60551762 shot peening

1. INTRODUCTION1. History Shot peening is not a new process. People have long known that pre -stressing or work-hardening metal could create harder and more durable metals. The process of was used in forging processes as early as the Bronze Age to strengthen armor, swords and tools. Gun barrels in the civil war were subject to peening to increase the hardness of Damascus steels, and the fillets of crankshafts in early European racecars were hand-peened with specially-made hammers by 1922. Of course, peening has evolved substantially in the late 20th and early 21st centuries, but the general idea remains the same. Shot peening the material with thousands of tiny balls of high -velocity shot works in much the same way as peening with a hammer did in medieval times.

2. Process Shot peening is a cold working process used to produce a compressive residual stress layer and modify mechanical properties of metals. It entails impacting a surface with shot (round metallic, glass or ceramic particles) with force sufficient to create plastic deformation, each particle functions as a ball-peen hammer. This process creates the same effect that a peening hammer does by causing outer surface to yield in tension. The material directly beneath it is subjected to high compressive forces from the deformation and tries to restore the outer surface to its original shape. By overlapping the surface ind entations, a uniform compressive layer is achieved at the surface of the material. The compressive layer squeezes the grain boundaries of the surface material together and significantly delays the initiation of fatigue cracking. As a result, the fatigue life of the part can be greatly increased. By this process less material is removed and less dust created.

Shot-peening is in fact a true machining operation which helps increase fatigue and stress corrosion resistance by creating beneficial residual surface stresses. It has many uses in industry, particularly in the manufacture of parts as1 ANALYSIS AND FABRICATION OF SHOTPEENING MACHINE

diffe e as he ical s ings roc ers aircraft parts welded joints trans ission shafts torsion bars etc Shot Peening allows metal parts to accept higher loads or to endure a longer fatigue life in service without failure In usual applications shot peening can be done without changing the part design or its material.

ANALYS S AND FAB

2 ATION OF S OTPEENING MACHINE

3. Working Metals fail under tension loads and not under compressive loads. The failure crack will usually initiate at the part surface where tension stresses are highest and a stress riser exists (scratch, dent, machine mark, etc). When parts which have been shot peened are loaded, the failure producing tensile stresses are thus reduced by the amount of the compressive stresses preexisting in the part surface. This lowering of the effective tensile stress will then allow the part to accept higher loading or to extend its service life significantly. When the depth of the induced compressive stress layer exceeds the depth of all surface discontinuities (stress risers) their ability to start a crack is effectively masked. The atoms in the surface of a piece of manufactu red metal will be under (mostly) tensile stresses left over from grinding, welds, heat treatments and other stressful production processes. Cracks promulgate easily in areas of tensile stress because the tensile stresses are already working to pull the atoms of the metal apart. By shot peening the material you introduce a layer of compressive stress by compacting the material. As the shot peening is performed, the atoms on the surface of the metal become crowded and try to restore the metal's original shape by pushing outward. The atoms deeper into the metal are pulled toward the surface by their bonds with the atoms in the compressive layer. These deeper atoms resist the outward pull creating internal tensile stress that keeps the part in equilibrium with the compressive stress on the surface.

3 ANALYSIS AND FABRICATION OF SHOTPEENING MACHINE

2) EQUIPMENT There are numerous types of shot-peening machines. Those can be divided into two categories, dependent on the method used for projecting the shot, compressed air or wheel, The choice between these two types of machine will depend on the quality of the shot-peening required and the type of part which is to be treated. Shot-peening machines can also vary considerably in the way the part is positioned in the stream of shot. There are thus drum-type machines for shot-peening parts in bulk, rotating table machin es for small parts in series, linear conveyors for helical springs, and overhead conveyors (see schematic representation).

Shot Peening systems are comprised of 6 basic subsystems: 1. Shot delivery method: A. By Compressed Air b. By Centrifugal turbines. 2. Recovery and cleaning. 3. Dust collection. 4. Peening Cabinet. 5. Part movement and support system. 6. Controls and instrumentation. 1. Abrasive delivery method There are two ways of accelerating the steel shot: a) By compressed air: This system is suitable for lower production applications where maximum flexibility is needed. These systems are very flexible in that the shot can be delivered horizontally through a rubber hose and nozzle assembly. This enables uses in finishing operations of steel frames and weldments thereby replacing hand tools. Because of this, an air blasting machine for a production line is expensive compared to the centrifugal wheel blasting machine. For example to deliver shot at a rate of 1100 kg per minute a 1650 Hp compressor and 334 ANALYSIS AND FABRICATION OF SHOTPEENING MACHINE

workers are needed using 10 mm diameter nozzles delivering 6.5 kg/cm2 and on the other hand the same task using centrifugal wheel turbines only re uires a total of 100 Hp distributed to between one or a multitude of turbines housed in the same machine. Only one or two operators are needed for such a shot blasting machine.

b) B cent ifu l tu bine: Centrifugal wheel blasting is the more common blast cleaning technique as well as the most economical and environmentally friendly method. The turbine delivers abrasive shot by centrifugal force in a specific and controlled direction, speed and quantity. Function of the turbine is similar to that of a fan or centrifugal pump. Shot blasting machines may use one or a multitude of turbines positioned in such a way that the abrasive blast pattern covers the entire surface of the material to be shot cleaned. The shape and size of the parts determine the number of turbines used in a machine. Power of the turbine motor is based on degree of cleaning needed and throughout speed of the material.

2. Ab si e Reco ery and cleanin system Recirculation and cleaning the abrasive shot is required to maintain a consistent cleaning operation. In conventional shot peening equipment after the shot hits the part it falls into the collection hopper under the machine. The shot is then carried by gravity or screw conveyor to a bucket elevator. The elevator carries the shot, removed oxides and other contaminates to an air wash separator located in the upper portion of the machine. A combination of baffles, strainers and plates separate these contaminate which are ineffective during the shot blast operation. The cleaned abrasive is contained in an upper hopper (feeding-box) and is subsequently fed into the shot turbine by gravity. The recirculating and cleaning capacity of the shot in each machine is related5 ANALYSIS AND FAB ICATION OF SHOTPEENING MACHINE

to the shot peening power used for the turbines. An incorrectly sized system will cause premature wear to the machine and decrease overall shot peeening effectiveness and shot consumption.

3. Peening Cabinet The machine cabinet contains dust and abra sive. A machine mounted dust collector reduces air pressure inside the machine thereby preventing dust from escaping into the shop environment. Material access openings in the entrance and exit of the machine must be designed and protected to prevent abrasive spillage. Cabinets are built from low carbon steel with an inner shell made of abrasive resistant materials including high strength alloy plates and thick rubber compounds. In the areas that are subject to direct high velocity shot, alloy steel plates (64 RC hardness) are used which have much more abrasion resistance than other more commonly used materials like manganese steel. 4. Dust collector system Dust produced during shot peening is withdrawn from the machinery cabinet and continuously recirculating abrasive by a dust collector. Typical dust collector design uses baffle filters or cartridges. The dust collector not only evacuates dust within the machine but also keeps the surrounding area clean and dust free. Changes in airflow will reduce collector efficiency and therefore result in lower dust extraction, loss of the cleaning power, and contribute to dust in the immediate production area. A properly designed and sized dust collector is therefore critical to the ongoing performance of the shot peening system. 5. System for holding and transporting parts for shot blasting Handling and transporting parts through the shot peening process will depend on several factors. For materials in large quantities (brake, pulleys, screwdrivers, etc.) tumblast machines are used. For larger and heavier pieces, (motor blocks, bicycles frames, bunch-welded parts, etc.) spinner hanger machines are used. For the shot peening of gears and other special


components, table and multi table machines are used. For cleaning pipes, plates, bars and wire, continuous machines are used. 6. Controls and instrumentation The system providing the control and instruction for the starting and stopping all functions such as, elevators, dust collectors, turbines, part handling system, ammeters and time meters for the turbine motors are all placed in a central console. The control panel is designed with sequential startup to assure the different systems are energized in the proper sequence. All systems can be automated for continuous processing that will increase production, reduce operator interaction and consistently maintain a particular surface specification.


3) SHOT MEDIA. To accomplish a shot peening job efficiently, shot of the correct material and size maintained in a condition free of cutting edges, should be used. Shot of peening is available in a variety of materials depending on the nature of the job in hand. Cast steel shot is generally used for peening ferrous components - leaf or coil springs, for example - or for the treatment of non-ferrous articles when the possibility of ferrous contamination is acceptable. It is however possible to blast components with angular non ferrous abrasive after peening with cast steel shot to remove any ferrous contamination. This is not generally done, since it would probably be cheaper to peen the article with glass beads in the first instance. Glass beads are widely used for peening non-ferrous components or where very low intensities may be required, for example on aluminum and its alloys, titanium and stainless steel, particularly in the aerospace industry where peening is widely used to increase the fatigue life on non ferrous components. The regularity of size and shape can be clearly seen in Fig. 5. Copper wire is less widely used for peening and consists of spring steel wire, or piano wire, shopped into lengths equal to its diameter. Before being used for peening it has to be blasted against a steel plate to blunt the sharp cut ends. Ceramic Beads Ceramic and inorganic abrasives and media include aluminum oxide, silicon carbide, zirconium, silicate, fused silica, boron carbide, synthetic diamond, CBN, tin oxide, tungsten carbide, and cerium oxide. The ceramic abrasives are dense abrasives with outstanding grinding and finishing performance on a variety of work piece materials. Hardness of ceramic and inorganic abrasives and media varies with specific composition. Diamond, boron carbide, and CBN are among the hardest materials and are used to grind and finish very hard ceramic and alloys.


Porcelain materials are used for both useful industrial and ornamental applications. Traditional porcelain is made from a mixture of feldspar, clay (and flint. Porcelains can be aluminum silicate, magnesium silicate, or aluminum magnesium silicate-based - depending on the raw materials or minerals selected. tainless Steel Shot - Made by techniques similar to cast steel shot. A more expensive medium and great care must be exercised to ensure that all the fabric lining of a blast machine is fully protected to prohibit the stainless quality of the shot be impaired by ferrous contamination.

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3.2. S APE The shot or media are pre-conditioned or rounded to radius or round off any sharp edges or corners that would otherwise scratch or mar the parts. Cut wire shot is normally conditioned before use as peening media.&

Balls / Beads (Spherical): Media is a bead, ball, or spherical shape. Shot - Cut Wire: Shot can consist of round, cut wire or irregular shapes. Shot usually consists of steel, stainless steel, or other metals. Shot Cast: Shot can consist of cast round shapes. Cast shot usually consists of steel, stainless steel, cast iron, or other metals. A range of sizes or diameters may occur during the atomization or droplet formation process. Cast shot may not be perfectly round unless conditioned - SAE specification allow up to 5% of the shot to be elongated, 10% with voids, 10% with shrinkage, and 5% with cracks.

c. Pneumatic h t peening systems There are three systems of using compressed air for shot peening purposes, in Which the shot I s projected from a blast nozzle. 1) .Induction - siphon method. 2). Induction - gravity method.10 ANALYSIS AND FABRICATION OF SHOTPEENING MACHINE( '

3) .Direct pressure. 1). Siphon Induction method is one in which the shot is aspirated to the nozzle. The method is uncomplicated and lends itself readily for use In hand cabinets. The only claim that may be made is the simplicity of the system that permits constant uninterrupted working. The one real objection is that most Installations do not incorporate shot Cleaning and shot grading which in the case of shot peening would not be tolerable. It is true that many parts requiring peening are ground and hardened and are in the finished state. In consequence they are classed as clean, so there is little Possibility of contamination of the shot, but all abrasives break down, particularly, glass beads which is a popular medium for use in this type of machine) and this fact which has been emphasized continually is not tolerable in shot-peening. 2) .The Gravity Induction This system offers improved efficiency, with the shot being elevated above the poin t of usage, from there being permitted to free fall to the nozzle where it is energized by the compressed air. This is a neat method of providing continuous operation by elevating the shot in the recirculation system. The essential method of incorporating screening, grading and cleaning may be included. 3). Direct Pressure This system is the most universally used. It is based upon the use of a pressure vessel in which the shot is fed under pressure t o the nozzle. A metering valve is incorporated in the system to adjus t the volume of abrasive into the air stream. There is a precise balance between the volumes of shot and the bore size of the nozzle. The value of this system is in the, adjustments that may be made positively to the air pressure. The pressure vessel as a unit may be incorporated into hand cabinets and blas t chambers.


4. SHOT PEENING PARAMETERS The shot peening process has to be a precisely controlled and repeatable process for optimum benefit. To achieve this, all its process variables must be identified and controlled. There are many fundamental parameters affecting the shot peening process. The most common are as follows: y Shot density; y Hardness and size of the shot; y Nozzle characteristics (diameter, deflection angle, length); y Air pressure: y Impact angle; y Distance from nozzle to work-piece; y Exposure time, number of passes; y Linear and rotational speed of work-piece relative to nozzle.

4.1. CONTROL PARAMETERS Controlled shot peening is different than most manufacturing processes in that there is no nondestructive method to confirm that it has been performed to the proper specification. Techniques such as X-Ray Diffraction require that a part be sacrificed to generate a full compressive depth profile analysis. To ensure peening specifications are being met for production lots, the following process controls must be maintained: Media Intensity Coverage Equipment


Peening media must be predominantly rounded. When media breaks down from usage, the broken media must be removed to prevent surface damage upon impact. Peening media must be of uniform diameter. The impact energy imparted by the media is a function of its mass and velocity. Larger media has more mass and impact energy. If a mixed size batch of media is used for peening, the larger media will drive a deeper residual compressive layer. This results in a non-uniform residual compressive layer and will correlate into inconsistent fatigue results.

Damaged Surface from Broken Shot Media

4. b. INTENSITY CONTROL Shot peening intensity is the measure of the energy of the shot stream. It is one of the essential means of ensuring process repeatability. The energy of the shot stream is directly related to the compressive stress that is imparted into a part. Intensity can be increased by using larger media and/or increasing the velocity of the shot stream. Other variables to consider are the impingement angle and peening media. Intensity is measured using Almen strips. An Almen strip consists of a strip of SAE1070 spring steel that is peened on one side only. The residual compressive stress from the peening will cause the Almen strip to bend or arc convexly towards the peened side. The Almen strip arc height is a function of the energy of the shot stream and is very repeatable.2

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0

4. a.)

ED A CONTROL

Typical Surface from Proper Media


There are three Almen strip designations that are used depending on the peening application: N Strip: Thickness = 0.031" (0.79 mm) A Strip: Thickness = 0.051" (1.29 mm) C Strip: Thickness = 0.094" (2.39 mm)

More aggressive shot peening utilizes thicker Almen strips. The Almen intensity is the arc height (as measured by an Almen gage) followed by the Almen strip designation. The proper designation for a 0.012" (0.30 mm) arc height using the A strips is 0.012A (0.30A). The usable range of an Almen strip is 0.004"0.024" (0.10-0.61 mm). The next thicker Almen strip should be called out if intensity is above 0.020" (0.51 mm). The intensity value achieved on an N strip is approximately one-third the value of an A strip. The intensity value achieved on a C strip is approximately three times the value of an A strip (N ~ 1/3A, C ~ 3A). Almen strips are mounted to Almen blocks and are processed on a scrap part (Figure 11-6) or similar fixture. Almen blocks should be mounted in locations where verification of impact energy is crucial. Actual intensity is verified and recorded prior to processing the first part. This verifies the peening machine is set up and running according to the approved, engineered process. After the production lot of parts has been processed, intensity verification is repeated to insure processing parameters have not changed. For long production runs, intensity verifications will be performed throughout the processing as required.14 ANALYSIS AND FABRICATION OF SHOTPEENING MACHINE

4. c. SATURATION (INTENSITY VERIFICATION) Initial verification of a process development requires the establishment of a saturation curve. Saturation is defined as the earliest point on the curve where doubling the exposure time produces no more than a 10% increase in arc height. The saturation curve is developed by shot peening a series of Almen strips in fixed machine settings and determining when the doubling occurs. Figure shows that doubling of the time (2T) from the initial exposure time (T) resulted in less than a 10% increase in Almen arc height. This would mean that the process reaches saturation at time = T. Saturation establishes the actual intensity of the shot stream at a given location for a particular machine setup. It is important to not confuse saturation with coverage. Coverage is described in the next section and deals with the percentage of surface area covered with shot peening dimples. Saturation is used to verify the time to establish intensity. Saturation and coverage will not necessarily occur at the same time interval. This is because coverage is determined on the actual part surface which can range from relatively soft to extremely hard. Saturation is determined using Almen strips that are SAE1070 spring steel hardened to 44-50 HRC.3 3

4. d. COVERA E CONTROL Complete coverage of a shot peened surface is crucial in performing high quality shot peening. Coverage is the measure of original surface area that has been obliterated by shot peening dimples. Coverage should never be less than 100% as fatigue and stress corrosion cracks can develop in the nonpeened area that is not encased in residual compressive stress.4


If coverage is specified as greater than 100 (i.e. 150 , 200 ) this means that the processing time to achieve 100 has been Increased by that factor. Coverage of 200 time would have twice the shot peening exposure time as 100 coverage.5 5 5 5 5 5

5) Residu l StressesResidual stresses are those stresses remaining in a part after all manufacturing operations are completed, and with no external load applied. These residual stresses can be either tensile or compressive. For example, a welded joint will contain high magnitude residual tensile stresses in the heat -affected zone (HAZ) adjacent to the weld. Conversely, the surface of induction hardened components may contain residual compressive stresses. In most applications for shot peening, the benefit obtained is the direct result of the residual compressive stress produced. It is well known that cracks will not initiate nor propagate in a compressively stressed zone. Because nearly all fatigue and stress corrosion failures originate at or near the surface of a part, compressive stresses induced by shot peening provide significant increases in part life. The magnitude of residual compressive stress produced by shot peening is at least as great as half the tensile strength of the material being peened. In most modes of long term failure the common denominator is tensile stress. These stresses can result from externally applied loads or be residual stresses from manufacturing processes such as welding, grinding or machining. Tensile stresses attempt to stretch or pull the surface apart and may eventually lead to crack initiation. Compressive stress squeezes the surface grain boundaries together and will significantly delay the initiation of fatigue cracking. Because crack growth is slowed significantly in a compressive layer, increasing the depth of this layer increases crack resistance. Shot peening is the most economical and practical method of ensuring surface residual compressive stresses.


The residual stress generated by shot peening is of a compressive nature. This compressive stress offsets or lowers applied tensile stress. Quite simply, less (tensile) stress equates to longer part life. A typical shot peening stress profile is depicted in Figure 1-5. Maximum Compressi e Stress This is the maximum value of compressive stress induced. It is normally just below the surface. As the magnitude of the maximum compressive stress increases so does the resistance to fatigue cracking. Depth of Compressi e Layer This is the depth of the compressive layer resisting crack growth. The layer depth can be increased by increasing the peening impact energy. A deeper layer is generally desired for crack growth resistance. Surface Stress This magnitude is usually less than the Maximum Compressive Stress. 5. . SU7

ATION OF APPLIED AND RESIDUAL STRESS

When a component is shot peened and subjected to an applied load, the surface of the component experiences the net stress from the applied load and Shot peening residual stress. Figure 1-6 depicts a bar with a three-point load that creates a bending stress at the surface. The diagonal dashed line is the tensile stress created from the bending load. The curved dashed line is the (residual) compressive stress from shot peening. The solid line is the summation of the two showing a significant reduction of tensile stress at the surface. Shot peening is highly advantageous for the following two conditions:


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Stress risers

High strength materials

Resultant Stress in a Shot Peened Beam with an External Load Applied

Stress risers may consist of radii, notches, cross holes, grooves, keyways, etc. Shot peening induces a high magnitude, localized compressive stress to offset the stress concentration factor created from these geometric changes. Shot peening is ideal for high strength materials. Compressive stress is directly correlated to a material s tensile strength. Higher the tensile strength, the more compressive stress can be induced. Higher strength materials have a more rigid crystal structure. This crystal lattice can withstand greater degrees of strain and consequently can store more residual stress.

5.2. DEPTH OF RESIDUAL STRESS


The depth of the compressive layer is influenced by variations in peening parameters and material hardness. Figure shows the relationship between the depth of the compressive layer and the shot peening intensity for five materials: steel 30 HRC, steel 50 HRC, steel 60 HRC, and 2024 aluminum and titanium 6Al-4V. Depths for materials with other hardness values can be interpolated. 5.3. RESIDUAL STRESS DISTRIBUTION The residual compressive stresses introduced by shot-peening are the parameters that influence the improvement in the operating performance of the part, to the greatest extent. It is obvious that, depend on the treatment conditions, the nature of the steel and the shot used. The distribution of the residual stresses introduced will vary. We have already seen that the depth of metal plasticized will increase with the projection velocity and the shot size. This phenomenon conditions the residual stress distribution and the stability of the stresses within the material, when the part is operating. The maximum residual stress level and the residual stress gradient will depend not only on the material from which the part is made but also the depth of metal affected.

5.4. RESIDUAL STRESS RELAXATION DURING FATIGUE TESTThe residual stress levels and distribution are generally altered when parts are subjected to fatigue loading. The problem is the then to find out what the magnitude and distribution of the stable" residual stresses are and to include them in our calculations. It is essential, therefore, to appreciate the stability of residual stresses as a function of the load applied. In design calculations, for a part, one can only consider the values of the stabilized19 ANALYSIS AND FABRICATION OF SHOTPEENING MACHINE

stresses, that is to say the values of those stresses that are likely to be actually present in the part during most of its operating life.

6) STRESS CORROSION CRACKINGStress corrosion cracking (SCC) failure is most often associated with static tensile stress. The static stress can be from applied stress (such as a bolted flange) or residual stress from manufacturing processes (such as welding). For SCC to occur three factors must be present: y Tensile stress y Susceptible material y Corrosive environment

7) METAL FATIGUEThe graph below compares metal fatigue strength with ultimate tensile strength for both smooth and notched specimens. Without shot peening, optimal metal fatigue properties for machined steel components are obtained at approximately 30 HRc (700 MPa). At higher strength/hardness levels, materials lose fatigue strength due to increased notch sensitivity and brittleness. With the addition of compressive stresses from shot peening, however, metal fatigue strength increases proportionately to increasing strength/hardness. For example, at a 52 HRc (1240 MPa), the metal fatigue strength of the shot peened specimen is 144 ksi (988 MPa), more than twice the metal fatigue strength of the unpeened, smooth specimen.


Comparison of peened and unpeened fatigue limits for smooth and notched specimens as a function of ultimate tensile strength of steel.

8)

ETHODS OF RESIDUAL STRESS

EASUREMENT

In x-ray diffraction residual stress measurement, the strain in the crystal lattice is measured, and the residual stress producing the strain is calculated, assuming a linear elastic distortion of the crystal lattice. Although the term stress measurement has come into common usage, stress is an extrinsic property that is not directly measurable. All methods of stress determination require measurement of some intrinsic property, such as strain or force and area, and the calculation of the associated stress. Mechanical methods (dissection techniques) and nonlinear elastic methods (ultrasonic and magnetic techniques) are limited in their applicability to residual stress determination. Mechanical methods are limited by assumptions concerning the nature of the residual stress field and sample geometry. Mechanical methods, being21 ANALYSIS AND FABRICATION OF SHOTPEENING MACHINE

necessarily destructive, cannot be directly checked by repeat measurement. Spatial and depth resolution are orders of magnitude less than those of x-ray diffraction. All nonlinear elastic methods are subject to major error from preferred orientation, cold work, temperature, and grain size. All require stress-free reference samples, which are otherwise identical to the sample under investigation. Nonlinear elastic methods are generally not suitable for routine residual stress determination at their current state of development. In addition, their spatial and depth resolutions are orders of magnitude less than those of x-ray diffraction. To determine the stress, the strain in the crystal lattice must be measured for at least two precisely known orientations relative to the sample surface. Therefore, x-ray diffraction residual stress measurement is applicable to materials that are crystalline, relatively fine grained, and produces diffraction for any orientation of the sample surface. Samples may be metallic or ceramic, provided a diffraction peak of suitable intensity and free of interference from neighboring peaks can be produced in the high backreflection region with the radiations available. X-ray diffraction residual stress measurement is unique in that macroscopic and microscopic residual stresses can be determined nondestructively.


(a) = 0. (b) = (sample rotated through some known angle surface. Principles of x-ray diffraction stress measurement.

). D, x-ray detector: S, x-ray source; N, normal to the

8.1. Principles of -Ray Diffraction Stress Measurement The figure shows the diffraction of monochromatic beams of x-ray at a high diffraction angle (2 ) from the surface of a stressed sample for two orientations of the sample relative to the x-ray beam. The angle , defining the orientation of the sample surface, is the angle between the normal of the surface and the incident and diffracted beam bisector, which is also the angle between the normal to the diffracting lattice planes and the sample surf ace. Law: n = 2d sin , where n is an integer denoting the order of diffraction, is the x-ray wavelength, d is the lattice spacing of crystal planes, and is the diffraction angle. For the monochromatic x-rays produced by the metallic target of an x-ray tube, the wavelength is known to 1 part in 105. Any change in the lattice spacing, d, results in a corresponding shift in the diffraction angle 2 . Figure l (a) shows the sample in the = 0 orientation. The presence of a tensile stress in the sample results in a Poisson's ratio contraction, reducing the lattice spacing and slightly increasing the diffraction angle, 2 . If the sample is then rotated through some known angle (Fig. 1b), the tensile stress present in the surface increases the lattice spacing over the stress-free state and decreases 2 . Measuring the change in the angular position of the diffraction peak for at least two orientations of the sample defined by the angle enables calculation of the stress present in the sample surface lying in the plane of diffraction, which contains the incident and diffracted x-ray beams. To measure the stress in different directions at the same point, the sample is rotated about its surface normal to coincide the direction of interest with the diffraction plane. Because only the elastic strain changes the mean lattice spacing; only elastic strains are measured using x-ray diffraction for the determination of macro stresses. When the elastic limit is exceeded, further strain results in dislocation motion, disruption of the crystal lattice, and the formation of micro stresses, but no additional increase in macroscopic stress. Although residual stresses result from no uniform plastic deformation, all residual macrostresses remaining after deformation are necessarily elastic.


9

Changes in the interplanar spacing d can be used with the Bragg s equation to detect elastic strain through a change in the Bragg scattering angle . 2dsin = n Giving

=

= -cot

8.2. Hole Drilling Met od y Principle: The undisturbed regions of a sample containing residual stresses will relax into different shape when the locality is machined, thereby providing data for back calculation of residual stress. y The process involves drilling a hole into a residually stressed body with a depth which is about equal to its diameter and small compared to the thickness of the test object. y Strain is measured using either a rosette of strain gauges; moire interferometry, laser interferometry based on a rosette of indentations or holography.

Hole Drilling: Equation= ( max + min ) A+ ( max min )B cos 2

Where A and B are hole drilling constants, and is the angle from the axis to the direction of maximum principal stress max .For the general case of a hole drilled in an infinite plate A and B must be calculated numerically.

9) SPECIF ING SHOT PEENINGFigure shows a splined shaft (shaded) installed with two bearings supporting the shaft inside an assembly. The outboard spline and adjacent radius would be likely fatigue failure locations from bending and/or torsional fatigue. In this


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case, engineering would specify shot peening (of the shaft) on the drawing as follows: y Area "A": Shot peen y Area "B": Overspray allowed y Area "C": Masking required

y The details on the print should read: Shot peen splined areas and adjacent Radius using MI-110H shot; 0.006"-0.009" A intensity. y Minimum 100% coverage in splined areas to be verified by PEENSCAN. y Overspray acceptable on adjacent larger diameter. y Mask both bearing surfaces and center shaft area. y Shot peening per AMS-S-13165. It is important to note that if Non-Destructive Testing is required, NDT should always be performed before shot peening.

Assembly Drawing of Spline Shaft Requiring Shot Peening

10) BENEFITSy y y yEnhances fatigue strength. Improves ultimate strength. Prevents cracking due to wear. Prevents hydrogen embrittlement.25 ANALYSIS AND FABRICATION OF SHOTPEENING MACHINE

y

y y y

y Optimal fatigue properties for machined steel components are obtained at approximately 700 MPa, any higher and the materials lose fatigue strength due to increased notch sensitivity and brittleness. When compressive stresses from shot peening are added fatigue strength increases proportionately to increased strength.


D

B

C

A

D

C

y y y y y y y y y

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 . 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.

11) APPLICATIONS

11.1. PEEN FORMING Peen forming is the preferred method of forming aerodynamic contours into aircraft wing skins. It is a dieless forming process that is performed at room temperature. The process is ideal for forming wing and empennage panel shapes for even the largest aircraft. It is best suited for forming curvatures where the radii are within the elastic range of the metal. These are large bend radii without abrupt changes in contour. Residual compressive stress acts to elastically stretch the peened side. The surface will bend or "arc" towards the peened side. The resulting curvature will force the lower surface into a compressive state. Typically aircraft wing skins have large surface area and thin cross sectional thickness. Therefore, significant forces are generated from the shot peening residual stress over this large surface area. The thin cross section is able to be manipulated into desired contours when the peen forming is properly engineered and controlled. 11.2. CONTOUR CORRECTION Shot peening utilizing peen forming techniques can be used to correct unfavorable geometry conditions. This is accomplished by shot peening selective locations of parts to utilize the surface loading from the induced compressive stress to restore the components to drawing requirements. 11.3. WORK HARDENING A number of materials and alloys have the potential to work harden through cold working. Shot peening can produce substantial increases in surface hardness for certain alloys of the following types of materials:


y y y y y y y

Stainless steel Aluminum Manganese stainless steels Inconel Stellite Hastelloy

11.4. PEENTEXSM Controlled shot peening also can be used to deliver a number of different, aesthetically pleasing surface finishes, architectural finishes that are consistent, repeatable and more resistant to mechanical damage through work hardening. 11.5. POROSITY SEALING Surface porosity has long been a problem that has plagued the casting and powder metal industries. Irregularities in the material consistency at the surface may be improved by impacting the surface with shot peening media. By increasing the intensity (impact energy), peening can also be used to identify large, near-surface voids and delaminations.

11.6. STRAIN PEENING Strain (or stress) peening offers the ability to develop additional residual Compressive stress offering more fatigue crack resistance. Whereas dual peening offers improvements at the outermost surface layer, strain peening develops a greater amount of compressive stress throughout the compressive layer. To perform strain peening, a component must be physically loaded in the same direction that it experiences in service prior to peening. Extension springs must28 ANALYSIS AND FABRICATION OF SHOTPEENING MACHINE

be stretched, compression springs must be compressed and drive shafts must be pre-torque. This will offer maximum (residual) compressive stress opposing the direction of (applied) tensile stress created during cyclic loading. The additional compressive stress is generated by preloading the part within its elastic limit prior to shot peening. When the peening media impacts the surface, the surface layer is yielded further in tension because of the preloading. The additional yielding results in additional compressive stress when the metal s surface attempts to restore itself.




CONCLUSION:Depending on the part geometry, part material, shot material, shot quality, shot intensity, shot coverage, shot peening can increase fatigue life from 01000%. Compressive stresses are beneficial in increasing resistance to fatigue failures, corrosion fatigue, stress corrosion cracking, hydrogen assisted cracking, fretting, galling and erosion caused by cavitation. The maximum compressive residual stress produced just below the surface of a part by shot peening is at least as great as one-half the yield strength of the material being shot peened


60551762 shot peening

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