Note: Descriptions are shown in the official language in which they were submitted.
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Description
Drive Train Cear of Alloy Steel
Technical Field
This invention relates generally to drive
train gears, and more particularly to a toothed alloy
steel gear having a preselected bainitic microstructure.
Background Art
Carburized and hardened alloy steel gears are
widely used for vehicle power trains in order to obtain
a sufficient resistance to surface pitting and high
bending loads, and thereby a generally desirable
service life. However, the heat treating and
processing of such gears takes a long time, uses a
considerable amount of energy and, accordingly, the
gears are expensive. Drastic quenching of the gears is
also often required, which results in considerable
distortion. Moreover, even with such extensive
processing, the microstructure of the gears is
inhomogeneous and the gears lack sufficient case
toughness at the desired high hardness levels.
Likewise, nitrided alloy steel gears are relatively
brittle at relatively high hardness levels, for
example, above a magnitude of about 58 on the Rockwell
C hardness scale (RC58), and do not exhibit a
relatively uniform metallographic structure.
The properties of some bainitic alloy steels
are very desirable as a substitute for the
above-mentioned martensitic steels. For example, in
high carbon steels low temperature bainite is more
ductile than martensite at the same hardness level.
But most prior art bainitic alloy steels have utilized
controlled amounts of potentially critical and/or
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expensive materials such as chromium and nickel.
Exemplary of the art in this area are the following
U.S. Patents Nos.: 3,418,178 to S.A. Kulin et al on
December 24, 1968; 3,303,061 to J.E. Wilson on February
7, 1967; 2,128,621 to B.R. Queneau on August 30, 1938;
3,298,827 to C.F. Jatczak on January 17, 1967;
3,348,981 to S. Goda et al on October 24, 1967;
3,366,471 to M. ~ill et al on January 30, 1968;
3,418,178 to S~A. Kulin et al on December 24, 1968;
3,528,088 to H. Seghez~i et al on September 8, 1970;
and 3,907,614 to s.L. Bramfitt et al on September 23,
1975. A considerable portion of the available carbon
is tied up by the critical alloy in the aforementioned
patent references so that a relatively coarse
crystallographic texture is formed that is detrimental
to the toughness and nardness of the final article.
Another disadvantage is that when such critical alloys
are utilized, the austenite transformation temperature
is generally raised, thereby requiring increased
amounts of energy and adding to the processing costs.
U.S. Patent No. 1,924,099 issued to E.C. Bain
et al on August 29, 1933 describes a process known as
austempering. Such process involves the steps of:
a) heating a steel article above an upper critical
temperature to assure a change in the morphology of the
article to substantially 100~ austenite; b) quenching
the article below approximatel,y 540 C. (1000 F.),
but above the temperature of martensite formation or
the so-called martensite start (Ms) line; and
c) holding the steel article at such an intermediate
temperature for a preselected period of time sufficient
to convert the morphology of the article to a form
other than 100% martensite. While time-temperature-
transformation (TTT) diagrams are known Otl various
prior art steels, the alloy compositions and
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austempering processes resorted to have suffered two
general deficiencies. Firstly, the quenching step has
involved cooling at a rate such that the transforming
start (Ts) curve o~ the alloy has been crossed and
undesirable upper transformation products such as
proeutectoid ferrite/carbide, pearlite, and upper
bainite formed. This is due to a major degree to the
- relatively critical time period of the alloy to the
usual nose portion of the transformation start curve.
The undesirable crossover of such nose portion results
in a loss of toughness and hardness. Secondly, the
heat treat holding times sufficient to obtain
substantially complete transformation have been too
long, for example, five hours or more. For general
commercial applications, such an extended holding
period is substantially impractical and represents a
considerable waste of energy and time.
Still another factor of major importance is
that as the body of the article to be produced
increases in thickness, its cooling rate decreases. As
a consequence, although thin nails or the like having
an appreciable proportion of lower temperature bainite
may have been made because such articles could be
rapidly cooled in a practical manner, it is believed
that the prior art TTT diagrams of the materials of
such articles woulcl not permit the practical production
of a substantially complete lower temperature bainite
morphology when the section thickness is increased to
about 12mm (1/2"), for example. Thus, the composition
taught by U.S. Patent No. 3,528,088 to H. Seghezzi et
al on September 8, 1970 is not desirable for an article
thicker than about 12~m (1/2") because the austempered
morphology of the article would vary nonuni~ormly
across its cross section away from substantially
complete lower bainite, and the hardness would
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undesirably drop below a magnitude level of about 55 to
58 on the Rockwell C scale. The anchoring device of
the Seghezzi patent also undesirably utilizes the
potentially critical element chromium. U.S. Patent No.
3,196,052 to K.G. Hann on June 28, 1961 is
representative of another alloy steel for a thin wire
that has substantially the same problems.
In some instances, such as in U.S. Patent No .
2,814,580 to H.D. Hoover on November 26, 1957,
10 processing of certain steel alloys has been
accomplished at a holding temperature which is above
the level of lower bainite formation. In other
instances, although a lower holding temperature may
have been established, austenite, pearlite, upper
bainite or martensite have been undesirably present to
an excessive proportion in comparison with the lower
temperature bainite, for example, above an individual
or collective proportion of about 10 Vol.%.
The present invention is directed to
overcoming one or more of the problems as set forth
above.
Disclosure of the Invention
In accordance with the present invention, a
drive train gear having a plurality of gear teeth for
meshing engagement with another gear is formed of alloy
steel having a predominantly homogeneous and
substantially complete low temperature bainite
microstructure including one of cementite precipitated
within the ferrite laths and epsilon carbide
precipitated within the ferrite lath boundaries, and
with the gear having a hardness of at least Rc56
therethrough.
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The drive train gear of the present invention
preferably has no pearlite or upper bainite and less
than 10 Vol.% of retained austenite in a substantially
complete low temperature bainite microstructure.
Moreover, the gear is made of an alloy steel consisting
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essentially of preselected amounts of carbon,
manganese, silicon, molybdenum, preferably but not
essentially boron, and the balance substantially iron.
Brief Description of the Drawings
Fig. 1 is a diagrammatic time-temperature-
transformation diagram for a first example bainitic
alloy steel gear of the present invention and including
a heat treatment processing route.
lO Fig. 2 is a second diagram of the type
illustrated in Fig. 1, of a second example bainitic
alloy steel gear made in accordance with the present
invention.
Best Mode for Carrying Out the Invention
The composition of the low temperature
bainitic alloy steel gear according to the present
invention preferably consists essentially of the
following elements in the proportions indicated:
Elements Broad Range Preferred Range Most Desirable
(Wt.~) (Wt.%)(Amount Wt.%)
Carbon 0.60 - 0.80 0.65 - 0.750.70
Manganese 0.4S - 1.000.60 - 0.70 0.65
25 Silicon 0.15 - 2.20 1.20 - 2.001.50
Molybdenum 0.40 - 0.700.50 - 0.60 0.55
Iron Remainder RemainderRemainder
Preferably, but I believe not essentially,
boron in the broad range of 0.0003 to 0.004 Wt.% is
controllably added to the above-designated
composition. More particularly, a boron range of 0.002
to 0.0035 Wt.~ is preferred, and the most desirable
amount is about 0.003 Wt.~.
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In the composition of the alloy steel carbon
(c) is present in the relatively high amounts indicated
to impart the desired strength and hardness throughout
the body of the article. Carbon is an austenite
former, and is present in the minimum amount stated to
assure that a relatively uniform through-hardness value
of a magnitude in excess of about RC56 on the
Rockwell C hardness scale can be obtained in the
finished gear. Below the value of about 0.60 Wt.% the
alloy would lack sufficient hardness and strength.
Above the value of about 0.80 Wt.~ the alloy would
become less ductile and/or too brittle, and the amount
of carbon present would undesirably contribute to the
formation of free carbides. Moreover, the range of
carbon set forth assures that substantially complete
transformation to low temperature bainite can be
positively obtained.
Manganese (Mn) is also an austenite former and
ferrite strengthener. Below the minimum established
value of about 0.45 Wt.% the strength and hardness of
the gear produced would be lower than that desired, and
there would not be enough manganese to tie up at least
some of the sulfur usually present in residual amounts
and to form manganese sulfide rather than undesirable
iron sulfide. Above the maximum established value of
about 1.00 Wt.~ the ductility of the gear would be
lowered excessively.
Silicon (Si) is also a ferrite strengthener
and is effective in the amounts indicated to assure the
desired tensile strength and hardness of the final low
temperature bainite alloy, as well as for grain size
control. Below a minimurn value of about 0.15 Wt.~
would be insufficient for deoxidation purposes and for
the desired level of hardness in the range of magnitude
abouve about RC56. Above a maximurn value of about
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2.20 Wt.% the toughness decreases to the point where
excessive embrittlement occurs, and graphite tends to
form.
Molybdenum (Mo) reduces graphitization, is a
ferrite strengthener, and provides the desired
hardenability characteristics to the low temperature
bainite alloy. In general, the stated amounts of
carbon, manganese, silicon, and molybdenum serve to
lower the martensite start (Ms) transformation
portion of the process route permitting the lower
bainite transformation to occur at a relatively low
holding temperature for increased hardenability of the
gear and at a savings in energy. Fur~hermore, these
four elements optimize the position of the
transformation start curve so that quenching does not
have to be achieved at an excessive rate. More
specifically, the nose portion of the transformation
start curve is thereby desirably located to the right
on the TTT diagram sufficient to allow quenching of
gears of thicker cross section at a more practical rate
that will minimize distortion of the gear and still
result in relatively uniform through-hardening
thereof. Below a minimum value of about 0.40 Wt.%
molybdenum hardness undesirably decreases and the
transformation start curve is too far to the left so
that quench rate problems arise. On the other hand,
above a maximum value of about 0.70 Wt.~ of molybdenum
the transformation completed curve is located too far
to the right on the TTT diagram, resulting in an
extended required holding time of above two hours and a
corresponding waste of energy.
Boron (B) improves bainite hardenability. The
addition of boron (B) is preferred because the boron
plus molybdenum plus silicon conserve these elements
and provide a more advantageous rightward position of
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the transformation start curve and to thereby permit
more practical cooling rates for gears of various
thickness during austempering. Boron and molybdenum
and possibly silicon retard the polygonal ferrite
reaction without retarding the bainitic ferrite
reaction. Furthermore, the boron acts as an
intensifier from the standpoint that it intensifies the
reaction of the other major elements. Boron is present
in the minimum amount indicated to enable the
proportions of molybdenum and/or manganese to be
disproportionately reduced for economy, while
simultaneously providing the desired morphology~
However, going above the maximum value of about 0.004
Wt.% is believed detrimental to toughness.
Some undesirable residual elements, such as
sulfur (S) and phosphorus (P) are usually present in
commercial steels. On the other hand, other residual
elements, such as copper (Cu), chromium (Cr), titanium
(Ti), etc. may also be present in relatively small
amounts with some degree of benefit. However, all of
these residual elements should be individually limited
to less than 0.30 Wt.%, and preferably limited to less
than 0.20 Wt.%.
A first example of the desired low temperature
bainite steel alloy has the following composition:
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Element Wt~%
C 0.62
Mn 0.64
Si 0.29
Mo 0.46
B 0.0011
Cr 0.07
Ni 0.03
Cu 0 03
P 0.02
S 0.006
Al ~ 0.01
V 0.001
Fe Remainder
A second example of the low temperature
bainite steel alloy has the following composition:
Element Wt.%
C 0.66
Mn 0.68
Si 1.35
Mo 0.43
B 0.0014
Cr 0.08
Ni
P 0.02
S 0.004
Al < 0.01
V 0.002
Fe Remainder
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A third example has the following composition:
Element Wt ~
C 0.76
Mn 0.62
Si 0.38
Mo 0.51
B 0.0021
Cu 0.12
Cr 0.09
Ti 0 059
Ni 0.04
S 0.027
P 0.013
V 0.011
Zr 0.005
Al ~ 0.01
Fe Remainder
A fourth example has the following composition:
Element W _
C 0.72
Mn 0.65
Si 2.02
Mo 0.54
B 0.0033
Cu 0.17
Ti 0.15
Cr 0.14
Ni 0.06
Al 0.047
S 0. 019
P 0.0]5
V 0.01~
Zr 0.006
Fe Ramainder
5~
The first example lower bainite alloy steel
embodiment set forth above had a TTT diagram as
illustrated in Fig. 1, including a transformation start
curve 10 and a transformation complete curve 12. A
5 heat treatment processing route 14, therefs~r, is also
shown, and it is to be noted that the processing route
desirably avoids intersection with a nose portion 16 of
the transformation start curve.
More particularly, the processing route 14 for
lO making an article of the first example composition
included the initial formation of a 76mm x 76mm (31 x
3") ingot and subsequently rolling and/or forging the
ingot down to a 38mm x 38mm (1-1/2" x 1-1/2l') bar. The
bar was heated to a preselected first temperature 18
15 within the austenite transformation temperature range.
For the first example composition the lower end of such
temperature range, often referred to as the upper
critical temperature, was about 770 C (1420 F),
so that the preselected first temperature was
20 established above that limit at about 820 C
(1510 F). Preferably the bar is heated in a salt
bath. In the instant example the bar was most
desirably heated in a nontoxic, electrically heated
chloride salt bath to the approximate preselected first
25 temperature point 18 noted in Fig. 1. The bar was
maintained at such temperature for about 5 to 10
minutes to assure a substantially complete austenite
microstructure.
The second step after heating the bar to the
30 preselected first temperature 18 is to relatively
rapidly cool or quench the heated bar as indicated in
Fig. 1 while missing the nose portion 16 of the
transformation start curve 10 particular to ~he allo~
steel composition of the present invention. If the
35 heated bar is quenched toward a preselected second
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temperature 26 too slowly, a significant portion of the
microstructure would be undesirably transformed to
pearlite because the processing route would pass
through a pearlite region 20 between curves 10 and 12
as indicated in Fig. 1. If it is quenched at a
slightly faster rate in a bath of a higher temperature,
then an undesirable upper bainite microstructure could
be formed because the processing route would pass
through an upper bainite region 22 as shown in Fig. 1.
Thus, it may be appreciated that it is imperative to
transform the austenite microstructure of the bar or
gear directly to a lower bainite microstructure by
choosing a preselected cooling rate 28 sufficient for
avoiding crossing of the transformation start curve 10
until reaching a preselected lower range of
temperatures. Preferably, the lower end of such lower
bainite range is defined by a preselected second
temperature 26 located within a band of temperatures
adjacent the Mx line 24 for the composition of
elements selected in order to maximum the final
hardness of the article. For example, I prefer that
the preselected second temperature 26 be limited to
less than about 15 C (30 F) above or below the
M line. The preselected second temperature should
be above the Ms line if it is desired to
substantially avoid transformation to martensite.
However, even if such temperature is below the Ms
line by the amount indicated, I believe that the
percentage of martensite formaticn will be limited to
less than about 10 Vol.~. The MS temperature for the
first example alloy was about 270 C (520 F), and
the preselected second temperature 26 chosen was
260 C (500 F~. The upper end of the lower
bainite range is defined by a preselected third
temperature 30 of about 350 C (660 F). If the
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preselected third temperature is raised to a higher
temperature, the alloy may be transformed at least in
part into an upper bainite microstructure with its
undesirable coarser grain structure. In view of the
S above-stated considerations, the bar is preferably
quenched in a second salt bath having the preselected
second temperature 26. In the instant example the bar
was quenched in a nontoxic, electrically heated
nitrate-nitrate salt bath at the preselected cooling
rate 28 indicated in Fig. 1. It is of note to
appreciate that the time scale along the bottom of Fig.
1 is of advantageous logarithmic form, so that in this
way the cooling rate 28 approximates a substantially
straight line throughout a significant portion of the
first 10 seconds or so of the processing route 14. In
the instant example the second salt bath was maintained
at a quenchant temperature of about 260 C
~500 F), and approximately 0.6 Wt.% water was added
to the salt bath for greater quench severity.
The third step of the processing route 14 is
to hold or maintain the bar at a relatively stable
temperature between the above-mentioned lower and upper
temperature reference lines 26 and 30 for a preselected
period of time just prior to the transformation start
curve 10 and thereafter to the transEormation complete
curve 12 to complete the transformation of the alloy
steel to a substantially complete low temperature
bainite microstructure. By this term it is meant that
there is less than 10 Vol.% of retained austenite,
substantially no pearlite, and less than about 10 Vol.%
transformation to martensite in the subject lower
bainite alloy steel. Since the hardness of the article
increases as the holding temperature approaches the
Ms line 24, it is desirable to maintain the bar or
article at or adjacent the preselected second
r
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temperature 26. In the instant example, such
isothermal heat treat transformation is complete when
the processing route 14 reaches the transformation
complete cureve 12. Thus, the transformation start and
complete curves 10 and 12 define the left and right
time-indicating boundaries of a lower bainite
transformation region 32, while the lines 26 and 30
define the lower and upper temperature boundaries of
the same region which varies in a range of about
250 C (482 F) to 350 C (660 F) for the
subject alloy. Advantageously, the time scale along
the bottom of the lower bainite transformation region
12 indicates that the length of holding time required
for the first example alloy steel is only about 1800
seconds. This is a great improvement over the extended
holding time period of prior art. On the other hand,
for reduced hardness applications the holding time can
be reduced to about 800 seconds by raising the
temperature of the second salt bath and the subsequent
holding temperature to a point adjacent the line 30 in
order to save energy and time.
The fourth step of the processing route 14 not
shown in the drawing, is to remove the article from the
second salt bath and allow air cooling thereof at
substantially ambient temperatures. Such step is taken
after the transformation complete curve 12 has been
breached by the processing route.
The second example of the low temperature
bainitic alloy steel set forth above was advantageously
so constructed as to move the transformation start and
transformation complete curves 10 and 12 to the right
when looking at the time-temperature-transformation
diagrams as may be noted by comparing the second
example diagram of Eig. 2 with that of Fig. 1. This is
35 advantageous for allowing the article or gear to be
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cooled at a slower or more practical rate by quenching,
as would be the case for an article having a thicker
cross section, and yet would still assure positive
attainment of a substantially complete lower
temperature bainite microstructure. The second example
had a relatively higher proportion of silicon, an upper
critical temperature of about 800 C (1470 F), and
a martensite start (M ) temperature of about 260 C
(500 F). Accordingly, the preselected first
temperature 18 to assure substantially complete
austenite formation of the second example was about
850 C (1560 F), the preselected second
temperature was about 260 C (500 F), and the
curves and regions corresponding to those of Fig. 1 are
identified with the same reference numerals with prime
indicators appended thereto.
In viewing Fig. 2, note that the length of the
holding time required at or adjacent the preselected
second temperature 26' for complete lower temperature
bainite transformation is still only about 2400
seconds, or two-thirds of an hour. More importantly,
the quench severity necessary to avoid the nose portion
16' has been appreciably reduced as may be noted by
comparing the broken line cooling rate 28 of the first
example alloy steel to the solid line cooling rate 28'
of the second example. The horizontal distance between
lines 28 and 28' is indicative of the extra time that
is available for the necessary quenching, and it is
apparent that much larger articles can be heat treated
30 along the processing route 14' than the route 14.
~ ardness readings taken of all four of the
lower bainite alloy steel examples .set forth above
varied generally in magnitude between 55 and 57 on the
Rockwell C hardness scale. However, by maintaining the
35 amount of carbon at about 0.70 Wt.% and silicon at
about 1.50 Wt.%, I believe that hardness levels of
about 59 on the Rockwell C scale can be consistently
attained after the stated heat treat process period of
less than two hours.
Notched tensile strength test specimens were
machined from bars of the first and second example
alloy steels having a 0.5" diameter (12.7mm)
cylindrical neck portion with a 60 V-notch centrally
thereabout, and with the notch having a depth of about
0.357" (9.07mm) and a notch radius of 0.006" (0.15mm).
The first and second bainite alloy steel test specimens
registered notched tensile strength measurements of
1174.5 Mpa (170,348 psi) and 1,332.43 Mpa (193,246
psi) when heat treated in accordance with the
processing routes 14 and 14' respectively. As a
comparison, test specimens of the same first and second
alloy steel compositions heat treated by quenching the
specimens from about the preselected first temperatures
18,18' into a hot oil bath at about 95 C (205 F),
cooling, and subsequently tempering in a furnace for a
period of one hour at about 250 C (480 F) were
made. Because of such different heat treatment these
comparison specimens exhibited a substantially complete
martensite microstructure and notched tensile strength
measurements of 752.66 Mpa (109,160 psi) and 962.27
Mpa (139,561 psi) respectively. Thus, the greatly
improved notched tensile strength of the bainite alloy
steel is apparent, with the second example (Fig. 2)
alloy composition exhibiting a substantial increase in
tensile strength over the first example (Fig. 1).
The amount of retained austenite in the first
example lower bainite steel alloy and the first example
comparison martensite specimen was not measurable by
X-ray diffraction analysis, while the second example
lower bainite steel alloy and the seconcl example
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comparison martensite specimen measured at 5.7 Vol.~
and 6.7 Vol.~ respectively. This indicates that the
increased amount of silicon in the second example
alloys tended to stabilize the austenite so that
proportionately more was retained, and also that the
amount of retained austenite can be expected to remain
below 10 Vol.~.
An examination of the microstructure of the
first example alloy steel indicated a predominantly
homogeneous lower bainite structure including cementite
(Fe3C) precipitated within the ferrite laths. In
comparison, the second example alloy steel with its
higher silicon content was of finer homogeneous lower
bainite form including epsilon carbide (Fe2C)
precipitated within the ferrite lath boundaries.
Industrial Applicability
In view of the foregoing, it can be
appreciated that the preferred through-hardened, low
temperature bainitic alloy steel exhibits physical
properties that are extremely useful for a wide number
of applications including gears, bushings, bearings and
the like. Particularly, it exhibits the potential for
use in a power train gear having a plurality of teeth
thereon for increasing gear static strength to a level
of magnitude of 50%, reducing gear distortion by a
level of magnitude of 75%, and maintaining equivalent
pitting resistance when compared to conventional
carburized and hardened steel gears at a minimal
increase in cost. The subject lower bainite alloy
steel is economical to produce, yet is aclaptable to
manufacturing procedures requiring no natural gas, for
example. Moreover, the entire therrnal trans~ormation
time is less than about two hours. This has been made
possible to a considerable extent by preselected
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combinations of alloying elements which have resulted
in shifting the transformation start curve 10 to the
right sufficient for allowing practically attainable
rates of cooling for the solution, in so positioning
the completion curve 12 as to reduce holding time, and
in lowering the Ms line 24 to achieve the desired
high level of hardness. For example, by the addition
of carbon alone the MS line is lowered about 35 C
(60~ F) per 0.01 Wt.% carbon to desirably allow the
transformation to be achieved at relatively lower
temperatures. Furthermore, the preselected amount of
boron indicated is believed to increase the time
available to lower the temperature of the article being
quenched to the transformation temperature, and the
amount of molybdenum has a significant effect in
reducing the holding time required to complete
isothermal transformation to lower bainite.
Significantly too, all of the preselected elements,
except boron, lower the Ms line.
Other aspects, objects and advantages of this
invention can be obtained from a study of the drawings,
the disclosure and the appended claims.
