Note: Descriptions are shown in the official language in which they were submitted.
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This invention rela-tes to thread rolled or thread
formed metal products produced from sintered powder metal ~P/M)
blanks and, in particular, to a method of producing thread rolled
PjM products from sintered cylindrical P/~ blanks.
State of the Art
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It is known to produce threaded products from cylindri-
cal wrought metal blanks by using a thread rolling die. ~hen
rolling a thread on a cylindrical workpiece or blank, the die
penetrates the surface of the blank to form the root of the
thread. This forces displaced material radially outward to orm
the crest and the major diameter of the thread. The diameter of
the blanks used for thread rolling wrought metals is usually
the pitch diameter of the finished thread.
Because thread rolling does not remove or compr~ss
material, an essential requirement in thread rolling is ~hat the
blank should not contain more than the correct amount of ~aterial
to form the finished thread, otherwise the dies tend to b~come
overloaded. If the diameter of the blank is less than the correct
amount, an incomplete thread form results.
It is very important that the outside diameter IO.D.)
of the blank be as accurate as possible. As the volume of the
thread above the pitch diameter ~addendum) of an American Standard
thread very nearly equals the volume of the material displaced
from below ~dedendum), it becomes clearly apparent that the
diameter of the blank approximates the pitch diameter of the
finished thread. Failure to control the blank diameter is one
of the biggest causes of premature die failure.
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A balanced thread is one in which the thread volume
above the pitch diameter is substantially equal to the thread
volume below the pitch diameter. Generally, it is recommended
' -that the diameter of the wrought metal blank be less than the
actual pitch diameter to allow for "grow room" before the maximum
allowable blank diameter is reached. Metal can be forced to flow,
but for all practical purposes it cannot be compressed.
Thus, as stated earlier, blank dimensions must be
accurately controlled. For example, the final thread diameter
tolerances are generally two to three times that of the starting
blank diameter tolerancesO So long as the wrought metal blank is
uniform and dimensionally accurate, thread rolling has the unique
inherent ability to maintain the accuracy OL the original set up
during long runs oE high speed production.
It would be desirable to provide a method of thread
rolling metal blanks which does not require highly dimensionally
accurate blanks and which is capable of providing high production
rates and a roll threaded product having a good combination of
physical properties.
Objects of the Invention
It is an object of the invention to provide a powder
metallurgy method for producing thread rolled metal products.
Another~object is to provide a thread rolled sintered
P/M product characterized metallographically by a structure in
which the rolled and mechanically formed threads from the root
to the crest of the threads are hlghly dense at and below the
surface thereof and exhibit a density of at least about 95~,
e.g., at least about 98--, of the actual density of the metal,
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with the core o e the threaded product below the threacls having
a porosity defined by an average density of about 75% to 92%,
generally about 80~ to 92~ r oE -the actual density of the metal.
These and other objects will more clearly appear when
taken in con~unc~ion with the following disclosure and the
accompanying drawings, wherein:
Fig. 1 is a portion of a sintered cylindrical P/M blank;
Fig. 2 shows the P/M blank portion in the thread rolled
condition;
Fig. 3 depicts schematically a typical balanced thread
in which the thread volume above the pitch diameter (addendum) is
equal to the thread volume below the pitch diameter (dedendum~;
Fig. 4A is a sintered P/M blank prior to thread rolling,
while Piy. 4B is the blank following thread rolling partially
broken away to show the internal structure, it being understood
that the structure is enlarged for purposes of clarity;
Fig. S is a representation of an unetched macrograph a-t
20 times magnification of a cross section of a thread rolled P/M
product showing the high density at the thread por-tions~ parti-
cularly high density sub-surface, and the rather high porosity at
the core of the thread rolled product;
Fig. 6 is the same as Fig.~ 5~ except that it depicts
the cross section of the thread rolled P/M product at 50 times
magnification;
Fig. 7 is a tensile test assembly employed i-~ determining
the shear strength of the threaded blank.
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Statement of_the Invention
An advantage of the invention is that the sintered P/M
blank need not be as accurately dimensioned as a wrouyht metal
~ blank in order ko achieve a quality end product. For example,
the P/M blank diameter is greater than the recommended blank
diameter of the wrouyht material, that is to say, greater than
the pitch diameter, so long as the startiny sintered P/M blank
is porous and has a density ranging from about 75% to 92% o~ the
actual density of the metal.
Work hardening and grain orientation of the P~M blank
is similar to that obtained with wrought material but to a lesser
degree due to porosity elimination by densification. A major
advantage in using P/M material is the elimination of embrittling
strength-reducing porosity in areas within and at the root section
where stress concentration is the greatest.
As stated earlier for the wrought material, once the
total tooth of the thread is "filled out", no further deformation
is permitted or tolerated; whereas, in the case of the P/M blank,
further deformation or compression may be achieved with the P/M
material merely by further closing the internal pores of the
blank. This further deformation increases the depth or zone of
densification, thus improving the shear strength of the P/M thread.
The fact that one may start with larger P/M blank
diameter and thread roll beyond the point of full thread formation
is due to the compressibility of the porous material. ,Thus, a
practical advantage of the invention is that a high precision
thread can be formed from a low precision preform or blank.
This enables a wide range of flexibility in the process since
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the dimensions of the blank need not be overly precise and are
easily within the capabilities of the P/M process without the
need for secondary operations~
Following thread rolling of the sintered P/M blank,
the threaded blank may be further sintered to improve its strength
followed by heat treatment, should the material employed be heat
treatable~ such as carbon steel.
A section of a typical cylindrical powder metal blank lO
is depicted in Fig. l. The blank is shown being threaded by die 11D
Fig. 3 is a schematic of a balanced thread 12, with pitch line 13,
addendum l4 and equal dedendum 15, the pitch line being midway
between the root and the crest of the thread.
Thus, one embodiment of the invention resides in a
method of thread rolling a cylindrical sintered P/M blank comprising,
forming a sintered cylindrical powder metal (P/M) blank of density
r~nging from about 75% to 92% of ,he actual density of said metal
blank having a selected diameter larger than the final pitch
diameter of a predetermined roll threaded product produced there-
from and not substantially exceeding the outside diameter of said
predetermined roll threaded product; the P/M diameter selected
being substantialIy inversely related to the density of said blank
over said range of 75~ to 92% of the actual density, the P~M
diameter selected being correlated to produce a substantially
full thread; and then thread rolling said sintered P/M blank using
a threading die corresponding to the gage of the predet~rmined
roll threaded product to be produced; thereby producing a threaded
product in which the density from the root to the crest of the
threads is at least about 95% of the actual density of the metal r
the material below the thread having a density ranging from about
75~ to 92% of the actual density of the metal.
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~y employing a blank diameter in excess of the inal
pitch diameter, a high degree of densifica-tion is assured a-t the
thread portion. This is shown in Figs. 4A and 4B. In Fig. 4A,
,~' a sintered P/M blank 16 is shown, with the pitch diameter indicated
by dotted line 17, the -threaded blank 16 produced being depicted
by Fig. 4B showing the work hardened threads 18 with flow lines 19
shown schematically and the interior porous section or core indi-
cated by the numeral 20. As stated earlier, the surface and sub-
surface area of the threads at the root and the crest is at least
about 95% dense and generally at least about g8% of the actual
density of the metal.
Representations of photomacrographs taken at 20 times
and 50 times magnification, respectively, are shown in Figs. 5
and 6 which depict cross sections of threaded P/M blanks in the
unetched condition.
Fig. 5 is a threaded sintered blank of a size 3f~ - 16
UNC that demonstrates the features of forming a powder me~al
blank. The initial blank diameter was 0.346 inch, whereas a
wrought blank must be sized at the pitch diameter of 0.331 inch.
The final pitch diameter of the formed P/M thread was mea~ured
to be 0.337 inch. A blank diameter of 0.015 inch oversized
resulted in a final pitch diameter only 0.003 inch oversized.
The difference is explained by the compressive nature of the
material.
Fig. 6 is the same part as Fig. 5 except,that, ~he
magnification is 50 times.
As will be noted in Fig. 6, the threads are very dense,
at both the roots and the crests (A) r the inner region or tooth
core "B" of the too-th being less porous than the original porosity
r r
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of the blank typified by re~ion "C". The core re~Jion "C" has a
density ranging from abo-ut 75% to 92% and yenerally 80% to 92%
of the actual density of the metal. Region "A" nearest the to.oth
surfaces has an actual density exceeding 95% and usually 98% of
the actual density of the metal. Densification occurs in region
"B" with the density between the extremes of "A" and "C"~
Details of the Invention
Powder Metal Compaction
The P/M blank may be made of various metal compositions;
for example, steell aluminum alloys, copper alloys, such as brass
and bronze; nickel-base alloys~ such as the alloy known by the
trademark Monel containing 60% nickel, 37% copper and such
residuals as silicon, manganese, etc., making up the balance,
among others.
The invention is particularly applicable to steel P/M
parts~ In the production of a sintered cyl.indrical blank of
steell a steel powder composition is cold pressed in a cylindrical
die dimensioned to produce the desired size. The composition is
compacted at a pressure of about 30 to 45 tons per square inch
and the resulting blank then sintexed under substantially non-
carburizing conditions in a non-oxidizing atmosphere, such as
cracked ammonia for about 20 minutes at a temperature of about
2000F to 2150F. The sintered blank has a density of about 75%
to 92% of the actual steel density and generally from about 80 or
85% to 92% of the actual density.
Powder Type and Alloy
The types of steel powder used are preferably selected
according to those which are economically attractive as well as
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those which are the most practical. The powder composit:ion may
comprise a mixture of elemental powders or comprise the final
alloy compositions.
/ Pre-alloyed powclers, however, are preferred such as
those produced by atomization from a liquid melt. To assure that
such powders are compactable, the carbon is omitted rom the
composition, the carbon being subsequently blended to the atomized
powder prior to compaction. Alternatively, the carbon can be
added after the blank has been sintered by carburizing the
sintered blank ~o the desired carbon level.
The invention is applicable to a wide variety of steels,
such as 52100-type steels, low nickel-molybdenum steels, molybdenum-
manganese steels, and khe like. Thus, for the purpose of this
invention, a steel is defined as a composition containing by
weight at least about 65% iron, about 0.3% to 1.5% carbon r and
the balance steel alloying ingredients.
Examples of steels which may be employed in -the inven-
tion are 4% Ni, 2% Cu, 0.6% C, and the balance iron; 1.5% Mo,
1% C, and the balance iron; 0.5% Mo, 0.5% Mn, 0~8% C~ and the
balance iron; 1~5% Cr, 0.5% Mo, 1.0% C, and the balance iron;
and 1~8% Ni, 0.5% Mo, 0.25% Mn, and 0.6% C, among other well-known
steel compositions.
As illustrative of the various embodiments of the
invention, the following examples are given:
Example 1
Sintered P/M cylindrical steel blanks were produced
measuring 7/8 inch long and 3 inches in diameter in accordance
with -the method described hereinbefore. The cylinders which had
an average density of about 6.6 g/cc had a composition of 0. 8o C
0.50Q6 Mn, 0.50% Mo, and the balance iron.
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The cylindrical blanks were cut, machined and ground
dry to thread rolling blanks for producing a 3/8-16 UNC thread.
The cut blanks ~ere ground to four di~ferent diameters, to wit:
O .33 ~ O .3 41 l~r O .346l~ and 0.356". Tne blanks were thread
rolled on a Reed thread rolling machine referred -to in the trade
by the designation as a Reed A22HB Cylindrical Die Thread Rolling
machine manufactured by the Reed Rolled Thread Die Co~, a division
of Litton Industries.
The machine was set to roll a full form thread in low
carbon steel wrought blanks which, by necessity, were machined to
the actual pitch diameter (0. 331 inch) of the finished thread
t 3,/8-16 UNc thread). Each of the P/~ cylinders were rolled at
the same setting using an oil coolant (lubricant). The details
of thread rolling are not discussed since thread rolling is well
known to those skilled in the art.
Following the thread rolling of each of the PjM blanks,
the threaded blanks were tested using the tensile test assembly
of Fig. 7 comprising two internally threaded collars or jaws 20,
21 into which both ends of the finished blank 22 are threaded as
shown. Load is then applied to both ends of jaws 20, 21 as shown
and the load at failure recorded. In the tests conducted, failure
occurred upon shearing of all the threads.
For an additional refèrence of comparison, machined P/M
threads were produced in some of the P~M blanks so that a compari-
son could be made between machined threads and roll-ed ~hreads of
the P/M blanks. The threads formed from P/M blanks of ~arious
diameters were measured. Densities measured were an average of
the entire formed part and are s-ta-ted as an average. Hardening
was performed by heating to 1565F for 20 minutes at temperature
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in an enclothermic atmosphere with the dew point adjusted to be
in equilibrium with the carbon content of the blank followed by
oil quenching and then tempering for 2 hours at 340F in air.
The following table summarizes t:he dimensional and
physical property data obtained. The various P/M threads are
designated as A, B, C, and D.
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- A study of the data in -this table reveals that full
thread form is achieved with the wrought blank whose blank diameter
was .331'l, whereas the P/~ blanks at .331 inch (A) and .341 inch
~B) still had incomplete thread forms demonstrating the compressible
nature of the P/M material. Full thread form is achieved for the
.346 and .356 inch tC and D) diameter P/M blanks. Forming the
oversized threaA blanks did not result in die racture or part
fracture as would have occurred with an oversized wrought blank.
The data of parts C and D furthermore demonstrate the
"forgiving" characteristic of the P/M materials. While the
diame-ter was increased by .010 inch, the final pitch diameter
increased only .002 inch, one-fifth that of the initial blank.
It was mentioned before that the final tolerance of a wrought
thread is 2 to 3 times the tolerance of the initial blank. These
results indicate the broad tolerance range possible with roll
threaded P/M blanks. In other words, the starting P/M blank does
not require the precision of a wrought metal blank~
The data also show that the density of the P/M thread
increases upon thread forming. It should be remembered, however,
that the increases shown are due to the elimination of porosity
near the tooth surfaces themselves, the central portions of the
blanks being unchanged from the original blank densities. The
average densities of parts B, C and D is 6.82 g/cc (86.7%), an
increase from the initial blank density of 6.56 g/cc (83.3%).
The fact that most of this increase is due to the concëntration of
porosity elimination near the thread root and tooth surface is
demonstrated clearly by the increases in shear strength realized.
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As will be noted, the cut threads provide much less
shear strength in both the non-heat trea~ed and the heat treated
condition. Thus, the roll threaded P/M blanks are markedly
superior ~o machined thread P/M blanks. Thread rolling increases
the "as rolled threaded" shear strength by over 40% of the shear
strength obtained with machined threads and in the heat treated
condition by over 75% of the shear strength obtained with heat
treated machined threads.
As stated earlier, it is important in roll forming
threads on PJM blank that the starting diameter of the blank be
larger than the final pitch diameter of the predetermined roll
thread product and not substantially exceed the outside diameter
(i.e., the major diameter) of the predetermined roll threaded
product, the P/M blank diameter selected being substantially
inversely related -to the densi-ty of the porous blank ranging from
75% to 92% of the actual density of the metal making up the blank.
For example, the higher the density of the blank, the smaller is
the selected diameter so lony as it is greater than -the final
pitch diameter of the roll threaded product and vice versa.
It is preferred that the blank diameter prior to thread rolling
be between the final pitch diameter and the major diameter of
the rolled -thread.
An approximate formula that can be employed in determining
the starting blank diameter of the P/M blank is given as follows~
blank diam. = pitch diam. -~ 2x thread height x (10~ - ~ensity)
100
In producing a 3/8 - 16 thread from a P/M blank having
a density of 83% of actual density, the following blank diameter
is employed;
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blank diam. = 0.331 + 2x.044 x (100-83)
100
= 0.331 -~ 2x.044 x 0.17
= 0.331 -~ 0.015
= 0.346 inch
It will be noted that the blank diameter calculated
corresponds to the same blank diameter indicated for P~M blank C
hereinabove which provided the desired results.
An example of a steel alloy for use in producing roll
threaded products is a steel known by the designation AIsr 4660
containing 1.8% Nir 0.5~ Mo, 0.25% Mn, 0.6% C, and the balance
essentially iron. The steel except for the carbon is produced
as an atomized pre~alloyed powder of particle size less than
100 mesh U.S. Standard. Carbon along with 3~4% wax is added,
the amount of carbon being sufficient to reduce any oxides present
and to provide a final carbon content of about 0.6%. The powder
mix produced this way will have a greater degree of compressibility.
P/M blanks are formed from the powder mix by cold
compression in a die at a compaction force of about 30 tons per
square inch ~TSI) and the blanks sintered at a temperature of
about 2050F for 20 minutes at temperature in an atmos~here of
dissociated ammonia. During sintering, the carbon diffuses into
the alloy quickly and uniformly resulting in a highly homogeneous
alloy.
While high carbon materials are normally considered too
brittle to deform, lt has been observed that porosity is more
completely removed in the deformed area when the material is
harder.
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~ l 6~1 4
Following the production of the sintered P/M blank,
the material is then thread formed. The forrning is set up to
inhibit or eliminate as far as i-t is possible a high stress J low
cycle, fati~ue Eailure rather than the limit imposed by tensile
ductility. This is achieved by carrying out the forming operations
in as few revolutions as possible; fewer t:han 5 revolutions is
preferred, and fewer than 10 revolutions is generally necessary.
Following thread forming, the blank is treated in
either of the following ways:
(1) The roll formed blank may be simply hardened by
~uenching in oil from the austenitizing tempera-
ture for the particular steel and then tempered
at a temperature from about 250F to 400F
(94C to 204C) for abou-t 1 hour to 4 hours; or
(2~ The roll formed blank may be resintered at a
temperature of about 2000~ to 2150F
(e.g., 2050~F) for about 20 to 60 minutes at
temperature followed by hardening as
described above.
In order to assure optlmum properties t it ls preferred
that the latter trèatment be used following roll forming of the
threads.
In summary, the invention provides as an article of
manufacture a sintered thread rolled P/M product c~ara~terized
by a porous core and highly densified threads, the density of
the threads at surface and sub-surface thereof from the root to
the crest being at least about 95%, pxeferably at least about ~8%,
of the actual density of the metal forming the sintered product,
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~ 3 C~551 ~
wi~h the density oE the porous core ranging from about 75~ to 92%
' and generally from about 80~ to 92%. The metals employed may be
selected ~rom the group consisting o steel, aluminum alloys,
,i copper alloys, nickel alloys/ etc~ Steel is preferred. A typical
composition range of steel is one containing at least about 6S~
iron, about 0.3% to 1.5% carbon, and the balance steel alloying
lngredients.
Although the present invention has been described in
coniunction with preferred embodiments, it is to be understood
that modifications and variations thereto may be resorted to
without departing from the spirit and scope of the invention
as those skilled in the art will readily understand. Such
modifications and variations are considered to be within the
purview and scope nt the invention and thè appended c:aim~,
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