INTRODUCTION
SHOT PEENING is a method of cold working in which compressive stresses
are induced in the exposed surface layers of metallic parts by the impingement
of a stream of shot, directed at the metal surface at high velocity under
controlled conditions. It differs from blast cleaning in primary purpose and in
the extent to which it is controlled to yield accurate and reproducible
results. Although shot peening cleans the surface being peened, this function
is incidental. The major purpose of shot peening is to increase fatigue
strength. The process has other useful applications, such as relieving tensile
stresses that contribute to stress-corrosion cracking, forming and
straightening of metal parts, and testing the adhesion of silver plate on
steel.
The
technique consists of propelling at high speed small beads of steel, cast iron.
Glass or cut wire against the part to be treated. The size of the beads can
vary from 0.1 to 1.3 or even 2mm. The shot is blasted under conditions which
must be totally controlled.
The main
advantage of this particular surface treatment is that it increases
considerably the fatigue life of mechanical parts subjected to dynamic
stresses. It has many uses in industry, particularly in the manufacture of
parts as different as helical springs, rockers, welded joints, aircraft parts,
transmission shafts, torsion bars, etc.
At a time
when the optimum characteristics are being demanded of mechanical assemblies,
shot-peening is a surface treatment method which is being increasingly chosen
by engineers. However, shot-peening technology is yet to be fully perfected and
the substantial changes produced in the treated material make it difficult at
the present time to put the best conditions into practical use.
SCIENTIFIC FUNDAMENTALS
2.1 DEFINITION
Shot
peening is a cold working process wherein a steam of shots are propelled at a high velocity onto the surface to
be treated under controlled conditions. The process is applied on engineering components to produce
a compressive residual stress on a layer of material in the surface and
subsurface regions in order to enhance resistance to metal fatigue and some forms of stress
corrosion.
2.2 PROCESS
During
the shot peening process, a stream of shots is propelled at a high velocity
onto the surface to be treated under controlled conditions. Each piece of shot
that strikes the material acts as a tiny peening hammer, creating a small
indentation or dimple on the surface. Sufficient energy is needed to create the
dimple, as the surface of the material must yield in tension. As the dimples
overlap with random impacts, the entire surface is effectively elongated,
driving the surface layer of deformed material into compression.
2.3 MECHANISM
The compressive layer is formed by a combination of surface and
subsurface compression developed by Hertzian loading combined with
lateral displacement of the surface material around each of the formed dimples.
Sufficient intensity causes plastic flow of surface metal at the instant of
contact, stretching it radially. The metal beneath this layer is stressed but not plastically
deformed. Thus, the large volume of material below the surface, which is in an
elastic state, tries to recover or regain its original shape, thereby
producing, below the dimple, a hemisphere of cold-worked material highly stressed
in compression. In the stress distribution that results, the surface metal has
induced residual compressive stress parallel to the surface, while metal
beneath has reaction-induced tensile stress. The surface compressive stress is
several times greater than the subsurface tensile stress.
2.4 EFFECIVENESS
The effectiveness of
the process depends on two principal parameters, namely the surface coverage
and the peening intensity. The randomly striking shots must uniformly spread
the effect all over the surface; surface coverage is the term used to describe
this aspect. The parameter associated with the depth and degree to which the
effect is imparted is the peening intensity. Surface coverage is a measure of
how completely an area has been hit by the myriad of impinging shot particles.
Without 100% coverage or saturation, the improvement in fatigue characteristics
or the full benefit produced by shot peening cannot be realized. Surface
coverage directly depends upon the exposure time, time to which a specific
area
is exposed to the impinging shots. As the exposure time increases coverage also
increases by a nonlinear function; 100%coverage is the theoretical limit and
difficult to obtain.The relative work done to the surface is called the peening
intensity. The depth up to which the effect can extend and the magnitude of
residual stress that can be induced are functions of the peening intensity and
the characteristics of the material treated: the higher the
intensity,
the higher the effectiveness. Because of the difficulty in quantitatively
measuring
the
actual coverage and peening intensity directly, a simpler comparative method
has been devised to measure these parameters. If a flat strip of metal is shot
peened on one side only it
will
curl slightly and produce a convex surface from the side that has been peened;
the degree of curvature is a measure of the peening intensity, with the strip
curling more at higher intensities. For reliability, reproducibility, and
comparison, measurements are done using a standard strip and this test is
popularly known as the Almen test, after the man who formalized this method.
2.5 EQUIPMENT
The equipment used in
shot peening is essentially the same as that used in abrasive blast cleaning,
except for certain auxiliary equipment made necessary by the more stringent controls
imposed in the shot peening process. The principal components of shot peening
equipment are a shot propelling device, which govern the shot velocity and the angle
of impingement, shot-cycling arrangements, and a work-handling device that
controls the coverage. The entire process is carried out in a closed enclosure
for safety as well as to facilitate shot recycling. All portions of equipment
that are exposed to the stream of shot are enclosed to confine the shot and
permit it to be recycled.
PROPULSION OF SHOT
Two methods of
propulsion of the shot are used widely in shot peening. One uses a motor-driven
bladed wheel, rotating at high speed and the other uses a continuous stream of
compressed air and entrains the shot along the flowing stream of air. In the
wheel method, shot is propelled by a bladed wheel that uses a combination of
radial and tangential forces to impart the necessary peening velocity to the shot.
The position on the wheel from which the shot is projected is controlled to
concentrate the peening blast in the desired direction. Among the advantages of
the wheel method of propulsion are easy control of shot velocity when equipped
with a variable speed drive, high production capacity, lower power consumption,
and freedom from the moisture problem encountered with compressed air.
The
air blast method introduces the shot, either by gravity or by direct pressure,
into a stream of compressed air directed through a nozzle onto the work to be
peened. Aside from being more economical for limited production quantities, the
air blast method can develop higher intensities with small shot sizes, permits
the peening of deep holes and cavities by using a long nozzle, consumes less shot
in peening small areas on intricate parts, and has lower initial cost,
especially when a source of compressed air is already available.
CYCLING OF SHOTS
Equipment for shot recycling consists of
devices for the separation and removal of dust and undersize shot from the used
shot mix. The shot will deform or fracture during use, leaving broken pieces
that can cause damage in the form of sharp notches upon impact with the
surface. The media must therefore be constantly screened to remove broken shot
and dust. In better quality shot blast equipment this is achieved by an
efficient air wash system that removes undersize particles by passing a
controlled air-stream upon the falling stream of peening media that has been precipitated
by centrifugal means in a cyclone. The effectiveness of the separator depends
on careful control of the velocity of the air. For final elimination of
oversize unwanted debris, the reusable particles pass through a vibrating
sieve. Shot-adding devices automatically replenish to maintain an adequate
quantity of shot in the machine at all times. They are equipped with a
capacitance switch or similar device to control the level of shot in the
storage hopper and to add shot, as required, from a supply hopper.
BENEFITS
- · Enhances fatigue strength
- · Improves ultimate strength
- · Prevents cracking due to wear
- · Prevents hydrogen embrittlement
- · Prevents corrosion
- · Prevents galling
- · Prevents fretting
- · Can increase gear life more than 500%
- · Can increase drive pinion life up to 400%
- · Can increase spring life 400% to 1200%
- · Can increase crankshaft life 100% to 1000% (Figure 6)
- · Can permit the use of very hard steels by reducing brittleness
- · Possible to increase the fatigue strength of damaged parts extending the wear
- · Increases lubricity by creating small pores in which lubricants can accumulate
- · Substitution of lighter materials can be possible without sacrificing strength and durability
- · Leaves a uniformly textured, finished surface ready for immediate use or paint and coatings
- · Can be used to curve metal or straighten shafts without creating tensile stress in a Peen forming process
- · Shot Peening can be used in a number of specialized processes such as flow treatment of pipes used to transport polymer pellets used in oil and gas industries. Polymer pellets will slide against the inside of a smooth pipeline, melt and form streamers or angel hair. These long polymer fibers will contaminate the pellet flow and clog up the transfer system. When the inside of the pipeline is roughened by shot peening, the polymer pellets bounce or roll instead of sliding along the inside of the pipe. The pellets contact with the side of the pipe is shortened, and formation of angel hair is prevented.
LIMITATIONS
Shot peening has few practical limitations in terms of the materials
or the size, shape, quantity, surface condition, and surface hardness of parts
that can be peened. Major limitations are not related to the mechanical aspects
of the peening process, but to subsequent processing, such as the effects of
machining and post-peening elevated temperature, that can nullify the
beneficial results of shot peening.
·
Size and Shape of Workpiece.
The size of the peening cabinet is usually the only
limitation on the size of workpiece that can be peened. To some extent, even
this limitation can be overcome by the use of portable mechanized peening
equipment. Provided the surface to be peened is accessible to the blast, the
shape of a workpiece is seldom a limitation. The peening of small radii in
fillets and thread roots is limited by the smallest available media size,
currently 0.0200 mm (0.001 in.) diameter glass beads. Sharp edges that must
retain their sharpness should not be peened.
·
Surface condition. Provided the work piece surface is free of gross contaminants, is
seldom a limitation in shot peening. Water, oil, and grease seriously
contaminate the shot and interfere with peening quality and effectiveness. An
as- forged surface usually shows greater improvement in fatigue strength than a
polished surface as a result of peening. Cast surfaces respond as well to
peening as wrought surfaces. Peened aluminum parts may be bright-dipped before
being anodized.
·
Temperature Limitations. Low tempering temperatures, such as those conventionally used for
carburized parts, have no adverse effect on peening stresses. Low-alloy steels
can be heated to about 175 to 230 °C (350 to 450 °F) for about a half hour
before significant decrease in the compressive stresses occurs. Steels intended
for elevated-temperature application usually withstand temperatures of 260 to
290 °C (500 to 550 °F) without undergoing a significant stress- relieving
effect; however, exposure at 540 °C (1000 °F) or above relieves induced
stresses in all high-temperature alloys. Exposure to temperatures above 175 °C
(350 °F) can eliminate the induced compressive stresses in some alloys of
aluminum.
APPLICATIONS
·
Shot
peening is used on gear parts, cams and camshafts, clutch springs, coil springs, connecting rods, crankshafts, gearwheels, leaf and suspension springs, rock drills, and turbine blades. It is also used in foundries
for sand removal, decoring, descaling,
and surface finishing of castings such as engine blocks and cylinder heads. Its descaling action
can be used in the manufacturing of steel products such as strip, plates,
sheets, wire, and bar stock.
·
Shot
peening is a crucial process in spring making. Types of springs include leaf
springs, extension springs, and compression springs. The most widely used
application are for engine valve springs
(compression springs) due to high cyclic fatigue. In an OEM valve spring
application, the mechanical design combined with some shot peening ensures longevity;
however, automotive makers are shifting to more high performance higher
stressed valve spring designs as modern engines evolve. In aftermarket high
performance valve spring applications, the need for controlled and multi-step
shot peening is a requirement to withstand extreme surface stresses that
sometimes exceeds material specifications. The fatigue life of an extreme
performance spring (NHRA, IHRA) can be as short as two passes down a 1/4 mile
drag racing track before relaxation or failure occurs.
·
Shot
peening may be used for cosmetic effect. The surface roughness resulting from
the overlapping dimples causes light to scatter upon reflection. Because peening typically
produces larger surface features than sand-blasting, the resulting effect is more
pronounced.
·
Shot
peening and abrasive blasting can apply materials on metal surfaces.
When the shot or grit particles are blasted through a powder or liquid
containing the desired surface coating, the impact plates or coats the
workpiece surface. The process has been used to embed ceramic coatings, though
the coverage is random rather than coherent.
CONCLUSION
In modern manufacturing
technology lots of surface treatments are used. Among all this shot peening is
more effective method for inducing the residual compression stresses. Shot
peening is differs from shot blasting and has various controlling parameter .The
parameters are controlled by shot peening machine according to need and
application. Shot peening is mainly useful for many mechanical components and
also for after some manufacturing processes. Shot peening process controlling
and effectiveness measured with help of Almen strips. This helps in better
process control. The process has other useful applications, such as relieving
tensile stresses that contribute to stress-corrosion cracking, forming and
straightening of metal parts, and testing the adhesion of silver plate on
steel. Now a days various advance development has been introduced in case of
shot peening process parameters.
REFERENCE
1.
Effect of shot peening coverage on fatigue limit in round bar of annealed
medium carbon steel† Junji Sakamoto1,2, Yong-Sung Lee1 and Seong-Kyun Cheong1,Department of Mechanical and Automotive
Engineering, Seoul National University of Science and Technology
2.
SHOT PEENING BY MAYURAM M. M., Department of Mechanical Engineering, Machine
Design Section, Indian Institute of Technology Madras, Chennai,
TN,
India
3. Influence of Carburization Followed by Shot
Peening on Fatigue Property of 20CrMnTi Steel Yu-kui Gao, Mei Yao, Qing-xiang Yang, Yan-hui Zhao, Feng Lu, and
Xue-ren Wu
4. EFFECTS OF SHOT PEENING ON FATIGUE PROPERTY IN
SICP/AL-MMC Yasuo Ochi, Kiyotaka Masaki , Takashi
Matsumura and Tatsuhiko Hamaguchi1 Department
of mechanical Engineering & Intelligent Systems, University of Electro- Communications, Tokyo, Chofu, Tokyo
182-8585, Japan
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