Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
5~3
S P E C I F I C A T I O N
This invention relates to the annealing of steels,
and more particularly to a process for the annealing of
steels to improve the forming and machining characteristics
of the steel.
Annealing is a well-known process in the treatment
of steels, and is used primarily to soften the steel so that
it can be machined or formed into a part having ~he desired
configuration in an economical manner. In general, annealing
is carried out by heating the steel in a furnace maintained
at the austenitizing temperature, from which the steel is
removed and cooled in a controlled manner. The steel is
heated to a temperature above the austenitizing temperature
(i - ' the A3 temperature) and then cooled so that the micro-
structure of the steel contains the so-called upper trans-
formation products, namely pearlite, blocky ferrite, spheroidal
carbides and combinations thereof. The upper transformation
products are to be distinguished from the equally well-known
lower transformation products, namely bainite, acicular ferrite
and martensite, in that the upper transformation products are
softer and more ductile than the lower transformation products.
Thus, for annealing to improve machinability, the goal in the
annealing process is to form upper transformation products to
the substantial exclusion of lower transformation products.
Annealing is frequently carried out with respect
to hot rolled steel, using large annealing furnaces. Just
the size of the furnaces required in terms of space requirements
and capital investment represents a significant drawback to
3 5~3
their use. As is well known to those skilled in the art,
there are several further disadvantages associated with the
use of such annealing furnaces. In the first place, furnace
heating efficiency ls generally quite low, with the result
that increasing fuel costs make it desirable to provide
a more efficient means of heating the steel. In addition,
furnace heating takes place by radiation, conduction and by
convection as the mechanism for heat transfer, thus necessitating
long cycles to insure that a load of steel in the furnace has
been subjected to uniform processing in a given heating cycle.
Such long cycles are themselves disadvantageous, for the
elevated temperatures used require the use of a known non-
o~idizing atmosphere (i.e., a protective atmosphere or
vacuum), which requires additional energy to produce.
It is thus desirable to avoid the use of such large
annealing furnaces provided that the physical properties of
the annealed steel fall within acceptable limits. It has
been proposed, as described in U. S. Patent Nos. 3,908,431,
4,040,872 and 4,088,511 to treat steels using various thermal
cycles by use of electrical resistance heating techniques.
Those techniques have the adva-ntage of providing very rapid
~eating of steel workpieces with high efficiencies, including
uniform heating over the entire cross section of the steel
workpiece.
Electrical resistance heating has been used in
annealing processes, as described in U.S. Patent No. 3,855,013.
In that process, a cold worked steel is rapidly heated by
electrical resistance heating to a temperature above the A
temperature, in which the ferrite present in the steel is
5~
converted to austenite. The carbides present do not dissolve in
the austenite thus formed, but remain as a separate phase. The
steel is then quenched to room temperature to convert the
austenite back to ferrite, in essence resul-ting in elimination
of the cold work in the steel leaving the carbides unchanged.
It has now been found that annealing of steels can be
significantly modified to provide steels having improved
ductility and toughness when the heating to a temperature above
the austenitizing temperature is rapidly carried out at a rate
such that most of the carbides dissolve in the austenite thus
formed, while leaving small particles of carbide in undissolved
form. The small carbide particles which remain are of sufficient
quantity to serve as nuclei for the growth of upper transformation
products during cooling, thereby resulting in an overall
acceleration of the annealing process.
The present invention is directed to providing an improved
process for the annealing of steels.
This invention also is directed to providing improved
steels which have been annealed to yield high ductility,
formability and toughness characteristics.
According to the present invention therefore, there is
provided a process for annealing steel comprising the steps of:
(a) rapidly heating a hypoeutectoid steel to a
temperature above the upper transformation temperature of the
steel to form austenite and carbides, with the rate of heating
being such as to cause dissolution of most of the carbides in
the austenite, leaving in undissolved form small particles of
carbide in an amount sufficient to serve as nuclei for the
precipitation of upper transformation products, and
(b) cooling the s-teel to precipitate said upper trans-
formation products and form an annealed steel having improved
ductility, formability and toughness.
-- 3 --
L5~3
The heating of the hypoeutectoid steel may be far less
than ten minutes and may be effected by passing an electrical
current through the steel.
More specifically, the present invention may be defined
as a process for annealing steel comprising the steps of:
(a) heating a hypoeutectoid steel uniformly across its
cross section to a temperature above the upper transformation
temperature of the steel to form austenite and carbides, with
the rate of heating being such as to cause dissolution of most
of the carbides in the austenite, leaving in undissolved form
small particles of carbides to serve as nuclei for the preci-
pitation of upper transformation products, and,
(b) cooling the steel to precipitate said upper trans-
formation products.
Figure 1 is a graph of temperature versus time,
representing the heating and cooling of two steel specimens to
show the temperature dependence of accelerated annealing in
accordance with the present invention;
Figure 2 is a graph of temperature versus time,
representing the heating and cooling of several steel specimens
to show the time dependence of accelerated annealing in accordance
with the present invention;
Figure 3A is a photomicrograph showing the micro-
structure of 4142 steel prior to processing in accordance with
the present invention (Heat A);
Figure 3B is a photomicrograph showing the micro-
structure of 4142 steel after heating to 1415F followed by
quenching (Heat A);
Figure 3C is a photomicrograph showing the micro-
30 structure of 4142 steel after heating to 1415F and air cooling
(Heat A);
Figure 4 is a graph of mechanical properties (hardness
-- 4
S~L3
~ and tensile strength~ versus austenitizing temperature of 8640
steel (Heat B);
Figure 5 is a schematic illustration of apparatus used
for annealing in accordance with the concepts of this invention;
Figure 6A is a photomicrograph showing the micro-
structure of 4140 steel prior to treatment in accordance with
the present invention (Heat C);
- 4a ~
` ?
~ ~,
5~L3
Figure 6B is a photo~icrograph showing the micro-
structure of 4140 steel after furnace annealing (Heat C);
Figure 6Cis a photomicrograph showing the micro-
structure of 4140 steel whicll has been annealed in accordance
with the process of this inven~ion (Hea~ C);
Figure 7 is a graph of Charpy impact energy
versus temperature for 4140 steel which has been furnace
annealed and annealed by the process of this invention (Heat C);
Figure 8 illustrates the results of machinability
testing (in the form of a plot of part growth versus parts
produced in running time)for 4140 steels which have been
furnace annealed and annealed in the process of tnis invention(Heat C)
Figure 9A is a photomicrograph showing the micro-
structure of 4140 steel prior to processing in accordance
with the present invention(Heat D);
Figure 9B is a photomicrograph showing the micro-
structure of 4140 steel after furnace annealing(Heat D);
Figure 9C is a photomicrograph of 4140 steel as
annealed by the process of this invention (Heat D);
Figure lQ is a plot of Charpy impact energy versus
temperature for 4140 steel which has been furnace annealed
and annealed by the process of this invention(Heat D);
Fi~ure 11 is a graph (in the form of time versus
speed as determined by a modified Taylor life test) of 4140
steel which has been furnace annealed and annealed by the
process of this invention (Heat D);
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51~
Figure 12A is a photomicrograph showing the micro-
structure of 8640 steel prior to processing in accordance
with the present invention (Heat B);
Figure 12B is a photo~licrograph showing the micro-
s~ructu-.e OI ~6~u Sl~i which nas be~n Iurn~c~ anneaieu ~-nea
B);
Figure 12C is a photomicrograph showing the micro-
structure of 8640 steel which has been annealed in accordance
with the process of this invention (Heat B);
Figure 13 is a graph of Charpy impact energy versus
temperature for 8~40 steel which has been furnace annealed
and annealed by the process of this invention (Heat B);
Figure 14A is a photomicrograph showing the micro-
structure of 6150 steel prior to treatment in accordance
with the process of this invention (Heat G);
Figure 14~ is a photomicrograph showing the micro~
structure of 6150 steel which has been furnace annealed (Heat G);
Figure 14C is a photomicrograph showing the micro-
structure of 6150 steel ~hich has been annealed by the process
of this invention (Heat G);
Figure 15 is a graph of Charpy impact energy versus
testing temperature for 6150 steel which has been annealed in
a furnace and 6150 steel which has been annealed in accordance
with the process of this invention (Heat G);
Figure 16A is a photomicrograph showing the micro-
structure of a 1144 steel prior to processing in accordance
with the process of this invention (Heat H);
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~ igure 16B is a photomicrograph showing the micro-
structure of the 1144 steel shown in Figure 16A after annealing
in a furnace (Heat H);
Figure 16C is a photomicrograph showing the micro-
structure of the 1144 steel after it has been annealed in
accordance with the process of this invention (Heat H);
Figure 17A is a pnotomicrograph showing the micro-
structure of an 86L20 steel prior to processing (Heat I);
Figure 17B is a photomicrograph showing the micro-
structure of the 86L20 steel of Figure 17A after it has been
furnace annealed (Heat I); and,
Figure 17C is a photomicrograph showing the micro-
structure of the 86L20 steel of Figure 17A after it-has been
annealed in accordance with the process of this invention (Heat I).
` The concepts of the present invention reside in the
discovery that high levels of ductility and toughness can be
achieved with hypoeutectoid steels by annealing where the
steel is rapidly heated to a temperature above the upper
transformation temperature to form austenite and retained
iron carbides, with the rate of heating being sufficiently
rapid such that most of the carbides are dissolved in the
austenite leaving small particles, usually spheroidal in form,
3 of carbide in an undissolved state. Following that heating,
the steel is cooled at a rate such that the small particles
of retained carbide which are undissolved in the austenite
serve as nucle:i for the growth of upper transformation products,
notably including pearlite and blocky ferrite as well as
fine spheroids of iron carbide. It has been found that
--7--
carbides thus formed on cooling are characterized by particle
sizes which are much finer than those formed during furnace
annealing, with the result that the refined microstructure
of the annealed steel provides improved ductility, formability
and toughness when compared to steel subjected to furnace
annealing.
The concepts of the present invention are applicable
to the processing of hypoeutectoid steels having a carbon
content ranging up to 0.7% by weight, and preferably
containing between 0.1 to 0.7% carbon by weight. Such
steels may contain relatively small quantities of the
common alloying elements such as chromium, molybdenu~,
nickel and manganese. By a widely used convention, a
steel containing less than 5% by weight of such alloying
elements is referred to in the art as a "low alloy steel".
Representative hypoeutectoid steels which can be used in
accordance with the present invention are shown in the
following table:
--8--
- ~I r ~ 15~3 ~ o
U ,
~1
I ~ ~t~t ~ ~t ~r ~1
¢ O o oo o o o ~ o
O o oo o o O I O
u~ oo OCO O
O ~ ~ l ~ O O ~
o o o o o o o o o
1~ C~ ~oC~ o~ ~ o Ln
C~
I o oo ~ o o o o o
æ ~ ~t ~ l o o
O o oo o O O I O
U~
O o oo o o o o o
U~ O o oo o o o ~ o
~ .
I O O O O O O O O ~
. I
~;
E~
~ ~ ~ ~ ~ ~ ~ ,~ ~
P~ O o o o o o o o o
O o o o o o o O o
O ~ CS~
O ~i o o o o O ~1 0
~ -I c~ ~1 ~t O ~t O
t ~ ~ ~t ~t ~ Lf) ~t C~
O o oo O o o o O
S~
~ J ~ C~ J o C~
t~S ~ ~ ~ l ~ O ~ ~O 1
O o o~ o o o O
~ ~ ~ ~~,~
- a~ c~lo o o oc~l o ~t O
~t ~t ~~t~ ~ u~
,~
~0 ~t~t ~t ~~D
C~ oO
Q)
X
~lS~L3
In the preferred practice of this invention~ the
steel is in the form of a workpiece which can be heated
separately so that the heating process can be precisely
controlled. For that purpose, it is frequently preferred
to employ workpieces in a form having a repeating cross
section such as bars, rods, tubes and the like.
In accordance with the preferred embodiment~ the
individual workpieces are rapidly heated by direct electrical
resistance heating or by electrical induction heating,
preferably while the temperature of the workpiece is monitored
by a suitable sensing device. The rapidity of the heating
process, while permitting the economic processing of large
quantities of workpieces, causes the austenitizing trans-
formation to proceed very rapidly. The most preferred
method for rapid heating in accordance with the present
invention is by direct electrical resistance heating. That
technique, described in detail by Jones et al, in United States
Patent No. 3,90~,431 involves a procedure whereby an electri-
cal current is passed through the steel workpiece; the electri-
cal resistance of the workpiece to the flow of electrical cur-
rent causes rapid heating of the workpiece uniformly throughout
its entire cross section.
In heating according to the technique of Jones et
al., the workpiece is connected to a source of electrlcal
current, with the connection being made at both ends of the
workpiece so t~at the current flows completely through the
workpiece. Because of the uniform flow of the current
through the workpiece, the temperature of the workpiece,
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r~s
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~5~S~
usually in the form of a bar or rod, increases uniformly,
both axially and radially. Thus, the interior as well as
the exterior of the workpiece are heated simultaneously,
without introducing thermal strains. In contrast, in a
conventional furnace, the exterior of the furnace load is
heated much more rapidly than the interior, with the result
that the steel near the exterior of the load is completely
transformed to austenite while the interior of the load may
not have, at the same point in time, undergone transformation
to austenite. Such furnace techniques thus involve a
significant disadvantage since it is difficult, if not
impossible, to control the rate of heating such that austenitic
transformation along with dissolution of only part of the
carbides is achieved. In other words, there is a tendency,
when heating in a furnace, to dissolve all of the carbide,
and hence fine particles of retained carbides are not
available to serve as nuclei for the formation of the upper
transformation products on cooling.
Ir the practice of this invention, the steel work~
piece is heated to a temperature above the A3 or upper
transformation temperature at a very rapid rate, usually
within the range of one second to ten minutes. The control
of the heating step may be effected within relatively narrow
limits by making use of the well-known endothermic character
of austenite transformation. As is now well established,
the temperature of the workpiece at the onset of austenitic
transformation remains constant, or even decreases slightly
for a period ranging from a few seconds to several minutes,
depending somewhat on the heating rate. A typical heating
curve for the austenitizing step used in the practice of
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s~
this invention is shown in Figure 1 of the drawing, which
is a plot of temperature versus time for heat A of 4142
steel. (The chemistry for that particular heat is shown
in Table 1, supra.) As can be seen from that figure, two
steel workpieces were rapidly heated in less than five
minutes to austenitizing temperatures of 1412F for sample
(1) and 1550F for sample (2). The heating curve indicates
that carbide dissolution is taking place when the rate of
temperature increase remains constant, that is at the so-
called heating arrest point. Care must be taken when that
point is reached to insure that all of the carbides do not
dissolve in the austenite thus formed. It is an important
concept of this invention that sufficient quantities of
carbide be retained in undissolved state to serve as
nuclei for precipitation of the upper transformation products
on cooling of the workpiece.
Thus, heating to the austenitization temperature
is continued for a short time above the heating arrest point.
During this time, austenitization of most of the structure
is completed, and the structure which exists at the point
of maximum temperature consists essentially of austenite
and undissolved carbide particles. Following the austeni-
tizing, the workpieces are allowed to air cool at their own
rate.
As shown in Figure 1, the steels exhibit a cooling
arrest (1220F for sample (1) and 880F for sample (2)) at
which point precipitation of transformation products begins.
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3L5~3
The cooling arrest point is thus determined by the austenitizing
temperature employed. In the case of sample (1), the
austenitizing temperature was such that the sample cooled
to for~ upper transformation products, while sample (2),
austenitized at the higher temperature, cooled to form lower
transformation products.
The complete data on this phenomenon is shown in
the following table:
TABLE 2
~L~EC~NICAL PROPERTIES OF 4142 -
AUSTENITIZED AND AIR COOLED SPECI~ENS
. . _ . . _
SPECIMEN Hardness Tensile Yield EL RA
-~~ (~hn) (ksi)- (ksi)
As Received Steel 311 153.0 77.3 14.1 37.2
Austenitized at
1412F (Time: 212 110.4 63.9 24.0 59.7
3 min.-36 sec.)
Austenitized at
1430F (Time: 235 127.1 70.4 18.5 41.6
3 min.-58 sec.)
Austenitized at
1480F (Time: 266 135.2 74.3 17.9 40.2
4 min.-26 sec.)
Austenitized at
1550F (Time: 2~2 147.3 95.9 16.1 42.2
5 min.-2 sec.)
The data show that the hardness of the steel increased
(accompanied by a decrease in ductility) as the austenitizing
temperature increased.
The foregoing data demonstrate that accelerated
annealing in accordance with the present invention is
dependent on austenitization temperature. If the temperature
is too high, all of the carbide dissolves, and hence no nuclei
-13-
~515~3
are available to accelerate the rate of precipitation of
upper transformation products.
It has also been found tha-t time affects the
annealing process of this invention as well, with longer
heatin~ ~i~es resuLting in ~3s~u~an ~ al? o th~ c~rbi~
particles present. This effect is shown in Figure 2, which
is another plot of temperature versus time for a series of
samples held for varying lengths of time prior to air
cooling. (The heating and cooling portions of this curve
have been cut off at 1100F so that only the temperature
where upper transformation products form are shown. The
hardness values for each sample are also shown in the graph.)
As shown in Figure 2, as the time at which the
various samples are held at the austenitizing temperature
is increased, the cooling arrest point decreases; as a
result, there is a tendency for lower transformation products
to form instead of the upper transformation products in
accordance with the practice of this invention. Thus, the
hardness values increase as the austenitizing time increased.
Indeed, after 12 minutes at the austenitizing temperature,
there is no cooling arrest within the temperature range
shown in Figure 2, and the hardest air-cooled specimens are
produced.
The :Eoregoing tests with 4142 specimens demonstrated
that the accelerated annealing phenomenon of this invention
is dependent upon both austenitizing temperature and
austenitizing time. To demonstrate how the accelerated
annealing phenomenon occurs, the microstructures of the steel
prior to austenitization, at the austenitizing temperature,
-14-
~515~L3
and after air cooling, were e~amined. The as received
structure and the air cooled structure could be examined
using standard metallographic techniques. To observe the
condition of the austenite at the austenitizing temperature,
a classical metallurgical quenching technique was used. A
specimen from Heat A was rapidly heated to 1415F and quenched
in agitated water. The parts of the structure tha-t were
austenite prior to the quench were converted to martensite.
Consequently, the austenitized structure could be observed
at room temperature with standard metallographic techniques
by using an etchant thaL would not reveal the martensite.
Figures 3A, 3B and 3C show the as received structure,
the austenitized-quenched structure, and the austenitized-
air cooled structure of samples from Heat A. The scanning
electron microscope (SE~I) was used for these photomicrographs
due to the fine nature of these structures.
This technique clearly revealed the structure o~
the steel beforç austenitizing, at the austenitizing
temperature and after air cooling.
It can be clearly seen from these photomicrographs
that the as received structure (before processing) was
austenitized during the rapid heating cycle, but some
particles of carbide remained undissolved in the austenite.
Since nuclei already existed in the austenitized structure,
there was no time required at the annealing temperature for
nucleation oE upper transformation products. The retained
carbide particles simply began to grow as the temperature dropped
below the Al temperature, and eventually pearlite began to
~15~3
grow from the carbide nuclei. Consequently, the time required
to anneal the steel was shortened considerably. Several other
grades of steel were tested in a similar manner, and, in
each case, it was discovered that the austenitized structure
consisted of austenite with fine spheroidal carbides. This
retention of carbide in the austenitized structure due to
rapid heating is believed to be the basis of the accelerated
annealing phenomenon of this invention.
The retention of carbide in the austenitized
structure of steel has been noted in the literature. However,
with slower heating, the amount of carbide retained in the
austenitized structure is small. Consequently, a steel which
is slowly heated to the austenitizing temperature is less
likely to display the accelerated annealing phenomenon.
Comparison tests with furnace austenitizing treatments and
rapid austenitizing treatments revealed that the accelerated
annealing phenomenon did not occur with furnace treatments.
In one of these tests, bars of 8640 from Heat B
were austenitized at various temperatures in a furnace and
allowed to air cool. Then another set of bars from the same
heat was austenitized with electric resistance heating and
allowed to air cool. The mechanical properties which
resulted from this treatment are sho~ in Figure 4. All of
the furnace treated specimens cooled to a relatively high
hardness. However, the rapidly heated specimens show a
noti-ceable transition between hard and soft air cooled
specimens. Table 3 shows the mechanical properties of one
set of specimens from this test. The furnace austenitized
specimen had mechanical properties very similar to those of
-16- .
~ ~ 5~ S~ ~
the as received steel while the rapidly austenitized
specimen was significantly softer. It is clear from that
data that 8640 responded to the rapid austenitization in
the same manner as the 4142 had responded. However, the
accelerated annealing phenomenon is more apparent in ~640
because ~his steel has lower hardenability than 4142.
TABLE 3
.~CH~ICAL PROPERTIES OF 8640 -
AUSTE~ITI~ED AND AIR COOLED SPECIMENS
S~ECI~iE~ Hardness Tensile Yield EL RA
(Bhn) ~ksi) (ksi)
As Received 8640 256 132.7 94.3 15.1 39.1
Furnace Austenitized
at 1500F, Air 254 132.1 96.2 15.5 47.0
Cooled (Time: 1 ltr~)
Rapidly Austenitized
at 1500F, Air 205 111.1 71.1 20.0 50.3
Cooled (Time: 4 min.)
The tests demonstrate that the accelerated annealing
phenomenon is sensitive to austenitizing temperature because
the specimens which were rapidly austenitized above 1550F
did not self-anneal. These tests also demonstrate that the
accelerated annealing phenomenon is dependent upon austenitizing
time as well because none of the furnace austenitized specimens
annealed during air cooling regardless of the austenitizing
temperature. Furnace treatments are simply too slow to
permit the accelerated annealing phenomenon to occur. The
relatively long time at the austenitizing temperature permits
the retained carbide to be dissolved or reduced in size to
the point where there is not enough carbide left to be
effective as nuclei for carbide growth during cooling.
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~ 5~ 3
In accordance with one variation in the prac~ice
of this invention, it is sometimes desirable to insure
uniformity in cooling rates in large batches of workpieces
being processed. If, for example, steel bars were simply
heated and piled in a rack to cool, the first bar might
cool at a much faster rate than the last, and hence lack
of uniformity withln a batch of steel processed at one time
might develop. Accordingly, to avoid lack of uniformity of
batches, use can be made of an insulated cooling queue of
the sort illustrated in Figure 5 of the drawing. When using
this type of equipment, it is possible to pass bars through
the queue with a dwell or residence time of, for example,
lO minutes. No external source of heat need be used in
equipment of that type,and hence no energy is consumed.
Tests with respect to uniformity of mechanical properties
have demonstrated that the insula~ed cooling queue is
effective.
Having described the basic concepts of the present
invention, reference is now made to the following examples,
which are provided by way of illustration and not by way of
limitation, of the practice of the present invention in the
annealing of steel bars having a length of 7 feet. In each
example, the steel was examined in three conditions, namely
as received or prior to any treatment, after furnace annealing,
and after annealing by the process of this-invention, with
comparisons having been made between the furnace annealed
steel and steel annealed by way of this invention.
EX~PEE 1
This example illustrates the annealing of a 4140
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steel from heat C as shown in Table 1.
Twenty bars of 4140 from Heat C were furnace
annealed using a roller hearth :Eurnace. The furnace
austenitizing temperature was 1550F, and the annealing
cvcle was a total of 16 hours long.
Twenty bars from the same heat were also annealed
by the process of this invention. Tlte austenitizing temperature
was 1450F, and each bar was austenitized in 33 seconds. The
total annealing time for all 20 bars was less than one hour.
Both lots of steel were cleaned, cold drawn and
straightened after annealing. Then the two lots were
e~tensively tested, and the steel that remained after testing
was used for a machinability test. Table 4 shows the mechanical
properties of the steel used in these tests. The mechanical
properties of the as received steel and`the as annealed steel
are sho~t for comparison purposes.
The steel annealed by the process of this invention
has a better combination of properties than the furnace
annealed steel. The hardness of the steel annealed by the
present process is slightly higher, but the significant
difference between the two products is the improved ductility
of the steel annealed by the process of the invention. The
elongation was 13.3% for the furnace annealed steel after
complete processing, and the elongation for the steel annealed
by this process was 17.0%. This is an improvement of 28%.
The reduction of area for the furnace annealed steel was
39.0% and the reduction of area for the steel annealed by the
-19 -
~ 1513
~resen~ 2rocess ~as ,9.~/O~ T;~is is an i~rovement oi 52%.
The elongation and reduction of area are a basis for
estimating tl~e ductility of a steel, and these improvements
over the furnace annealed steel indicate a dramatic i~provement
in ductility and formability.
TABLE 4
?.`IEC~ ICAL PROPE~TIES OF Ll 40 - HEAT C
Hardness Tensile Yield EL RA
SPECI.EI~ (Bhn) (ksi)(ksi)
As ~eceived 4140315 151.0120.5 13.0 39. 4
Furnace ~nnealed197 99.746 . 6 21.5 40 . 8
Annealed by the
Present Process 217 108.058.7 23.5 60.9
Furnace Annealed `~
& Cold Drawn 226 111.581.2 13.3 39.0
Annealed by the
Present Process
& Cold Drat~n 23~ 119.493.9 17.0 59.3
The reason for the improved ductility of the product
annealed by the present process can be clearly seen in the
microstructure of these steel samples. Figures 6A, 6B, and
6C show the microstructures of samples from this heat of 4140
in three conditions: as received, furnace annealed, and annealed
by the process. The as received structure consists of lower
transformation products: upper bainite and acicular ferrite.
The furnace annealed structure consists of pearlite and ferrite.
The steel annealed by the present process has a structure which
consists essentially of ferrite, pearlite and fine carbide
spheroids. Ferrite areas are not distinct, and the ferrite
contains spheroidal carbides. Also, the grain size is finer
for the steel annealed by the present process. It is the fine
nature of this structure which gives the steel its improved
ductility and formability over the coarse furnace annealed
structure.
-20-
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The fine microstructure also gives the annealed
product improved toughness. Figure 7 shows the Charpy
impact curves for bars taken from the two lots of annealed
steel. The steel annealed by the present process has a
lower transitlon ,emperature, and an upper shelf energy
which is almost three times that of the furnace annealed
steel. Improved toughness is valuable for applications
where the par~ is machined or formed, and then only surface
hardened. In such applications, improved core toughness
would give the part higher resistance to rracture.
To demonstrate that the improved toughness and
ductility of steel annealed by the instant process did not
adversely affect its machinability as compared to that of
furnace annealed steel, a comprehensive machinability test
was carried out. The screw machine test was selected
because it tests the machinability of the steel with
several different types of tools. Figure 8 shows the
results of machinability testing of the two annealed lots
of 4140 from Heat C. ln this type of test> part growth
is ~easured and plotted against time or the number of parts
produced. Steels which machine well have part growth curves
which are relatively flat and near the time axis. Steels
which machine poorly have curves which have steep slopes.
The part growth curves shown in Figure 8 indicate that the
two annealed steels machined about the same. The steel
annealed by the present process was slightly better than
the furnace amlealed steel, but the difference is not
considered significant.
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EXAMPLE 2
This example illustrates the annealing at a 4140
steel from neat D.
~ Jenty bars from Heat D were furnace annealed using
a 16 hour cycle with an austenitization temperature of 1550F.
Then 20 additional bars from the same heat were annealed by
the present process. For this treatment each bar was
austenitized at 1500F in approximately 36 seconds, and the
entire lot was annealed in less than one hour.
Both lots were then descaled, cold drawn and straightened.
Extensive testing was conducted on each lot, and the steel -
that remained after this testing was used for a machinability
test. The mechanical properties of the as received steel,
the furnace annealed steel and the steel annealed by the new
process are shown in Table 5. Once again, the steel annealed
by the present process had greater ductility and was slightly
harder than the furnace annealed steel. Figures 9A, 9B and
gC show the microstructure of this heat of steel in three
conditions: as received, furnace annealed, and annealed by
the present invention, respectively. Just as before, the
steel annealed by the present process has a microstructure
which is much finer than the furnace annealed steel.
TABLE 5
~ECHANICAL PROPERTIES OF 4140 ~ HEAT D
. _ .... .. . . _
SPECI~EN Hardness Tensile Yield EL RA
~E~- (ksi)--- (ksi~ (%) ~
As Received 4140 311 151.4 109.6 13.3 41.9
Furnace Annealed
and Cold Drawn 231 119.1 lQ2.312.1 42.8
Annealed by the
Present Process
and Cold Drawn 241 127.4 105.913.8 53.1
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Figure 10 shows the Charpy impact curves for the
furnace annealed steel and -the steel annealed by the present
process. The superiority of the steel annealed by the present
?rocess is again obvious. The transition temperature is lower
and the upper shelf energy is higher for the steel annealed
by ~he present process.
Machinability testing of the two annealed lots of
steel was accomplished using a Modified Taylor Life test.
In this type of machinability test, the bar is turned at
various speeds and feeds until the machining tool fails. Then
the data points representing the time to failure at various
speeds are plotted on log-log paper. The result is a straight
line which represents the relationship between machining speed
and time to tool failure. Figure 11 shows the results of this
type of machinability testing on the two annealed lots produced
from Heat D. The two lines cross, indicating that there is
some difference between the way these two annealed steels
machined. However, at the lower machining speeds, where
alloy steels are usually machined, the steel annealed by
the present process is slightly better. ~nce again, even
though the steel annealed by the present process was harder,
tougher, and more ductile, it machined better than the furnace
annealed steel. These differences in ductility, toughness and
machinability are in the aggregate, a significant inprovement
in the mechanical properties of this steel.
EXAMPLE 3
Ten bars of 4140 steel from Heat E were furnace
annealed using a 16 hour cycle with an austenitization
temperature of 1550F. Then ten bars from the same heat were
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annealed by the present process using an austenitization
temperature of 1450F. The bars were each austenitized in
35 seconds, and the entire annealing cycle was 45 minutes
long. Table 6 shows the results of this processing. The
specimens made in this test were not cold drawn after
annealing. This heat responded to annealing by the present
process almost exactly as the other heats had responded.
The superior ductility of the steel annealed by the process
of this invention is apparent from the data in Table 6.
TABLE 6
~ECHA~ICAL PROPERTIES OF 4140 - HEAT E
SPECI~EN Hardness Tensile Yield EL RA
(Bhn) (ksi) (ksi) (%) (%)
As Received 4140 269 141.6 99.2 15.7 50.2
Furnace Annealed
Steel 186 103.7 49.6 19.7 42.6
Annealed by the
process of this
invention 194 104.5 97.1 24.6 60.5
EXA~PLE 4
15 bars of 4142 from Heat F were annealed using a
furnace. The austenitizing temperature for the furnace
treatment was 1550~F and the cycle was 16 hours long. Then
15 more bars from the same heat were annealed by the process
of this invention. An austenitizing temperature of 1450F
was used, and each bar was austenitized in 60 seconds. The
entire cycle was less than one hour long. Table 7 shows the
mechanical properties of the steel in three conditions: as
received, furnace annealed, and annealed by the process of
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this invention. ~nce again, the steel annealed by the present
process had superior ductility as compared to that of the
furnace annealed steel.
TABLE 7
MECHANICAL PROPERTIES 3F 4142 - HEAT F
SPECI~IEN Hardness Tensile Yield EL RA
(Bhn) (~si) (ksi)~ /O)
As received 4142 268 141.0 99.115.5 46.5
Furnace Annealed
Steel 194 101.8 50.3 20.846.6
Annealed by the
present process196 102.3 66.2 25.068.3
EXA~lPLE 5
Ten bars of 8640 from Heat ~ were annealed using
the roller hearth furnace. The furnace austenitizing temper-
ature was 1550F and the furnace cycle was a total of 16 hours.
Then ten bars from the same heat were annealed using the
p.esent process. The austenitizing temperature was 1450F,
and each bar was austenitized in 35 seconds. The total
annealing cycle with the process of this invention was
approximately 30 minutes. Table 8 shows the mechanical
properties of the steel in three conditions: as received,
furnace annealed and annealed by the instant process. Once
again, the steel annealed by the process of this invention
had significantly better ductility than the furnace annealed
steel.
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TABLE 8
~EC~NICAL PROP~RTIES OF 8640 - HEAT B
SPECI~`~N Hardness Tensile Yield EL 7~A
(Bhn)~ksi) (ksi~ (%~ (~/o)
As P~eceived 8640 258 132.1 102.8 16.7 46.9
Fu~nace .~nnea~ed
Steel 176 98.9 49.3 22.8 47.0
Ar.nealed by the
?resent process180 9~.9 60.4 28.1 64.6
The microstructures of the as received steel, the
furnace annealed steel, and the steel annealed by the process
of this invention are shown in Figures 12A, 12B and 12C. The
steel annealed by the present process had a more spheroidal
st-ucture than the furnace annealed steel, and it was somewhat
finer. This difference in microstructure is similar to what
was observed in the 4140 tests. The 8640 was also tested
for toughness using the Charpy impact test. The results of
impact testing of the two annealed lots is shown in Figure 13.
Once again the steel annealed by the present process had far
su?erior toughness. (It should be noted that the annealed
~6~0 was not cold drawn prior to testing. Consequently, it
was some~.7hat softer and tougher than the 4140 heats that were
mentioned earlier.)
EX.~PLE 6
Twenty bars of 6150 from Heat G were annealed using
a roller hearth furnace. The furnace austenitizing temperature
was 1550F and the cycle was 16 hours.
Then twenty bars from the same heat were annealed
with the process of this invention using an austenitizing
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te~perature of 1500F. Each bar was austenitized in 34
seconds, and the total annealing time was approximately
one hour. Table 9 shows the mechanical properties of
the as received steel, furnace annealed steel, and steel
annealed by the present process.
The two annealed lots were then cold drawn and
straightened to duplicate one type of typical commercial
processing. The cold drawn and straightened properties are
also given in Table 9. For this particular grade, the steel
treated with the present process was slightly harder than
the furnace annealed steel, but it was still more ductile.
This superior ductility is evident both before and after
cold drawing.
TABLE 9
MECHANICAL PROPERTIES OF 6150 - HEAT G
SPECIMEN Hardness Tensile Yield EL RA
(Bhn) (ksi) (ksi) ~ (%)
As Received 6150 299151.0 111.6 12.0 37.8
Furnace Annealed
Steel 195 100.853.0 20.7 44.1
Annealed by the
present process 225 109.881.2 25.5 66.4
Furnace annealed
cold dra~
straightened 240 120.486.0 9.6 3-l.3
Annealed by the
present process
cold drawn
straightened 263 130.9103.8 13.0 48.3
The microstructures of the as received steel, the
furnace annealed steel, and the steel annealed by the present
~5~L5~L3
process are shown in Figures 14A, 14B and 14C, respectively.
The steel annealed by the present process has a finer carbide
structure than the furnace annealed steel. Charpy impact
testing was also accomplished on the two annealed samples
and the results are shown in Figure 15. The curves shown
are for the 6150 after cold drawing. Once again the steel
annealed by the present process had a finer microstructure,
improved ductility and improved toughness as compared to the
furnace annealed steel.
EXAMPLE 7
Several bars of 1144 from Heat H were furnace annealed
using a five hour cycle. The austenitization temperature
was 1550F for the furnace treatment.
Then five bars from the same heat were annealed
using the present process. The austenitization temperature
was 1450F and the annealing time for the five bars was 20
minutes. Each bar was austenitized in 30 seconds.
Table 10 shows the mechanical properties of the
as received steel, the furnace annealed steel and the steel
annealed by the process of the invention. In this case,
the hardness of the steel annealed by the present process
was very near that of the furnace annealed steel. As with
the previous examples, the steel annealed by the present
process has superior ductility. Figures 16A, 16B and 16C
show the microstructure of this steel in three conditions:
as received, furnace annealed, and annealed by the present
process, respectively.
S~L3
TABLE 10
`i~ECI~NICAL PROPERTIES OF 1144 - HEAT H
-
S.~'IPLE Hardness Tensile Yield EL RA
~n) ~ksi)- (ksi~ (~,') (v/O)
As Received Steel
Hot Rolled l144 190 99.0 60.9 19.5 41.4
Furnace Annealed
162 ~9.5 51.3 23.1 41.1
~nnealed by the
present process
165 89.1 57.4 25.0 48.5
EXA~PLE ~
Several bars of 86L20 from Heat I were furnace
annealed using a five hour cycle. The austenitizing
temperature used for the furnace anneal was 1625F.
Then 15 bars from the same heat were annealed using
the present annealing process. The austenitizing temperature
used was 1600F, and each bar was austenitized in 31 seconds.
The total annealing cycle was 47 minutes.
Table 11 shows the mechanical properties of the as
received steel, the furnace annealed steel, and the steel
annealed by the present process. For this grade of steel,
the improvement in ductility for the steel annealed by the
present process is relatively small. Also, the hardness of
the steel annealed by the present process was rather high.
The reason for these differences is clear from the photo-
micrographs of the structures of this heat of ste~l (Figures
17A, 17B and 17C). The grain size of the steel annealed by
the present process is much finer tnan the grain size of the
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furnace annealed steel. In a low carbon steel like 86L20,
the fine grain si~e which results from the new annealing
process is the dominant factor. There is not enou~h carbon
in the steel 'or the carbides to play a dominant role, and
the grain si7e effect makes the steel annealed by the
present ~rocess so~ewhat harder than the furnace annealed
product. Consequently, only marginal impovements in
ductility were achieved with the present annealing process.
TABLE 11
~ECH~iICAL PROPE~TIES OF 86L20 - HEAT I
S~IPLE Hardness Tensile Yield EL P~A
(~hn) (ksi) (ksi) ~ (%)
As Received 86L20 172 83.2 53.3 25.2 62.4
Furnace Annealed 141 74.9 49.5 29.7 62.1
Annealed by tne
present process 160 81.9 60.1 30.0 65.2
The foregoing examples demonstrate that the present
annealing process is applicable to a wide variety of carbon
and alloy steels. Each grade that was tested responded to
the present annealing process in about the same way. For
each alloy, a finer carbide morphology was produced which
gave the steel improved ductility, formability and toughness.
It is important to note that these improved properties were
achieved with no loss of strength or loss of machinability.
This combinaLion of improved ductility, formability and
toughness with no loss of machinability is an unexpected
phenomenon. Usually when ductility and toughness increase
at a given harclness level, the machinability decreases.
However, the new annealing process creates a structure which
does not follow this general trend.
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It will be apparent from the foregoing that the
present invention provides a significant improvement in
the annealing of hypoeutectoid steel. It affords improved
energy efficiency through the use of direct electrical
resistance heating, and,at the same time, eliminates the
need for long controlled cooling cycles of the sort that
have been required in the furnace annealing of steels. In
addition, the process of this invention eliminates the need
for protective or non-oxidizing atmospheres of the sort
required with furnace annealing procedures heretofore
used by the prior art.
It will be apparent that various changes and
modifications can be made in the procedures of carrying
out the present invention as well as the equipment employed
without departing from the spirit of the invention, especially
as defined in the foIlowing claims.
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