times.Cr (wt %).times.(0.45.div.C
(wt %)) is carburized such that the carbon concentration of its
carburized surface layer is adjusted to 0.6 to 0.9 wt %; and the
steel material is subjected to quenching and tempering at
300.degree. C. or less subsequently to the carburization process,
or alternatively the steel material is once cooled after the
carburization process and then subjected to treatments of
re-heating hardening and tempering at 300.degree. C. or less so
that a hardness of HRC 58 or more is ensured by the tempering
process at 300.degree. C.
Inventors: |
Takayama, Takemori; (Osaka,
JP) ; Hamasaka, Naoji; (Osaka, JP) ; Morioka,
Noriko; (Osaka, JP) |
Correspondence
Address: |
FRISHAUF, HOLTZ, GOODMAN & CHICK, PC
220 5TH AVE FL 16
NEW YORK
NY
10001-7708
US
|
Assignee: |
KOMATSU LTD.
Tokyo
JP
|
Family ID: |
31884535 |
Appl. No.: |
11/154919 |
Filed: |
June 16, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11154919 |
Jun 16, 2005 |
|
|
|
10641362 |
Aug 13, 2003 |
|
|
|
Current U.S.
Class: |
148/319 |
Current CPC
Class: |
C23C 8/22 20130101; C23C
8/80 20130101 |
Class at
Publication: |
148/319 |
International
Class: |
C23C 008/22 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 21, 2002 |
JP |
2002-240967 |
Claims
What is claimed is:
1. A rolling element which is made from a steel material containing
at least 0.15 to 0.35 wt % C; further containing either 1.0 to 3.0
wt % Si or 0.35 to 1.5 wt % Al or alternatively, 0.5 to 3.0 wt %
(Si+Al); and further containing one or more alloy elements selected
from the group consisting of Mn, Ni, Cr, Mo, V, Cu, W, Ti, Nb, B,
Zr, Ta, Hf, and Ca, unavoidable impurities such as P, S, N and O,
and balance essentially consisting of Fe; said steel material being
prepared so as to satisfy the relationship described by:
5.ltoreq.4.3.times.Si (wt %)+7.3.times.Al (wt %)+3.1.times.V (wt
%)+1.5.times.Mo (wt %)+1.2.times.Cr (wt %).times.(0.45.div.C (wt
%)), and which is formed by carburizing said steel material such
that the carbon concentration of a carburized surface layer of the
steel material is adjusted to 0.6 to 0.9 wt %; quenching the steel
material subsequently to the carburization process and then
tempering the steel material at 300.degree. C. or less, or
alternatively cooling the steel material once after the
carburization process and then applying treatments of re-heating
hardening and tempering at 300.degree. C. or less to the steel
material so that a hardness of HRC 58 or more is ensured by the
tempering process at 300.degree. C.
2. The rolling element according to claim 1, wherein, in said steel
material, the amount of Cr is limited to no more than 1.4 times the
amount of Si, and one or more elements selected from the group
consisting of 0.35 wt % or less Mo, 0.4 wt % or less V, 1.0 to 2.5
wt % (Mn+Ni) are added in an amount which satisfies the
relationship described by: -0.146.times.Si (wt %)+0.03.times.Mn (wt
%)-0.024.times.Ni (wt %)+0.075.times.Cr (wt %)+0.043.times.Mo (wt
%)+0.133.times.V (wt %).ltoreq.0.
3. The rolling element according to claim 1, wherein said steel
material contains 1.5 to 2.5 wt % Si or (Si+Al) and less than 2.0
wt % Cr to prevent precipitation of cementite during the
carburization process.
4. The rolling element according to claim 1, wherein said steel
material having an Al content of 0.3 wt % or more contains 0.3 to
1.5 wt % Ni.
5. The rolling element according to claim 2, wherein said steel
material contains 1.5 to 2.5 wt % Si or (Si+Al) and less than 2.0
wt % Cr to prevent precipitation of cementite during the
carburization process.
6. The rolling element according to claim 2, wherein said steel
material having an Al content of 0.3 wt % or more contains 0.3 to
1.5 wt % Ni.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a Divisional application of U.S.
application Ser. No. 10/641,362, filed on Aug. 13, 2003.
TECHNICAL FIELD
[0002] The present invention relates to rolling elements produced
by carburizing and quenching, bright hardening or induction
hardening. More particularly, the invention relates to gears and
rolling elements such as bearings, races and rollers, the gears
being made from steel which provides significantly improved
resistance to softening caused in low-temperature tempering at 300
to 350.degree. C. and having high pitting resistance in the tooth
flanks hardened by carburizing/quenching, bright hardening or
induction hardening.
BACKGROUND ART
[0003] Up to now, gears produced by applying carburizing/quenching,
carburizing/carbonitriding/quenching to SCr-based, SCM-based or
SNCM-based low carbon steel have been commonly employed in the
reducers of construction machines and earth-moving machines, since
high contact fatigue strength (200 kgf/mm.sup.2 or more) is
considered to be an important factor. For ring gears used under the
condition of comparatively low interface pressure (up to 150
kgf/mm.sup.2), gears produced by applying thermal treatment such as
bright hardening or induction hardening to carbon steel or
SMn-based middle carbon steel (0.45 to 0.6 wt % C) are used.
[0004] For the reducers of construction machines and earth-moving
machines, less expensive gears having higher strength and higher
resistance to interface pressure are required, in view of the
recent tendency to higher output power and compactness.
[0005] Construction machines and earth-moving machines often stride
obstacles such as rocks and structures during travelling and drill
the obstacles while making a turn, and therefore, the gears of the
reducer used for running and turning such machines receive
impulsive load. This is a serious problem of damage to carburized
quenched gears.
[0006] Bright-hardened or induction-hardened gears have higher
toughness than carburized quenched gears, but are more likely to
cause pitting or scuffing when they are used under high interface
pressure such as noted above.
[0007] The invention is directed to overcoming the problem of the
conventional carburized, quenched gears and induction-hardened
gears which exhibit poor impact resistance when they have
insufficient contact fatigue strength. Taking account of the fact
that the contact fatigue strength of a gear used under a
rolling/sliding contact condition highly depends on whether or not
it has sufficient temper softening resistance against an increase
up to 300.degree. C. in the temperature of the tooth flanks during
operation, the invention aims to provide various types of rolling
elements such as carburizing and quenching gears for use under high
interface pressure, which elements are made from a steel material
to which a large amount of Al and/or Si (Al and Si can effectively
increase resistance to softening caused by tempering at 300.degree.
C.) has been added and which elements have a temper hardness of HRC
58 or more after tempering at 300.degree. C. The invention further
aims to provide, through proper combined additions of Al and Ni to
the above steel material, rolling elements which can exhibit high
toughness in spite of their high hardness.
[0008] The present invention has been directed to overcoming the
poor pitting resistance of gears hardened by bright hardening or
induction hardening and therefore aims to provide inexpensive
rolling elements such as high induction hardened gears which have
been improved in temper softening resistance so as to have a temper
hardness of HRC 54 or more at 300.degree. C. and pitting resistance
equivalent to that of carburized quenched gears, by more proper
additions of Si, Al, V, Mn, Cr, Mo and Ni.
DISCLOSURE OF THE INVENTION
[0009] SNCM815, SCM420, SCr420, SMnB420 steels which had been
subjected to carburizing and quenching were preliminarily tested in
terms of rolling contact fatigue strength (pitting resistance)
under the condition of rolling/sliding at interface pressures of
375 to 220 kgf/mm.sup.2. As a result, it was found that the
interface pressure at which pitting appeared after 107 rotations
was 210 kgf/mm.sup.2 and the X-ray half value width of the
martensitic phase of the outermost layer of the rolling contact
surface in which pitting occurred under each pressure was reduced
to 4 to 4.2.degree., and significant softening was observed at the
outermost layer of the rolling contact surface.
[0010] An S55C carbon steel which has been subjected to quenching
and tempering so as to have HRC 61 to 62 was preliminarily tested
in terms of rolling contact fatigue strength at an interface
pressure of 250 kgf/mm.sup.2. As a result, it was found that the
interface pressure at which pitting appeared after 10.sup.7
rotations was about 180 kgf/mm.sup.2 and the X-ray half value width
of the martensitic phase of the rolling contact surface in which
pitting occurred under an interface pressure of 250 kgf/mm.sup.2
was reduced to 3.6 to 4.2 similarly to the above-described
carburized, case-hardened steels.
[0011] A preliminary test was also conducted on an eutectoid carbon
steel (0.77 wt % C) to check its rolling contact fatigue strength.
As a result, it was found that the interface pressure at which
pitting appeared after 107 rotations was about 230 to 240
kgf/mm.sup.2 which was substantially the same as the rolling
contact fatigue strength of the aforesaid carburized, case-hardened
steels having substantially the same carbon content. It was also
found that a decrease due to variation in rolling contact fatigue
strength was observed in the carburized case-hardened steels
because of the presence of an intergranular oxidation layer and a
slack quenching layer in the rolling contact surface.
[0012] A preliminary test was conducted on an eutectoid carbon
steel (0.82 wt % C), whose rolling contact surface had been
subjected to induction hardening, to check its rolling contact
fatigue strength and it was found that the interface pressure at
which pitting appeared after 107 rotations was about 260 to 270
kgf/mm.sup.2 and this eutectoid carbon steel had higher rolling
contact fatigue strength than the former eutectoid steel (0.77 wt %
C) because of fine cementite particles dispersing in the
martensitic phase of the rolling contact surface.
[0013] From the viewpoint of the dispersion of fine cementite
particles, SUJ2 containing about 1.0 wt % C and 1.5 wt % Cr was
quenched at 840.degree. C. and then tempered to have HRC 62.5. The
rolling contact fatigue strength of this steel was checked by a
preliminary test and it was found that the interface pressure at
which pitting appeared after 10.sup.7 rotations was about 270
kgf/mm.sup.2 which was approximately the same as that of the above
eutectoid steel and that the X-ray half value width of the
martensitic phase of the rolling surface in which pitting occurred
under an interface pressure of 250 kgf/mm.sup.2 was reduced to 4.2
to 4.50 similarly to the carburized, case-hardened steels described
above.
[0014] Further, carbon steels having a carbon content of 0.46,
0.55, 0.66, 0.77 and 0.85 wt % respectively were quenched from a
temperature of 820.degree. C. and tempered at 100 to 350.degree. C.
for 3 hours. Then, the hardness and X-ray half value width of each
steel were checked. It was found from the test result and studies
using, as a reference, published data on these steels (e.g.,
"Materials" issued by Society of Materials Science, Japan, Vol. 26,
No. 280, P26) that the hardness when the X-ray half value width of
the martensitic phase is 4 to 4.20 corresponds to a temper hardness
of about HRC 51 to 53. Taking account of the fact that the surface
carbon concentrations of the carburized, case-hardened steels were
adjusted to about 0.7 to 0.9 wt %, the tempering temperature was
found to be about 300.degree. C.
[0015] It is obvious from the preliminary tests described above
that the outermost surface of a tooth flank is tempered and
softened by heat generated at the time when the gears come into
engagement under high interface pressure so that pitting occurs,
and that a 300.degree. C. -temper hardness of HRC 53 or more is
necessary, as an index, for obtaining the same level of pitting
resistance as that of the carburized quenched gears.
[0016] It has also been understood from the comparison between the
300.degree. C.-temper hardness of the carburization-hardened layer
of the SCM420 steel which has undergone carburizing/quenching and
the 300.degree. C.-temper hardness of the eutectoid carbon steel
which has undergone quenching that since virtually no improvement
in temper softening resistance can be attained by additions of Cr
and Mo, a new alloy design intended for increasing temper softening
resistance during low-temperature tempering at about 300.degree. C.
is necessary in order to achieve pitting resistance equal to or
more than that of the carburized, quenched gears by bright
hardening or induction hardening. Also, dispersion of fine
cementite particles or the like in the martensitic phase has proved
effective as seen from the cases of eutectoid carbon steel (0.82 wt
% C) and SUJ2 which were improved in rolling contact fatigue
strength.
[0017] As a gear design value which provides pitting resistance
equal to or higher than the pitting resistance obtained by the
carburizing/quenching (interface pressure Pmax=230 kgf/mm.sup.2 or
more) described above, the hardness which can withstand fatigue
caused by pulsating shear stress (R=0) which is 0.3 times the value
of interface pressure may be set based on the theoretical analysis
of Hertz's contact pressure. Its calculated value is approximately
HRC 53.4 which coincides with the hardness (HRC=53) obtained from
the X-ray half value width of the martensitic phase of the rolling
contact surface in which occurrence of pitting was observed in the
above-described preliminary test. Since pitting occurs at the time
when the temperature of the outermost portion of the rolling
contact surface increases to about 300.degree. C. owing to the
friction heat generated by rolling/sliding, it has been found that
a highly pressure-resistant gear having interface pressure
resistance equal to or higher than that of the carburized quenched
gears can be developed by setting 300.degree. C.-temper hardness to
HRC 54 or more which can withstand Pmax=230 kgf/mm.sup.2.
[0018] As will be described in Example 2, the 300.degree. C.-temper
hardness of the martensitic phase of a carbon steel containing 0.1
to 1.0 wt % carbon is described by:
HRC=36.times.{square root}{square root over ( )}C(wt %)+20.9
[0019] After checking, based on the above hardness, the influences
of various alloy elements upon the hardness of the martensitic
phase after tempering at 300.degree. C., it has become apparent
that the hardness of the martensitic phase after tempering at
300.degree. C. is represented by:
HRC=(36.times.{square root}{square root over (C)}(wt
%)+20.9)+4.3.times.Si (wt %)+7.3.times.Al (wt %)+3.1.times.V (wt
%)+1.5.times.Mo (wt %)+1.2.times.Cr (wt %).times.(0.45.div.C (wt
%))
[0020] It should be noted that the coefficient (in the case of Si
for instance, this coefficient is 4.3 .DELTA.HRC/wt %) proportional
to the weight percent of each alloy element of the above equation
indicates the temper softening resistance of the alloy element.
[0021] In the invention, the content (wt %) of each alloy element
constituting the above steels is defined as follows based on the
above-described gear materials and thermal treatment designs.
[0022] To sum up, there is provided a rolling element according to
the invention which is made from a steel material containing at
least 0.15 to 0.35 wt % C; further containing either 1.0 to 3.0 wt
% Si or 0.35 to 1.5 wt % Al or alternatively, 0.5 to 3.0 wt %
(Si+Al); and further containing one or more alloy elements selected
from the group consisting of Mn, Ni, Cr, Mo, V, Cu, W, Ti, Nb, B,
Zr, Ta, Hf, and Ca, unavoidable impurities such as P, S, N and O,
and balance essentially consisting of Fe; the steel material being
prepared so as to satisfy the relationship described by:
5.ltoreq.4.3.times.Si (wt %)+7.3.times.Al (wt %)+3.1.times.V (wt
%)+1.5.times.Mo (wt %)+1.2.times.Cr (wt %).times.(0.45.div.C (wt
%)), and
[0023] which is formed by carburizing the steel material such that
the carbon concentration of a carburized surface layer of the steel
material is adjusted to 0.6 to 0.9 wt %; quenching the steel
material subsequently to the carburization process and then
tempering the steel material at 300.degree. C.- or less, or
alternatively cooling the steel material once after the
carburization process and then applying treatments of re-heating
hardening and tempering at 300.degree. C. or less to the steel
material so that a hardness of HRC 58 or more, more preferably, HRC
60 or more is ensured by the tempering process at 300.degree.
C.
[0024] While the 300.degree. C. temper-hardness of the carburized
layer of an SCM-based carburized, quenched material is usually
within the range of from HRC 53 to HRC 56, the 300.degree.
C.-temper hardness is set to HRC 58 or more in the invention on the
ground that: (i) an improvement in pitting resistance can be
clearly observed and (ii) taking account of the fact that the
percentage of compactness when a mechanical reduction gear is
downsized by one lank is 25 to 30% and the contact fatigue strength
of the gear in this case is no less than 1.15 times the contact
fatigue strength of the conventional gear (230.fwdarw.265
kgf/mm.sup.2), the 300.degree. C.-temper hardness is HRC 58 or
more.
[0025] An addition of about 1 wt % Al is apparently preferable for
improvement of pitting resistance, because 15 to 25% by volume of
the residual austenitic phase existing, for example, in the
carburized layer of an SCM-based steel is reduced to 10% by volume
or less, which has the effect of increasing the hardness of the
carburized layer of the surface (.DELTA.HRC=2).
[0026] It is also apparently preferable for rolling elements such
as gears having higher strength to apply mechanical pressurization
treatment such as shot peening or roller burnishing to the tooth
flanks, dedenda and tooth bottoms of the gear with the intention of
improving the strength of the tooth flanks and the bending strength
of the dedenda, whereby a distinct compressive residual stress is
generated. It is apparent that the elements to which such treatment
is applied are also within the scope of the invention.
[0027] The above-described carburization is usually carried out at
900.degree. C. or more. Where Si and Al are contained in high
concentration as described earlier, the dual-phase
(.alpha.+.gamma.) state develops in a reheating condition in the
raw material composition part having low carbon content and
positioned deeper than the carburized layer, and quenching starts
from this condition so that the strength of the inside of the
carburized layer decreases. This drawback can be overcome by
setting carburized case depth taking account of the distribution of
bearing stress and the distribution of bending stress. In addition,
it is economically disadvantageous to increase carburized case
depth and therefore, the A3 transformation temperature is adjusted
by adding the austenite stabilizing elements C, Mn and Ni in
combination with the ferrite stabilizing elements Si and Al as
described earlier, thereby controlling the temperature of
carburization to a typical carburization temperature of 950.degree.
C. or less.
[0028] In the case of a steel material containing either 1.0 to 3.0
wt % Si or 0.35 to 1.5 wt % Al or alternatively, 0.5 to 3.0 wt %
(Si+Al) to increase temper softening resistance, an addition of 3
wt % Si increases the A3 transformation temperature by about 170
degrees (see FIG. 1) and an addition of 1.5 wt % Al also increases
it to the same extent, where the carbon content is 0.20 wt %.
Therefore, the upper limits of the amounts of Si and Al are set to
3.0 wt % and 1.5 wt %, respectively. According to a third aspect of
the invention, Mn and/or Ni is added in the range of 1.0 to 2.5 wt
% (Mn+Ni) with the intention of restraining the temperature of
quenching by lowering the A3 transformation temperature through
proper additions of the austenite stabilizing elements such as C,
Mn, Ni and Cu.
[0029] Since carbon and nitrogen are extremely effective as an
austenite stabilizing element (see FIG. 1), the lower limit of the
original carbon content of the steel material is preferably 0.15 wt
% from the above viewpoint and the upper limit of the carbon
content is preferably 0.35 wt % with which the hardness of the raw
material composition part inside the carburized layer after
quenching and tempering does not exceed HRC 55. More preferably,
the lower limit of the carbon content is 0.2 wt %.
[0030] In addition, since nitrogen often reduces the temper
softening resistance of Al, it is necessary to prevent penetration
of nitride from the carburized or carbonitrided gas atmosphere and,
therefore, creation of Al nitrides. In view of this, the N content
of the carburized layer is set to at least 0.1 wt %.
[0031] According to the invention, there is provided a rolling
element which is made from a steel material containing at least
0.15 to 0.35 wt % C; further containing either 1.0 to 3.0 wt % Si
or 0.35 to 1.5 wt % Al or alternatively, 0.5 to 3.0 wt % (Si+Al);
and further containing one or more alloy elements selected from the
group consisting of Mn, Ni, Cr, Mo, V, Cu, W, Ti, Nb, B, Zr, Ta,
Hf, and Ca, unavoidable impurities such as P, S, N and O, and
balance essentially consisting of Fe; the steel material being
prepared so as to satisfy the relationship described by:
5.ltoreq.4.3.times.Si (wt %)+7.3.times.Al (wt %)+3.1.times.V (wt
%)+1.5.times.Mo (wt %)+1.2.times.Cr (wt %).times.(0.45.div.C (wt
%)), and
[0032] which is formed by carburizing the steel material such that
the carbon concentration of a carburized surface layer of the steel
material is adjusted to 0.9 to 1.5 wt %; applying treatments of
reheating hardening and tempering at 300.degree. C. or less to the
steel material after cooling to a temperature equal to or lower
than Al temperature from a state where no cementite precipitates in
the surface layer during the carburization process, so that fine
cementite particles having a size of 1 .mu.m or less are dispersed
within the tempered martensitic phase of the carburized surface
layer and a hardness of HRC 60 or more, more preferably, HRC 62 or
more is ensured by the tempering process at 300.degree. C.
[0033] The reason why the lower limit of the carbon content of the
surface area of the carburized layer is set to 0.9 wt % is that
eutectoid carbon concentration is markedly decreased by additions
of Si and Cr and undissolved cementite is stably formed in an
amount of 3% by volume or more when the carbon content is 0.9 wt %
or more. The reason why the carbon content is limited to 1.5 wt %
or less is that if the carbon content exceeds 1.5 wt %, coarse
cementite particles (3 .mu.m or more) will be unavoidably created
owing to aggregation of cenentite particles, so that there arises
the high risk of a decrease in the bending strength of the gear. In
addition, for carrying out carburization with a high carbon
concentration of 1.5 wt % or more without causing precipitation of
coarse cementite particles in the surface layer during the
carburization process, it is necessary to increase carburization
temperature to about 1100.degree. C. which is practically difficult
because of the limitation in terms of equipment.
[0034] Since the carburization which provides a surface carbon
concentration of 0.9 to 1.5 wt % is carried out in a high carbon
potential condition with a carbon activity (ac) of about 1 and is
preferably carried out in the high temperature region (1000.degree.
C. or more), there is the possibility of precipitation of coarse
cementite particles in the surface layer during the carburization,
and therefore, carbon potential needs to be controlled with high
accuracy. However, it is extremely difficult to control high carbon
potential carburization carried out at a temperature of
1000.degree. C. or more. Focussing on the fact that Cr contained in
a steel material promotes precipitation of coarse cementite
particles, the invention is arranged such that precipitation of
cementite is prevented even in high carbon potential carburization,
by reducing the amount of Cr to 0.5 wt % or less or by limiting the
amount of Cr to no more than 1.4 times the amount of Si.
[0035] More precisely speaking, the effects of Mn, Ni, Mo and the
like should be taken into account and it is preferable to consider
the relationship described by:
-0.146.times.Si (wt %)+0.03.times.Mn (wt %)-0.024.times.Ni (wt
%)+0.075.times.Cr (wt %)+0.043.times.Mo (wt %)+0.133.times.V (wt
%).ltoreq.0
[0036] Practically, it is preferable to set the amount of Si or
(Si+Al) to 1.5 to 2.5 wt % and to limit the amount of Cr to 2.0 wt
% or less.
[0037] As discussed earlier, the invention is intended for
dispersion of fine cementite particles on condition that reheating
hardening treatment is applied to the steel and, therefore, the
temperature of the reheating hardening treatment is equal to or
more than the Al transformation temperature. In the case where Si
and Al are contained in high concentration, the (.alpha.+.gamma.)
dual phase state develops under the reheating condition within the
raw material composition part having low carbon content and
positioned deeper than the carburized layer and quenching starts
from this condition so that the strength of the inside of the
carburized layer decreases as described earlier, however, it is
apparent that this problem can be solved by setting the carburized
case depth taking account of the distribution of bearing stress and
the distribution of bending stress. Since increasing of the
carburized case depth is disadvantageous in view of cost, it is
preferable to adjust the A3 transformation temperature by
controlling the amounts of carbon, Mn and Ni in compliance with the
amounts of Si and Al so that the temperature of the reheating
hardening process is set to 950.degree. C. or less.
[0038] Where the temperature of the reheating hardening process is
set to a value as high as 850 to 950.degree. C., it is difficult to
fine cementite particles dispersing in an ordinary SCM-based
material (0.75 wt % Mn, 1 wt % Cr, 0.15 wt % Mo) so as to have a
size of 1 .mu.m or less. Therefore, the steel of the invention
contains at least 0.4 wt % or less V which condenses to a
significant degree within cementite when austenite and cementite
are in the equilibrium state. It should be noted that the
distribution coefficient of each alloy element defined by a
distribution coefficient KM (KM=the concentration of M element in
cementite (wt %).div.the concentration of M element (wt %) in
austenite) is given by:
KV=12.3, KCr=6.4, KMo=3.5, KNi=0.22, KSi, Al.apprxeq.0
[0039] According to the invention, there is provided a rolling
element which is made from a steel material containing at least
0.35 to 0.60 wt % C; further containing either 1.0 to 3.0 wt % Si
or 0.35 to 1.5 wt % Al or alternatively, 0.5 to 3.0 wt % (Si+Al);
and further containing one or more alloy elements selected from the
group consisting of Mn, Ni, Cr, Mo, V, Cu, W, Ti, Nb, B, Zr, Ta,
Hf, and Ca, unavoidable impurities such as P, S, N and O, and
balance essentially consisting of Fe; the steel material being
prepared so as to satisfy the relationship described by:
5.ltoreq.4.3.times.Si (wt %)+7.3.times.Al (wt %)+3.1.times.V (wt
%)+1.5.times.Mo (wt %)+1.2.times.Cr (wt %).times.(0.45.apprxeq.C
(wt %)), and
[0040] which is formed by tempering the steel material at
300.degree. C. or less after quenching treatment such as induction
hardening so that a hardness of HRC 55 or more is ensured for a
hardened surface layer of the steel material by the tempering
process at 300.degree. C.
[0041] An essential condition of the invention is that the hardness
of the steel after tempering at 300.degree. C. subsequent to
quenching is HRC 55 or more. To satisfy this condition, the steel
material is preferably hardened by quenching so as to have a
hardness of about HRC 58 or more and therefore the lower limit of
the carbon content is set to approximately 0.35 wt %. Where the
temper softening resistance which corresponds to a 300.degree.
C.-temper hardness of HRC 53 is ensured by sole additions of Si or
Al, it becomes necessary to add 2.5 wt % or more Si or 1.47 wt % or
more Al as seen from the foregoing equation, which causes the
temperature of quenching to be as high as 900.degree. C. or more
(see FIG. 1). It is obviously more preferable for the invention to
properly control the amount of carbon which is an extremely
effective austenite stabilizing element, thereby restricting an
increase in quenching temperature and to set the amount of carbon
to 0.43 wt % or more in order to ensure stable hardness after
quenching.
[0042] A preferable upper limit of the amount of carbon is 0.6 wt %
or less when taking account of quenching crack susceptibility at
the time of induction hardening. It is understood from simple
calculation that where carbon is added within the range of from 0.4
wt % to 0.6 wt %, proper amounts of Si and Al in the case of sole
addition are 1.0 wt % or more and 0.6 wt % or more,
respectively.
[0043] Further, it is apparent that, in order to obtain the
substantially same level of contact fatigue strength as the average
tooth flank strength of the carburized, quenched gears, the
300.degree. C.-temper hardness is preferably HRC 55 or more, and
the upper limit of the amount of carbon is preferably 0.55 wt %
when taking account of susceptibility to quenching cracks caused by
water or an aqueous quenching liquid used in the induction
hardening.
[0044] As an effective way of producing a gear member capable of
withstanding high interface pressure, the above-described high
carbon-concentration carburization is applied to the gear material
of the invention such that fine cementite particles having a size
of 1 .mu.m or less are dispersed in the surface layer. For
preventing quenching cracks occurring during the reheating
hardening process or the induction hardening process, it is
preferable to utilize an aqueous quenching liquid or quenching oil
having a high concentration of a polymer element.
[0045] To attain higher contact fatigue strength than that of the
above-described carburized quenched gears, the inventors have
developed a rolling element such as a gear which is made from a
steel material containing at least 0.60 to 1.50 wt % C; further
containing either 1.0 to 3.0 wt % Si or 0.35 to 1.5 wt % Al or
alternatively, 0.5 to 3.0 wt % (Si+Al); and further containing one
or more alloy elements selected from the group consisting of Mn,
Ni, Cr, Mo, V, Cu, W, Ti, Nb, B, Zr, Ta, Hf, and Ca, unavoidable
impurities such as P, S, N and O, and balance essentially
consisting of Fe; the steel material being prepared so as to
satisfy the relationship described by:
5.ltoreq.4.3.times.Si (wt %)+7.3.times.Al (wt %)+3.1.times.V (wt
%)+1.5.times.Mo (wt %)+1.2.times.Cr (wt %).times.(0.45.div.C (wt
%)), and
[0046] which is formed by tempering the steel material at
300.degree. C. or less after quenching treatment such as induction
hardening so that a hardness of HRC 58 or more is ensured for a
hardened surface layer of the steel material by the tempering
process at 300.degree. C.
[0047] Since there is no need to dissolve all the cementite in the
austenite, the heating temperature of the induction hardening
process can be set to a value in the dual phase (austenite and
cementite) coexisting region having an Al transformation
temperature of 950.degree. C. or less and the concentration of
carbon dissolved in the austenite can be set to a smaller value
under this condition. By this arrangement and utilization of a
quenching oil or an aqueous polymer quenching liquid as a quenching
medium, quenching crack susceptibility can be reduced.
[0048] Where fine cementite particles are dispersed by high
frequency heating and quenching, undissolved cementite is unlikely
to be coarsened because the dispersion is carried out in a short
time (within several minutes) by rapid heating, and, therefore, the
addition of V is not indispensable. However, the addition of V is
useful from the viewpoint of further fining the structure prior to
the induction hardening process and the additions of Cr, V, Mo and
Mn is useful from the same view point.
[0049] In addition, in the case of a rolling element such as a gear
used under higher interface pressure, it can be assumed that the
rolling contact surface is exposed to higher temperature, and V
exhibits remarkable temper softening resistance A HRC (350.degree.
C.:4.6, 400.degree. C.:6.1, 450.degree. C.:9.2). Therefore, the
amount of V is set to 0.05 wt % or more with which the effect of V
becomes conspicuous. The upper limit of the amount of V is set to
0.4 wt % for the reason that the effect of V can be effectively
utilized when it is added in this amount in the case where the
maximum quenching temperature is 950.degree. C.
[0050] Although it is favorable to positively add Mo, because Mo
exerts remarkable temper softening resistance (350.degree. C.:2.4,
400.degree. C.:3.23, 450.degree. C.:4.9) in the higher temperature
region, the upper limit of the amount of Mo is set to 0.35 wt %
from the economical viewpoint.
[0051] Where the additions of large amounts of Si and Al cause
graphite precipitation in the process of manufacturing the steel
material or in the thermal treatment of the invention, there is a
likelihood of a significant decrease in strength. Therefore, it is
apparently preferable for the invention to add 0.2 to 0.5 wt % Cr
which significantly stabilizes at least cementite and prevents
graphitization.
[0052] The rolling element such as a gear contains one or more
elements selected from 0.3 to 1.5 wt % Mn, up to 0.35 wt % Mo and
0.0005 to 0.005 wt % B, on the ground that the induction hardened
steel material is not required to have high hardenability since the
induction hardening treatment is a heating process in which heat is
applied to only the part to be quench-hardened and its
neighborhood, unlike furnace heating.
[0053] The present inventors have already reported in Japanese
Patent Application No. 2002-135274 that considerably high toughness
can be achieved by the coexistence of Al in the aforesaid amount
and 0.3 to 2.5 wt % Ni and excellent Charpy impact characteristic
can be obtained by it, particularly, in high-hardness martensitic
structures containing 0.6 wt % carbon and 1.2 wt % carbon. Ni
contributes to a dramatic improvement in the impact load resistance
of a gear and is therefore apparently beneficial as a gear
material. Since the addition of Ni increases the cost of the steel
material, the amount of Ni to be added is limited to 1.5 wt % or
less in the invention.
[0054] If the carburization temperature, reheating temperature and
induction hardening temperature become too high in the invention,
there may arise the problem of coarsening austenite crystal grains.
In this case, it is obviously desirable to add the known elements
called "crystal grain fining elements" such as Ti, Nb, Zr, Ta and
Hf in an amount ranging from 0.005 to 0.2 wt %.
[0055] There will be hereinafter summarized the function of each of
the alloy elements constituting the above-described rolling
elements of the invention.
[0056] Si: 0.8 to 3.0 wt %
[0057] Si is an element which significantly enhances temper
softening resistance in tempering at a low temperature of
300.degree. C. to 350.degree. C. The mechanism of enhancing temper
softening resistance is such that softening is prevented by further
stabilizing .epsilon. carbides which precipitate at low temperature
and causing cementite precipitation to occur in a higher
temperature region.
[0058] (1) Induction Hardened Gears
[0059] Since the softening resistance .DELTA.HRC of Si per 1 wt %
in tempering at 300.degree. C. is 4.3 and the 300.degree. C.-temper
base hardness obtained from 0.6 wt % carbon is HRC 48.8, the amount
of Si for ensuring a 300.degree. C.-temper hardness of HRC 53 is
about 1.0 wt % and the amount of Si when it coexists with 0.15 wt %
Al is about 0.8 wt %. On this ground, the lower limit of the amount
of Si is set to 0.8 wt % and more preferably to 1.5 wt % to enhance
its function.
[0060] While the upper limit of the amount of Si is set to 3.0 wt %
in order that the Ac3 transformation temperature does not exceed
900.degree. C. and the quenching temperature is prevented from
increasing more than is necessary where the amount of carbon is
within the aforesaid range of from 0.35 wt % to 0.6 wt %, the upper
limit of the amount of Si is preferably 2.5 wt % or less in the
case where the lower limit of the amount of carbon contained in the
steel material for induction hardened gears is 0.4 wt %.
[0061] (2) Carburized and Quenched Gears
[0062] The range of Si content noted earlier can be substantially
suitably applied to the upper and lower limits of the amount of Si
to be added for ensuring a 300.degree. C.-temper hardness of HRC 60
in the process in which carburization is applied to the surface of
the rolling element such as a gear to increase its surface carbon
content to 0.6 to 0.9 wt % and then, quenching and tempering at
300.degree. C. or less are carried out.
[0063] Also, the range of Si content noted earlier can be
substantially suitably applied to the upper and lower limits of the
amount of Si added for ensuring at least a 300.degree. C.-temper
hardness of HRC 62 or more in the process in which carburization is
applied to the surface of the rolling element such as a gear,
thereby increasing the surface carbon content to 0.0.9 to 1.5 wt %
and then, reheating hardening treatment is applied so as to
disperse fine cementite particles in the rolling contact surface,
followed by tempering at 300.degree. C. or less.
[0064] To increase the surface carbon content of the rolling
element to 0.9 to 1.5 wt % without precipitating cementite in the
surface layer during the carburization process, the carbon activity
in the carburization process carried out at a high temperature of
930 to 1100.degree. C. needs to be increased. In this case,
precipitation of coarse cementite (3 to 15 .mu.m) (excessive
carburization) mainly due to the addition of Cr element is likely
to occur, resulting in a considerable decrease in the strength of
the gear. In the invention, this problem is solved by positively
adding Si which prevents excessive carburization while the amount
of Cr is limited so as not to exceed 1.4 times the amount of Si,
and, more precisely, by using a steel material which meets the
relationship described by
-0.146.times.Si (wt %)+0.03.times.Mn (wt %)-0.024.times.Ni (wt
%)+0.075.times.Cr (wt %)+0.043.times.Mo (wt %)+0.133.times.V (wt
%).ltoreq.0
[0065] Where the above steel material is used, the vacuum
carburization method wherein carburization is carried out with a
carbon activity of 1 can be employed. This method is extremely
beneficial for producing a rolling element such as a gear, because
high temperature carburization at 1100.degree. C. or less can be
carried out at low cost, and its function of inhibiting coarse
cementite precipitation is advantageous for increasing the strength
of the rolling element such as a gear.
[0066] Since Al has the strong deoxidization function as well as
the strong function of expelling P and S from the grain boundary, P
and S being impurities contained in steel, Al is useful for
cleaning steel materials. Further, it has been confirmed that Al
enhances temper softening resistance more than Si does
(.DELTA.HRC=7.3) in low temperature tempering. In the light of the
above facts, the invention is designed such that where Al is solely
added, the amount of Al is 0.35 to 1.5 wt % and where part of Si is
replaced with 0.15 to 1.5 wt % Al, the total amount of Si and Al is
0.5 to 3.0 wt %. Since Al is a stronger ferrite stabilizing element
than Si as noted earlier and has the function of increasing the Ac3
temperature about 1.6 times higher than Si does, the maximum amount
of Al is set to 1.5 wt % (=2.5 wt %/1.6) or less.
[0067] It has been reported in Japanese Patent Application No.
2002-135274 that remarkable toughness can be achieved by the
coexistence of the above amount of Al and 0.3 to 2.5 wt % Ni, and
excellent Charpy impact characteristic can be achieved,
particularly, in high-hardness martensitic structures containing
0.6 wt % carbon and 1.2 wt % carbon. Ni contributes to a remarkable
improvement in the impact load resistance of a gear and is
therefore apparently beneficial as a gear material. Since the
addition of Ni increases the cost of the steel material, the amount
of Ni is limited to 1.5 wt % or less in the invention.
[0068] Since Mn not only exhibits remarkable desulfurization but
also stabilizes austenite as noted earlier and further has the
beneficial effect of improving the hardenability of steel, Mn is
added in a proper amount according to purposes. Taking account of
the fact that, in a steel containing 0.35 to 0.6 wt % carbon,
austenite is satisfactorily stabilized by carbon, the lower limit
of the amount of Mn is set to 0.3 wt %.
[0069] Mo is a useful element as it improves the hardenability of
steel and restrains temper brittleness, and therefore it is
desirable for the invention to add Mo in an amount of 0.35 wt % or
less which is at the same level as that of ordinary case-hardened
steels.
[0070] In cases where the tooth flanks of a gear are
quench-hardened by induction hardening, only the surface layer
which has been heated to a temperature equal to or higher than the
Ac3 transformation temperature by high-frequency heating may be
quench-hardened, and therefore the gear material is not required to
have hardenability (DI value) higher than the hardenability (3.0
inches) of the ordinary carbon steel level. This means that
inexpensive steel materials can be employed. In the invention, the
addition of Mn and Cr is further suppressed and the addition of
alloy elements such as Si, Al, Ni, Mo and V is controlled to obtain
a DI value of 3.0 inches or less.
BRIEF DESCRIPTION OF THE DRAWINGS
[0071] FIG. 1 is a phase diagram showing the effects of various
alloy elements which constitute Fe3Si.
[0072] FIGS. 2(a) and 2(b) illustrate a test specimen used for a
roller pitting test.
[0073] FIG. 3 is a graph showing the result of a preliminary test
for checking roller pitting resistance.
[0074] FIG. 4 is a graph showing the comparison between the
measured values and calculated values of temper hardness.
[0075] FIG. 5 is a graph (1) showing the pitting resistance of
steels according to the invention.
[0076] FIGS. 6(a) and 6(b) show patterns for vacuum carburization
quenching treatment.
[0077] FIG. 7 is a graph (2) showing the pitting resistance of
steels according to the invention.
[0078] FIG. 8 is a graph (3) showing the pitting resistance of
steels according to the invention.
[0079] FIGS. 9(a) and 9(b) are photographs of the metallographic
structures of the carburized layers of test specimens No. 5 and No.
G2.
BEST MODE FOR CARRYING OUT THE INVENTION
[0080] Referring now to the accompanying drawings, rolling elements
of the invention will be hereinafter described according to
preferred embodiments of the invention.
EXAMPLE 1
The Pitting Resistance of Quenched, Tempered Carbon Steel and
Carburized, Quenched, Case-Hardened Steel
[0081] (Preliminary Test)
[0082] In this example, a roller pitting test was conducted with
the test specimen shown in FIG. 2 and the pitting resistance of
various quenched, tempered carbon steels and carburized, quenched,
case-hardened steels was checked to investigate the rolling contact
fatigue strength of the tooth flanks of gears. Table 1 shows the
chemical compositions of the various carbon steels and
case-hardened steels used in this example. These steel materials
were respectively shaped into the small roller test specimen shown
in FIG. 2(a) and the test specimens No. 1, 2 and 4 were subjected
to water quenching after heating at 820.degree. C. for 30 minutes,
and then tempered at 160.degree. C. for 3 hours, followed by
testing. The specimen No. 3 was quench-hardened, at its rolling
contact surface, using a 40 kHz high-frequency power source after
thermal refining and then subjected to tempering similarly to the
above specimens. No. 5 was cooled to 850.degree. C. after
carburization (carbon potential=0.8) at 930.degree. C. for 5 hours.
Then, it was kept at 850.degree. C. for 30 minutes and quenched by
a quenching oil having a temperature of 60.degree. C., followed by
the same tempering treatment as described above.
1 TABLE 1 C Si Mn Ni Cr Mo NOTE No.1 0.55 0.23 0.71 S55C No.2 0.77
0.21 0.74 EUTECTOID CARBON STEEL (1) No.3 0.85 0.22 0.81 EUTECTOID
CARBON STEEL (2) No.4 0.98 0.27 0.48 1.47 SUJ2 No.5 0.19 0.22 0.75
0.97 0.15 SCM420H
[0083] The large roller test specimen shown in FIG. 2(b) was
prepared by applying water quenching to the SUJ2 material of No. 4
after heating at 820.degree. C. for 30 minutes and then tempering
it at 160.degree. C. for 3 hours. The roller pitting test was
carried out in such a way that the small and large (loaded) rollers
were rotated at speeds of 1050 rpm and 292 rpm respectively, while
being lubricated with #30 engine oil having a temperature of
70.degree. C., and a load is imposed on the rollers with a slip
ratio of 40% and interface pressures ranging from 375 to 220
kgf/mm.sup.2.
[0084] FIG. 3 collectively shows the number of repetitions which
causes occurrence of pitting under each interface pressure. In FIG.
3, a lifetime line (indicated by solid line) is shown, which is
formed by connecting the respective minimum numbers of repetitions
of the reference carburized case-hardened steel tested under the
various interface pressures. If the interface pressure when the
number of repetitions which causes occurrence of pitting is
10.sup.7 times was defined as rolling contact fatigue strength, the
pitting resistance was found to be about 210 kgf/mm.sup.2. When
checking the pitting resistance of each test specimen in the same
way, it was found that No. 1=175 kgf/mm.sup.2, No. 2=240
kgf/mm.sup.2, No. 3=260 kgf/mm.sup.2, and No. 4=260 kgf/mm.sup.2.
It was further found that the pitting resistance of the carburized
case-hardened steels varied to a somewhat large extent because of
intergranular oxidation which had occurred during the carburization
of the rolling contact surface, the presence of a slack quenched
layer, and a large amount of residual austenite. It was found from
the comparison in terms of the average number of repetitions which
causes pitting that the pitting strengths of the carburized
case-hardened steels do not differ from that of the test specimen
No. 2.
[0085] The X-ray half value width of the martensitic phase of the
rolling contact surface of each test specimen in which pitting had
occurred under an interface pressure of 250 kgf/mm.sup.2 was
checked. As a result, it was found that No. 1=3.6 to 4.0.degree.,
No. 2=4 to 4.2.degree., No.3=4.2 to 4.4.degree., No.4=4.3 to
4.6.degree. and No.5=4 to 4.2.degree..
[0086] Further, the test specimens Nos. 1 to 5 which had undergone
the above-described thermal treatment were tempered at 250 to
350.degree. C. for 3 hours and then, the X-ray half value width of
the rolling contact surface of each test specimen in which pitting
had occurred was checked. As a result, the half value width of each
specimen under the above condition was found to be coincident with
the half value width when tempering was carried out at 300.degree.
C. It was also found to be substantially coincident with the
relationship between the temper hardnesses and half value widths of
carbon steels having various carbon concentrations which was
reported in "Material" Vol. 26, No. 280, P26.
EXAMPLE 2
Checking of Temper Softening Resistance
[0087] Table 2 shows the alloy compositions employed in this
example. Thermal treatment was carried out in such a way that after
heated at 810 to 870.degree. C. for 30 minutes, each test specimen
was subjected to water cooling and then tempering at 300.degree. C.
or 350.degree. C. for 3 hours. Thereafter, the Rockwell hardness
HRC of each test specimen was checked and the effect of the
addition of each alloy element on the hardness was analyzed.
2TABLE 2 TPNo. C Si Al Mn Ni Cr Mo V B No.6 0.45 1.45 0.46 1.49
0.52 0.14 0.0018 No.7 0.49 1.45 0.46 1.01 1.03 0.15 0.0019 No.8
0.47 0.31 0.46 2.01 1.03 0.15 0.0019 No.9 0.49 0.29 0.45 1.5 1.49
0.23 0.0019 No.10 0.36 1.77 0.6 0.62 0.11 0.0026 No.11 0.45 0.95
0.68 0.01 1.29 0.5 0.0029 No.12 0.39 0.93 1.02 0.08 0.97 0.95 0.5
No.13 0.43 0.26 0.44 1.01 0.48 0.001 No.14 0.47 0.25 0.4 1.01 1.05
0.0018 No.15 0.46 1.5 0.4 1 0.51 0.002 No.16 0.45 0.24 0.4 1.02
0.48 0.31 0.0011 No.17 0.45 1.46 0.39 0.96 0.98 0.001 No.18 0.41
0.25 0.35 1 0.49 0.0017 No.19 0.52 2.3 0.57 0.11 No.20 0.98 0.27
0.48 1.47 No.21 0.55 0.23 0.71 No.22 0.77 0.21 0.74 No.23 0.45 0.21
1.26 0.53 1.51 0.21 No.24 0.6 0.25 0.97 0.93 0.98 1.04 0.35
[0088] In a preliminary experiment, the hardness of a carbon steel
containing 0.1 to 1.0 wt % carbon and 0.3 to 0.9 wt % Mn was
checked to be utilized as base data for the analysis of the effect
of each alloy element. As a result, it was found that the hardness
of this steel was approximated by the following equations.
HRC=34.times.{square root}{square root over ( )}C (wt %)+26.5
(tempering temperature=250.degree. C.)
HRC=36{square root}{square root over ( )}C (wt %)+20.9 (tempering
temperature=300.degree. C.)
HRC=38.times.{square root}{square root over ( )}C (wt %)+15.3
(tempering temperature=350.degree. C.)
[0089] After analyzing the effect of each alloy element based on
the hardnesses of the carbon steels noted above, it was found that
the temper softening resistance .DELTA.HRC in the case of tempering
at 300.degree. C. for instance could be described by the following
equation.
.DELTA.HRC=4.3.times.Si (wt %)+7.3.times.Al (wt %)+1.2.times.Cr (wt
%).times.(0.45.div.C(wt %))+1.5.times.Mo (wt %)+3.1.times.V (wt
%)
[0090] It was found from this result that Al exerted temper
softening resistance 1.7 times higher than that of Si and was
therefore extremely effective as an element for improving rolling
contact strength.
[0091] FIG. 4 shows the degree of coincidence of the temper
hardness obtained from the result of the above analysis with the
temper hardness obtained from actual measurements. It will be
understood from FIG. 4 that temper hardness can be accurately
estimated with the variation range of HRC.+-.1. The 300.degree. C.
-temper hardness of the carburized layer (0.8 wt % carbon) of
SCM420 (No. 5) of Example 1 is indicated by mark .star. in FIG. 4
and well coincident with the calculated value.
EXAMPLE 3
An Improvement in Pitting Resistance by Use of Steel Materials
Having Excellent Temper Softening Resistance 1
[0092] Table 3 shows the alloy components of the steel materials
used in this example. The test specimens No. P1 to No. P10 were
subjected to tempering at 160.degree. C. for 3 hours subsequently
to quenching at 850 to 920.degree. C., whereas the test specimens
No. 11 and No. 12 were subjected to induction hardening under the
same high frequency heating condition as in Example 1. A roller
pitting test was conducted on these test specimens.
3 TABLE 3 300.degree. C. C Si Al Mn Ni Cr Mo V B calculated HRC
No.P1 0.34 0.21 1.47 1.17 0.17 0.11 53.96 No.P2 0.39 1.49 0.49 0.51
0.34 0.05 53.91 No.P3 0.41 1.51 0.72 0.32 0.15 51.09 No.P4 0.41 1.5
0.71 0.32 0.16 0.3 51.99 No.P5 0.45 0.18 1.26 0.53 0.5 0.21 55.94
No.P6 0.55 1.51 0.71 0.15 0.16 54.48 No.P7 0.61 1.21 0.75 0.14
54.34 No.P8 0.62 0.21 1.24 0.53 0.12 59.31 No.P9 0.45 1.02 1.26
0.49 0.12 58.78 No.P10 0.61 0.25 1.47 0.93 0.98 1.04 0.35 62.27
No.P11 0.83 1.01 0.31 0.55 0.96 0.38 62.11 No.P12 1.21 0.2 0.52
0.52 1.01 0.51 0.4 66.62
[0093] The test for checking pitting resistance was carried out
under substantially the same condition as in Example 1 and the test
result is shown in FIG. 5. In FIG. 5, the pitting occurrence line
obtained in Example 1 is indicated by solid line and the pitting
occurrence line obtained in Example 3 is indicated by broken
line.
[0094] It was found from the above result that the pitting
resistance of the rolling contact surface can be dramatically
improved by the sole addition of Al or Si or the combined addition
of Al and Si and found from the comparison between the test
specimens No. P3, P4, P11 and P12 that the pitting resistance of
the rolling contact surface can be dramatically improved by the
addition of V.
[0095] A remarkable improvement in pitting resistance was observed
in the test specimens No. 11 and No. 12 which had been induction
hardened such that fine cementite particles were dispersed in the
martensitic phase of the rolling contact surface.
[0096] Table 3 shows the 300.degree. C. temper hardness obtained
from the calculation and it will be understood that this temper
hardness has good conformity to the interface pressure at which
pitting occurs after 10' times repetitions, the interface pressure
being calculated from this temper hardness.
EXAMPLE 4
An Improvement in Pitting Resistance by Use of Steel Materials
Having Excellent Temper Softening Resistance 2
[0097] This example is intended to increase interface pressure
strength by carburizing and quenching treatment. Table 4 shows the
alloy components of the test specimens. Two kinds of
carburizing/quenching treatments as shown in FIGS. 6(a) and 6(b)
were applied. The treatment shown in FIG. 6(a) is vacuum
carburization carried out at 950.degree. C. (with the intention of
achieving a carbon concentration of 0.8 wt % in the carburized
layer) with methane gas free from N2 gas and the treatment shown in
FIG. 6(b) is carried out at 1,020.degree. C. (with the intention of
achieving a carbon concentration of 1.3 wt % in the carburized
layer). The test specimens for the high-carbon concentration
carburization (1.3 wt % C) were subjected to quenching and
tempering after reheating at 900.degree. C. for 30 minutes.
4TABLE 4 C C Si Al Mn Ni Cr Mo V B No.5 0.19 0.22 0.75 0.97 0.15
No.G1 0.22 0.83 0.72 0.96 0.16 No.G2 0.21 1.48 1.18 0.45 0.21 0.41
No.G3 0.26 0.81 0.37 1.21 0.49 0.52 0.19 No.G4 0.26 0.21 1.01 1.09
1.51 0.15 0.37 No.G5 0.34 0.21 1.47 1.17 0.17 0.11 No.G6 0.22 0.24
0.99 1.29 1.01 1.02 0.36
[0098] A roller pitting test was made under the same condition as
in Example 1. The test result is shown in FIGS. 7 and 8. FIG. 7
shows the result of the test using the test specimens which were
subjected to carburizing/quenching/tempering treatment intended for
a carburized surface layer containing 0.8 wt % C. Compared with the
result of the reference specimen No. 5, No. G3 to No. G6 containing
Al have presented an obvious improvement. It has been proved by the
results of No. G2 and No. G3 that the sole addition of Si
definitely leads to an improvement where the amount of Si is
approximately 1.0 wt % or more.
[0099] FIG. 8 shows the result of a pitting test conducted on the
test specimens No. 5, No. G2 and No. G4 which were subjected to
reheating/quenching/tempering treatment such that the carburized
surface layer had a carbon content of 1.3 wt % and such that
cementite particles were dispersed in the tempered martensitic
phase of the rolling contact surface. Compared with the reference
specimen No. 5, they were significantly improved in interface
pressure strength.
[0100] FIGS. 9(a) and 9(b) show structural photographs of the
carburized surface layers of the test specimens No. 5 and No. G2.
In No. G2, the cementite particles dispersing in the martensitic
phase and, therefore, the martensite were fined by the addition of
V, which obviously contributed to the significant improvement in
interface pressure strength.
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