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| More
recently material specifications have changed dramatically and
peening is now frequently specified to greatly extend the safety
margin in highly stressed aircraft components or to allow the use
of lighter, less robust components in high performance engines.
Many other applications exist and the list extends readily. |
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| Vapormatt
Turbine Blade Peening Machine & turbine blade
component. (left). |
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| What Is Peening
? |
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| Peening is
the technique which changes the characteristic of a metal surface
to increase its' fatigue strength, it is achieved by bombarding the
surface with a high velocity stream of pre-selected round
balls. This produces a permanent stretching of the surface
causing a plastic flow of surface fibres. |
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| The crystalline
grains are re-oriented to a shallow depth which resists flow
fracture, since the slightly compressed layer is somewhat stronger
than the material below this zone. |
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| Metal
samples showing increased peening effect. |
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| Fibres beneath the
top layer retain elasticity and the resulting equilibrium leaves
the surface in compression and the lower levels in tension. The
compressed layer extends 0.005 to 0.010 inch below the surface. |
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| Since
fatigue failures generally originate from tension stress and not
from compression stress the effect is a considerably greater
fatigue strength. |
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| Peening
also super-imposes a random surface texture over the original
surface which is usually directional (i.e. directional machining
marks or polishing scratches, etc.). Surface cracks are less
likely to propagate along a random texture. |
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| Uses of Peening |
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| Any part
subject to twisting or bending stress is likely to benefit from
peening. The process is ideal when applied to irregular
shapes where heat treatments may cause distortion. |
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| Peening
is now frequently used as an alternative to polishing where
directional scratches can propagate minute cracks. It is
ideal for the treatment of fillets, grooves or unsupported edges
or anywhere where sudden changes in form occur. |
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| (Such changes
are commonly noted as fatigue fracture causes). |
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| Auto
gear processing machine. (above). |
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| TYPICAL
PEENING APPLICATIONS |
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Turbine
disc root peening. |
General
peening of complete springs. |
Turbine
blade peening application. |
Gear
teeth peening application. |
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Hole
surrounds, bores. |
Internal
and external corners. |
Internal
and external radii. |
Threads,
grooves, application. |
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| Over-Peening |
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| Over-peening
can weaken or damage a component either by so over-stretching that
its durability is exhausted (when surface splitting or cracking
may become evident) or by erosion of surface material to
unacceptable levels. Peening operations should always be
controlled and monitored to a precise specification. |
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| Specification
Of Peening Levels |
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| The degree
of peening can be measured and is specified as Arc Height
or Deflection. (See also "Peening
Techniques" below). |
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| The process
specified will take into account the shape, nature and duty of the
component and will be used either to eliminate failure in existing
designs, to allow components to be used more safely, to allow
component performance increases, or to permit size and weight
reduction for new designs. |
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| Measurement
and test will establish the ball size, its' material make-up,
operating velocity and coverage. |
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| Light
peening using micron sized glass balls at relatively low pressures
may be specified for light alloys, delicate components or for low
levels of induced stress. |
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| Heavy peening using large steel
balls at high velocity would be selected for steel castings,
forgings and similar rugged subjects. |
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| Specifications
usually arise from a combination of theory, previous experience
and component testing. |
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| Barrel
processing machine. (above). |
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| Peening Techniques |
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| It has been
stated that "peening, even carried out in an uncontrolled and
indifferent manner, will probably have some good
effect." However since the reliance of peening is often
of great importance it is necessary to control its' application to
strict limits. The factors influencing good techniques are
:- |
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| 1).
Intensity of peening. |
| 2).
Angle of impact. |
| 3).
Standard of peening media. |
| 4).
The achieved Arc Height or Deflection. |
| 5).
Coverage of the component. |
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1 |
Peening
Intensity |
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Literally the
intensity of the blast which is the combined factors of
velocity, hardness and the condition of the media or shot. |
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| 2 |
Angle of
Impact |
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Usually the aim
is to achieve an angle of 90 degrees to the component
surface. This is obviously not always possible due
to component shape and the fact that the media is not
usually ejected at a single angle. However peening
is most effective at 90 degrees and that ideal should be
the aim. |
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| 3 |
Standard
Of Media |
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PEENING
MEDIAS : |
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Stainless
Steel Shot |
Ceramic
/ Zirconia Beads |
Glass
Beads |
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Specifications
cover size, hardness and shape. Separation systems
become all important in maintaining these standards and so
will be required to eliminate dust, metal fragments,
broken or misshapen shot and shot which falls below
specified size range. |
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| In dry systems,
separation relies mainly on providing an expansion chamber
in which air velocity falls to a level at which full sized
shot will fail to be recirculated, while other matter
including undersize broken material is carried into a
collection hopper or bag. |
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| More sophisticated systems
cater for roundness (by using spiral separators) and size
(by the use of vibratory screening). |
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Vibratory sieve for media grading. |
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In wet systems,
glass bead conditioning uses similar techniques so that by
slowing down the water flow the larger beads settle, whilst
dust and debris (including oil and grease on occasions) is
"floated" off using weirs or positive
filters. The detrimental effect of worn or even misshapen beads in a wet system is much less
disadvantageous due to the "cushioning" effect
of water. Excellent light peening without damage to
fragile components can be undertaken using glass beads in
a wet system. |
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| 4 |
Arc Height
(Deflection) |
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A measuring
system - now used internationally - was introduced by J.O.
Almen and consists of the use of test strips produced to a
very tight specification for thickness, flatness and
hardness. Three types are used ; "A" and
"C" for varying degrees of shot peening, and
"N" specifically introduced for lighter duty
glass bead peening ; both wet and dry. |
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The
specifications are :- |
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Type |
Thickness |
Flatness |
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N |
0.031
+/- 0.001 inches. |
+/-
0.001 inches. |
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A |
0.054
+/- 0.001 inches. |
+/-
0.001 inches. |
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C |
0.094
+/- 0.001 inches. |
+/-
0.001 inches. |
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S.A.E. 1070 cold
rolled spring steel, 3 inches long x 0.75 inches wide,
uniform hardness Rockwell C44-50. |
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| During blasting
the strips are retained on a metal block by four set
screws. For large complicated components it may be
necessary to fix a series of blocks to record arc heights
on various planes, at different angles and on curved
surfaces. During production peening, strips should
be processed at regular intervals, measured and retained for
record purposes. |
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| Deflection
or
intensity is expressed as the measured curvature as shown
on the gauge, followed by the strip type "A",
"C", or "N". For example, 0.006
gauge reading on an "N" strip would be shown as
0.006N or 6N. 0.009 on an "A" strip as
0.009A or 9A. |
| The
gauge used for measuring
curvature consists of a
clock gauge calibrated in
0.001 inch
increments. the
spindle passes through the
base onto which are
mounted magnetic contracts
to hold the |
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| almen
strip firmly in
position.
With an un-used
Almen strip held
by the magnets the
gauge clock is set
to zero.
After peening the
strip is replaced
so that the
spindle registers
the deflection
from flatness
(curvature) as a
direct reading
taken from the
original
zero. The concave
face is used so as
to eliminate
variations caused
by surface
roughness. |
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Turbine
blade showing almen strip
holders. (above). |
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almen strip.
(above). |
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| 5 |
Coverage
(Saturation) |
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This
relates to the time of exposure of a component to a given
blast stream. A number of test strips can be peened
under a constant blast stream for different exposure
times. A graph of the achieved deflections can then
be produced as below :- |
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The
graph shows that short initial exposure times show a
relatively sharp increase in arc height and that as
exposure times extend the increase in arc height reduces
to become constant (saturation). |
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Checking
Coverage |
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To
ensure adequate coverage of a component, its' peening
program should be specified and peening should then take
place together with appropriate Almen strips.
When the operation is complete, the arc heights should be
recorded and the programme repeated for double the
exposure time. The arc heights achieved should,
when compared to the originals, indicate a less than 20%
increase. This being so, then the original
coverage is considered adequate and intensity is being
achieved. |
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| Good
Peening Practice |
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| The action
of peening is rapid and when correctly applied will ensure that
arc height is reached very quickly. It should always be the
aim to achieve this situation by avoiding the use of shot or beads
of too small a diameter, setting pressures too low and operating
with excessive gun to work distances :- |
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| Continued
increase of arc height usually indicates faulty setup. The
aim should be to gain a relatively low increase in arc height
after the "flattening off". |
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| The
condition of peening media is a vital factor in good peening
practice. Careful check of surface finish can play a part in
monitoring media since worn or broken spheres tend to produce a
duller, matt finish than would be produced by good spheres.
Protection of the surface and particularly of threads, sharp edges
and shoulders is greatly enhanced in wet systems where harsh
impact of bead to component is cushioned by water. |
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| Peening
Processes And Equipment |
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Metallic
shot (steel balls) constitute the most widely used peening media
when high intensity "A" and "C" readings are
sought. Centrifugal wheel throwing and air operated machines
are available each with separation, grading and dust control. |
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Over-peening
is a danger and care must also be taken to ensure that
contamination (scale, oxides, fragments of shot, etc.) is not
embedded in the component surface. Use of steel shot on
non-ferrous metals can result in cross-contamination. It is
now possible to achieve "A" and "C" strip
levels in wet blast machines using ceramic or stainless steel
balls. |
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Dry air
blast systems use non-metallic beads for medium operations and the
sophisticated range of equipment available includes monitoring
devices for media control, separation of broken beads and a wide
range of dust control units. Dust can be a problem and the
component must be dry and free of oil or grease before blasting. |
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| To provide
an environmentally improved system completely free of dust, a
highly controllable process and one in which even the most
delicate aerofoil or similar surfaces can be treated safely, wet
blast equipment has been extensively developed. |
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| Although the
wet process is ideal for use with aluminium or light alloys it may
be used with ceramic or stainless steel beads to produce
"A" and "C" levels of intensity. |
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| No prior
degreasing or cleaning of the component is necessary and a very
smooth finish can be maintained. Finishes are significantly
better than those achieved in dry processes for the same intensity
and can often eliminate the need for subsequent finishing
operations. |
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| Pumps are
now able to feed a series of blast nozzles with |
| a consistent and
high concentration of blast media to |
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| Auto
turbine blade processing machine. |
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water. Fine control is
possible and a wide variety of peening levels can be obtained from
equipment adjustment and without the need to constantly change
media size or type. |
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Centrifugal
separation is used to remove waste material and broken
beads. The cut-off point of cyclones can be adjustable to
reduce the unwanted material to less than 10% of the entire
charge. Automatic replacement by measured addition of new
beads is also available. |
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For very
critical work it is possible to monitor the solids to water ratio
of the flow from pump to guns and to use the signal so received to
activate automatic glass bead top up. |
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| Auto
turbine disc processing machine. (above). |
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| Wet blast
machines, like their dry and shot blast counterparts, may be used
in manual, semi or fully automatic forms to suit application and
work rate. |
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| The current range of Vapormatt peening equipment
includes fully programmable CNC machines with 3 axis gun movement
and worktable positioning to an accuracy of +/- 0.5 degrees. |
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| Why
Vapormatt And Wet Peening ? |
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| 1 |
Vapormatt
Wet Peening To Avoid Dust |
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Dust is at least
as serious a problem in peening as it is in cleaning. Many
peened components have to be cleaned after dry blasting to
remove embedded dust. |
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| 2 |
Vapormatt
Wet Peening Reduces Glass Bead Consumption |
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Bead breakdown
is reduced by the cushioning effect of water. This
means reduced operating costs, reduced risk of abrading
and enables peening specifications to be maintained over
longer periods. |
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| 3 |
Vapormatt
Wet Peening For Superior Finishes |
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Better, cleaner
and brighter finishes are achieved by wet blasting at the
same peening intensity. This frequently avoids the
need for a subsequent cleaning or finishing operation. |
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| 4 |
Vapormatt
Wet Peening To Avoid Prior Degreasing |
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If a component
requiring peening is contaminated with oil, grease, wax,
release agents, resins, dyes, etc. it may be simultaneously
cleaned, degreased and peened. |
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| 5 |
Vapormatt
Wet Peening An Alternative To Conventional Steel Shot
Peening |
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Dry shot peening
using steel shot on light alloys, stainless steels or
titanium can result in ferrous cross contamination.
Use of ceramic beads or stainless steel shot in a
Vapormatt Machine is a viable alternative. Relatively
high "A" and "C" levels and all
"N" levels are attainable. |
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| 6 |
Vapormatt
Wet Peening For Delicate Control And Consistent Results |
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Variations of a
tenth of a peening point can be consistently achieved
because wet blasting is not reliant on compressed air for
producing a rich consistent media flow. The air
supply is available entirely for control purposes. |
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| 7 |
Vapormatt
Wet Peening For Environmental And Health Reasons |
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Abrasive dust
from any dry system will inevitably cause some atmospheric
pollution. The dust is abrasive and can damage fine
instruments or machine tools as well as creating an
unpleasant atmosphere in which operatives have to work
(and breathe). |
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Author
: G. Wallis. |
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©
Vapormatt Limited |
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Connecting
Rods
