Note: Descriptions are shown in the official language in which they were submitted.
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Description
METHOD AND APPARATUS FOR PROVIDING A LAYER OF
COMPRESSIVE RESIDUAL STRESS IN THE SURFACE OF A PART
Technical Field
This invention relates to a method and an apparatus of providing a
layer of compressive residual stress in the surface of a part and, more
particularly, to an improved and novel method and apparatus of shot
peening.
Background
Surface residual stresses are widely known to have a major effect
upon fatigue and stress corrosion performance of metallic parts. Residual
stresses, such as tensile residual stresses, add to the applied stresses
imposed on a part in service and can lead to more rapid fatigue or stress
corrosion failure. Compressive residual stresses have been shown to have
the effect of countering applied tension and have been used to generally
improve the life of a part by reducing its overall stress state and by
retarding
fatigue and stress corrosion crack initiation and growth. A variety of surface
enhancement methods, such as shot peening, gravity peening, laser
shocking, deep rolling, low plasticity burnishing, split sleeve cold expansion
and similar mechanical treatments, have been developed to induce a
beneficial layer of compressive residual stress along the surface of a part.
The depth and magnitude of such residual stress and diffraction peak
broadening distributions produced by such surface enhancement treatments
are typically measured using x-ray diffraction methods.
Shot peening has been commonly used in industry, particularly in the
automotive and aerospace industries, as the preferred method of inducing
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compressive stress in the surface of a part. During the shot peeving
process, metallic, glass, or ceramic pellets are projected, mechanically or
through air pressure, such that they impinge on the surface of a work piece.
The parameters used to shot peen the work piece are selected by
determining the time required to achieve a specified "Almen intensity" which
is determined from arc heights representing the deflection due to residual
stresses induced in a thin standard steel Almen strip. The "coverage" of the
shot peeving process is determined by examination of the surface of the
work piece at magnification to ensure that essentially the entire surface has
been impacted at least once by projected pellets. This condition of an
entirely impacted surface is defined to be 100% coverage and is achieved by
shot peeving using fixed peeving parameters in a measured time as
designated herein as 1T. For a given peeving apparatus and peeving
parameters (including shot size, hardness and flow rate), the shot peeving
processing time to achieve a fixed percent coverage is commonly taken as
proportional to the time required to achieve 100% coverage.
Until now, it has been believed that the surface of the work piece must
be essentially entirely impacted by shot (i.e. entirely covered by impact
craters or dimples) during the shot peeving process and shot to at least
100% coverage in order to achieve a consistent and desirable depth and
magnitude of residual compression. Indeed, many military and industrial shot
peeving standards recommend shot peeving to a minimum of 100%
coverage, and often require 125% to 200% coverage, in order to achieve
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reliable fatigue and stress corrosion life improvement. Most of the published
fatigue data supporting the 100% minimum coverage has been developed
using fully reversed axial loading or bending with a stress ratio
(R=Smin/Smax) of -1Ø
Unfortunately, it has been shown that such conventional shot peeving
induces a high degree of surface deformation and cold working which
increases with increasing shot peeving coverage. This relatively large
amount of cold working leaves the surface susceptible to rapid thermal
relaxation. Further, such cold working has also been found to increase the
yield strength of the surface and leaves the residual stress layer within the
surface susceptible to mechanical relaxation in the event of deformation
following shot peeving.
Accordingly, a need exists for a method for shot peeving the surface
of a part to induce a layer of residual compressive stress therein to improve
the part's fatigue and stress corrosion performance and also renders the
surface less susceptible to thermal and mechanical relaxation than parts
treated by convention shot peeving.
Disclosure of the Invention
The present invention is a new and novel method and apparatus of
providing a layer of compressive residual stress in the surface of a part and,
more particularly, provides an improved and novel method and apparatus of
shot peeving that induces a desired amount of residual compressive stress
within the surface of the part that is less susceptible to thermal and
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mechanical relaxation fihan that obtained with convention shot peening.
Further, the present invention is a new and novel method and apparatus of
shot peening that provides the required compressive residual stress
magnitude and depth as well as fatigue strength as provided by conventional
shot peening processes, but with reduced processing times and reduced cold
working.
In a preferred embodiment of the invention x-ray diffraction
determinations of residual stress and sine broadening measurements of cold
work are used to determine the minimal amount of coverage required to
achieve a desired depth and magnitude of compression with a minimal
amount of processing time and surface cold working.
In another preferred embodiment of the invention the novel method of
the present invention utilizes the steps of determining the depth and
magnitude of compressive residual stress and the percent of cold working by
x-ray diffraction for a range of shot peening coverage; developing the shot
peening parameters, including Almen intensity and coverage for a given shot
peening operation necessary to induce the desired compressive residual
stress and surface cold working; and determining the shot peening time
required to achieve the desired Almen intensity and coverage.
In another preferred embodiment of the invention, the shot peening
time required to achieve the desired coverage is determined using low
magnification optical examination of the surface.
In another preferred embodiment of the invention, the method includes
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using test coupons or actual components shot peeved with a range of
coverages from nominally less than about 10% to more than 100% to
determine the required shot size, hardness, and Almen intensity.
In another preferred embodiment of the present invention, the part is
5 shot peeved for a period of time necessary to produce the minimal percent
coverage for achieving the desired depth of compressive residual stress.
In another preferred embodiment of this invention the part is shot
peeved for the minimal amount of time needed to achieve the maximum
possible surface compressive residual stress.
In another preferred embodiment of this invention the part is shot
peeved for a minimal amount of time and coverage to minimize the amount
of surface and subsurface cold working to achieve a desired degree of
thermal stability.
In another preferred embodiment of this invention the coverage
employed during the shot peeving process is selected to achieve a desired
amount of cold working for achieving a given degree of thermal stability at a
given elevated temperature.
Another preferred embodiment of the invention is an apparatus
comprising means for projecting a plurality of pellets against a surface of a
part; means for controlling the amount of coverage; and means for optically
examining the surface of the part and means for taking residual stress and
line broadening measurements along the surface of the part.
In another preferred embodiment of the invention, the apparatus
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further comprises means for electronically storing said measurements.
In another preferred embodiment of the invention, the means for
taking residual stress and line broadening measurements along the surface
of the part comprises x-ray diffraction means.
Various objects and advantages of the invention will be apparent from
the following description, the accompanying drawings, and the appended
claims.
Brief Description of the Drawings
To provide a more complete understanding of the present invention
and further features and advantages thereof, reference is now made to the
following description taken in conjunction with the accompanying drawings, in
which:
FIG. 1 represents metal surfaces that have been peened to various
coverages;
FIG. 2 illustrates surface residual stress-depth distributions for various
coverage levels for shot peened 4340 steel plate before thermal exposure;
FIG. 3 illustrates surface percent cold work-depth distributions for
various coverage levels for shot peened 4340 steel plate;
FIG. 4 illustrates surface residual stress-depth distributions for various
coverage levels for shot peened 4340 steel plate after thermal exposure for
475°F (246°C)/24 hr.;
FIG. 5 illustrates cold work-depth distributions for various coverage
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levels for shot peeved 4340 steel plate after thermal exposure;
FIG. 6 illustrates bending fatigue lives at 1240 MPa (180 ksi), R = 0.1,
for electropolished, low stress ground and shot peeved 4340 steel plate
peeved to the coverage indicated;
FIG. 7 illustrates high-cycle fatigue results for shot peeved 4340 steel
plate, 38 HRC, at 20%, 100% and 300% coverage;
FIG. 8 illustrates surface residual stress-depth distributions for various
coverage levels for shot peeved IN718 plate before thermal exposure;
FIG. 9 illustrates surface percent cold work-depth distributions for
various coverage levels for shot peeved IN718 plate;
FIG. 10 illustrates surface residual stress-depth distributions for
various coverage levels for shot peeved IN718 plate after thermal exposure
for 525°C (977°F)/10 hr.;
FIG. 11 illustrates cold work-depth distributions for various coverage
levels for shot 525°C peeved IN718 plate after thermal exposure for
525°C
(977°F)/10 hr.;
FIG. 12 illustrates high-cycle fatigue results for shot peeved IN718
plate, 30 Hz, at 79.3%, 98% and 200% coverage; and
FIG.13 is a schematic representation of the apparatus of the present
invention for inducing a layer of compressive residual stress in the surface
of
a part.
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Best Mode for Carrying Out the Invention
The present invention is a new and novel method and apparatus for providing
a layer of compressive residual stress in the surface of a part and, more
particularly, to an improved and novel method of shot peeving that uses x-ray
diffraction residual stress and line broadening measurements of cold work to
determine the minimal amount of coverage required to achieve a desired
depth and magnitude of compression, such as that produced with 100%
coverage, with a minimal processing time and surface cold work.
The present method utilizes a method of determining the minimum
amount of shot peeving coverage necessary to achieve a desired depth and
magnitude of compressive residual stress with reduced surface cold work. It
has been unexpectedly found that essentially the same depth of the
compressive layer and even higher surface compression, can be obtained by
shot peeving a work piece to substantially less coverage with correspondingly
shorter processing times than obtained by conventional shot peeving. The
method of the present invention includes determining the minimum coverage
necessary for a part thus is reducing the time and cost of the shot peeving
process. By minimizing coverage, less cold working of the surface is
achieved by reducing the number of shot impacts. It has been found that
reducing the amount of cold working of the surface during the shot peeving
process improves the stability of the compressive layer at elevated
temperatures and reduces loss of compression due to mechanical overload
in the event of deformation in service.
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The invention can be better understood by reference to the following
illustrative examples. It should be understood that the method of the present
application may be used for any metallic material having a high enough
strength that fatigue and/or stress corrosion cracking would be of issue.
Accordingly, the examples are meant to illustrate the invention and not to
limit the scope of the invention in any way.
Example 1:
Example 1 is shown using aircraft quality 4340 steel plate (.5 in.
(1.27cm) thick) per AMS 6359F (Aerospace Material Specification, Society of
Automotive Engineers, United States, 1993). The material composition of
4340 steel is shown in Table 1.
Table 1
Steel Comaosition
C Mn P S Si Cr Ni Mo Fe
0.40 0.6~ 0.015 0.015 0.23 0.79 1.70 0.23 95.9
For peening trials, specimens of .5 in. (1.27 cm) thick and about 33 X 38 mm
(1.3 x 1.5 in.) were cut from the steel plate with the longer dimension
oriented
along the rolling direction. After hardening and tempering to 38 HRC
hardness, the specimens were reduced to 9.5 mm (0.375 in.) thickness by
low stress grinding. Tensile properties resulting from heat treatment were
1164 MPa (169 ksi) ultimate tensile strength and 1089 MPa (158 ksi) 0.2%
offset yield strength.
Example 2:
Example 2 is shown using nickel based super alloy IN718 plate (.5 in.
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(1.27cm) thick. The material composition of IN718 is shown in Table 2.
Table 2
5 _ IN718 Com ~osifiion
Ni Cu Fe Cb+Ta Mo Ti AI
53.0 18.0 18.0 5 3.0 1.0 0.5
For peening trials, specimens of .5 in. (1.27 cm) thick and about 33 X 38 mm
(1.3 x 1.5 in.) were cut of the IN718 plate with the longer dimension oriented
along the rolling direction. After solution treated and aged to 44 - 45 HRC
10 hardness, as typically done for use at elevated temperature high strength
applications, such as in engine applications, the specimens were then
reduced to 9.5 mm (0.375 in.) thickness by low stress grinding. Tensile
properties resulting from heat treatment were 1192 MPa (173 ksi) ultimate
tensile strength and 1433 MPa (208 ksi) 0.2% offset yield strength.
Peeving for both Example 1 and Example 2 were performed using
direct air pressure at 482 kPa (70 psi.) through a single 4.7 mm (3/16 in.)
diameter nozzle aligned to give an 80-degree incidence angle from
horizontal. Specimens were mounted on a rotary table running at 6 RPM at a
vertical distance of 305 mm (12 in.) from the nozzle outlet. Carbon steel
CCW14 conditioned cut wire shot was used at a controlled flow rate of 1.36
kg/min (3 Ib/min). The intensity achieved was 0.22 mm A (0.009 in. A).
Coverage was then determined by optical observation at 20X magnification.
The time to achieve 100% coverage was defined as the peeving exposure
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time at which essentially no undimpled areas remained in an approximately
2.5 cm (1.0 in.) square area in the center of the specimens. Undimpled
areas were easily observed using surface texture contrast between the
original ground surface and shot impacted areas. Fractional and multiple
coverages were taken as ratios of the time for 100% coverage.
As used herein, coverage is defined in terms of the fraction of area
impacted. Assessing coverage as the fraction of the area impacted using
optical examination is inherently subjective, but does include the effect of
the
work piece mechanical properties, and is the method adopted by most shot
peeving standards (Aerospace Material Specifications, AMS 2403L, AMS-S-
13165, Society of Automotive Engineers, United States 1992 and 1997;
Surface Vehicle Recommended Practice, SAE J443, Society ofAutomotive
Engineers, United States, 1984; Military Specifications, Shot Peeving of
Metal Parts, MIL-S-113165C, United States, 1989).
For the Examples, 100% coverage was achieved in 5.0 minutes
(intermitted peeving in the turn table) while only 2.0 minutes was required
for
saturation of the Almen strip under the same peeving conditions (A factor of
2.5 difference). To avoid ambiguity, the number of shot impacting the
sample per square mm at 100% coverage was quantified by direct
measurement of total collected shot as 336 shot/mm2. in the Examples, the
coverage calculated from the dimple diameter and total impacts (Abyaneh,
M., Kirk, D., "Fundamental Aspects of Shot Peeving Coverage Control, Part
Three: Coverage Control Versus Fatigue", ICSP6, pp. 456-463, 1996) was
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is
99.8%,
Residual stress measurements were conventionally made using x-ray
diffraction from the shift in diffraction peak position using Cr Ka radiation
(Prevey, P.S., Metals Handbook, ASM International, United States, 1986, v.
10, pp. 380-292; Hilley, M.E. ed., SAEJ784, 1971; Noyen, I.C. and Cohen,
J.B., Springer-Verleg, United States, NY, 1987). Subsurtace data were
conventionally obtained by alternately measuring the residual stress and then
electropolishing to remove surface layers. This process can be automated
using residual stress profiling apparatus such as disclosed in U.S. Patent No.
5,737,385. Residual stress measurements made as a function of depth from
the peened surface were corrected for relief resulting from layer removal and
for penetration of the x-ray beam into the subsurface stress gradient. An
irradiated area of nominally 5 x 5 mm (0.2 x 0.2 in.) was used for residual
stress measurement, providing the arithmetic average residual stress over
the area of an estimated 8400 shot impacts at 100% coverage.
Determinations of cold work resulting from peening were conventionally
made by relating diffraction peak breadths to the equivalent true plastic
strains (Prevey, P.S., "The Measurement of Subsurface residual Stress and
Cold Work Distributions in Nickel Base Alloys," ASM International, 1987, pp.
11- 19). This distribution of cold work as a function of depth was obtained
from diffraction peak breadth measurements and made simultaneously with
the residual stress measurements.
Following residual stress and cold work determinations, specimens
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used in Example 1 were thermally exposed at 246°C (475°F) for 24
hours to
simulate high temperature use typically encountered for steel. Specimens
used in Example 2 were thermally exposed at 525°C (977°F) for
100 hours to
allow relaxation such as typically encountered in an engine application.
Residual stress and cold work determinations were then repeated to
determine if thermally induced relaxation had incurred.
Fatigue testing in four-point bending mode was conducted, at room
temperature (22°C) for Example 1 and at 525°C (977°F) for
Example 2,
under constant load amplitude sinusoidal loading at 30 Hz and stress ratio, R
= Smin / Smax, of 0.1. The R-ratio was chosen to avoid compressive overload
and the resulting immediate reduction of the compression introduced by shot
peeving. Bending fatigue specimens were machined with a trapezoidal cross
section to ensure fatigue failure from the peeved surfaces. The specimen
geometry and test fixturing provided a nominally 1.25 cm (0.5 in.) wide by
2.54 cm (1.0 in.) long surface area under uniform applied stress. The central
gage sections of fatigue specimen test surfaces were finished by low stress
grinding and peeving using the same techniques as for specimens in the
peeving coverage trials.
Example 1 Results:
Referring to FIG.1, representative metal surfaces are shown that have
been peeved, as described above, to various coverages. Defined coverage
was based upon the time ratio to achieve 100% dimpling of the surface area.
It should be apparent to those skilled in the art that the percent of area
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covered at 80% (0.8T) coverage approached that of 100% (1T) coverage. As
shown, the arrow in the photograph for 0.8T identifies a relatively small
undimpled area visible when viewed optically at 20X magnification. The
undimpled areas of the specimens peened for less than 0.8T are obvious in
appearance. The overall appearance of surfaces peened for times, 2T and
4T, did not change relative to that peened for time T.
FIG. 2 illustrates the residual stress-depth distributions that were
obtained in the example for the various coverage levels, including the
distribution for the as-ground surface before peening. Except at the lowest
coverage level, 3% (0.03T), classical shot peening distributions resulted,
whereby residual compressive stress magnitudes reached a subsurface
maximum and decreased gradually until small tensile stresses occurred at
greater depths. For 3% coverage levels, the maximum compression is
shown to have occurred at the upper surface, or at a very slight depth below
the upper surface. The form of the subsurface residual stress distribution for
a 3% coverage level was shown to conform to finite element models of the
stress developed in regions between dimples when impact areas are widely
separated by twice the dimple radius (Mequid, S.A., Shagal, G. and Stranart,
J.C., Analysis of Peening of Strain-Rate Sensitive Materials Using Multiple
Impingement Model, Int. J, oflmpactEng., 27 (2002) 119 -134). Since x-ray
diffraction results provide an average stress over mostly un-impacted
material at the 3% coverage level, it would be apparent to one skilled in the
art that the data confirms the FE prediction that even the regions between
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impacts are in compression. The residual stress distributions for coverage
levels less than 20% (0.2T) exhibited systematic changes with coverage,
whereby increasing coverage in this range resulted in increasing compressive
stress magnitude at given subsurface depths and an increase in the total
5 depth of compression. Beyond a 20% coverage level, there were no further
significant changes in stress magnitude at a given depth, other than at the
surface, or in total depth of compression. Surprisingly, compression at the
surFace tended to decrease with increasing coverage above 20%.
Referring to FIG. 3, cold work-depth distributions produced at various
10 coverage levels of the example are shown. Consistent with residual stress-
depth distributions, systematic changes in cold work-depth distributions
occurred wifih increasing coverage levels up to 20% (0.02T). Beyond that
level, no systematic changes occurred with increasing coverage. Cold work
values for the lower coverage levels were lower than at higher coverages
15 only to a depth of about 0.05 mm (0.002 in.).
Referring to FIGS. 4 and 5, residual stress and cold work-depth
distributions obtained after thermal exposure at 246°C (475°F)
for 24 hours
are shown. The exposure temperature was chosen based upon specification
AMS 13165 (Aerospace Material Specification, AMS-S-13165, Society of
Automotive Engineers, United States, 1997) regarding maximum
recommended exposure temperature to avoid residual stress relaxation in
shot peened steels. Comparison with pre-exposed results (FIGS. 2 and 3)
revealed changes in both residual stress magnitudes and cold work.
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Relaxation of both residual stress and cold work occurred at depths less than
0.05mm (0.002in.) with the greatest percent changes occurring in surface
values. Reduction of surface residual stress magnitudes ranged from 20 -
30%, and percent reduction of surface cold work ranged from 40 - 70%.
There was no systematic trend with coverage in these reductions although
the reductions decreased with depth from the surface, and initial cold work
level, to about 0.05 mm (0.002 in.) for all coverage levels. Beyond 0.05 mm
depth, where the initial cold work level was less than nominally 5%, there
were no significant changes in residual stress or cold work.
IO If should now be apparent to one skilled in the art that cold work from
shot peeving, even at less than 100% coverage, is sufficient to induce
significant residual stress relaxation in surface and near surface layers at
modest temperatures. Accordingly, where such reduction cannot be
tolerated, surface enhancement techniques, such as low plasticity burnishing,
laser shock, or coverage controlled shot peeving to provide adequate
compression with minimum or controlled levels of cold working may be used.
FIG. 6 shows the example results of limited initial fatigue testing.
Significant surface and near surface compressive residual stresses were
associated with the low stress ground condition. As shown, fatigue life for
this condition was intermediate between lives for peeved specimens and the
electro-polished specimen ("ELP"), which had no residual stresses. Optical
fractography revealed that subsurface fatigue origins occurred in all peeved
specimens and in the low stress ground specimen. No crack initiation sites in
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peened specimens were associated with undimpled surface areas
irrespective of coverage. Therefore, the undimpled surface areas appear to
be in compression. These results indicate the beneficial effect of peening
relative to unpeened conditions. Unexpectedly, it has been found that for
R>0 loading, the full benefit from peening can be realized at less than 100%
coverage. This result is particularly unexpected in view of conventional
teaching that fatigue life will decrease dramatically when coverage drops
below 100%.
S-N curves for a range of coverage were prepared to verify the
unexpected finding that uniform fatigue strength is independent of coverage.
Because the residual stress depths and magnitudes were found to be
comparable for any coverage greater than 20%, samples were prepared with
20%, 100% and 300% coverage levels. The fatigue results, as shown in FIG.
7, surprisingly indicated that there is no loss of fatigue life or strength
for
coverage as low as 20%. It was found that the fatigue performances for 20%
and 100% coverage levels are essentially equal given the experimental
uncertainty for the limited number of samples tested. Testing also showed
that a coverage level of 300% will produce consistently shorter life and a
slightly lower endurance limit than coverage levels of either 100% or 20%.
When fatigue testing of shot peened surfaces is conducted in fully
reversed loading, (R = -1.0), the compressive half-cycle superimposes a
compressive applied stress on the already highly compressive shot peened
surface. The compressive surface then yields in the first few cycles of
testing
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resulting in rapid relaxation of the compressive surface layer. Surface
residual stress measurements after fatigue testing revealed that even at
alternating stress levels below the residual stress-free material endurance
limit, the surface compressive stress can be reduced to 70% of the original
level in the first half-cycle in fully reversed loading. Residual stress
measurements on failed samples showed no significant change in surface
compression after 130 and 220 x 103 cycles at R = 0.1 and Sma~ of 1240 MPa
(180 ksi) for either the 100% or 20% coverage samples, respectively.
The tests performed have demonstrated that complete coverage of a
workpiece is not required to produce full benefits of shot peening in 4340
steel, 38 HRC, peened to 0.22mm (0.009 in) intensity when fatigue tested in
tension-tension loading (R = 0.1 ). A coverage level of as little as 20%
(0.2T)
provided fatigue performance equivalent to full coverage under conditions
employed in the examples.
Example 2 Results:
FIG. 8 illustrates the residual stress-depth distributions that were
obtained in the IN718 example for the various coverage levels. As shown
with Example 1, except at the lowest coverage level, 5% (0.03T), classical
shot peening distributions resulted, whereby residual compressive stress
magnitudes reached a subsun'ace maximum and decreased gradually until
small tensile stresses occurred at greater depths. For 5% coverage levels,
the maximum compression is shown to have occurred at the upper surface.
As in Example 1, since x-ray diffraction results provide an average stress
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over mostly un-impacted material at the 5% coverage level, it would be
apparent to one skilled in the art that even the regions between impacts are
in compression. The residual stress distributions for coverage levels less
than about 10% (0.1 T) exhibited systematic changes with coverage, whereby
increasing coverage in this range resulted in increasing compressive stress
magnitude at given subsurface depths and an increase in the total depth of
compression. Beyond a 10% coverage level, there were no further significant
changes in stress magnitude at a given depth, other than at the surface, or in
total depth of compression.
Referring to FIG. 9, cold work-depth distributions produced at various
coverage levels of the example are shown. As in Example 1, systematic
changes in cold work-depth distributions to have occurred with increasing
coverage levels up to 20% (0.02T). Beyond that level, no significant
systematic changes occurred with increasing coverage
Referring to FIGS. 10 and 11, residual stress and cold work-depth
distributions obtained after thermal exposure at 525°C (977°F)
for 10 hours
are shown. The exposure temperature was chosen to simulate typical high
temperature applications, such as in engine applications, often encountered
with parts formed from IN718 metal. As shown, higher surface compression,
nearly equal depth to 100% and with excellent thermal stability, can be
obtained with just 10% coverage.
As previously shown, it should now be apparent to one skilled in the
art that cold work from shot peening, even at less than 100% coverage, is
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sufficient to induce significant residual stress relaxation in surFace and
near
surface layers at relatively modest temperatures. Accordingly, where such
reduction cannot be tolerated, surface enhancement techniques, such as low
plasticity burnishing, laser shock, or coverage controlled shot peeving to
5 provide adequate compression with minimum or controlled levels of cold
working may be used.
It should alto be apparent that the method of the present invention
can be used for a variety of parts including nickel based super alloy turbine
blades, disks, and other parts that typically operate in hot environments.
10 FIG. 12 shows the example results of high cycle fatigue testing for
peeving times of about .4T, 1T and 2T needed for 79%, 98% and 100%
coverage, respectively. As shown, the performance trends obtained for
IN718 are substantially the same and indeed show better results than that
demonstrated for the 4340 steel of Example 1 (FIG. 7).
15 Accordingly, it has been unexpectedly found during studies of the
residual stress and cold work distributions produced by different amounts of
coverage on a variety of steel, nickel, titanium, and aluminum alloys, that
the
depth and magnitude of compression generally attributed to 100% coverage
can be achieved with as little as about 20% coverage in some alloys. The
20 depth and magnitude of compression produced by 100% coverage can be
essentially equaled by shot peeving to much lower coverage. It has also
been found that the maximum surface residual stress may be achieved at
less than 100% coverage.
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It should now be understood that the method of the present invention
provides benefits over conventional shot peening particularly in applications
where compressive overload occurs. Further, shot peening to only the
reduced coverage required to achieve the necessary compression provides a
means of substantially reducing the time and therefore the cost of the shot
peening process. An additional benefit of the reduced coverage shot
peening is less cold working of the surface during processing which is known
to improve both the thermal and mechanical stability of the compressive
residual stresses developed. This may be easily accomplished by using
larger shot than typically used when 100% coverage is required. Such use of
larger shot will provide deeper compression and reduced cold work without
loss of fatigue performance as well as improved surface finish. As previously
stated, reducing cold working will also provide improved thermal stability of
the induced compressive layer.
The method of this invention therefore provides a means of
determining the minimal percent coverage required to optimize the
compressive residual stress distribution produced while minimizing the
amount of cold working and the time and cost of processing.
The novel method of the present invention utilizes the steps of
determining the depth and magnitude of compressive residual stress and the
percent cold work, preferably by x-ray diffraction, for a range of shot
peening
coverage; developing the shot peening parameters, including Almen intensity
and coverage for a given shot peening application; and determining the shot
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peeving time required to achieve 100% coverage.
In a preferred embodiment of the invention, the method can include
the step of using test coupons or actual components shot peeved with a
range of coverages, from less than about 10% to more than 100% using the
shot peeving apparatus, shot size, shot hardness, and Almen intensity that
will be employed during the production process. It has been found that a
logarithmic progression of coverage levels, such as 5%, 10%, 20%, 40%,
80%, 100%, 200% and 400% is suitable.
In another preferred embodiment of the invention, the method
comprises the step of using x-ray diffraction monitoring of residual stress
and
cold work through diffraction peak broadening to determine the optimal
coverage for a given material, shot peeving size and intensity, and
application.
In another preferred embodiment of the present invention the method
further includes the step of inducing a layer of compressive stresses in the
surface of the part by shot peeving the surface for a period of time to
produce the minimal percent coverage necessary to achieve the depth of
compressive residual stress required.
In another preferred embodiment of this invention the method includes
the step of controlling the time of shot peeving and coverage to the minimum
time needed to achieve the maximum possible surface compressive residual
stress.
In another preferred embodiment of this invention the method includes
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the step of controlling the amount of coverage needed to achieve a minimum
amount of surface and subsurface cold working to achieve a desired degree
of thermal stability.
In another preferred embodiment ofthis invention the method includes
the step of controlling the amount of coverage to produce not more than a
certain amount of cold working in order to achieve a given degree of thermal
stability at a given elevated temperature.
Referring to FIG. 13 an apparatus 100 for performing the method of
the invention is show comprising a projection means 102 for projecting a
plurality of pellets 104 against a surface 106 of a work piece 108; means 110
for controlling the time and coverage of the pellets 104, optical means 112
for
optically examining the surface 106 of the work piece 108 and; measurement
means 114 for taking residual stress and line broadening measurements
along the surface 106 of the work piece 108. As schematically illustrated, the
projection means 102 is preferably mounted to a conventional positioning
device 116 for properly positioning the projection means 102 to direct the
pellets 104 against the surface 106 of the work piece 108. As previously
discussed herein, the size and the material comprising the pellets 104, the
force by which the pellets 104 are projected, and the amount of coverage will
depend on the material forming the work piece 108 and the final application
of the part and the desired penetration of the residual compressive stress
induced therein. The size and material comprising the pellets 104, the
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projecting force, and the amount of coverage will also depend on the desired
penetration of residual compressive strength and on the material
composition, material properties, and dimensions of the work piece 108 and
the application of the final part.
The apparatus 100 of the present invention can be manually or
automatically operated. As schematically illustrated, the apparatus 100 can
include a controller 118 for automatically controlling the positioning device
116 and, thus, the direction and velocity of the pellets 104. The controller
118 can include a microprocessor, such as a computer operafiing under
computer software control. In one embodiment, the positioning device 116
includes belt andlor gear drive assemblies (not shown) powered by
servomotors (not shown), as is known in the art. The controller 118 can be in
operable communication with the servomotors of the positioning device 116
through suitable wiring (not shown).
One or more sensors (not shown), including, but not limited to, linear
variable differential transformers or laser, capacitive, inductive, or
ultrasonic
displacement sensors, which are in electrical communication with the
controller 118 through suitable wiring, can be used to measure the spacing
and angle of the projection means 102 above the surface of the work piece
108, and, thus, the motion of the projection 102. Similarly, shaft encoders in
servo systems, stepper motor drives, linear variable differential
transformers,
or resistive or optical positioning sensors can be used to determine the
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position and projection angle of the tool along the surface 106 of the work
piece 108. When inducing compressive residual stress along the surface
106 of a work piece 108, the work piece 108 is preferably secured to a work
table by means of a clamp or similar device. The apparatus 100 is positioned
5 relative to the work piece 108 such that the projection means 102 is
positioned above to the surface 106 of the workpiece 108. The projection
means 102 projects pellets 104 against the surface 106 of a work piece 108
to achieve the desired coverage and induce a layer of compression within the
surface 106. According to another embodiment (not shown), the projection
10 means 102 is fixed and the work piece 108 which is moved relative to the
projection means 102.
According to another embodiment of the present invention, the
measurement means 114 is an x-ray diffraction means. As previously
disclosed conventional x-ray diffraction techniques are used to analyze the
15 surface 106 of the work piece 108 to determine a desired coverage,
penetration depth, as well as the amount of cold working and surface
hardening necessary to optimize the material properties of the work piece
108. The x-ray diffraction means also operates to take residual stress and
line broadening measurements along the surface of the work piece. The
20 measurement means 114 is in electrical communication with the controller
118 and operates to relay information to the controller 118 for controlling
the
projection means 102.
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In another preferred embodiment of the invention, the apparatus 100
further comprises memory means 120 that is in electronic communication
with the optical means 112 and/or the measurement means 114 and/or the
positioning device 116 for storing measurement information.
It should now be understood to those skilled in the art that the present
method and apparatus provides a means for implementing a controlled shot
peeving method to achieve the desired magnitude and depth of compression
with minimal cold working of the surface and with a minimal amount of
processing time and cost. The method also permits determination of the
minimal percent coverage required to produce the desired depth and
magnitude of residual compression and minimal cold work for a given
component, material, geometry, and application.
Accordingly, while the method and apparatus described constitutes
preferred embodiment of the inventions, it is understood that the invention is
not limited to the precise method and that changes may be made therein
without departing from the scope of the invention which is defined in the
appended claims.
The method of the subject invention further provides a novel and
effective means of reducing the coverage required during conventional shot
peeving while retaining the beneficial depth and magnitude of compression
and the corresponding benefits of improved fatigue life and reduced stress
corrosion cracking. By minimizing the coverage, the time and therefore cost
of shot peeving processing of components can be reduced to a fraction of
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the current practice of using at least 100% coverage. It has been
unexpectedly found, that the shot peeving coverage can be reduced to the
minimum amount that still provides essentially the same residual stress depth
and magnitude as 100% coverage, as determined by x-ray diffraction
measurement.
It has also been found that reduction of coverage to this minimal level
does not reduce the fatigue performance of shot peeved of steels, such as
4340 steel, and also improves the retention of compression at elevated
temperatures for the nickel-based alloys, such as super alloy IN713.
Accordingly, contrary to the current practice and teaching, the method of the
subject invention produces a compressive layer of residual stress in the
surface of a work piece while deliberately minimizing the cold working and
the time and cost of such processing without degrading fatigue performance.
It should be understood to those skilled in the art that while the
invention describes a process of shot peeving, the method and apparatus
described herein may also be utilized with other similar processes, such as
gravity, ultrasonic, and needle peeving.
As previously described, the apparatus for performing the method of
the invention provides means for projecting a plurality of pellets against a
surface of a part; means for controlling the time and coverage of the pellets,
means for optically examining the surface of the part; and means for taking
residual stress and line broadening measurements along the surface of the
part.
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In another preferred embodiment of the invention, the apparatus
further comprises means for storing said measurements.
In another preferred embodiment of the invention, the means for
taking residual stress and line broadening measurements along the surface
of the part comprises x-ray diffraction means.
It should now be understood to those skilled in the art that the present
method and apparatus provides a means for implementing a controlled shot
peening method to achieve the desired magnitude and depth of compression
with minimal cold working of the surtace and with a minimal amount of
processing time and cost. The method and apparatus of the present
invention also permits determination of the minimal percent coverage
required to produce the desired depth and magnitude of residual
compression and minimal cold work for a given component, material,
geometry, and application.
It should also be understood that the method and apparatus of the
present application can be utilized for a variety of applications,
particularly for
applications where components are subject to shot peening damage.
Applications include parts having laps or folds that may lead to fatigue
initiation, such as edges of bolt holes and bores that typically get
excessively
peened from multiple directions, nickel base alloy turbine disks and titanium
alloy compressor and fan disks. In addition, applications may include those
that are typically time and cost prohibited to shot peen to 100% coverage,
such as automotive applications like connecting rods and rocker arms. The
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method and apparatus of the present application may also be used for
applications where the use of large shot would provide deeper compression
but 100% coverage would be time and cost prohibited or for applications
where lower cold work provides lower generalized corrosion rates while still
producing the compression required to reduce or eliminate stress corrosion
cracking. Such applications include, but are not limited to, nuclear
weldments, steam generator U-bends, and similar piping and welds. It
should be understood however, that the method and apparatus of the
present application are not limited to the above described applications.
Although this invention has been primarily described in terms of
specific examples and embodiments thereof, it is evident that the foregoing
description will suggest many alternatives, modifications, and variations to
those of ordinary skill in the art. Accordingly, the appended claims are
intended to embrace as being within the spirit and scope of the invention, all
such alternatives, modifications, and variations.
What is claimed is:
