Note: Descriptions are shown in the official language in which they were submitted.
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Dkscription
Nitriding Grade Alloy Steel
And Article Made Therefrom
Technical Field
This invention relates generally to an alloy
steel and more particularly to a nitriding grade alloy
steel and articles made therefrom.
Background Art
Nitrogen case hardening, more commonly
referred to as nitriding, increases surface hardness,
wear resistance, and resistance to certain types of
corrosion and surface stresses that improve the fatigue
resistance of a nitrided part. Accordingly, nitrided
alloy steel articles are often used for gears,
couplings, shafts and other applications that require
resistance to wear and high stress loading.
One group of alloy steels suitable for
nitriding have a composition as follows:
Carbon 0.21 - 0.26% by weight
Manganese 0.50 - 0-70%
Aluminum 1.10 - 1.40%
25 Nickel 3.25 - 3.75%
Chromium 1.00 - 1~30~
Molybdenum 0.20 - 0.30%
Silicon 0.20 - 0.40
Iron and acceptable
trace elements Balance
A more economical group of hardenable alloy
steels that have been nitrided after heat treating are
the AISI/SAE 4100 series alloy steel In particular,
AISI/SAE 4140H alloy steel has been found to be useful
in the manufacture of various gears that require a
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combination of high surface hardness and core
hardness. AISI/SAE 4140~ alloy steel has a specified
composition as follows:
Carbon 0~37 - 0^44
Manganese 0~65 - 1.10
silicon 0.15 - 0.35
Chromium 0.75 - 1.20
Molybdenum 0.15 - 0.25
Iron and acceptable
trace elements Balance
Typically, parts having the above composition
are first forged, or rolled from billets, and are
quenched and tempered, then machined and nitrided.
Although AISI/SAE 4140H alloy steel has been useful in
certain nitriding applications, it also has some
disadvantages. For example, this steel contains
molybdenum, an expensive alloying element. Further, it
has been found that articles having the AISI/SAE 4140H
composition are prone to quench cracking and therefore
generally require an oil quench. Still further, the
nitrided case hardness of AISI/SAE 4140~1 is generally
limited to about Rockwell C (Rc) 55 or less.
The present invention is directed to
overcoming the problems set forth above. In
particular, an alloy steel according to the present
invention provides a composition that is economical,
adaptable to a variety of quench mediums, maintains
high core hardness ater tempering and has improved
nitriding characteristics. The initial cost of the
3~ steel is reduced due to the deletion of molybdenum.
Within quite broad dimensional sizes, articles
manufactured of the alloy steel composition of the
present invention may be quenched in either a water or
oil medium. E~rther, after tempering, such articles
retain a useful core hardness within a controlled range
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of from Rc 25 to Rc 32 depending upon tempering
temperature. Also, it has baen found that for
equivalent case depths, nitriding time of articles
having the new alloy composition may be decreased by
as much as 40% compared to articles having the
AISI/SAE 4140H composition.
Disclosure of the Invention
In accordance with one aspect of the present
invention a nitriding grade alloy steel has a
composition, by weight percent, of 0.20 to 0.40
carbon, 0.50 to 1.60 manganese, 0.40 to 1.50 chromium,
0.07 to 0.30 aluminum, 0.03 to 0.20 vanadium, with the
balance being essentially iron and incidental
impurities, which after heating, quenching, tempering,
machining and nitriding, has a surface hardness,
measured on the Rockwell 15-N scale, of at least 89
and a core hardness of at least Rockwell C25 after the
article has been quenched, tempered and nitrided.
The alloy steel composition according to the
present invention provides a unigue combination of
hardenability, resistance to loss of hardness during
tempering and greatly enhanced response to nitriding.
These characteristics are achieved by the use of
small, carefully controlled amounts of aluminum and
vanadium. Further, the unique properties are achieved
without the need for expensive nickel or molybdenum
additions and hence provides an economical material.
Brief Description of the Drawings
Fig. 1 is a graph illustrating the improved
hardenability characteristics of a nitriding grade
alloy steel according to the present invention;
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Fig. 2 is a graph illustrating the improved
nitride response and resistance to loss of hardness
during tempering of the alloy steel; and,
Fig. 3 is a graph comparing the nitriding
response of the alloy steel according to the present
invention with a typical nitriding grade alloy steel.
Best Mode for Carryinq Out the Invention
Carbon contributes to the attainable hardness
level as well as the depth of hardening. In accordance
with the present invention, the carbon content is at
least 0.20% by weight to maintain adequate core
hardness after tempering and is no more than about
0.40% by weight to assure resistance to quench cracking
and an adequate response to nitriding. It has been
found that it the carbon content is more than about
0.34~ by weight, water quenching may cause cracking or
distortion in complex~shaped articles and, in such
cases, a less drastic quench medium such as oil may be
required. Therefore although a broad range of 0.20 -
0.40% carbon by weight is contemplated, a more
desirable range is 0.24 - 0.34% carbon.
Advantageously, alloy steel articles formed from the
more desirable or preferred ranges described herein may
be either water or oil quenched, whichever is more
convenient.
Manganese contributes to the deep
hardenability and is therefore present in all
hardenable alloy steel grades. The disclosed alloy
steel contains manganese in an amount of at least 0.50%
to assure adequate core hardness and contains no more
than about 1.60% to prevent cracking. In addition to
the permissable broad range of 0O50% to 1~60% by
weight, a narrower range of manganese from 1.00% to
1.30% is preferred to maintain unifGrmity of heat treat
response.
Chromium contributes to the hardenability of
the present steel alloy and is also an excellent
nitride former thereby enhancing nitriding
characteristics. To best realize these effects a
minimum of 0.40~ chromium is required, and preferably
at least 0.90% chromium should be present. Ib avoid
embrittlement, the amount of chromium should be limited
to a maximum of 1.50~, and preferably no more than
about 1.20%.
Aluminum, an essential ingredient of the
present invention, contributes to hardenability and is
a good nitrider wormer. As will be shown by examples,
aluminum should be presen-t in an amount of at least
0.07%, and preferably at least 0.10~. If aluminum is
present in an amount less than about 0.07~, not only is
there little observable improvement in either
hardenability or nitride response but also, the
benefits are very inconsistent. It has also been found
that while aluminum in amounts greater than 0.30% are
beneficial to nitrideability, the tendency for case
embrittlement also increases. Accordingly, it is
desirable to maintain an upper limit of no more than
0.30% aluminum and preferably no more than about
0.20%. It has been discovered that the present alloy
steel having aluminum in the designated range, permits
a wide range of quench practices and consistently
improves hardenability.
Vanadium is also an essential ingredient in
the present alloy steel composition, and must be
present in an amount of at least 0.03% to realize a
consistently measurable enhancement of case and core
hardness. Vanadium in amounts greater than 0.20~ does
not significantly enhance the nitride response or the
hardenability of the material. For these reasons the
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limits of vanadium are at least 0.03% and no more than
0.20%; and preferably from 0.05% to 0.10~ to make the
best economic use of this ingredient.
It has been found that the unique combination
of aluminum and vanadium, within the specified ranges,
greatly contributes to improved nitride response,
thereby decreasing required nitriding time and
increasing case hardness and depth. Further, the
unique combination of aluminum and vanadium, within the
specified ranges, contributes to hardenability and
temper resistance.
The remainder of the alloy steel composition
is essentially iron except for nonessential or residual
amounts of elements which may be present in small
amountsO For example silicon in the recognized
commercially specified amounts is used for deoxidation
of the molten steel. Eor this purpose silicon may be
present in an amount of at least 0.10~. Sulphur, which
in small amounts may be beneficial in that it promotes
machining, is allowable in an amount of no more than
about 0~10~, and preferably no more than 0.04% to avoid
loss of ductility. Likewise, if desired, lead may be
added in an amount up to about 0.15~ to improve free
machining characteristics of the material. Phosphorous
in an amount over .05% may cause embrittlement, and
preferably the upper limit should not exceed 0.035
Other elements generally regarded as incidental
impurities may be present within commercially
recognized allowable amounts.
Manufactured articles, such as shafts,
couplings and gears, having the above stated
composition, are preferably formed by forging or
rolling and then hardened by heating to a temperature
of about 870C (1600F) for a period of about one
hour and quenched in either water or oil followed by
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machining to a desired final dimension. In the past,
nitriding grade alloy steel compositions generally
required oil quenching. The less restrictive
requirement for the present material with respect to
quench practice is a result of the contribution that
aluminum makes to both nitrideability and
hardenability, particularly in combination with the
lower carbon limits of the preferred range. The
increased freedom to select quench medium is therefore
a valuable benefit of the present invention.
EXAMPLE 1
The marked influence of aluminum, and in
particular the combined benefit of both aluminum and
vanadium, in providiny increased hardness is shown
below in Table I and illustrated graphically in Fig.
1. The test samples were all made from the same base
heat and have substantially identical measured amounts
of carbon, manganese, chromium and silicon, all of
which are within the above described required ranges.
Varying amounts of aluminum and vanadium were added
during four taps of the base heat. The product ox
these taps was rolled into 32 mm ~1.25 inch) squares
before Jominy bars and nitriding specimens were
obtained. Test samples 1 and 3 contain relatively low
amounts of aluminum, and test samples 2 and 4 contain
aluminum in an amount close to the upper end of the
preferred range. Vanadium, in an amount representing
the lower end of the preferred range, was added to test
samples 3 and 4. The test samples were heated and
quenched in accordance with the standard ASTM End
Quench Test for Hardenability ox Steel (~2553 and the
following hardness values were measured in 1/16 inch
(1.59 mm) increments from the quenched ends.
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1AB E I
Measured Test Test Test Test
Chemi~ Bar #1 Bar #2 Bar #3 Bar #4
C 0.34 0.34 0.35 0.36
5 Mn 0.52 0.57 0.54 0.54
Cr 1.04 1.05 1.04 1.04
Al 0.056 0.18 0.072 0.18
V 0.046 0.048
Si 0.33 0.34 0.32 0.32
Mo* 0.01 0.01 0.01 0.01
Cu* 0.02 0.02 0.02 0.02
Ni* 0.03 0.03 0.03 0-03
Hardness - Rc**
Distance Erom
Quenched End
1/16
Inch mm
15 2 3.2 54 1/2 53 52 1/4 53
4 6.4 50 52 48 52 1/4
6 9.5 42 ]/2 50 1/2 41 50 3/4
8 12.7 36 1/2 48 36 46 3/4
15.g 33 42 3/4 33 43 3/4
12 19.0 31 1/4 40 1/4 30 1/2 4~ 1/2
14 22.2 3~ 35 3/4 29 3/4 38
2016 25.4 29 3/4 33 1/2 29 35 3/4
18 28.6 28 1/4 32 1/4 28 3/4 34 1/2
31.8 ~5 3/4 31 1/2 28 3/4 32 1/4
22 34.9 23 1/4 31 1/4 27 1/2 31 3/4
24 38.1 22 31 1/4 26 31 1/2
26 41.3 21 27 3/4 24 1/4 32 1/4
28 44.4 -- 25 1/4 22 1/4 31
47.6 -- 23 1/4 20 1/2 31
2532 50.8 -I 22 1/4 -- 31 1/2
* Mo, Cu and Ni were measured to assure that they
were within allowable incidental amounts.
** Two Jominy traverses were taken on opposite sides
of the test bar and averaged.
The graphical presentation of the above data
in Fig. 1 illustrates the improved hardenability
characteristics of test samples 2 and 4, having 0.18~
aluminum over test samples 1 and 3 which respectively
f
g_
contain only 0.056% and 0.072% aluminum. While it is
recognized that aluminum in amounts less than that
present in test samples 2 and 4 may contribute somewhat
to hardenability, it has been found that the influence
of aluminum in amounts less than about 0.07% is
inconsistent. Since aluminum readily combines with
oxygen and nitrogen, the amount of aluminum available
for hardening will depend on melt practices which
influence the amount of free oxygen and nitrogen in the
steel. For these reasons, 0.07% is considered to be
the practical lower limit for consistent hardenability.
EXAMPLE 2
After heating and oil quenching, test bars for
nitriding from the four taps were tempered. The
nitriding grade alloy steel of the present invention
can be tempered at relatively high temperatures without
adversely reducing core hardness. Each of the four
tests bars, having the composition as identified in
Table I above, were heated to a temperature of 593C
~1100 F) and held at that temperature for three hours.
After tempering, samples from the test bars
were nitrided in an ammonia atmosphere at a temperature
of about 526C (980F) for 28 hours. The hardness
measurements, taken after nitriding the samples of the
four taps corresponding respectively to test bars 1-4
described above in Table I, are shown below in Table II
and illustrated graphically in Fig 20 The hardness
measurements were determined by taking a Tukon
microhardness traverse of the nitrided sample and
converting the Knoop hardness measurements to
equivalent values on the Rockwell C Scale.
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TABLE II
Distance Hardness (Rc - Converted from Knoop)
From
Nitrided Test Test Test Test
SurfaceImm) Sample 1 Sample 2 Sample 3 Sample 4
0.05 54 61 56 63
O.lO 56 59 59 62
0.15 52 57 57 59
0.20 50 54 56 58
0.25 47 49 53 56
0.30 43.5 47 50 53
0.40 36 39 40 40.5
0.50 32 35 36 37
0.60 30 32 34 34
0.75 30 30 32 32
l.00 30 30 32 32
This example lllustrates the contribution of
aluminum, and especially the unexpected benefit of the
combination ox aluminum and vanadium to hardness, both
at the surface and in the core. Test sample 2,
containing 0.18% aluminum and no vanadium, shows much
higher surface hardness (3 to 7 points Rc) and
substantially the same core hardness as test sample l
which likewise contains no vanadium and has a lower
aluminum content (0.056%). Both of the test samples
containing about .05% vanadium, tests samples 3 and 4,
have higher hardness than corresponding samples l and 2
which respectively have essentially the same base
composition and aluminum addition but contain no
vanadium. The highest hardness was measured on test
sample 4 which contained ~18% aluminum and 0.048%
vanadium. The lowest hardness was measured on test
sample l which had no vanadium and the lowest aluminum
content, 0.056%.
Additional samples having compositions
corresponding to test samples 1-4 above were tempered
at 538C ~1000), 649C ~1200F) and 704C
Il300F~ The same results were observed as those in
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the above sample which was tempered at 593C
(1100F)o In each case, sample 4 containing 0.18~
aluminum and about 0.05~ vanadium had higher surface
hardness, and at least as hiyh core hardness, than
samples 1-3 which contained lower amounts of aluminum
or vanadium.
EXAMPLE 3
Three additional test samples were prepared to
measure the response to nitriding. Nitriding response
is measured by the amount of time that a test sample
must be held at an elevated temperature in an
atmosphere containing raw ammonia to develop a
predetermined nitride case depth. The measured
composition of the three test pieces and their
respective physical properties are listed below in
Table III. The first test piece was formed of AISI/SAE
4140 material. This material has been commercially
used by applicant and is generally recognized as a
desirable nitriding grade alloy steel in heat treated
and tempered, nitrided article applications. the
second test piece, identified in Table III as test
sample 5, contains aluminum and vanadium in the
preferred range of the present invention. The thlrd
test piece, test sample 6, is similar to sample 5
except that sample 6 has added aluminum to increase the
aluminum content to near the maximum amount desired in
the more broadly contemplated range of the present
invention. Two sets of the three above-described test
pieces were prepared. Both sets were heated, quenched
and tempered as previously described for the test
samples in Table II. After tempering, one set of the
test pieces was nitrided in an atmosphere of raw
ammonia gas with a dissociated ammonia carrier gas at a
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temperature of 526C (980F) for a period of 21
hours. The second set of test pieces was similarly
nitrided for a period of 48 hours. These time periods
were selected to assure a minimum case depth of 0.20 mm
and 0.30 mm respectively in the control AISI/SAE 4140
test piece. After nitriding, -the two sets of three
test pieces were removed from the nitriding furnace and
nitrided depth was determined by a Tukon microhardness
traverse on a metallographic section of each test
piece. The measured composition and hardness values
are listed below in Table III, and the respective case
depths as a function of nitriding time is illustrated
graphically in Fig. 3.
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Table III
AISI/SAE Test Test
Chemistry 4140 Sample #5 Sample #6
C 0.39 0.31 0.31
Mn 0.71 1.12 1.13
Cr 0.98 0.93 0.g4
Al 0.025 0.152 0.260
V -- 0~06 0.06
Si 0.25 0.35 0.36
S 0.012 0.010 0.010
P 0.022 0.025 0.025
Mo 0.14 0.03* 0-03*
Cu 0.10* 0.12* 0.12*
Ni 0.06* 0~09* 0 09*
21 HOUR NITRIDING TIMF.
SURFACE HARDNESS
Rockwell
1515-N 86 91 92
CORE HARDNESS **
Rockwell C 28 28 28
CASE DEPTH***
mm 0.27 0.33 0.35
48 HOUR NITRIDING TIME
SURFACE HARDNESS
Rockwell
15-N 87 90 92
CORE HARDNESS **
25Rockwell C 28 27 28
CASE DEPTH***
mm 0.38 0.47 0.50
* allowable residual amounts; these elements were
not present as the result of controlled, planned
or intentional additions.
** core hardness was measured as a Vickers Hardness
number (DPH) and converted to a corresponding
value on the Rockwell C scale.
*** case depth is defined as the distance from the
surface at which a Vickers hardness of 423 was
35measured.
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From the graph shown in Fig. 3, it can be seen
that the 0.27 mm case depth of the prior art AISI/SAE
4140 material after 21 hours nitriding time can be
realized in about 15 hours with the composition of the
present invention. Similarly, a 0.38 mm case depth in
the new material, having the preferred range of
aluminum, can be reached in only about 25 hours whereas
the prior art material required 48 hours. Thus it can
be seen that nitriding time can be significantly
reduced Eor articles having the composition of the
present invention. For a case depth of 0.3 mm the
nitriding time can be reduced on the order of 40
thereby effecting significant cost savings.
Industrial Applicability
The present invention is particularly useful
in the manufacture of oil or water quenched, nitrided
articles such as gears, shafts, bushings and similar
parts where maximum nitriding response is desired.
Further, the alloy steel composition of the present
invention is economical to produce in that it does not
require expensive alloy additions. Still further, the
new alloy steel is especially desirable in applications
where nitrided articles and first hardened and then
subsequently tempered and machined to final dimensions
prior to nitriding. When applied in such instances,
the new alloy steel provides a unique combination of
hardenability, retention of high core hardness after
tempering, and additional processing cost savings as a
result of the material's excellent nitriding response.
Other aspects, objects and advantages of this
invention can ye obtained from a study of the drawings,
the disclosure and appended claims.
