Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
This invention relates generally to an improved method of
obtaining fully dense, carburized, powdered metal, low alloy steel parts,
the most common of which are the powdered metal equivalents of AISI 4000
and 4600 wrought steel series and particularly those having sintered
carbon levels in the range of 0.22% to 0.37% by weight. More specifically,
this invention is related to the improved method oi carburizing powdered
metal briquetted preform during the sintering step, or alternatively
successively thereafter, and precedent to the forging step. Prior to our
invention such powdered metal low alloy steel parts were first brought
from a sintered preform to full density by a forging operation such as
shown in U.S. Patent No. 3,772,935, owned by assignee of the present
invention; and subsequently, carburized by methods conventional for wrought
steels. Such conventional heat treat methods include both liquid and gas
carburizing. Where gas carburizing is used, either a batch type or
continuous rurnaco may bc used. The parame~crs to be controlled to
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achieve carburization of a fully dense part of speciflc hardness, case
depth and carbon gradient are generally well kno~vn, as described, for
example, ln Metals Handbook, Eighth Edition, Vol. 2, pp. 67-114, published
by the American Society ior Metals.
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SUMMARY OF THE INVENTION
Our invention is to carburize the briquette, either during
slntering, or successively thereafter in a two zone operation, and before
the further consolidation of the preform to fully dense condition and final
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shape, i.e. by iorglng. The advantage oi 90 doing over the conventional
carburizing techniques for any one particular part are substantial and
include:
(a) the carburizing is obtained more quickly because the
preiorm is at the most at only 70 to 90% of its full density, and there-
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i'ore the carbon being liberated by the carbon rich carburizing gas
-i penetrates the preform more rapidly than in conventional wrought carburiz-
; ing techniques;
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~ (b) carburizing is further hastened by reason of the fact
,j the preform case depth can be.substantially less than the required case
depth of the final forged part provided one orientates the preform in the
1~ 20 forging die so that by compression and flow the case depth at the crltical
~ wall of the part ïs incrcascd.
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Additionnl ndvnnta6es accruing over thc prior art, all of
which perhaps should be regarded as ancillary to the above are substantial
rt-duction in capital investment for equipment, greater utillzation oi'
plant space, longer carburizing equipment life, and less carburizing gas,
heat and other utllities as well as less manpower, to achieve the same
results as presently obtained wlth conventional methods.
.
A BRIEF DESCRIPTION OF THE DRAWINGS
- Fig. 1 shows a schematic layout of the overall process of
the subject invention beginning with the step of blending and concluding
wlth post-forging steps of quenching and stress reliei'.
Fig. 2 shows a cross-sectional side view of a transmission
stator clutch cam manui'actured in accordance with the sub~ect invention
and after i'orging,
Fig. 3 shows a cross-sectional side view o~ the transmission
~ 15 stator clutch cam taken along section line A-A of Fig. 2.
`~ Fig. 4 shows an expanded side view of the sintered carburized
preform in the forging die prior to forging.
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DETAILED DESCRIPTION OF THE INVENTION
~eferring to Fig. 1, the inventive process includes the steps
20of blcnding 1, prcssing or briquctting 2, sintering nnd cnrburizing 3, 3',
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~ 105'~668
forging 4, quenchlng 5 and stress relicving 6.
The blending step comprises blending either the alloyed
me~tal powder, or a combination of metal powders which together make up the
desired steel alloy powder, with graphite and die lubricant, for example
acrawax. The purpose of the graphite addition is to raise the carbon
content of the preform, as is well known. And the purpose of acrawax is
to act as a die lubricant for the preform, as is well known. Any other
equivalent additives could be added for those mentioned,
The pressing or briquetting step 2 comprises pressing the
blended powder lnto a low-density, semi-final shape. In the example
illustrated in Flg. 4 this semi-final shape resembles a ring.
The sintering and carburizing step comprises in our preferred
embodiment the simultaneous sintering and carburizing o~ the preform as
~ shown in ~tep 3, which can be re~erred to as a single furnace single zone
~ 15 operation. As an alternative, as shown in dotted lines, the sintering
? and carburizing of the preform can be accomplished in separate successive
steps, shown in 3'. This process may be referred to as a single furnace
two-zone operation. With either embodiment lt is contemplated one should
: use a single conventional sinte ing furnace of the horizontal, continuous
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feed typc equipped with the utilities and controls neccssary to provide
a ca uriz1ng ~as atDospùere.
With the single furnace, single zone process 3, standard
slntering conditions, namely temperature and time, sre maintained within
S the ~urnace and there ls provided a carburizing gas atmosphere throughout,
either within or without forced circulation of the gas, l.e. by internal
fan
With the single furnace, two zone, process 3', the first zone
is primarily for purposes of sintering and the second zone for carburizing.
As is well known, conventional sintering $urnaces, as are contemplated for
use herewith comprise a plurality of zones including, in order, a preheat
zone i'or burning out lubricants and anywhere from one to three separate
hot zones ior sintering depending on the choice of the operation. For
purposes of describing our invention, the first zone referred to is the
~- 15 hot zone, regardless if a sintering zone, whether it Fomprises a multiple
oi' zones. In such case the temperature in the i'irst zone will be in the
range of 2000 F to 2080 F and the parts held a period of time sufficient
to achieve the desired degree of sinter; while in the second zone the
carburizing temperature will range from 1500 F to 1800 F, again depending
: on the part specification, gas and other parameters.
It is also within the scope of our invention to accomplish
thc sintering and carburizing step 3' ln two separate furnaces, onc for
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sintering nnd the other for subsequent carburizing.
Not one of the aforementioned embodiment is considered
partlcularly desirable over the other since in each case one must consider
the particular part specifications to arrive at the most efficient
operation. However, where close control of case depth or carbon and
hardness gradient is required, it will generally be more sdvantageous
to u the two zone ~lnter and carburlzlng procezz step 3'. Speclrlc
examples are given hereafter.
After sintering and carburizing, the preform is forged, as
shown at 4, to its final shape and then quenched 5. Forging is done at
preform temperature ranging generally from 1600-1750F, The temperature
oi the forged part ls then allowed to substantially stabilize before the
part is quenched as shown at 5, preferably in a quenchant such as oil.
The generally preferred forge/quench process is described more fully in
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co-pending patent application-Ur~. Serial No. ~C~C81 assigned to the
assignee of the sub~ect invention. A final step of stress relieving,
shown as numeral 6, may also be desirable for particular application.
¦~ The sult i9 e fully denze, full carùurlzed powdered metal part o~ Rc60
minimum hardness on the exterior,- a hardness gradient as required, and a
tough inner core for strength characteristics. The known primary
application for the invention is in the production of nutomotive trnns-
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mlssion parts such as the low-reverse position ovcrrunning clutch cam and
l the stntor clutch cam. Additionally, gears and antifriction bearing
¦ components commonly must meet these same requirements.
¦ A typical stator clutch cam is shown in Fig. 2, wherein the
¦ race lO into which the clutch rollers ride constitutes the critical wall
l surface sub~ect to wear and accordingly requires high hardness.
¦ According to a second feature of the invention, the preform
is designed with an upset ratio substantially greater than one to one so
, that metal flow as distinguished from metal powder densification of the
prei'orm in the forging die takes place. This technique together with
proper selection o~ geometry of the pre~orm in the die cavity is used to
cause the metal to flow in the area oi' the critical wall thickness ~hich
in turn efi'ects an increase in the desired case depth. This happening,
although discovered by accldent, is used to advantage by carburizing the
preform during the sintering operation to a lesser depth than required in
the iinal i'orged product and making up the difference during forging.
The result is improved efficiency during the sinter-carburizing step with
no loss of efficiency elsewhere in the process. It has also been determined
that the metal flow itself enhances the overall strength of the forged part.
To illustrate what is meant by "upset ratio" there is shown in full lines
~n Fig. 3 the section A-A of tha forged stator clutch cam of Fig. 2.
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Superimposcd tllereon and shown schematically in dashed lines ls a preform
p' of width b' and thickness a' in accordance with the invention and
halving an upset ratio substantially greater than one to one; while shown
~, ln dotted lines ls a preform p" of width b" and thlckness a" which
represents a conventional upset ratio slightly greater than one to one,
Each preform p' and p" has a plan surface area designated as Ap' and Ap ,
repsectively, as shown in Fig. 2. The upset ratio for the example given
~ is: I
Af - Ao
Ao
WHEREIN: A~ is the plan area of the forged part;
,' Ao is the plan area of the part before forg-
~, ing; and
1 15 WHEREIN: Ao = Ap,
¦ In pr tlce, an upget ratlo o2 generally 40% has been found to be deslrable.
An acceptabl,e upset ratio range would be 10% to 80%. Of course, in the
' example of Figs. 2 and 3 referred to, the preform height a' must be
-,~, increased to such extent that the volume of preform p' is the same as
that of the conventional preform p" to insure full densification. The
' conventional upset ratio is usually referred to as a tolerance of
,,;~ 1 / 1 in order to assure full densification of 99.6 - 100% of
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, , wrought density.
A suitable conventional forging die ~s shown ln Flg. 4 wherein
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there is shown a die 20 having corc 221 a lower punch 2~ and upper punch 26.
The = er p~=ch 22 a=d die 20 de~l=e the dle cavity 28 I=to whi=h 1s
placed the preform p'. The die cavity is notably of greater width than
the preform as referred to above. Upon lowering of upper punch 26 with a
force in the order of 60-90 tons per square inch, full consolidation is
achieved.
Despite the i'act Fig. 4 is not intended to be to scale, it
is intended to be shown that width s of the die cavity is greater than
width b' of preform p', and that as depicted by dotted reference line r
preform p' is sized or orientated with respect to die cavlty 28 such that
substantially all lateral metal flow occurs between critical wall lO and
the adJoining dle surface o~ core 22, ,
- Having described the invention generally, specific examples
15 ll nrs 1 e= here1nbslow 1= Tab1s I.
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T A B L E
PART 1 PART 2
PHYSICAL DESCRIPTION
O,D, (inches) 4,52 4,52 3,64
I,D, (inches) 3,52 3,52 2,75
HEIGHT (inches) 1,22 1,22 ,590
SPECIALS (Tooth Hrd.) I,D, AND O.D, CASE HARDENED
E.B, WELD AREA LO~V CARBON
PLAN SURFACE AREA (in, ) 6,52 6,52 4,24
SINTER (@ #/hr,) 400 400 400
POWDER METAL COMPOSITION 4620 4620 46F27
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SINTERING AND CARBURIZING
CONDITIONS:
ENDOTHERMIC GAS (ft /hr) 2400 2400 2400 2400
TEMPERATURE (F) 2050+15 2050~15 1600 to 2050+15
. 1700
TIME (minutes) 30 30 10 to 20 30
DEW POINT (F) -8 to -18-5 to tO to -10 to -18
-18 ~10
NO, OF FURNAOE S ONE TWO ONE
NO. OF ZONES ONE TWO ONE
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~ FORGE ~
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~` 20 PRESSURE (Tons/in, ) 70 to 90 70 to 90 70 to 9O
~- TEMP. AT FORGE ( F) 1600-1750 1600-1750 1600-1750
QUENCH ~4 to 8 sec, ~4 to 8 sec, ~4 to 8 sec,
~- dwell before dwell before dwell before
quench quench quench
-~ BATH COMPOSITION Park AAA Oil Park AAA Oil Park M A Oil
BATH TEMPERATURE ( F) 140-180 140-180 140-180
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; 25 FINAL
CHARACTERISTICS Core Hrd. Core Hrd, Core Hrd,
HARDNESS. 31 to 48 Rc 31 to 48 Rc 31 to 48 Rc
Surface 62 Rc Surface 62 Rc Surfnce 62 Rc
CASE DEPTI{ (inches~ O.Q60 min. .090 min.
0.080 max. 0.120 max,
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Part No. 1 wns ~ low-reverse position overrunning clutch
cam and o~ the came general shape as Part No. 2, the stator clutch cam
shown in Fig. 2.
As will be noted from Table 1, Part 1 was sub~ected to both
a slngle furnace, single zone sintering and carburizing step and a two
furnace two zone sinter-carb~rize step. While the single zone step is
advantageous for acquiring conventional case depth, the two zone sinter-
carburize step yields a deeper case depth without prolonging the time or
elevating the temperature requlred to achieve desired degree of sinter,
and more importantly allows for better control of case depth and carbon
gradlent bécause the carburizlng is iinished at lower temperature.
Using conventional batch carburizing techniques, namely
carburizing a$ter forging, this same part would have required 6-12 hours
-~ carburizing time at a furnace temperature o~ 1650-1750F, plus an
additional two hours diffusion at lower temperature.
During forging of Part No. 2, the required case depth for
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wear surface 10 was e~ffrinches. The preform a', b' was carburized to a
.;~ o c~ o, o 6~
~; case depth of 0.10 inches minimum and the increase to O.C0 inches minimum
~` was achieved by forging using an upset ratio of 40%.
While the invention has been described in connection with a
pre~erred embodiment and specific examples thereof, it is not intended to
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llmit thc inventlon to any pnrticulnr form set forth, but, on the
contrary, it ls intended to cover such alternatives, modifications and
equivalents as may be lncluded ~vithin the spirit and scope oi' the
lnvention as defined by the appended claims.
