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Curtiss-Wright Surface Technologies

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Shot Peening Success Stories

Shot Peening

Shot peening has proved its effectiveness in extending the service life and enhancing the performance of metal components by protecting them against fatigue, fretting fatigue, stress corrosion cracking and a variety of other failure mechanisms. To learn about the success of shot peening in specific applications, click on one of the links below:

Aerospace Applications

Shot peening has proved its effectiveness in extending the service life and enhancing the performance of metal components by protecting them against fatigue, fretting fatigue, stress corrosion cracking and a variety of other failure mechanisms. Following are some of these success stories for aerospace applications:


Auxiliary Power Unit (APU) Exhaust Ducts

One particular type of Auxiliary Power Unit APU is used to provide power to an aircraft when it is on the ground with the main engines turned off. The tubular exhaust ducts are a high temperature 8009 aluminum alloy welded in an end-to-end design.

Tension-tension fatigue tests measured fatigue strength of 23 ksi (156 MPa) at 3,000 cycles in the as-welded condition. Glass bead peening of the welds resulted in a 13% fatigue strength improvement to 26 ksi (180 MPa).

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what is shot peeningNASA Langley Crack Growth Study

Engineers at NASA performed a study on crack growth rates of 2024-T3 aluminum with and without shot peening. The samples were tested with an initial crack of 0.050" (1.27 mm) and then cycle tested to failure. (It should be noted that the United States Air Force damage tolerance rogue flaw is 0.050" (1.27 mm)).

It was found that crack growth was significantly delayed when shot peening was included. As the following results demonstrate, at a 15 ksi (104 MPa) net stress condition the remaining life increased by 237%. At a 20 ksi (138 MPa) net stress condition the remaining life increased by 81%.

This test reflects conditions that are harsher than real world conditions. Real world conditions would generally not have initial flaws and should respond with better fatigue life improvements at these stress levels.

(Note on sample preparation: A notch was placed in the surface via the Electro-Discharge Machining (EDM) process. The samples were loaded in fatigue until the crack grew to ~ 0.050" (1.27 mm). If samples were shot peened, they were peened after the initial crack of 0.050" (1.27 mm) was generated. This was the starting point for the above results.)

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Turbine Engine Fan Disks

In 1991, the Federal Aviation Authority (FAA) issued an airworthiness directive that required inspection for cracks in the low pressure fan disk. At the time, over 5,000 engines were in use on business jets in the United States and Europe. The FAA required that engines that did not have lance (shot) peening following machining in the fan blade dovetail slot be inspected. Those engines having fan disks without lance peening were required to reduce service life from 10,000 to 4,100 cycles (takeoffs and landings). Disks that were reworked with lance peening per AMS 2432 (Computer Monitored Shot Peening) prior to 4,100 cycles were granted a 3,000 cycle extension.

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what is shot peeningTurbine Engine HP Compressor Rotors

Two leading companies in the manufacture of jet turbine engines jointly manufacture high pressure compressor rotors. Separate pieces are machined from forged titanium (Ti 4Al-6V) and then welded together. Testing produced the following results: Initially, shot peening was used as additional "insurance" from failure. After many years of failure free service, coupled with innovations in shot peening controls, shot peening has been incorporated as a full manufacturing process in engine upgrades.
* In aircraft engine terminology one cycle equals the ramp up required for one take-off of the aircraft for which the engine is configured.

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Automotive Applications

Shot peening has proved its effectiveness in extending the service life and enhancing the performance of metal components by protecting them against fatigue, fretting fatigue, stress corrosion cracking and a variety of other failure mechanisms. Following are some of these success stories for automotive applications:


Diesel Engine Crankshafts

Four-point-bending fatigue tests were carried out on test pieces from a diesel engine crankshaft. The material was Armco 17-10 Ph stainless steel. The required service of this crankshaft had to exceed one hundred million cycles. Fatigue strength of unpeened and shot peened test pieces were measured at one billion cycles. The fatigue strength for the unpeened material was 43 ksi (293 MPa) versus 56 ksi (386 MPa) for the peened material. This is an increase of ~ 30%.

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what is shot peening

High Performance Crankshafts

Crankshafts for 4-cylinder high performance engines were failing prematurely after a few hours running on test at peak engine loads. Testing proved that gas carburizing and shot peening the crankpins gave the best fatigue performance. Results from nitriding and shot peening also demonstrated favorable results over the alternative to increase the crankpin diameter.

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Automotive Torsion Bars

The automotive industry has used hollow torsion bars as a means of weight savings. Shot peening was performed on the outer diameter where the highest load stresses occur. On heavy duty applications (four wheel drive utility trucks, sport utility vehicles, etc.) cracks can also occur on the inner diameter (ID), which also experiences torsional loads.

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Chemical & Petrochemical Applications

Shot peening has proved its effectiveness in extending the service life and enhancing the performance of metal components by protecting them against fatigue, fretting fatigue, stress corrosion cracking and a variety of other failure mechanisms. Following are some of these success stories for chemical and petrochemical applications:


what is shot peeningFabrication of Chemical Handling Equipment

Shot peening has been utilized as a cost savings measure for construction of chemical handling equipment. In cases where ammonia or chloride based solutions were to be contained, a lower cost Stress Corrosion Cracking (SCC) susceptible material was selected with shot peening rather than a more expensive non-SCC-susceptible material. Even with the additional shot peening operation, construction costs were less than using the more expensive alloy. The table demonstrates the effectiveness of shot peening in combating SCC for those stainless steel alloys listed. A load stress equivalent to 70% of the materials yield strength was applied.

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what is shot peening Fatigue of Offshore Steel Structures

A Norwegian research program concluded that the combination of weld toe grinding and shot peening gave the largest improvement in the life of an offshore steel structure. This corresponds to more than a 100% increase in the as-welded strength at one million cycles. Other research shows that the improvement in weld fatigue strength from shot peening increases in proportion to the yield
strength of the parent metal.

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Metal Finishing

Shot peening has proved its effectiveness in extending the service life and enhancing the performance of metal components by protecting them against fatigue, fretting fatigue, stress corrosion cracking and a variety of other failure mechanisms. Following are some of these success stories for metal finishing applications:


what is shot peening Al 7050-T7651 High Strength Aluminum

Fatigue specimens were prepared from high strength Al 7050-T7651. All four sides of the center test portion were shot peened. Fatigue tests were conducted under a four-point reversed bending mode (R = -1). The S-N curve of the shot peened versus non-shot peened alloy is shown in the accompanying table. It was found that shot peening improved the fatigue endurance limit by approximately 33%. Even in a regime where the stress ratio is between the yield strength and the endurance limit, the fatigue strength increased by a factor of 2.5 to almost 4.

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what is shot peening Anodized Aluminum Rings

Aluminum (AlZnMgCu 0.5) rings with external teeth were tested for comparison purposes with anodizing and shot peening. The rings had an outside diameter of ~ 24" (612 mm) and a tensile strength of ~ 71 ksi (490 MPa). The (hard) anodizing layer was ~ 0.0008" (0.02 mm) thick. Bending fatigue tests were conducted to find the load to cause a 10% failure probability at one million cycles. The table shows the results.

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what is shot peening High Density Powder Metal Gears

Tooth root bending fatigue studies were performed using pulsator tests to compare a reference wrought gear steel to a 7.5 g/cm3 powdered metal gear. Both gears were 3.5 mm module consisting of 25 teeth and case hardened to 60 HRC. The wrought gear was a 16MnCr5 steel and the powdered metal gear was Fe-3.5Mo alloy content. In the accompanying graph the powdered metal gear results are depicted with the blue curves. The endurance limit improved ~ 35% with the addition of shot peening. The endurance limit improved from ~ 95 ksi (650 MPa) to ~ 128 ksi (880 MPa). The endurance limit of the shot peened powdered metal compares very closely with the non-peened 16MnCr5 material. Due to the significant cost savings of powdered metal, the shot peened powder metal gear may be a suitable replacement to the more expensive wrought steel gear. Shot peening was performed at 0.013" A (0.32 mm A) intensity for all samples.

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what is shot peening Sulfide Stress Cracking

Hydrogen sulfide (H2S) is commonly encountered in sour gas wells. Certain metal alloys when exposed to H2S will experience a significant decrease in fatigue strength. The test results shown in the table illustrate the response of precipitation hardened 17-4 stainless steel exposed to H2S with and without shot peening.

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Processing Applications

Shot peening has proved its effectiveness in extending the service life and enhancing the performance of metal components by protecting them against fatigue, fretting fatigue, stress corrosion cracking and a variety of other failure mechanisms. Following are some of these success stories for processing applications:


Food Industry Cheese Molds

The cheese/dairy industry has found that uniform dimples provide a surface that can advantageously replace other surface treatments. The textured surface from shot peening often has a lower coefficient of sliding friction that is necessary for cheese release properties on some food contact surfaces. The dimples act as lubricant reservoirs for fat or other substances allowing the cheese product to slide easier through the mold on the peaks of the shot peening dimples. Testing has shown that shot peened finishes meet or exceed necessary requirements for cleaning in terms of microbial counts. This is due to the rounded dimples that do not allow bacteria to collect and reproduce. Sharp impressions left from grit blasting, sand blasting or broken media have proven to be less cleanable and have a much greater tendency for bacteria to collect and reproduce. Both glass beaded and stainless steel media have been used successfully in this application.

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what is shot peening Pneumatic Conveyor Tubing

Pneumatic conveyor tubing can be up to 10 inches in diameter and is usually made of a stainless or aluminum alloy. It is used to transport plastic pellets at the facilities of molding companies or at various production, blending and distribution sites. Transported pellets degrade when contact is made with internal piping surfaces. The velocity of the pellets results in friction, heat and lost production. Using a variation of its Peentexsm process that produces directional dimpling, Metal Improvement Company can achieve a directionally textured surface that significantly reduces the formation of fines, fluff and streamers that can account for millions of pounds of lost and/or contaminated production each year. Directional shot peening has been found to be much superior to other internal treatments of the tubing, is often more economical and can be applied on-site. The directional surface finish has the added benefit of work hardening (when stainless or aluminum piping is used), thus extending the life of the surface treatment. The table shows test results from six different internal pipe treatments. A lower value of fines per 100,000 lbs conveyed is desirable. The directional shot peening resulted in one third of the fines of the next closest finish.

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Transportation Applications

Shot peening has proved its effectiveness in extending the service life and enhancing the performance of metal components by protecting them against fatigue, fretting fatigue, stress corrosion cracking and a variety of other failure mechanisms. Following are some of these success stories for transportation applications:


Train Emergency Brake Pin

A brake pin is part of a hydraulic brake assembly used in a mass transit train system. The undercut sections near the chamfered end were designed to fail in the event of axial overload. During the investigation of premature failures it was found that a bending load also was occurring. The combined axial and bending load when simulated in test caused fatigue failure between 150,000 - 2,600,000 cycles. After shot peening was added to the brake pin all test specimens exceeded 6,000,000 cycles without failure.

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Turbomachinery / Power Generation Applications

Shot peening has proved its effectiveness in extending the service life and enhancing the performance of metal components by protecting them against fatigue, fretting fatigue, stress corrosion cracking and a variety of other failure mechanisms. Following are some of these success stories for turbomachinery / power generation applications:


Feedwater Heaters

Large thermal fatigue cracks were discovered in eight high pressure feedwater heaters used in a power generation application. These units operated in both an elevated temperature and thermal fatigue environment. Startups and shut downs caused thermal fatigue. Steady state operation was at 480-660 °F (250-350 °C). The cracks were circumferential in the weld heat affected zone between the water chamber and tube sheet. Fatigue cracking was attributed to years of service and 747 startups and shutdowns of the unit. This caused concern about the remaining life of the units. The cracked locations were machined and shot peened. Subsequent inspections showed that no additional fatigue cracks developed after five years of service and 150 startup and shutdown cycles.

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Diaphragm Couplings

Metal diaphragm couplings are often used in turbomachinery applications. These couplings accommodate system misalignment through flexing. This flexing, or cyclic loading, poses concerns for fatigue failures. Researchers concluded that the Electro-Chemical Machining (ECM) process produces parts that are geometrically near-perfect. However, they found under scanning electron microscope observation that small cavities sometimes developed on the surface of a part as a result of ECM. These cavities apparently generate stress concentrations that lead to premature failures. Shot peening after ECM was able to overcome this deficiency and has significantly improved the endurance limit of the diaphragm couplings.

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what is shot peeningTurbomachinery Blades And Buckets

A very common fretting environment is the dovetail root of turbomachinery blades. Shot peening is commonly used to prevent fretting failures of these roots. Blade roots have a characteristic fir tree shape. The tight mating fit coupled with demanding loading conditions require that the surfaces be shot peened to prevent failure associated with fretting. Many turbine and compressor blade roots are shot peened as OEM parts and re-shot peened upon overhaul to restore fatigue debits otherwise lost to fretting. The discs or wheels that support the blades should also be peened.

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This technical information has been contributed by
Curtiss-Wright Surface Technologies

Click here to find suppliers

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