U.S. patent number 3,857,741 [Application Number 05/342,700] was granted by the patent office on 1974-12-31 for steel product having improved mechanical properties.
This patent grant is currently assigned to Republic Steel Corporation. Invention is credited to Frank A. Hultgren, Richard A. Kot.
United States Patent |
3,857,741 |
Hultgren , et al. |
December 31, 1974 |
**Please see images for:
( Certificate of Correction ) ** |
STEEL PRODUCT HAVING IMPROVED MECHANICAL PROPERTIES
Abstract
Steel products, which have been subjected to a desired
deformation, are characterized by superior strength, including
markedly improved toughness or tensile properties or both, as a
result of procedure whereby the steel is made temporarily
superplastic at an elevated temperature and is deformed while in
such state. This procedure for converting steel, notably ordinary
and alloy grades of low carbon, ferritic character, to a
superplastic state, e.g. affording very high ductility, and for
deforming such superplastic steel in a desired manner, embraces:
rapidly heating a body of steel to a temperature, advantageously in
the alpha-plus-gamma phase field, where the steel is then found,
over a brief interval, to experience a transitional state of severe
microstructural instability and to be characterized by
superplasticity; and applying stress to the body in such interval
to effect the desired deformation. The new products, which are
compositionally of the character required for the process, are
found to have much higher strength than the original steel.
Inventors: |
Hultgren; Frank A. (Burton,
OH), Kot; Richard A. (Parma, OH) |
Assignee: |
Republic Steel Corporation
(Cleveland, OH)
|
Family
ID: |
26921102 |
Appl.
No.: |
05/342,700 |
Filed: |
March 19, 1973 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
227045 |
Feb 17, 1972 |
3723144 |
|
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|
98674 |
Dec 16, 1970 |
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Current U.S.
Class: |
148/320;
148/648 |
Current CPC
Class: |
C21D
7/13 (20130101) |
Current International
Class: |
C21D
7/00 (20060101); C21D 7/13 (20060101); C21d
009/48 (); C22c 039/14 () |
Field of
Search: |
;148/36,37,12 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Stallard; W.
Attorney, Agent or Firm: Cooper, Dunham, Clark, Griffin
& Moran
Parent Case Text
This patent application is a continuation-in-part of our copending
application Ser. No. 227,045, filed Feb. 17, 1972, now U.S. Pat.
No. 3,723,194 and of our application Ser. No. 98,674, filed Dec.
16, 1970, now abandoned, with which said application Ser. No.
227,045 was copending.
Claims
We claim:
1. A steel product having a selected shape produced by deformation
of a body of steel which has a compositional character that is
normally ferritic and is such that as exhibited by the equilibrium
phase diagram for said compositional character there is an
alpha-gamma-transition temperature value capable of providing
temporary microstructural instability, said steel product having
substantially greater impact strength than said body of steel, said
steel product being ferritic and having a microstructure composed
essentially of ferrite grains characterized by a coarse cell
substructure within each of the grains, and said steel product
being produced by: subjecting said body of steel to rapid heating,
at a rate of at least about 10.degree. F. per second, to the
aforesaid temperature value, holding the body at said value for
providing high ductility in said body during an interval of
microstructural instability while transformation occurs in the
steel from alpha iron at least partially to gamma iron, and
subjecting the body to rapid deformation to said selected shape by
applying stress thereto while the body is held at the said
temperature value in said interval.
2. A steel product as defined in claim 1, in which the steel
thereof is essentially a carbon standard steel having a carbon
content in the range up to about 1%.
3. A steel product as defined in claim 1, in which the steel
thereof has a carbon content in the range up to about 0.5%.
4. A steel product having a selected shape produced by deformation
of a body of steel which has a compositional character containing
less than 0.8% carbon, that is normally ferritic and is such that
it exhibits a phase field, in a range of elevated temperature
values, which at equilibrium contains both ferrite and austenite,
said steel product having substantially greater impact strength
than said body of steel, said steel product being ferritic and
having a microstructure composed essentially of ferrite grains
characterized by a coarse cell substructure within each of the
grains, and said steel product being produced by: subjecting said
body of steel to rapid heating, at a rate of at least about
10.degree. F. per second, to a temperature in the aforesaid range,
such that upon reaching such temperature the steel experiences
temporary microstructural instability, while phase transformation
is occuring, and subjecting the body to rapid deformation to said
selected shape by applying stress thereto while the body is
undergoing said instability at said temperature.
5. A steel product as defined in claim 4, in which the steel
thereof is essentially a carbon standard steel of hypoeutectoid
character having a carbon content in the range up to about
0.5%.
6. A steel product as defined in claim 5, in which the temperature
to which the body of steel is heated is about 1,450.degree. F.
7. A steel product having a selected shape produced by deformation
of a body of steel which has a compositional character that is
normally ferritic and is such that as exhibited by the equilibrium
phase diagram for said compositional character there is an
alpha-gamma-transition temperature value capable of providing
temporary microstructural instability, said steel product having
substantially greater strength in at least one of the properties of
impact strength and tensile strength than, and at least as great
strength in each of said properties as, said body of steel, said
steel product being ferritic and having a microstructure composed
essentially of ferrite grains characterized by a coarse cell
substructure within each of the grains, and said steel product
being produced by: subjecting said body of steel to rapid heating,
at a rate of at least about 10.degree.F. per second, to the
aforesaid temperature value, holding the body at said value for
providing high ductility in said body during an interval of
microstructural instability while transformation occurs in the
steel from alpha iron at least partially to gamma iron, and
subjecting the body to rapid deformation to said selected shape by
applying stress thereto while the body is held at the said
temperature value in said interval.
8. A steel product as defined in claim 7, in which the steel
thereof is essentially a non-alloy carbon steel having a carbon
content in the range under 0.8%.
9. A steel product as defined in claim 7, in which the steel
thereof has a carbon content in the range up to about 0.5%.
10. A steel product as defined in claim 7, in which the steel
thereof is essentially a non-alloy carbon steel of hypoeutectoid
character having a carbon content in the range up to about 0.5%.
Description
BACKGROUND OF THE INVENTION
This invention relates to new and stronger steel wherein such as
result from new procedure whereby or wherin steel is converted to a
superplastic state, e.g. a state of unusually high ductility, and
in a more specific and particularly important sense, the invention
is concerned with such products derived from the new procedure for
making steel and deforming the metal by application of stress to
achieve desired characteristics of dimension, shape or the like
which may involve a large extent of such deformation. Thus for
example elongation by stretching is contemplated up to an extent of
the order of 100% or more and similarly large or difficult changes
of shape or dimensions by operations such as pressing, drawing,
coining or the like. The improvements are related to a wide variety
of low carbon steels including ordinary or so-called standard
steels and alloy steels of many conventional compositions, and
other normally non-austenitic steels such as a number of stainless
steel grades.
Superplasticity is a phenomenon known to be attainable in some
metals, that has been used or proposed mostly for certain
non-ferrous alloys, such as zinc-aluminum compositions, and
aluminum-copper, lead-tin, and tin-bismuth alloys. In general, the
techniques used to achieve this state of extraordinarily high
ductility, in such alloys, have been directed to establishing an
extremely fine grain or microstructure, as by recrystallization
with or without precipitation effects; a stable condition of
superplasticity is then attained for the desired large extent of
deformation. Another principle sought to be utilized is to take
some advantage of change of phase in metals characterized by
different phase conditions, one expedient being to apply heat and
cooling so that the metal object is cycled slowly back and forth
through a phase change region while stress is applied over a
relatively long time, to effectuate deformation.
Although it is readily seen that the attainment of superplasticity
or extreme ductility in steel would permit an expedited and
simplified mode of fabricating a variety of articles where large
changes of dimension or shape are required from an initial piece or
blank, or where an article could be made in one piece to have a
complex configuration of relatively thin section, not heretofore so
produced, there has been no practical success, as presently known,
in achieving superplasticity with steels of ordinary types,
particularly those mentioned above.
Whether attempted by utilization of the techniques described for
non-ferrous metals or by some similar or other approach,
superplasticity of practical and commercially useful nature does
not appear to have been reached in any ordinary steels, especially
those that at room temperatures or other normal temperatures of use
are essentially non-austenitic. A principal difficulty is that when
superplastic steel bodies have been established, capable in theory
of very large elongation under stress, the strain rate attainable
for such elongation (i.e. without rupture) has been extremely low,
e.g. of the order of 0.0001 to 0.1 per minute. It will be
understood that strain rate is a measure of the rate or speed of
elongation, determined as the ratio (e.g. in inches per inch) of
the increment of extension to the original unextended length, per
unit of time. Strain rates as low as 0.1 per minute, and notably
the still lower rates mentioned above are such as to have
essentially no practical or commercial value. Formation of a single
article to have anything like a deformation of the order of 100%
would require a time interval of at least many minutes, and most
likely of the order of an hour or hours, which is manifestly not
feasible.
It will be understood that references to deformation and elongation
as related to desired superplasticity or ductility, are herein
concerned with substantially uniform changes of length or section
over the entire extent of the article subject to deformation. This
is in contrast to the ordinary situation of steel where, for
example, an application of tension, as in standard tensile testing
of a bar or rod, results first in moderate degree of uniform
stretch, and then produces a phenomenon known as necking, whereby a
localized reduction of sectional area occurs, producing a very thin
region at a single place, further stress then leading to rupture at
such locality. The present procedure is related to superplasticity
in the sense that deformation or reduction of cross-sectional area
occurs substantially uniformly throughout the length of a piece
being stretched, without necking or other appreciable local
consequences -- i.e. short of ultimate rupture at the ultimate
limit of superplastic elongation.
The invention is therefore designed to afford what are believed to
be novel characteristics in steels, particularly to achieve a
significant condition of very high ductility, and further and most
particularly, to provide steel of new properties from these novel
forming or shaping operations, producing dimensional changes or
other changes of configurations, in a single step, not heretofore
generally possible. As will be understood, a primary feature of the
invention is to attain the stated superplasticity with the
characteristic of a practical strain rate, e.g. upwards of one per
minute and indeed desirably in the range of 100 to 1,000 per
minute.
SUMMARY OF THE INVENTION
The present invention relates to the products of the process herein
described, having unusual advantage in respect to strength, notably
toughness as measured by Charpy V-Notch impact values. According to
the process, the steel is deformed upon establishing
superplasticity in it by a relatively simple treatment which
develops a high order of microstructural instability in the metal,
and permits rapid application of deforming stress in substantial
coincidence with such instability.
An essential feature of the process involves heating the steel, as
from room or other ordinary low temperature, extremely rapidly to
an elevated temperature, specifically a temperature where the
alpha-to-gamma transformation can then proceed while the steel is
thereafter held at such temperature for a short but convenient
interval. In particular, the temperature is advantageously selected
as one which would afford, at equilibrium, a combination of the
alpha and gamma phases of iron, e.g. such as characterizes the
well-known alpha plus gamma phase in the conventional
temperature-versus-carbon equilibrium phase diagram for ordinary
steel. That is to say, it has been found that instead of attempting
to achieve a stable or temporarily stable condition of very fine
grain size or the like, or instead of endeavoring to cycle the
metal somehow through variations in equilibrium condition between
one phase and another, it has been found that the attainment of a
severely unstable microstructure -- specifically by the procedure
of rapid heating upward to a selected elevated value and then
holding at said value for a short interval while relatively rapid
deformation is effected -- an unexpectedly very high ductility is
attained. When formed, moreover, the metal of the article promptly
proceeds to an equilibrium or stable condition, and the resulting
piece, e.g. upon cooling in any desired manner, reattains
substantially the characteristics of the selected steel composition
with its normal mechanical and other desired properties, or even in
some instances superior properties. There is thus no need for any
special step or operation to terminate or destroy the superplastic
condition, after the forming step.
In many instances, the process of the invention can be
circumstanced or modified to take advantage of two cooperating
mechanisms or phenomena that coact to establish a special degree or
extent of superplasticity. Thus by first subjecting the steel to
drastic cold reduction, as for example cold rolling to an extent of
70% or preferably higher, e.g. 90% reduction, the process is
capable, indeed inherently by virtue of the range of temperatures
selectable for the above-described unstable state, of providing a
correlated characteristic of instability, with respect to
recrystallization and with phase-change instability.
This special procedure may be explained by reference to examples,
as in the use of ordinary low carbon steel, e.g. AISI 1006 or AISI
1018, respectively 0.06% and 0.20% carbon. The steel, after the
usual first stages of production and hot rolling, is cold rolled to
90% reduction, and then the desired piece, without annealing, is
rapidly heated, as at a rate of 100.degree. to 200.degree. F. per
second, to a temperature within the alpha plus gamma phase area,
advantageously midway in such area, this being the area where the
ferrite phase begins substantial transformation to the austenite
phase and indeed where at equilibrium, e.g. over a period of time,
the structure would consist of both ferrite and austenite. In
accordance with the procedure of the invention, the article is held
at the selected temperature for a short interval, say five seconds,
and then subjected to rapid deformation, as by drawing, stretching
or pressing, to the desired large extent. Conveniently the total
time of continuing deformation may be relatively short, of the
order of a few seconds or less.
In this way a high degree of microstructural instability occurs in
the piece as it is held at the selected temperature, notably after
the stated holding interval, and the instability is characterized,
according to present understanding, by the initiation and
continuance of extensive recrystallization simultaneously with the
rapid, vigorous, phase transformation from ferrite toward and to
austenite. During and as a heretofore unappreciated result of these
conditions of severe instability, a substantial state of
superplasticity, i.e. a condition of very high ductility, is found
to occur and thus to permit the desired deformation.
As stated, the invention is nevertheless basically advantageous,
according to present understanding, without the cold reduction of
the metal and thus can be applied to steel produced without cold
reduction or to steel which has been cold reduced and annealed,
even though in such cases reliance for superplasticity is then
placed primarily on the high degree of transformation instability
after rapid heating to the selected temperature. It is presently
believed, moreover, that in at least some instances the
superplastic property may be enhanced by what can be called a
dynamic recovery brought about by the fact of elongation or other
deformation -- i.e. a secondary (second occurring) phenomenon or
mechanism which is advantageously occurring in all cases.
As used with any of a large variety of steels, most advantageously
low carbon steels but also alloy steels and likewise non-austenitic
stainless steels, e.g. the compositions classed as ferritic and
martensitic in the 400 series, the procedure can be effective for
the production of many highly or intricately deformed shapes, with
great economy and efficiency.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 5 inclusive are graphs where various significant values
of deformation properties are plotted against testing temperature,
being temperatures to which specimens of a selected steel, AISI
1006, were very rapidly heated and then held, in accordance with
the invention, these graphs showing the unusual results attained at
the temperature of severe microstructural instability, as
follows:
FIG. 1 being a plot of elongation to fracture, against testing
temperature, for the steel which had been cold worked to reduction
of 50%;
FIG. 2 being a plot of uniform reduction in area (i.e. non-necking
reduction) against testing temperature, again after 50% cold
work;
FIG. 3 being a plot of maximum deformation load against testing
temperature (50% cold work);
FIG. 4 being a plot of elongation to fracture, like FIG. 1, but for
specimens which had not been cold worked; and
FIG. 5 being a plot of elongation to fracture, like FIG. 1, for
groups of specimens that had respectively been cold worked 70 and
90%.
FIGS. 6 to 11 inclusive are graphs of deformation properties
against testing temperature, similar to FIGS. 1 to 5, for specimens
of another steel, AISI 1018, as follows:
FIG. 6 being a plot of elongation to fracture, for specimens having
50% cold work;
FIG. 7 being a plot of uniform reduction in area, 50% cold
work;
FIG. 8 being a plot of maximum deformation load, 50% cold work;
FIG. 9 being a polot of elongation to fracture, for specimens with
no cold work;
FIG. 10 being a plot of elongation to fracture, for groups of
specimens that had respectively been cold worked 70 and 90%;
and
FIG. 11 being a plot of maximum deformation load, for specimens
with no cold work;
FIG. 12 is an iron-carbon equilibrium phase diagram, plotting
temperature against carbon content for pure iron-carbon systems,
showing selection of temperatures especially suited for practice of
the invention, e.g. for the hypoeutectoid compositions there are
two intermediate boundaries shown which predict mixtures of 75%
alpha, 25% gamma and 50% alpha with 50% gamma, each so labeled in
the diagram;
FIG. 13, illustrating deformation properties in accordance with the
invention, is a plot of uniform reduction in area against testing
temperature for specimens of stainless steel grade .430, both in
hot rolled condition and after 60% cold work;
FIG. 14 is a plot of maximum deformation load against testing
temperature for specimens of stainless steel grade 430, as in FIG.
13.
FIG. 15 is an equilibrium phase diagram for stainless steels of the
400 series, plotting temperature against weight percent chromium,
exemplified at a carbon level of 0.05%.
DETAILED DESCRIPTION
More particularly explained, the procedure is notably applicable to
so-called hypoeutecoid compositions, meaning compositions where the
carbon content is less than the eutectoid value, e.g. 0.8% in
ordinary low carbon steel. Preference for this low carbon range is
related to the fact that in the ordinary equilibrium diagram
plotting carbon percentage and temperature for such steels, FIG.
12, there is an area (commonly designated by the letters alpha plus
gamma, and here bounded by the gamma, alpha and alphacementite
regions) in a temperature range from generally about 1,350.degree.
F. to varying higher points up to about 1,650.degree. F. (about
725.degree. to 900.degree. C.), where a stable structure can be
reached having a composition consisting of both alpha and gamma
phases of iron. It will be understood, however, that the procedure
of the invention in no sense involves the attainment of such
equilibrium for providing superplasticity, but on the contrary
resides in the discovery that a severe microstructural instability,
which in turn is found to exhibit the desired superplasticity,
occurs when the steel has been heated rapidly from low temperature
(meaning any temperature sufficiently below the selected value as
not to involve transformation and as not to involve
recrystallization where there has been substantial cold reduction)
up to the selected value and then holding at said value for
development of such instability and for employment of it, so to
speak, in deformation under stress. A particularly important
consideration is that in this alpha-plus-gamma zone, the
instability is severe, and though brief, lasts long enough to
accommodate the desired, rapid operation of deformation.
The new steel products, e.g. characterized by greatly increased
strength as compared with the untreated steel, have compositions as
described herein for suitability of application of the process,
including the hypoeutectoid steels. The products have much improved
impact strength, or better yield and tensile strength, or enhanced
properties in both respects, and reveal a distinctive
microstructure. Further details of the procedure of treatment and
deformation are here given, followed by data about the special
properties of the resulting products.
Thus the invention is specially applicable to steels, such as the
above mentioned hypoeutectoid low carbon standard steels, where the
equilibrium diagram exhibits and area of coexistence of two phases,
e.g. both ferrite and austenite, or as more generally represented
on such phase diagram, an area representing, at equilibrium, a
combination of alpha iron and gamma iron. Whereas in ordinary
steels the eutectoid is a composition containing about 0.8% carbon,
corresponding characteristics are shifted or different in alloy
steels and notably in the non-austenitic stainless steels, e.g.
so-called ferritic or martensitic steels of the 400 series.
Nevertheless, it is apparent that an austenite-ferrite area exists
in a wide variety of steels, notably of low carbon content, and can
be availed of for optimum practice of the invention, by
corresponding selection of a terminal and holding temperarture
after the stated fast heat-up, for achieving a high order to
temporary instability.
To a degree, however, it is also conceived that the procedure can
be utilized with other steels, advantageously of low carbon content
(e.g. not more than 1%), where a value of elevated temperature can
be selected and maintained such as to yield a state of
transformation instability which, even though very short, may be
sufficient to permit deformation under stress. As will be
understood, however, selection and maintenance of such temperature
may require special care, notably to avoid so rapid a phase
transformation as not to permit practical utilization of the
instability of microstructure in accordance with the principles
explained above. Moreover, if advantage is also to be taken of the
action of recrystallization while it is going on -- such action
being conceived as the conversion of large, elongated crystal
structure resulting from cold reduction, to a fine, equi-axed
crystal form -- care must be taken to time the occurrence of such
action with the occurrence of the transformation instability.
The procedure can afford an unusual forming operation effective for
production of a variety of articles such as greatly elongated
parts, cuplike elements, or similar tubular casing or pieces closed
at one end having cylindrical or other shapes, various other hollow
articles such as refrigerator shells, hub caps, automobile body
elements, and the like. In general, these are parts which may have
heretofore required a series of forming steps, sometimes a long
series, as by successive stages of cold forming, e.g. pressing,
drawing or other deformation, interspersed with annealing steps.
Inasmuch as the strain rate afforded by the invention generally
resides in the range of 100 to 1,000 per minute and inasmuch as the
property of superplasticity is found to be accompanied by a marked
decrease of the required deformation load, the process results in
an unusually rapid and highly economical operation for fabricating
finished or semi-finished structures in one or no more than a few
forming steps, from a wide variety of steels of recognized
commercial type, including steels of essentially inexpensive
character such as the ordinary low carbon compositions.
Referring further to specific examples of the invention, an
extensive series of tests was carried out with specimens of
ordinary carbon steels as noted above, being AISI 1006 (0.60%
carbon) and AISI 1018 (0.20% carbon), both well below the
eutectoid. The 1006 steel was aluminum killed, but presence or
absence of such condition is not understood to bear significantly
upon the invention. The steels had the usual manganese contents
(below 0.39% and 0.67% respectively) and were entirely conventional
nonalloyed compositions, with usual limits of phosphorus and
sulphur. Ordinary processing with hot rolling had been followed in
each case.
For test purposes, cylindrical specimens were prepared, 1/4 inch
diameter by 8 inches long, by machining from suitable pieces. Where
cold working was employed prior to test, the pieces were of
material which had been reduced by cold rolling in conventional
manner, without intermediate or final anneal. SPecifically,
specimens were employed for which the metal had zero cold working
and cold working to reductions of 50, 70 and 90% respectively.
These percentages refer to reduction in thickness, the cold rolling
being effected with the necessary series of passes, preferably in
the same direction, for production ease. In order to take full
advantage of the occurrence of recrystallization in practice of the
invention, the selected degree of cold working should be performed
without intermediate or subsequent anneal; any previous treatment
(not employed in these tests) which may have involved a cold work
and anneal, is disregarded. For the invention, cold working can be
achieved in any suitable manner, as by rolling, forging or like
operations.
All of the tests in these series involved heating the specimen
rapidly, i.e. at a rate of 120.degree. F. per second or thereabout,
to a test temperature, where the specimen was held for a
predetermined short time, which was 5 seconds for the 1006
specimens and 10 seconds for the 1018 specimens, and then each
specimen (while remaining at such temperature) was subjected to
rapid deformation, e.g. at a cross head speed of 90 inches per
minute. The equipment used, for heating, holding and deformation,
was an apparatus commercially available for test purposes (under
the name of Gleeble), designed for other tests of high temperature
properties as in welded structures.
In general, the tests involved determination of deformation
characteristics at various test temperatures, being the selected
temperature to which the specimen was heated, and at which it was
held, and the results demonstrated the occurrence and utilization
of superplasticity in accordance with the invention, being the
effect of severe microstructural instability when the temperature
was in the austenite-ferrite range for these steels. For best
representation of superplasticity, all deformation tests in these
examples were of tensile characteristics, and all deformation was
effected by application of tensile stress alone, exerted between
elements gripping the end portions of the specimens. It was fully
apparent, indeed confirmed by other investigation, that the
extraordinarily high ductility revealed by these tests was
accomplished by other, unusually superior properties of
deformability, as in pressing to deep or complex shapes, full
conformity with intricate die configuration, and the like.
One index of superplasticity was taken to be the extent of
elongation to fracture, under tensile stress. For simplicity of
comparison among specimens of identical shape and size, identically
held by the tension-applying elements of the test apparatus, this
elongation was simply measured as the total increment of length, in
inches, exhibited by the stretched specimen, to the point of
fracture. The fact that in each case the specimen necked down, just
prior to fracture, at the fracture locality did not affect the
comparative significance of the results, it being fully apparent
that the large values of increment in length obtained at the test
temperatures of the invention were fairly representative of a large
comparative increase in neck-free elongation of the free central
portion of the test specimen which had an effective gauge length of
about one inch.
FIG. 1 shows the result of such tests for various specimens of the
1006 steel that had been cold worked 50%, the specimens which were
prepared from the cold worked metal being rapidly heated to various
test temperatures as shown in the figure, held and subjected to
stress deformation as described above. As will be seen, a very
pronounced maximum of elongation to fracture was achieved for
temperatures in the range of about 1,375.degree. to about
1,550.degree. F., with optimum results at approximately
1,500.degree. F. These values coincide with the alpha-plus-gamma
region of the equilibrium diagram (see FIG. 12), the value of
1,500.degree. F. (about 815.degree. C.) being substantially that
for which at equilibrium, e.g. after prolonged holding and
stabilization the metal (0.06% carbon) would consist of
approximately 75% ferrite and 25% austenite from the figure for
pure iron-carbon. The actual phase boundaries for commercial alloys
will be shifted due to the presence of Mn, P, S, Al, Si, etc.
Another and somewhat more significant value was measured as
indicated in FIG. 2, being the extent of uniform reduction in area,
obtainable under tensile elongation, at the several test
temperatures. Specifically this is in effect a measure of the
maximum cross-section reduction which is reached in each case
before necking occurs. This is believed to be an effective measure
of superplasticity, or of high ductility, inasmuch as the purpose
of the invention is to achieve a high order of deformation, without
necking or equivalent localized effect in the workpiece subjected
to the desired deformation. The value actually determined in these
tests is a function of the ratio of the original cross-sectional
area of the test rod to the final area (just prior to necking),
conveniently a logarithmic value e.sub.D, defined as
e.sub.D = 2 1n (D.sub.o /D.sub.f)
where D.sub.o is the initial diameter and D.sub.f is the final
diameter, with factor 2 providing the conversion to a ratio of
areas. The values of e.sub.D obtained at various test temperatures
are shown in FIG. 2, where the test specimens (of 1006 steel) were
the same as in FIG. 1. Percent elongation is definable in reference
to e.sub.D data; thus e.sub.D values of 0.4 to 0.8 represent
uniform elongation of about 50 to 125%. Normal uniform elongation
for these steels is about 25-30% or less.
It will be noted that in FIG. 2 a sharp maximum of useful result
again appeared at about 1,500.degree. F., with useful high values
also apparent at adjacent points within the range of the
austenite-ferrite region. Specifically the measured quantity
e.sub.D peaked to values varying from 0.4 to above 0.5 (i.e. above
65% elongation) at the effective temperature range from values of
0.1 (10% elongation) and less both below and above it.
A further and extremely desirable characteristic of superplasticity
is a large reduction in required deformation load, i.e. stress
required to deform the workpiece. With the specimens of the 1006
steel which had received 50% cold work, measurements were made of
the maximum load needed for tensile deformation at various test
temperatures, i.e. the maximum reading of applied load reached
during the brief interval of deformation. Results were as plotted
in FIG. 3, showing a low minimum of such deformation load at about
1,500.degree. F., with a useful range of superplasticity generally
similar to that exhibited in the preceding figures. Specifically,
the required tensile load dropped to well below 1,000 pounds,
indeed close to 500 pounds, from values in the range of 2,000 to
3,000 pounds at lower temperatures. Other investigation has
indicated that the maximum tensile load likewise rises at higher
temperatures than as shown in this graph; certain limitations of
the test equipment prevented extension of this and other curves
about 1,600.degree. F. in these sets of tests.
Tests of elongation to fracture, similarly made and measured as
those reported in FIG. 1 and following identical procedure of
treatment of the specimens were also made of specimens of the 1006
steel where the prior treatment of the metal was different from
that in FIG. 1. Specifically, tests on specimens of steel that had
received no cold working are reported in FIG. 4, showing a
significant maximum of ductility at essentially the same value,
i.e. about 1,500.degree. F. Like tests on specimens of which the
steel had received relatively large amounts of cold working
(without anneal) are plotted in FIG. 5, the solid circles
representing specimens of steel had been cold reduced 70%, and the
triangles being specimens of metal that had received cold reduction
to 90%. The superiority in elongation to fracture, at and adjacent
to the optimum temperature of 1,500.degree. (reaching increments
well above 2 inches) is very great, being especially so for the 90%
cold worked metal.
The second example of the invention is illustrated by the tests
similarly made with specimens of the 1018 steel as explained above.
For metal that had been cold worked to 50% reduction, FIG. 6 shows
the values of elongation to fracture (maximum nearly 2 inches),
while FIG. 7 correspondingly shows the extent of uniform reduction
in area (maximum e.sub.D, 0.6 to 0.8+), and FIG. 8 indicated the
maximum deformation load (lowest 1,000 pounds), all for various
test temperatures. The curves and results agree qualitatively with
those in FIGS. 1, 2 and 3, indicating a very large increase in
ductility and a large decrease in required tensile stress, at the
optimum temperatures for this alloy, specifically a temperature in
the range of about 1,350.degree. to 1,450.degree. F., notably
temperatures around 1,400.degree. F.
Again, tests of the 1018 steel which had not been cold worked at
all and which had received higher degrees of cold working were
made, under conditions otherwise the same as those for FIGS. 6.
Specifically, in FIG. 9, the steel without cold working showed a
fairly sharp maximum of elongation to fracture at around
1,400.degree. F. (peak value about 2 inches), while very high
values were achieved in the steels tested in FIG. 10, the solid
circles being 70% cold worked material and the triangles 90% cold
worked material (peak values well over 2 inches), the optimum
temperatures, around 1,400.degree. F., being clearly similar to
those revealed in FIGS. 6 to 9.
Corresponding to FIG. 8, tests of maximum deformation load (for
tensile stress) were made as reported in FIG. 11, relative to the
1018 specimens (see FIG. 9) of which the metal had received zero
cold working. A minimum value (about 1,000 pounds, as against other
values up to 3,000 pounds), being an extremely desirable attribute
of superplasticity, appears in the vicinity of the optimum
temperature of 1,400.degree. F.
From all of these tests, which in effect represent practice of the
invention at the optimum and near-optimum temperatures, and from
other investigations by test, it has been demonstrated that the
present procedure affords unusually high ductility, representative
of the desired superplastic state, and thus permits feasible and
rapid deformation to large uniform elongations, such as 50% and
well above, and usually of the order of 100% and greater, in
optimum conditions. Effective results are achieved with material
which has received no cold working at all, but significantly
superior advantage is demonstrated for metal cold worked to a
reduction of about 70% or more. It will be seen that the process is
eminently practical, requiring only a rapid heating of the piece up
to the selected deformation temperature, then holding the piece at
such temperature for a brief time to develop severe microstructural
instability, and rapidly performing the desired deformation, indeed
under relatively low stress.
Summarizing the above examples and taking into account other
experimental work of similar sort relative to the invention, a
presently preferred practice, for obtaining excellent deformability
at high strain rates in the case of ordinary carbon steels having a
carbon content below 0.8% is as follows: the piece is heated
rapidly at a rate of 100.degree. to 200.degree. F. per second to a
selected temperature in the two-phase field, preferably a
temperature which on holding to the point of stabilization would
produce a microstructure of about 50% ferrite and 50% austenite.
Thereupon after a short holding time, e.g. 5 to 10 seconds, the
piece is deformed under stress, conveniently so at a strain rate
upwards of 10 per minute and advantageously in the range of 100 to
1,000 per minute. Maximum superplasticity appears to be obtained if
the steel has previously recieved a significant cold reduction,
e.g. 70% and upwards, and preferably about 90%. For the AISI 1006
steel, the selected temperature (apparent optimum 1,500.degree.)
can be in the range of 1,475.degree. to 1,550.degree. F., while a
preferred value for AISI 1018 is about 1,400.degree. to
1,425.degree. F. It may be noted in reference to the above tests as
involving 90% cold reduction, this aim was not in fact precisely
reached, i.e., reduction was only about 85%, but it is clear that
the results can properly be stated as those characterizing the
effect of approximately 90% reduction.
It is essential that the treatment temperature be reached by
heating the metal up from a much lower value, conveniently from
room temperature or usually at least from a value several hundred
degrees F. below the transformation point. Indeed where
recrystallization effects are to be significant, the heat-up must
progress from a temperature well below any annealing value. The
rate of heating is preferably quite rapid, as in the range
mentioned above, although it is understood that substantially lower
rates can be useful, especially where the onset of severe
instability does not occur before reaching the desired temperature.
In general heat-up rates should be at least 10.degree. F. per
second, more suitably 20.degree. F. per second and upwards; a good
range appears to be from 50.degree. to 300.degree. F. per second.
While there is no theoretical upper limit, convenience is served by
rates below 500.degree. per second; and indeed in some cases a
lower degree of superplasticity has inexplicably been noted with
extremely fast heat-up times, e.g. above 1,000.degree. F. per
second. The time taken, from room temperature, has normally been of
the order of 10 to 12 seconds, with little change in result at
times as high as 60 seconds or as short as 1 or 2 seconds.
Correlation of the heat-up rate with occurrence of
recrystallization (if desired) from cold worked condition is
relatively simple to determine, usually in the light of known or
ascertainable characteristics for annealing the selected steel.
In most cases, some holding time will be required, suitable values
for the low carbon steels having been indicated above. In other
situations the optimum time is at most a matter of simple test. For
example, after selecting a conveniently rapid heat-up time and
selecting an appropriate terminal temperature from the phase
diagram for the metal (where the minimum is usually about
1,330.degree. F. for ordinary steel) a short series of tests, e.g.
utilizing several different holding times and then applying
deformation at the desired strain rate, will readily reveal the
holding interval for best results. Indeed selection of the
treatment temperature itself may be correlated with the strain rate
needed for a given type of deformation or other needs, in that
within an available range of temperature, the useable range of
strain rates appears to rise with temperature.
Once the parameters have been selected, as to treatment
temperature, heat-up rate and holding time, practice of the
invention is a convenient and expedited operation. Suitable
provision is made for heating the workpiece rapidly, as by
induction heating, and then by appropriate thermal control,
maintaining the piece at selected temperature for the desired time,
including the holding interval and the very brief period required
for actual deformation. The latter task may be performed in any
suitable fashion, with equipment adapted to hold the piece at the
desired temperature, e.g. as part of the heating equipment, or to
receive the piece from such equipment into whatever dies (e.g.
preheated) or other elements may be needed for engaging and for
mechanically exerting stress in the desired forming operation.
Thereafter, treatment of the shaped article may be of any sort
desired, essentially without regard to the fact that a condition of
superplasticity has temporarily existed. Indeed such condition,
including the stated instablity, will usually disappear at once,
more or less simultaneously with the completion of forming or very
briefly thereafter. The piece can be allowed to cool slowly or
rapidly and can be subjected to heat treatments or other operations
as may be conventionally desirable for the ultimate service of the
article and for agreement with the metallurgical properties of the
selected alloy. Indeed it is found that in a number of cases the
resulting article has properties superior to those of articles
formed by conventional procedure (such as repeated cold working and
annealing), in being characterized by a, better toughness, smoother
or otherwise improved surface quality, and indeed in the case of
stainless steel, a lessening or avoidance of the surface shape
characterized as roping.
If the nature of the required fabricationn is such that a single
deformation is insufficient, it will be understood that the piece,
after cooling, can be further worked in the same manner, as by
again rapidly heating and holding at the temperature of
microstructural instability, while a second deforming operation is
performed, which may, of course, be similar to or different from
the first. While in such repetition or repetitions it may not be
possible to take advantage of recrystallization effects from a cold
worked state, substantial superplasticity is nevertheless
attainable.
Although the invention is not dependent on theoretical
considerations, it is believed that the superplasticity is at least
in substantial part occasioned by the change in structure (while it
occurs) of some of the material from the body-centered cubic
structure of alpha iron, which is the low temperature phase, to the
face-centered cubic structure of gamma iron, which is the high
temperature phase. Thus there is a driving force, so to speak,
which produces such transformation and which may be deemed to
contribute to the severity of instability. At the same time where
the material has been heavily cold worked, the concurrent
recrystallization affords a like driving force in converting the
elongated crystals to very small equiaxed ones. Indeed tests have
been made by quenching the deformed product immediately after such
deformation, and examination of the microstructure has indicated a
finer ferrite grain, including a coarse substructure as discussed
below.
It is also believed that even where there has been no previous cold
working, the deformation, during transformation instability, may be
accompanied by a so-called dynamic recovery, which enhances the
unstable condition. The preference for operation in the two-phase
field is that the instability is there considerably prolonged, so
as to afford full opportunity for deformation time and for
avoidance of difficulty close control of time and temperature.
Indeed it may be noted that exact treatment temperature values do
not usually have to be maintained, e.g. in the sense that if the
theoretical optimum is 1,500.degree.F., the selected temperature
can actually be in a range, as from 1,475.degree. to 1,550.degree.
F.
The invention is applicable, as stated above, to a variety of
steels, which are normally non-austenitic, i.e. which are not
austenitic at room temperature as commonly processed, and
especially in such steels having a two-phase region of austenite
plus ferrite at specific elevated temperatures. These include, in
addition to the ordinary carbon steels (most advantageously up to
about 0.5% C), any of a wide variety of alloy steels, of which a
few examples of compositions that are appropriate for making
articles of the sort for which the invention is especially
suitable, are grades 4340, 8620 and 10B20, the latter being a boron
steel corresponding to grade 1020. It is likewise conceived that a
number of high alloy grades can be treated. Particular utility is
contemplated for stainless grades of the 400 series as mentioned
above, these being generally straight chromium compositions, with
chromium content ranging from 11 to 27%, and generally lacking
nickel as an intended alloying element, except for one or two
grades with nickel up to 2.5%. Specific examples of stainless steel
are AISI types 409, 410, 430, 434 and 436, which have carbon
contents up to about 0.2%. All of the above steels, whether of
ordinary, alloy or stainless types, and including those which are
conventionally classed as martensitic, may be generically deemed to
be normally ferritic compositional character.
The forming operations conceived as useful include drawing and
stretching, pressing, coining and similar die procedures, and
indeed a variety of operations often attempted in the cold state,
such as those of the nature of heading, upsetting, and like impact
methods. Not only is there obtainable a very large extent of
deformation, preferably upwards of 100% in uniform elongation, but
a letter conformity with intricate die configurations, thus
permitting single step fabrications in many cases not heretofore
attained. Indeed parts or shapes which could not at all be produced
in one piece may now be so made. As also stated, suitably high
strain rates are realized, well above one or preferably 10 per
minute, and indeed up to 1,000 per minute or higher if
feasible.
A further % of the invention is represented by application of the
process to stainless steel of the 400 series, i.e. so-called
straight chromium compositions, particularly stainless steel type
430 having a nominal chromium content of 145 to 18%. For these
tests a commercially produced 430 grade stainless steel was used,
containing, by weight, chromium 16.25% and carbon 0.073%, with the
usual maximum limits for incidental elements (e.g. Mn and Si each
under 1%).
The tests were performed in essentially the same way as in the
other examples above. The steel had been produced by ordinary
processing, including hot rolling to appropriate plate thickness. A
part of this plate material was subjected to cold rolling to a
reduction of 60% (without subsequent annealing), and another
portion was left in the as-received condition, i.e. with no further
work after hot rolling. Cylindrical specimens of both types of
material were prepared, 1/4 inch in diameter and 71/2 inches long,
providing a grip separation of 41/2 inches. The same test equipment
was employed as in preceding examples, having provision for heating
the specimen rapidly to a selected temperature, and then allowing
it to be held at such temperature while deformation was effected,
i.e. by stretching. The desired determination was with respect to
superplastic conditions, as represented by stretch deformation to a
maximum reduction of cross-section before occurrence of necking. In
particular, the tests regarded as significant for this steel,
within the competence of the equipment described above, were
measurements of uniform reduction in area at various test
temperatures, and likewise the deformation loads required, i.e.
stresses needed to deform the work piece.
The results of the tests, illustrating the effectiveness of the
invention in this type of stainless steel, are shown in FIGS. 13
and 14. Each plotted point represents a separate test, being the
same test for the two figures, wherein a specimen was subjected to
rapid heating, was held at the indicated temperature and was
promptly deformed while at such temperature and specifically, while
it remained in such state of microstructural instability, if any,
as occurred. In each case the specimen was heated at a rate of
about 125.degree. F. per second, requiring a time of about 12.5
seconds to reach the test temperatures in the optimum localities of
the range. The specimen was held for 10 seconds at the selected
temperature and then immediately subjected to deformation by axial
stretching, while remaining st temperature, at a cross head speed
of 90 inches per minute. In each of the figures, the plotted
triangles represent the as-received specimens, with no cold
working, and the circles represent specimens which were
characterized by 60% cold reduction.
As illustrated in FIG. 13, the tests showed that when the specimens
were heated to a temperature in the range of about 1,470.degree. F.
to about 2,100.degree. F., they exhibited highly desirable values
of e.sub.D as defined above, particularly values of the order of
0.4 or better, indicating deformation with uniform reduction in
area, and no necking, to elongation of about 50% or higher, in
contrast to normal uniform elongation for 430 stainless steel, of
not more than about 20%. It was evident that at these temperatures
the metal was temporarily in a condition of microstructural
instability while alpha to gamma transition was occurring.
The cold worked specimens showed significantly higher values of
e.sub.D, understood to result from the contribution of the
occurrence of recrystallization to the total microstructural
instability that afforded the superplastic state. FIG. 13 expressly
shows that the increase in ductility was roughly three-fold from
the situation of specimens at 1,200.degree. F., which is understood
to be approximately the same as at room temperature. It is also
understood that if the microstructural instability were not
present, i.e. as by holding the metal for a long time at a given
temperature so that the microstructure was stabilized before
deformation, the values of e.sub.D would be represented
approximately by the dotted line curve in FIG. 13.
FIG. 14 shows the values of maximum load required to deform the
specimens at the selected temperatures. As in the case of other
steels mentioned above, a drastic decrease in deformation load
requirement was exhibited over the temperature range of
superplasticity, including a three-fold decrease between
1,200.degree. F. and 1,580.degree. F., thereby further
substantiating the applicability of the invention to the steel, in
attainment of this additional advantageous property. Strain rates,
as will be understood, were well within the preferred ranges
indicated above, the test operations being at the same cross head
speed as in the other examples.
It will be noted in FIG. 13 that the improved elongation
characteristics were found over a plateau of temperatures from
about 1,470.degree. F. to 2,100.degree. F., and rose substantially
at much higher temperatures. Similarly the deformation load
requirements decreased, as shown in FIG. 14, over the plateau
temperature range indicated in FIG. 13 and beyond. It will be
understood, however, that the major significance of these tests is
the improvement experienced in the lower temperature values of the
plateau range, particularly in that higher temperatures are less
commercially desirable, and indeed at exceptionally high values
would become very difficult to utilize. At the same time, increases
of temperature above about 1,580.degree. F. do not significantly
improve the ductility until values of about 2,100.degree. F. are
reached. In consequence optimum temperatures for the selected steel
are represented by the range of about 1,500.degree. to about
1,700.degree. F., preferably 1,550.degree. to 1,600.degree. F.
The above tests, as illustrated in FIGS. 13 and 14, afford abundant
confirmation that the invention is applicable to alloy steels such
as the stainless grades of the 400 series and that the described,
large increase in ductility or superplasticity is attained for a
brief but sufficient interval when the steel has been heated
rapidly to a convenient temperature in the alpha-gamma phase
transformation range, with the aid of concomitantly occurring
recrystallization when the metal has been cold worked. The
difference in nature of results at very high temperatures, and thus
of the shapes of the curves in FIGS. 13 and 14, from those for
example in FIGS. 2, 3, 7, 8 and 11, is understood to be accounted
for by the different characteristics of the alloy compositions, as
indicated by a typical example of a phase diagram for these
stainless grades, shown in simplified form in FIG. 15. In this
graph, which is essentially a plot of equilibrium phase conditions
various temperatures, for various weight percent contents of
chromium, and which for convenience of illustration is specifically
such a plot for alloys containing 0.05% carbon (it being understood
that generally similar configurations pertain to other carbon
levels, with specific differences in shape and location of phase
boundary lines), it will be seen that there is a so-called gamma
loop as indicated at 20.
More particularly, in FIG. 15 the gamma loop is surrounded, in
effect, by an alpha plus gamma band 21, representing a region for
occurrence of the same kind of transition as is involved in the
triangular area of FIG. 12. Regions below these areas 20 and 21,
e.g. as generally indicated at 22, can be considered equivalent to
the alpha or alpha-cementite regions at the bottom of FIG. 12, i.e.
where the ferritic type of microstructure prevails, as
distinguished from the basic austenitic structure within the gamma
loop 20. It will be understood that the upper portion of the
complete diagram, where the boundaries of regions 20 and 21 return
to the zero chromium axis, is omitted from FIG. 15 as being of no
concern here, and likewise certain special characteristics are
omitted relative to regions below the band 21, or partly within it,
where, depending on temperature and chromium content, there are
varying proportions of one or more iron-chromium carbides. For
simplicity the latter features of composition have been merely
indicated by the letter "K," it being understoood that at various
localities, "K" comprises one or more of various iron-chromium
carbide compositions.
As will be recognized from phase diagrams of the sort shown in FIG.
15, ferrite-austenite transition regions can exist over a large
range of temperatures for at least some alloys, thus presumably
accounting for the phenomena at very high temperatures in FIGS. 13
and 14, while nevertheless affording practice of the invention at
conveniently lower temperatures for all these alloys, as explained.
Moreover, although the equilibrium phase diagrams, including the
position of the gamma loop, vary for different alloys of these
types and specifically for different carbon levels, and the phase
boundaries may also shift somewhat with heating rate, and although
a complete transformation to austenite may not be attainable,
alpha-gamma transition regions appropriate for present purposes are
available in generally all cases, e.g. for the variations in
chromium content in the 400 series and for variations in carbon
level, from extremely low values up to 0.5% or higher. In other
words, while the driving force for austenite transformation on
rapid heating into the two-phase field is more complicated in these
alloys than for ordinary carbon steel, this circumstance does not
alter the basic applicability of the present invention.
As indicated, these results demonstrate the attainment of the
desired superplastic state with the ferritic grades of stainless
steel, it being understood (as explained above) that the term
ferritic or "normally ferritic" is herein employed to include those
particular grades which are usually martensitic in the as-produced
cold state, in that the reversible ferrite-martensite
transformation occurs at lower temperatures than the terminal
values of the present process, which consequently involves rapid
heating, in all cases, upward through the ferrite region to the
point of alpha-gamma transition. As shown by FIG. 13, advantage may
be taken of the instability of occurring recrystallization in these
stainless steels in the same general way as for the plain carbon
grades. The extent of cold working required to effectuate this
contribution to high ductility appears to vary with steel
compositions, but it will be understood that the fact of such
contribution and the percentage of cold work needed can be readily
determined by tests of the sort illustrated in FIG. 13, for any
given steel. In other words, although the advantageous effects of
recrystallization are not presently deemed essential (where the
instability by phase transition can occur), they appear to be
generally capable of determination and realization in desired
cases.
The products of the invention resulting from significant
deformation while the steel is held in the temporary state of
microstructure instability have been found to possess mechanical
properties, especially as to toughness or impact strength, and also
as to yield and tensile strength, superior to the original steel.
The toughness, moreover, can be very much better than would
characterize the same steel not treated by the process but instead
directly subjected to cold deformation.
These results have been demonstrated by tests, of which
representative examples are here set forth. The material employed
was a steel hot rolled to plate, e.g. such as employed as skelp for
making welded pipe, having a thickness of 1/2 inch and a
composition as follows: 0.1% C. 0.95% Mn, 0.06% Cb, 0.04% Al
(aluminum killed), rare earth additions (less than 0.05% total),
and incidental elements, including very low sulfur (0.006%) and
silicon below 0.01%. This was a commercial skelp material, which
can be considered a substantially non-alloy carbon steel, available
under the identification X-52, and the three plates used had yield
strengths in the range from 57,000 to 66,000 psi. Massive
specimens, 0.5 .times. 0.5 .times. 7 inches, were prepared and used
in the above described Gleeble apparatus. In each instance of
treatment of a specimen it was heated rapidly to one or another of
three selected temperatures, viz. 1,350.degree., 1,450.degree. or
1,550.degree. F., i.e. at a rate faster than 10.degree. F. per
second, specifically at about 50.degree. F. per second. Some of the
specimens were held for 10 seconds at temperature and then (while
fully superplastic) were deformed, i.e. by stretching, to
approximately 20% reduction in area of cross section, uniformly
along the axis of stretch with full advantage of the
superplasticity. In other cases the specimen was held for 300
seconds and then similarly deformed. In each instance the deformed
object was thereafter air cooled. Control specimens were subjected
to the same thermal cycles without deformation.
From all specimens, both deformed and control, suitable test
specimens were prepared, e.g. for hardness and impact testing, and
also for measuring tensile properties as described further below.
The specimens for Charpy toughness (impact tests) were prepared,
for convenience, at two-thirds of the usual size, but will be
understood that the comparative results obtained are fully
significant, since all tests were made at the reduced size.
Test specimens were also prepared from the plate as received, from
the same received material which was reduced 10 to 15% in thickness
by cold forging.
The hardness tests showed that there was no significant
deterioration in hardness by the procedure wherein the steel was
deformed in superplastic state, especially in the test of material
which had been deformed respectively at 1,350.degree. and
1,450.degree.. The specimens remained in the Rockwell B hardness
range of 80-90, whether measured with respect to the transverse or
longitudinal orientation, relative to the direction in which the
received plate had been hot rolled. When the deformation was
effected at 1,550.degree. F., the hardness departed to or a little
below the lower end of the above mentioned range. There was some
variation among various specimens, but results clearly demonstrated
that the product of the high temperature deformation during
microstructural instability, i.e. the ultimate cooled product,
maintained good hardness.
The Charpy impact tests showed very substantial improvement in
impact strength, especially in respect to the steel which had been
deformed while maintained in the upper superplastic state at
1,450.degree. or 1,550.degree.. As is conventional in toughness
testing, readings were made of the prepared V-notched test bars,
while held at room temperatures, i.e. 70.degree. F. and also at a
suitably cooled temperature, -20.degree. F., the measurements being
in foot pounds of energy required to break the bar. These tests
were made with specimens oriented both longitudinally and
transversely as to the original direction of hot rolling of the
plate. The same original plate material, untreated by the process,
showed Charpy values of 60-67 pounds at 70.degree. F. (and 4-16 at
-20.degree.), in the longitudinal direction. After holding 10
seconds at 1,350.degree. and deforming to an area reduction of
17-22%, the Charpy values at 70.degree., longitudinal, were 68-74
foot pounds. When the deformation was effected at 1,450.degree.,
e.g. for 24% reduction in area, these longitudinal values were
86-95 foot pounds. At 1,550.degree. with similar treatment
effecting reduction of approximately 15%, the ultimately
cooled-down specimens, tested for impact, yielded values of
approximately 118 foot pounds at 70.degree..
In the situation of longitudinal Charpy measurements for the low
temperatures, the material treated at 1,350.degree. showed
essentially no improvement, but at 1,450.degree. there was very
substantial improvement in the longitudinal direction. Thus where
reduction in area had been 17.6%, tests at -20.degree. F. showed
that the Charpy value was 75 foot pounds and where the reduction
was 23.8, the impact value was 57 foot pounds. This large
improvement was also exhibited in the material which had been
formed at 1,550.degree., the results, for 15-18% reduction being
100-105 foot pounds at -20.degree., longitudinally.
In the transverse direction, the nature of the results was similar
to that in the longitudinal direction, in that the impact or
toughness values improved significantly, especially where the
material had been deformed at 1,450.degree. and also at
1,550.degree., there being also some improvement in the
+70.degree.readings for product that had been reduced during
holding at 1,350.degree.. Whereas specifically, in tests at
70.degree., the original material showed impact values (transverse)
of about 47 foot pounds, the values after the 1,450.degree.
treatment (10 second holding) were 62 and 68 foot pounds
respectively for material reduced 25 and 10.6%. For the material
processed at 1,550.degree., with reduction of 15-18%, the Charpy
impact values (transverse) at 70.degree. were about 75 foot pounds.
In the measurements at -20.degree. F., there was a correspondingly
substantial improvement in toughness, e.g. in the transverse
direction. Whereas the original material had Charpy values of 18.5
to 28.5 foot pounds at -20.degree. (transverse), the product that
had been processed at 1,450.degree. with 18-22% reduction showed
transverse impact values in the range of 34-43 foot pounds. Where
the processing was at 1,550.degree., for a reduction of 14-17%, the
impact value of the product at -20.degree., transverse, was
approximately 61 foot pounds.
Results similar to those reported above were found in material that
was processed and deformed at 1,450.degree. and 1,550.degree. with
a holding time of 300 seconds before deformation.
It is noted that specimens which had been given the same thermal
cycle, but without any deformation during the interval of temporary
instability, also showed toughness improvement, but, of course,
these specimens did not have the advantage of having been deformed
with unusual facility to the desired, ultimate state.
Tests were also made to determine the effect on toughness of cold
forging similar plate material, i.e. as distinguished from the
deformation in accordance with the present invention. In these
tests, the Charpy V-notch values decreased very markedly in both
longitudinal and transverse directions, and at both +70.degree. and
-20.degree. temperatures, when the material was cold reduced, by
cold forging, to an area reduction in the range of above 10 to 15%.
In one instance longitudinal value dropped from over 100 foot
pounds to 40 foot pounds at 70.degree. F. and from upwards of 70
foot pounds, down to 26 foot pounds or less at -20.degree. F. In
the transverse direction, the drop was from about 60 to 33 foot
pounds, transversely, at 70.degree. and from 40 foot pounds or more
to about 10 pounds at -20.degree.. These results emphasized the
significance of the impact or toughness strength improvement
achieved by the present invention, in the production of deformed
steel articles. In other words, as distinguished from articles
prepared by cold working, the deformation to a desired shape by the
present process, yields products of very greatly superior
toughness. Instead of a substantial decrease in this strength
value, the new products have a large increase over the original,
hot-rolled material. It will be appreciated that these operations
permit the attainment of products having excellent properties,
without special alloying or other special operations heretofore
deemed necessary.
Upon practice of the defined process, with a significant
deformation of the material, e.g. more than 5% reduction in area,
or very preferably at least 10%, the steel product, as ultimately
tested after air cooling, has unusually high properties, without
loss in other respects. The deformation can be performed in any
suitable manner, by drawing, forging, rolling or the like, and it
will now be apparent that special uses of the invention are to
obtain products that may not otherwise be obtainable, e.g. rolled
high-strength, low-alloy steel at gauges smaller than heretofore
possible with hot rolling, while avoiding the loss of impact
strength and ductility that usually occurs in cold rolling.
Similarly, steel can be rolled by this process to achieve precision
bar products, of dimensional precision, with good strength, as
contrasted to currently imprecise hot-rolled bar products that now
must be further processed by cold drawing to achieve dimensional
precision but in such case suffer loss of impact strength and
ductility. As stated, the improved products of the invention are
obtainable for a variety of steels, as explained above, and for a
wide variety of deforming operations and ultimate shapes or
configurations.
Suitable tensile test specimens were prepared from the specimens of
X-52 steel which had been subjected to the present procedure and of
which other pieces were used for impact tests. These tensile
specimens were given standard tensile tests, determining ultimate
tensile strength and also yield strength, which in the case of this
steel could be measured at the upper and lower points, i.e. upper
and lower yield strengths. The tensile measurements were made with
respect to the longitudinal and transverse aspects of the steel,
for specimens which had been processed at the temperatures stated
above, namely 1,350.degree., 1,450.degree. and 1,550.degree. F.
Mostly the tests were related to the steel which had been held for
ten seconds at the selected temperature and then deformed during
microstructural instability, i.e. to reductions in area in the
range of 11-24%.
These tests demonstrated that generally over the range of
temperatures of deformation, the steel maintained its original
tensile properties or became characterized by improved tensile
properties -- comparing untreated specimens with those which had
been treated by the process and cooled as above described. Whereas
the original material in longitudinal direction showed upper and
lower yield strengths of approximately 57.5 ksi (thousands of
pounds per square inch) and ultimate tensile strength of 74.5 ksi,
the treated material showed roughly about the same values for
deformation at 1,550.degree. F., with somewhat better values at
1,450.degree. F. and with still higher values at 1,350.degree. F.,
i.e. ranging to lower and upper yield points of about 70 and 75 ksi
and ultimate tensile strength of 83.5 ksi. At the intermediate
temperature of 1,450.degree. the three values were respectively
about 66, 72 and 78 ksi.
In the transverse direction a similar situation was found, where
the original material had upper and lower yield points of 65 ksi
and ultimate tensile of 79 ksi. Here the material treated at
1,550.degree. F. showed a lower yield point of about 64, upper
yield point about 72 and ultimate tensile about 72 ksi. Likewise at
1,450.degree., in the transverse direction, the steel so treated
showed lower yield, upper yield and ultimate tensile strength of
about 69, 76 and 80 ksi respectively, while for the steel treated
at 1,350.degree. these values rose to about 74, 81 and 85 ksi. With
increase of yield and ultimate strengths, the present elongation
and percent reduction of area on yield decreased, but the
properties in these respects remained satisfactory.
The tensile properties of the treated steel and the impact
properties can be considered together in that, for example, over a
treating range of 1,350.degree. to 1,550.degree. F., the impact
properties are generally highest for highest temperature of
treatment, whereas tensile properties are generally highest for the
lowest temperatures in the range. Nevertheless, both the impact
properties and the tensile properties are at least substantially
equal to those of the original steel over the temperatures of the
process. As a result, the product can be generally characterized as
having superior properties in one or the other of these respects
and at least as good properties in each respect, in comparison with
the original body of steel. Moreover, it will now be seen that by
selecting the temperatures of treatment, i.e. for development of
superplasticity, the product can be produced to have a selected
improvement. This can be either improvement in both impact and
tensile properties, as with these steels processed at 1,450.degree.
F., or the product can be made to have greatly increased tensile
properties with essentially the orginial toughness, or greatly
improved impact properties with the orginal tensile strength, these
selections being respectively at the lower and upper ends of the
processsing temperature range.
Some other tests indicated that the hold time at the elevated
temperature of instability (superplasticity) had some bearing on
the ultimate properties of the deformed article, in the sense that
increasing the hold time to 300 seconds tended to decrease the
tensile properties slightly at the lower temperatures of treatment,
while the longer hold times appeared to afford some further
increase of impact strength.
Inasmuch as the yield strength values and the ultimate tensile
values generally always varied in substantially the same way, i.e.
as to increase or decrease or lack of change, with processing or
changes of conditions, this area of mechanical properties can be
simply identified as tensile strength for convenient identification
herein.
Finally, it is found that the steels processed in accordance with
the present invention are characterized by a distinctive
microstructure, which is believed to be new and unique. FIGS. 16
and 17 are photomicrographs of sections of steels that have been
processed according to the invention and are representative of the
new products. Specifically, these are photographs taken
respectively at 200X and 400X magnification of suitably prepared
specimens, FIG. 16 being the 1006 steel, originally in an annealed
state and deformed at 1,495.degree. F., and FIG. 17 being the 1018
steel which had been cold worked 50% and which was deformed at
1,425.degree. F. This new structure is understood to represent the
ferrite grains, which now contain an unusual substructure, i.e. a
coarse, well-defined cell structure within the grains, as shown in
each of these views. The microstructure also includes carbon-rich
or carbide-containing areas (dark etching areas in the photographs)
identified as an austenite decomposition product.
The specimens employed, which are helium gas quenched from the
above-quoted deformation temperatures, were processed and
photographed by conventional methods, for revealing the ferrite
grain structure. Standard metallographic techniques for specimen
preparation, which include grinding and polishing, were employed.
After etching in a nital solution, the specimens were examined
using normal bright field optical illumination. As will be
understood, the photomicrographs in FIGS. 16 and 17 show
representative areas and were taken at the above-stated
magnifications.
It is to be understood that the invention is not limited to the
specific operations and materials herein described, but may be
carried out in other ways without departure from its spirit.
* * * * *