U.S. patent number 3,661,655 [Application Number 05/090,342] was granted by the patent office on 1972-05-09 for metallic articles and the manufacture thereof.
This patent grant is currently assigned to North American Rockwell Corporation. Invention is credited to Louis J. Hrusovsky.
United States Patent |
3,661,655 |
Hrusovsky |
May 9, 1972 |
METALLIC ARTICLES AND THE MANUFACTURE THEREOF
Abstract
The manufacture of articles from non-austenitic metallic
materials by a technique in which the article is formed to
essentially its final configuration; cooled; and, while cooled,
worked to produce residual compressive stresses in one or more
surface portions of the article.
Inventors: |
Hrusovsky; Louis J. (Bloomfield
Hills, MI) |
Assignee: |
North American Rockwell
Corporation (Pittsburgh, PA)
|
Family
ID: |
22222374 |
Appl.
No.: |
05/090,342 |
Filed: |
November 17, 1970 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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681487 |
Nov 8, 1967 |
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Current U.S.
Class: |
148/578;
148/580 |
Current CPC
Class: |
C21D
9/02 (20130101); C21D 7/04 (20130101) |
Current International
Class: |
C21D
9/02 (20060101); C21D 7/04 (20060101); C21D
7/00 (20060101); C21d 001/62 (); C21d 009/02 ();
C21d 007/06 () |
Field of
Search: |
;148/12,125,12.4 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Rutledge; L. Dewayne
Assistant Examiner: Stallard; W. W.
Parent Case Text
RELATION TO OTHER APPLICATIONS
This application is a continuation-in-part of application Ser. No.
681,487 filed Nov. 8, 1967, and now abandoned.
Claims
What is claimed and desired to be secured by Letters Patent is:
1. The method of manufacturing a spring leaf or the like,
comprising the steps of: forming said leaf to essentially its final
configuration from a blank of non-austenitic steel; pre-stressing
the leaf in the direction of normal service loading; cooling said
leaf to a temperature not substantially in excess of 0.degree. F;
and, while said leaf is cooled and prestressed, working at least
one surface of said article to generate residual compressive
stresses therein and thereby increase the fatigue life and stress
corrosion resistance of said leaf.
2. The method of claim 1, wherein said spring leaf surface is cold
worked by shot peening.
3. The method of claim 1, wherein the surface of the spring leaf
which is worked is the tension surface.
4. The method of claim 1, wherein the spring leaf is cooled by
immersing it in a bath of liquid air or liquid nitrogen or dry ice
mixed with alcohol or mineral spirits.
5. The method of claim 1, wherein said spring leaf is cooled to a
temperature in the range of from about zero to about
-320.degree.F.
6. In a method of increasing the resistance to fatigue of a spring
leaf fabricated of a non-austenitic metallic material that has been
heat treated and formed to shape, the steps of grinding at least
the tension surface and the adjacent side edges of the spring leaf;
chilling the spring leaf to a temperature not substantially above
0.degree. F.; and strain peening the spring leaf at least on said
tension surface and said side edges.
7. In the method making a taper leaf spring wherein a
non-austenitic steel blank is heat treated and taper rolled, the
sequential steps of chilling the taper rolled leaf below about
0.degree. F. and mechanically working at least the surface that is
to be the tension surface in the finished spring.
8. In the method defined in claim 7, said taper leaf spring being
stressed in the normal operational direction while being shot
peened.
9. A method of increasing the fatigue life and stress corrosion
resistance of a non-austenitic steel spring leaf subject to random
high frequency operational reversing stresses in which said spring
leaf is heat treated to a temperature of about 1,550.degree. F.
formed to shape and tempered at around 900.degree. F., thereafter
chilled to a temperature appreciably below ambient temperature,
and, while chilled, mechanically worked upon a selected surface
area.
10. The method of claim 9, wherein said surface area is
mechanically worked by shot peening.
Description
BACKGROUND, SUMMARY AND OBJECTS OF THE INVENTION
The present invention relates to the manufacture of metallic
articles and, more particularly, to the manufacture of metallic
articles having improved fatigue life and stress corrosion
resistance..sup.1 (.sup.1. For a discussion of stress corrosion,
see Suss, "Stress Corrosion--Causes & Cures," MATERIALS IN
DESIGN ENGINEERING April 1965, pages 102-105, 146, and 148.) The
principles of the present invention are particularly adaptable to
the manufacture of articles which, under operating conditions, are
subjected to dynamic loading producing cyclic, high magnitude
bending or torsion stresses such as leaf and torsion bar springs
for vehicles. The principles of the present invention will
accordingly be developed primarily by relating them to such
applications of the invention. By doing so, however, it is not
meant to limit the scope of the invention, which is intended to be
defined only by the appended claims.
In my novel method for manufacturing metallic articles such as
those identified above, the article is formed to essentially its
final shape from a non-austenitic metal or alloy. The article is
then cooled, typically to a temperature of zero degrees Fahrenheit
or lower. While it is cooled, one or more surface portions of the
article are worked--by shot or strain peening, for example. The
article is then allowed to return to ambient temperature.
The working of the cooled metal induces residual compressive
stresses in the surface portion or portions of the article which
are worked. Due to the generation of such stresses, articles
produced in accord with the principles of the present invention
have improved fatigue life and improved stress corrosion
resistance.
It is essential, in the present invention, that the article be
formed to substantially its final shape before it is subjected to
the cold temperature working step. The residual compressive
stresses generated by this step exist only in the outermost surface
portions of the treated parts of the article. Consequently, if
these surface portions are ground or machined or otherwise removed
after the cold temperature working, the benefits of this treatment
will be lost. Furthermore, steps such as forming a bar into a coil
spring after working at cold temperatures will relieve the residual
compressive stresses and will, in fact, even generate deleterious
residual tensile stresses.
The working of metals at low temperature has heretofore been
proposed in U.S. Pat. Nos. 2,527,287 to Ziegler and in Ziegler et
al, "Hardening of Austenite Stainless Steels by Mechanical Working
at Sub-zero Temperatures," American Society for Testing Metals
Proceedings 50, pages 861-881, (1950)..sup.2 (.sup.2. A similar
process is described in U.S. Pat. No. 3,795,219 to Angel. The basic
difference between what is disclosed in the foregoing references
and the present invention is that Ziegler applied the cold
temperature working technique solely to austenitic steels while the
present invention is concerned solely with the treatment of
non-austenitic materials. That is, I have disclosed herein for the
first time the novel use of cold temperature working to improve the
properties of non-austenitic metallic materials.
When austenitic steels are cooled to the temperatures contemplated
by the present invention, there is a change of crystalline
structure from austenitic to martensitic, thereby increasing the
hardness of the material. By shot peening or otherwise working the
material at this reduced temperature, it can be caused to retain
the martensitic structure and consequent hardness upon being
returned to ambient temperature. It is abundantly clear that this
is the result sought in the Ziegler process since the material thus
worked is then subjected to a high temperature aging treatment to
further increase its hardness.
Further, in the Ziegler process, there is no generation of residual
compressive stresses by the working at reduced temperatures. This
is because the phase change from austenite to martensite is
accompanied by an increase in volume so that any compressive
stresses generated by the cold temperature working are
automatically relieved. Furthermore, the subsequent high
temperature aging treatment would relieve such stresses even if
they were generated in the cold temperature working.
As there is no generation of residual compressive stresses in the
Ziegler process, there is no improvement in the fatigue life or
stress corrosion resistance of the treated materials (at least
stress corrosion resistance may in fact be impaired to some
extent). Thus the major change in the physical properties of
materials so treated is an increase in hardness although increases
in tensile strength, yield stress, and proportional limit are also
claimed.
To contrast the technique of the present invention with the Ziegler
process, there is no phase change in the former since it is applied
only to non-austenitic metals and alloys..sup.3 (.sup.3. In fact,
if the presence of austenite is suspected, materials which are to
be treated by the process disclosed herein are cooled to a sub-zero
temperature to convert the austenite to martensite and then heat
treated to temper the freshly formed martensite before they are
cooled and worked.) There is no increase in hardness, and no
terminal aging step is employed to increase this property. Most
important, however, there is a generation of residual compressive
stresses in the worked surface portions of the material with a
consequent improvement in fatigue life and stress corrosion
resistance.
As indicated above, shot peening is one technique which may be used
to work metals in accord with the process described herein. Shot
peening to improve the fatigue life of metal articles has been long
known in the metal working industry, and shot peening the surfaces
of leaf springs, coil springs, torsion bar springs and the like
operationally stressed members to improve their endurance limits is
described in U.S. Pat. No. 1,947,927 to Vorwerk. Shot peening of
only the tension surface of a leaf spring is described in U.S. Pat.
No. 2,608,752 to Schilling. However, in all of the prior art
methods the shot peening operation is performed at or above room
temperature usually immediately after tempering when the metal of
the spring leaf may still be hot or is cooling or has cooled to
room temperature.
It has also been proposed to grind or polish the tension surface
and adjacent edges of a spring leaf as a step after heat treatment
but prior to strain peening to eliminate surface defects which in
part may offset the beneficial effect otherwise obtainable from the
shot peening treatment. Such improved processes are disclosed in
U.S. Pat. No. 3,238,072 and copending applications Ser. Nos.
449,485 and 449,486, both filed Apr. 20, 1965; and the present
invention may be advantageously applied to the foregoing processes
to further improve endurance of the spring leaf.
In the manufacture of leaf springs for vehicle suspensions, which
is one field to which the present invention may be advantageously
applied as indicated above, the problem of fatigue failure has been
and always will be of great concern; and various metal treating
methods have been suggested and are still being practiced to
improve the resistance to fatigue.
It has been well established that fatigue generally initiates at
that surface of the spring leaf where the stress is greatest and
boundary conditions unfavorable. This surface is conventionally
designated as the tension surface of the leaf, and premature
fatigue failure can be attributed to such stress raisers as edge
indentations, cracks, non-metallic inclusions, surface laps and
folds and other irregularities at or immediately below the tension
surface. Present attempts to counteract these detrimental surface
conditions include such measures as surface grinding or polishing
and shot peening. However, even if shot peened under strain,
maximum effect of the shot peening treatment was not always
obtainable in spring leaves operating with a certain stress
range.
The beneficial effect of shot peening is further influenced by the
fact that as the shot impinge upon the surface of the metal member
they produce depressions in the surface causing plastic flow of
surface metal. In instances, as when "over peening," the
depressions may become deep and develop into cracks or folds or
sharp edges may be produced around some of the depressions, thus
actually forming stress raisers which it was originally intended to
combat by the shot peening treatment. In the present invention
plastic flow of the surface metal during shot peening is inhibited
to reduce or eliminate the adverse effects of "over peening" and
prevent excessive plastic metal flow since the metal member is
maintained in a chilled condition (preferably at a sub-zero
temperature) while being shot or strain peened. In this regard, it
is generally known that hardness and yield and tensile strengths of
steel increase with successively lower temperatures, although this
increase is transitory and is lost when the steel returns to room
temperature. The increases in these strengths along with the
increased hardness considerably augment the shot or strain peening
effect on chilled steel because plastic metal flow is largely
inhibited.
In regard to other metal treating processes heretofore proposed
which involve chilling of the metal to be treated, the cooling of
high speed steels for cutting tools to sub-ambient temperature to
improve their productivity and life is described in an article by
P. Gordon and M. Cohen in the publication, A. S. M. Transactions,
September, 1942, Volume 30, pages 569-591. Chilling of the metal to
sub-zero temperatures has also been found to be useful in the
welding and metal flame cutting art to relieve the weld or cutting
area of undesirable tensile stresses and thus improve the strength
of the weld. This method is described in U.S. Pat. No. 2,824,818 to
Swenson, and U.S. Pat. No. 3,282,748 to Martens. However, these
localized treatments had no effect on the overall fatigue life or
stress corrosion resistance of the metal article itself but only
served to strengthen the weld and the immediate weld or flame
cutting area.
SUMMARY OF INVENTION
The present invention proposes a movel manufacturing process for
metal members, especially those subjected to dynamic loading such
as spring leaves, torsion bars, coil springs and similar articles
to considerably increase resistance to the fatigue and stress
corrosion of these members in operation. This is accomplished by
essentially combining suitable chilling of the metal, which must be
non-austenitic, with cold working while the metal is chilled with
unexpectedly advantageous results, the foregoing being a major
object of the invention.
The novel method of the invention may consist in one aspect of
forming a heat treated, non-austenitic steel or like ferrous metal
blank to the shape of a spring leaf, refrigerating the formed
spring leaf to a temperature well below the temperature of the
ambient atmosphere and preferably below 0.degree. F. and mainly
below -40.degree. F., and immediately shot peening at least one
surface of the refrigerated spring leaf to generate residual
compressive stresses in the spring leaf surface.
Accordingly, another important object of the invention is to
considerably increase the fatigue life and stress corrosion
resistance of a steel leaf spring by refrigerating and immediately
subjecting the chilled spring leaf at least on the tension side to
a shot peening operation.
Another object of the invention is to provide a novel spring leaf
manufacturing method in which a taper rolled and heat treated
ferrous metal spring leaf is chilled to a temperature between
0.degree. and -320.degree. F., strained in the direction of normal
service loading to introduce a pre-stress in the leaf, and shot
peened at least on the tension surface and usually the adjacent
side edges as well while the spring leaf is in stressed condition
and the metal is refrigerated.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a graph containing curves showing the cycles to failure
of samples peened at temperatures ranging from +80.degree. F. to
-80.degree. F. and to which the indicated stress was applied;
FIG. 2 is a diagram showing actual points of failure on solid lines
and extrapolated dotted line curves illustrating the relationship
between peening temperature and fatigue life of the metal article
at various stress ranges;
FIG. 3 is a hardness curve diagram of various hardnesses in
relation to the respective compressive residual stresses vs.
hardness depths;
FIG. 4 is a typical S-N diagram commonly used in fatigue testing to
plot the results of these tests;
FIG. 5 is a diagrammatic illustration in side elevation of a
typical, single tapered spring leaf for a vehicle suspension;
FIG. 5a is a section through the constant width spring;
FIG. 6 shows the spring leaf of FIG. 5 being chilled;
FIG. 7 shows the chilled spring leaf of FIG. 6 in strained
condition while being shot peened on the tension surface;
FIG. 8 is an enlarged microphotograph showing the condition of a
portion of the tension surface of a spring leaf shot peened at room
temperature;
FIG. 9 is a microphotograph similar to FIG. 8 but illustrating the
condition of the area shot peened at about -320.degree. F.;
FIGS. 10 to 12 are strip chart records of surface roughness
measurements taken on a leaf spring after shot peening at various
temperatures.
DESCRIPTION OF PREFERRED EMBODIMENTS
For illustrative purposes the present invention will be described
in detail herein as being applied as a processing step in the
manufacture of leaf springs for vehicles. However, as stated above,
it will be understood that the invention may be applied to a
variety of other metal articles or parts as well and particularly
to coil springs, torsion bars, drive shafts, axle shafts and
similar members which are subject to dynamic loading and/or
relatively high repeating stresses during operation. Repeating
stresses as referred herein include stresses not completely
reversed; that is, from substantially zero to maximum allowable
stress in each cycle.
In the manufacture of spring leaves, the shot peening or strain
peening operation is usually carried out as one of the last steps
in the manufacturing process. For example, a spring leaf is usually
made by providing a blank which has been cut from a length of
spring steel, formed to shape (such as by taper rolling), heat
treated, camber quenched and tempered. After that -- in accordance
with the present novel method disclosed herein -- the partially
finished spring leaf is chilled to 0.degree. F. or below and
immediately shot or strain peened while in refrigerated condition.
Strain peening is shot peening of a loaded stressed spring.
Subsequent fatigue tests by cycling showed that the spring leaves
which had been shot or strain peened at these low temperatures had
a considerably increased life within a particular stress range used
for cycling as compared to spring leaves cycle tested within the
same stress range which had been shot or strain peened in a
conventional manner; that is, near or above room temperature.
The invention includes the discovery that cold working the leaf
spring while it is chilled generates residual compressive stresses
in the surfaces which are worked with a consequent beneficial
increase in the fatigue life and stress corrosion resistance of the
spring leaf. It has been discovered that, since the improvement in
fatigue life and stress corrosion resistance are increased with
successively lower temperatures, the spring leaf to be shot or
strain peened should be chilled as much as practically possible and
as may be determined by the operational characteristics of the
metal involved and the particular leaf spring member operating
within a desired stress range. This can be accomplished in normal
shop operations and with presently known cooling methods and
equipment; and it is believed that the low limit is probably about
-320.degree., which is the temperature of liquid nitrogen.
Leaf spring manufacturing processes in which the method of the
invention may be advantageously included comprise the manufacture
of single tapered leaf springs as described in U.S. Pat. No.
3,145,984 issued Aug. 25, 1964 to W. A. Hallam for "Single Taper
Leaf Spring." A length of spring steel may be processed in
accordance with the method disclosed in United States Pat. No.
3,238,072, issued Mar. 1, 1966 to R. R. Green et al for "Method of
Making Taper Leaf Springs." This involves the sequential steps of
hot taper rolling, end forming, heat treating, camber quenching,
tempering and grinding of the proposed tension surface and the
edges adjacent the tension surface. Thereafter, the partially
finished single taper leaf spring is shot peened at least on the
tension side and adjacent edges, the leaf usually being strained in
the direction of applied service loading while being shot peened
(stress or strain peening).
It has been established that shot peening increases the fatigue
strength of a metal leaf spring; and, since the weight of the
spring is inversely proportional to the square of the strength of
the spring material, it follows that, if the strength is increased
by shot peening, the weight of the spring leaf can be reduced
proportionately, thus making possible the single taper leaf
spring.
The increase in strength due to shot peening is believed to be due
to the entrapment of beneficial residual compression stresses near
the shot peened surface which in effect is equivalent to some
pre-loading so that any applied tension stresses during operation
will be reduced to a safe level by the countereffect of the
entrapped residual compression stresses. An increase in the
residual compression stresses is effected when the metal member is
loaded statically during the shot peening operation to introduce
compressive stresses at the surface being shot peened, and thus a
larger amount of these beneficial residual stresses are trapped in
the sub-surface layers during shot peening.
The present invention provides a novel method to even further
unexpectedly increase the fatigue life of leaf springs (or other
metal members) by preceding the steps of shot or strain peening by
the step of effectively chilling the metal prior to such peening
and maintaining the chill temperature during the peening operation.
This novel technique has the further advantage that it improves the
stress corrosion resistance of the treated leaf spring.
In summary, both loading and chilling contribute to increasing the
residual compressive stresses at the tension surface, and shot
peening of the chilled tension surface under strain traps those
residual compressive stresses so that they are available to oppose
tension stresses incident to normal operation and thus increase the
spring life and efficiency.
The invention in a preferred embodiment will now be explained more
in detail as applied to a leaf spring manufacturing process.
With reference to FIGS. 5 to 7, in practice and as applied to the
manufacture of leaf springs for vehicles, a practical operation
would consist of the following steps: After a spring leaf 10 has
been processed up to the camber quench operation the leaf is
tempered and immediately thereafter chilled to a desirable very low
temperature, the value of which would be determined by the
operational characteristic of the spring leaf when in duty. As will
be more fully explained later, a spring leaf having an operational
characteristic within a medium stress range such as in light duty
operations as in passenger cars need not be chilled to as low a
temperature as heavy duty leaf springs such as springs operating on
higher stress levels. The chilling or refrigeration of the spring
leaf may be accomplished by any one of a variety of methods such as
by surrounding the spring leaf with dry ice, immersion of the leaf
in a bath 12 of dry ice diluted with alcohol or mineral spirits as
illustrated in FIG. 6, or by immersion of the leaf in liquid air or
liquid nitrogen for maximum low temperatures. For medium low
temperatures, it may suffice to simply place the spring leaf in a
refrigerator unit for a certain length of time to attain a
sufficiently low temperature. In any instance, the particular
method of chilling is not critical and usually depends primarily on
available facilities. In the case of a cambered leaf spring as
illustrated, the leaf is chilled in cambered condition prior to
being strained for shot peening.
After the spring leaf has reached the desired low temperature it is
shot peened while in a chilled state. In some instances it may be
shot peened while the refrigeration is maintained. Where the leaf
is refrigerated in a bath it is removed from the bath and
immediately shot peened. In the case of stress peening as
illustrated in FIG. 7, spring leaf 10 is strained by applying a
static load at the ends 14 and 16 of the leaf in the direction of
normal service loading as indicated by the arrows. In order to
maintain the static load during peening the leaf may be suitably
clamped down at its ends as shown at 18.
The peening operation may be carried out with standard shot peening
equipment (schematically illustrated at 20) for a sufficient time
and intensity to thoroughly work and completely cover the surface
to be peened. In general, no particular method is required, but it
will be preferred to employ larger size shot at higher intensities
with successively lower metal temperatures because of the
increasing transitory surface hardness to allow the shot forces to
penetrate the sub-surface layers. Although the shots themselves may
be chilled and maintained at the same temperature as the article
being shot peened, this is not necessary since it has been
determined that the impact of shots kept at room temperature
affected the temperature of the chilled metal article only to a
negligible degree. In some tests, the temperature of the article
was raised only about 15.degree. F. by the creation of heat due to
the impinging of the room temperature shot, which was found to be
tolerable.
In regard to the manufacture of spring leaves herein illustrated by
example, it will usually suffice to shot or strain peen only the
intended tension surface 22 of the spring leaf and its adjacent
side edges 24 which are usually scarfed as shown in FIG. 5a and
have been previously ground smooth.
After the chilled spring leaf has been shot or strain peened, it
may be further processed according to known manufacturing
methods.
In this invention residual compression stress is operative in
unexpectedly increasing the beneficial effect on the metal obtained
by shot or strain peening, and this advantageous condition is
retained when the metal member returns to and works in ambient
temperature.
During the course of various fatigue tests in connection with the
manufacture of shot or strain peened vehicle leaf springs the
following procedures were followed:
Leaf spring samples were prepared from SAE 5160 spring steel by the
foregoing methods including austenitizing the samples at a
temperature of 1,550.degree. F. The samples were than quench
formed, cambered in oil, and tempered at 900.degree. F. so that the
treated sample had a martensitic structure.
For the shot or strain peening operation No. 28 cast steel shot
were used at an intensity of between 0.016 and 0.020 Almen A2. For
strain peening, some of the cambered samples were strained prior to
shot peening by stretching the samples flat; that is, by applying
and maintaining a load at the ends of the spring samples in the
direction of normal service loading. The samples were then cooled
or chilled to various degrees ranging from +70.degree. F. (room
temperature) to -320.degree. F. and immediately thereafter shot or
strain peened.
The shot or strain peened samples were then fatigue tested by
cycling at various stress ranges including the design stress under
which the particular leaf spring was intended to operate in an
actual application. The fatigue testing was done by unidirectional
cycling from rebound to maximum load.
Exemplary test results have been tabulated in the following tables:
---------------------------------------------------------------------------
TABLE I
Fatigue Cycles at Indicated Stress Levels
Shot Peened Shot Peened Shot Peened Temperatures 30/140 Ksi 45/155
Ksi 60/170 Ksi
__________________________________________________________________________
+70 264,000 224,000 196,000 +30 398,000 310,000 -- 0 3,017,000* --
-- -50 3,262,000* -- 236,000 -75 1,067,000* 1,080,000* 325,000 -100
1,099,000* -- 1,132,000* -320 1,116,000* 2,000,000* 1,162,000*
__________________________________________________________________________
(*Tests stopped before failure occurred)
---------------------------------------------------------------------------
TABLE II
Fatigue Cycles At 65/190 Ksi
Strain Peened Strain Peened Temperatures at 120 Ksi at 170 Ksi
__________________________________________________________________________
+75 295,000 777,000 0 1,446,000* 3,325,000* -50 -- 1,040,000* -100
1,218,000* 3,390,000* -320 3,370,000* --
__________________________________________________________________________
(*Tests stopped before failure occurred)
From the above tables in connection with the curves and diagrams in
FIGS. 1 and 2 it will be seen that the samples shot peened and
strain peened at ambient temperature had by far the shortest
fatigue life within any of the stress ranges. The samples which had
been chilled to -320.degree. F. and then shot peened and strain
peened showed marked increase in fatigue life within the low and
medium stress range over the unchilled samples with the greatest
increase being found in the low stress range and indicating
infinite life there. However, in the higher stress ranges the most
marked improvement was found in the samples which had been strain
peened. The samples which were chilled to about -100.degree. F.
showed the most marked increase within all three of the stress
ranges used in the cycle tests with the highest increase being
shown again when the samples were strain peened under a static load
of around 160 Ksi. It appears that the greatest increase in fatigue
life takes place in highly stressed strain peened steel between
0.degree. and -40.degree. F.
Temperatures down to -100.degree. F. are readily obtained by using
mixtures of dry ice and alcohol or mineral spirits. Liquid
nitrogen, which is commercially available, provides for chilling to
-320.degree. F.
It can be deduced from the foregoing that the actual realization of
increases in fatigue life produced by the method of the invention
may depend on the maximum operating stress to which the
"chill-peened" part is subjected. There appears to be a definite
relation between operating stress and fatigue life. As exemplified
by the typical S-N curve shown in FIG. 4, which is plotted with the
magnitude of the applied stress or load (S) as ordinate and the
number of cycles (N) for fracture as abscissa, it will be seen that
the fatigue life of any specimen is not a linear function of the
applied stress. In other words, it will be noted that, as the
operating stress decreases, the fatigue life increases at a
geometrical rate. As the applied stress approaches the "knee" of
the curve, a condition becomes present where a very small change in
stress causes an indefinitely large change in fatigue life. Shot
peening has the effect of raising the "knee" of the curve, and this
holds true in stress peening as well as in the improved method of
"chill peening." Thus, an extraordinary increase in fatigue life
can be realized for any specimen tested in that region of applied
stress.
Applying the foregoing discussion to the invention and with
reference to the diagrams in FIGS. 1 and 2, it will be noted that
the minimum chilling temperature for peening to obtain maximum
increase in fatigue strength is dependent on the maximum applied
stress. As seen from the diagram in FIG. 2, for the lower stress
range a chilling temperature of around -50.degree. F. will be found
to be sufficient since it will be noted that a successively lower
temperature to -100.degree. F. did not produce any further
measurable increase in fatigue life. In the medium and high stress
ranges and when stress peening, a lower temperature of at least
-100.degree. F. or preferably even lower will be necessary to
obtain a comparable increase in fatigue life.
The phenomenon of attaining increased fatigue life when shot or
stress peening at successively lower temperatures is not fully
understood but it is believed to be partly dependent upon the
increased transitory hardness of the material at the low
temperatures.
Referring to the hardness curve diagram in FIG. 3, which was
prepared from cross sections of metal at room temperatures, it will
be seen that an increase in hardness produced a marked increase in
the magnitude of residual compression stresses at at steadily
decreasing depths. In other words, with increasing hardness while
the material is worked, maximum residual compression stresses will
be concentrated in a shallow subsurface layer where they are most
beneficial. And, in order to obtain full effect of the shot peening
treatment, it is desirable to concentrate a maximum amount of
residual compression stresses immediately below the surface to be
peened.
Test results indicated that the outer skin of the peened surface
developed a saturation value of compressive stress very early in
the peening operation. Continuing peening results in an increase of
entrapped compressive stresses just below the surface and a
deepening of the compressive layer. Thus, with reference to the
diagram in FIG. 3, the compressive stresses which are entrapped by
the shot peening operation at a hardness of 53 Rc (Rockwell C)
scale range from about 75 Ksi to a peak of about 140 Ksi at a depth
of 0.006 inch, which is within the range of average depth of the
compressive layer directly affected by the shot peening treatment.
Below that depth compressive stresses steadily decrease towards
transferral into tensile stresses. It will be seen that at an
increased hardness to 57.5 Rc the compressive residual stresses
range from 100 Ksi at the surface to a peak of about 175 Ksi at a
shallower depth of approximately 0.003 of an inch. This indicates
that with an increase in the hardness of the material while it is
worked a larger amount of compressive stresses may be entrapped
with a simultaneous reduction in peening effort.
As can be noted from the diagram in FIG. 3, a decrease in the
hardness of the material while it is worked causes a marked
decrease in the magnitude of compressive stress at an increase in
depth with the peak of the reduced stresses at a depth beyond the
average depth normally affected by shot peening.
Residual compression stresses normally have a magnitude of about 50
percent of the yield strength or yield stress of the metal. As the
transitory yield stress of the metal increases with the increase in
transitory hardness, the maximum residual compression stress
increases at a higher rate beyond the nominal 50 percent of the
yield stress which is illustrated in the table below based on the
curve diagram in FIG. 5.
max. Residual % of Hardness Yield Stress Compr. Stress Yield Stress
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38.5 156,000 83,000 53% 45 191,000 108,000 57% 53 237,000 140,000
53% 57.5 271,000 173,000 64%
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The realization of the foregoing is herein combined with the fact
that the chilling of metal produces a temporary increase in
hardness beyond that at room temperature and thus increases the
magnitude of residual compressive stresses in the sub-surface
compression layer, which accounts for the remarkable increase in
fatigue strength obtained by the present novel shot peening method
as well as the increase in corrosion resistance.
When impacted by shot, an increasingly smoother surface is produced
at decreasing temperatures in comparison to articles shot peened at
higher temperatures. This beneficial condition of diminishing
surface roughness in association with decreasing shot peening
temperatures further increases the fatigue strength of the metal
part since it effectively inhibits excessive plastic metal flow at
the surface during shot peening and thus prevents the formation of
stress raisers and minimizes the possible adverse effect resulting
from overpeening.
With reference to FIGS. 8 to 12, several samples of metal parts
were shot peened at various temperatures whereafter surface
roughness measurements were taken and recorded on a strip chart
recorder. The height of roughness is specified in micro-inches as
the arithmetical average of the absolute deviations from the mean
surface. This is graphically shown in the charts of FIGS. 10 to 12
from which it can be seen that the specimens shot peened at room
temperature produced a considerably rougher surface with deeper
valleys and peaks than the specimens shot peened at below zero
temperatures. At the lowest temperature of -320.degree. F. shot
peening produced an almost perfectly smooth surface. The deep
crevices produced by shot peening at ambient temperatures are
believed to be partly responsible for the lower fatigue life of
metal parts shot peened at or above ambient temperatures.
FIGS. 8 and 9 illustrate magnified surface protions of shot peened
leaf springs which show that springs "chill-peened" at an ambient
temperature as in FIG. 8 produced a pebbly appearance on the
surface characterized by countless small indentations reproduced as
high peaks and valleys in the roughness measurement chart of FIG.
10. On the other hand, leaf springs which were "chill-peened" at
-320.degree. F. as in FIG. 9 showed a remarkably smooth surface
indicating that the impinging shot were not able to erase earlier
grinding marks (shown as lines) from the surface.
In conclusion, the fatigue tests conducted within various operating
stress ranges conclusively determined that the fatigue life of
metal parts which in operation are subjected to relatively high
dynamic load stresses, especially unidirectional stress at high
frequencies, will be considerably enhanced by shot or strain
peening when the metal is chilled sufficiently below room
temperatures, preferably below 0.degree. F., and advantageously
below -40.degree. F. and as low as -320.degree.F. The extent of
fatigue life improvement which can be realized is governed by the
degree of reduction in temperature. In general, the lower the
temperature, the greater the improvement when kept within its
normal range of applied stress.
Tests conducted on other than non-austenitic ferrous metals (such
as iron or spring steel) showed similar beneficial results for such
non-austenitic materials as aluminum, magnesium, titanium, brass,
and stainless steel.
To demonstrate the effect of the present novel method on commercial
aluminum, specimens of an aluminum alloy 7075-T6 were cut from bar
stock, heat treated, and artificially aged. The specimens were then
subjected to a shot peening treatment at three temperature ranges;
namely 70.degree. F., -70.degree. F., and -320.degree. F. The shot
peening was conducted with No. 28 cast steel shot at an intensity
of Almen A.sub.2 0.sup.. 017.
Thereafter the specimens were fatigue tested by subjecting them to
unidirectional cycling at the following stress ranges:
10 - 50 Ksi (67 percent of tensile yield strength)
10 - 60 Ksi (80 percent of tensile yield strength)
The following table indicates the obtained test results:
Shot Peening Average Cycles To Failure Within Temperatures A Stress
Range of:
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F. 10-50 Ksi 10- 60 Ksi 70.degree. F. 439,000 134,500 +70.degree.
F. 1,631,500 155,000 +320.degree. F. 1,316,500 252,000
__________________________________________________________________________
it will be noted that the specimens shot peened at below ambient
temperatures showed a marked increase in fatigue life with the most
pronounced effect being found in the lower stress range when
chilled to -70.degree. F. Further chilling below that temperature
did not result in any further increase in fatigue life. The same
could also be observed in the higher stress range where chilling to
-70.degree. F. was also found to produce a larger increase in life
than the specimens shot peened at -320.degree. F. This would
suggest an ideal chilling temperature to obtain maximum increase in
life of somewhere between -70.degree. F. and -320.degree. F. for
this particular alloy.
The above tests also showed that the magnitude of the applied
stress dictates the degree of increase in fatigue life realized by
the present novel method. For example, operating at a maximum
stress level of 50 Ksi or 67 percent of the yield strength of the
aluminum, specimens shot peened at -70.degree. F. realized a 270
percent increase in fatigue life over specimens shot peened at
ambient temperature. It should be noted however, that, in practical
applications aluminum components are usually designed to operate at
stress levels considerably lower than 50 Ksi or 67 percent of the
yield strength employed as the low stress range in this test.
Micro hardness measurements on cross sections of the specimens
showed the shot peened surface to be strain hardened to a depth of
0.010 to 0.015 inch.
The present invention may be embodied in other specific forms
without departing from the spirit and essential characteristics
thereof; therefore, the present embodiments are to be considered as
illustrative only and not restrictive, the scope of the invention
being indicated by the appended claims rather than by the foregoing
description, and all changes which come within the meaning and
range of equivalency of the claims are therefore intended to be
embraced thereby.
* * * * *