Thomas N. Fritts

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SURFACE FINISHING AND SURFACE CONDITIONINGBY SHOT PEENING

Thomas N. Fritts CMFGTRepair Development Engineer

Cooper Airmotive t

PROCESS DESCRIPTION AND EFFECTS

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The primary benefit of shot peening is the generation ofan even compressive stress pattern and the effective elimina­tion of microscopic defects in the thincsurface shell of thepart.

Shot peening is more effective in reducing fatigue fail­ures in parts subject to cyclic loading~ Failures originatein surface areas under tension load and· cracks will propagatefrom a surface defect or other stress riser. Shot peeningprevents these failures by creating an .even compressive stresslayer in the surface of the part.

As each individual particle of shot strikes the metalsurface, it produces a slightly rounded depression. Plasticflow and deformation of the surface occur at the instant ofimpact and the edges of the depression may rise slightly abovethe original surface.

As the part is loaded, its critical surface area will notdevelop tensile stresses until the peen. induced compressivestresses are first overcome; this permits an increase in theallowable stress level and the service life of the part. Theeffect of shot peening toward the improvement of the·surfaceintegrity of the part is also important. No matter how care­fully a part is manufactured, it will exhibit some surfaceimperfections. These may be localized areas of tensilestresses or phase transformations from machining or grindingas well as pits, scratches and other sUfface defects.

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Thomas N. Fritts

In a completely peened part the compressive stress layermay have a depth of 0.001 to 0.020 inch, depending upon thepeening impact and the hardness of the work surface (Fig. 1).The tensile stress in the under layers being within the yieldpoint of the fibers, an equilibrium of stresses results andthe surface fibers are said to be in residual compression whensurface layer must return to zero stress before the fibers aresubjected to a te~sile stress. This tensile stress will bemuch lower than the surface compressive, stress because it isdistributed in a comparatively thicker core.

Most failures in machine parts have their beginning atstress risers' on the surface. These ar~ the result of a ten­sile stress or tearing apart of the metal fibers. Metal pa!tssubjected to frequent cyclic stressing or frequent reversal ofstress, tension and compression, or reverse twisting or tor­sional stressing, may ultimately fail through a fracturebeginning at the surface. This 'type of~failure is known asfatigue failure. .

The magnitude of residual stress that can be induced byshot peening is limited to about half the yield strength inhard metals. This can be extended to near full yield strengthby subjecting the surface to tensile stress while peening.Strain peening is commonly used on leaf: springs and in thepeen forming of aircraft wing contours. Heat treatment ormachining operations which would remove: the induced compres­sive stress layer are not feasible afte~ shot peening. Lighthoning or lapping is permitted on steellbut is usually limitedto one-tenth of the test strip arc height.

SHOT PEENING USES AND ITS APPLICATION

This process is used extensively on gear teeth, driveshafts, torsion bars, axles, ball studs', high strength fas­teners and oil well drilling equipment." Often it is appliedonly at a critical area such as a fille't radius. OVersprayof shot is often permitted. If not, simple masking techniquesare usually sufficient. New uses of ~hot peening in~lude theprevention of fatigue failures in automotive wheel rims andrailroad car wheels. Peening will reduce the notch sensitiv­ity of hard steels. When applied to highly stressed partsprior to chrome plating, peening acts t? prevent any cracksor imperfections in the chrome deposit from spreading into theparent part.,

Thomas N. Fritts

High performance engines may be built from 'standardengine parts if the parts are shot peened prior to assembly.Peening is used on aluminum die cast transmission housings orgearboxes to prevent "leakers" or loss of lubricant throughporous wall areas. Bearing surfaces and radius areas of enginecrankshafts are peened to reduce fatigu~ cracking from repeatedstress reversals, as well as to retain lubricants--the shotindentations acting as minute reservoirs. The aerospaceindustry has long' used shot peening for.the improvement offatigue life of landing gear and other $tructural members.It is used extensively on jet engine parts such as compressorand turbine rotor components, including compressor blades,turbine blades, gears and hubs as well ~s many other parts.Some of this work is done with glass beads.

Stress corrosion is a problem which has become criticalin recent years as a result of the use of higher strengthmaterials. Failures are caused by the complex interaction ofcorrosive (environmental) attack and su~tained tensile stressat the surface of a metal. Sustained static loadings arepresent in such structures as aircraft landing gear, which arestressed by internal hydraulic pressure while at rest on theground.

Shot peen forming of aircraft skins has been successfullycarried out for a number of years. Only recently, however,has the full potential of this forming system been realized,eliminating the need for costly stretch;and forming dies.

,1

Peening is sometimes used to straighten parts which mayhave bent in heat treatment, etc. Rings which are out ofround can sometimes be corrected by peening. Machined bulk­heads and other large structural shapes;. which twist or deformas a result of machining operations, c~ sometimes be straight­ened by selective peening. The technique used in such a caseis to select and peen a critical area which, .. if expanded orelongated, will straighten the part.

Recently, as a solution to the proplem in the overhaul offan jet engines where the mid-shaft was .bowed or bent beyondserviceable limits, selective shot peening was used to restoreeight shafts to the required .003 to .005 F.I.R. Originally,these shafts were out of tolerance in the rarige of .. 031 to.085. The price per part savings is approximately $43,000.00.

Thomas N. Fritts

Shot peening has been poorly under~tood by many designengineers and its application has often been left to thosewho do commercial shot peening. On the other hand, theresults of extensive testing to determine the best peeningapplication for a specific type of part .often are not widelydisseminated or are withheld as proprie~ary information.

RECAP OF SHOT PEENING USES

1. Increase in fatigue life2. Reduction of stress corrosion cracking3. Reduction of fret corrosion fal1ures4. Straightening5. Forming6. Reduction of tensile stresses after grinding7. Prior to chrome plating to increase fatigue strength8. A testing mechanism for electro-plated surfaces9. Reduction of porosity in casti~gs

10. A slight expansion of undersized parts11. Treatment to improve oil retentive properties of

the surfaces12. Improvement of decarborized steel surfaces13. Deburring14. Decorative texturing of surfac~s

TYPES AND SIZES OF S~OT

Shot used for shot peening can be ~ade from severaldifferent materials. These include cast steel shot, cast ironshot, malleable iron shot, chilled iron shot, stainless steelshot and glass beads.

Shot can be identified by a standardized numbering system.Si.zes range from S-70 to S-1320. The shot number is coded toindicate the approximate diameter of the individual pellets inthousandths of an inch. For example, S~70 shot is 0.007" indiameter. S-280 shot is 0.028" in diam~ter. Standard sizespecifications for cast shot has been established by the SAEand is shown in Figure 2.

The life of steel shot varies inversely with the shotdiameter as shown in Figure 3. It must also be rememberedthat not only is the average life of small diameter shotlonger in number of impacts, but thenurober of pellets perpound is greatly increased as the shot diameter is reduced.The number of shot pellets per pound is shown in Figure 4.

Thomas N. Fritts

The ideal peening application would use perfectly roundshot of uniform size with each particle -having the same hard­ness and density and applied at the same velocity and impactangle. In practice, of course, this is impossible.

Selection of the shot size depends ,upon thickness of thework, the appear~ce desired, the size of fillets and theintensity desired. At a given velocity,' smaller shot willgive a lower intensity and larger shot will give a higherintensity. More particles per pound provides wider coverageand is therefore more economical. Peen -forming of heavysections may employ steel balls up to 1/4" in diameter orlarger'. The use of the larger balls pr(f;vides a smootherfinish.

Most peening as in manufacturing aerospace and otherapplications is done with cast steel shot sized to SAErecommended practice J444 with a hardness range of 40 to 50Rockwell C. Some aerospace companies and the u.s. Air Forcehave developed shot specifications which require closer con­trol of size, -hardness, allowable non-rounds and unacceptableshapes.

The use of glass beads as a peening medium has increasedin recen~years. This material is appl~cable to thin sectionsrequiring low peening intensities. Steel shot will leaveferric contamination of aluminum. This:can be removed by aciddipping and in some cases by over-peening with glass beads.Because glass beads are inert, they wil~not contaminate thealuminum or titanium and other exotic space materials.

SHOT VELOCITY AND IMPACT -_ANGLE

Air pressure or wheel rpm are varied to change shotvelocity. Since only the shot traveling at ~he highest velocityin a given application will produce the·maximum peening inten­sity, any shot at lower velocities has the effect of extendingthe time required to reach desired intensity. Figure 4 showingthe number of pellets per pound versus ~hot size illustratesthe economical advantage of using the smallest size shot whichwill accomplish the task.

MEASUREMENT OF SHOT PEENING INTENS ITY -- PROCESS CONTROL

Shot peening intensity, or the effect of shot peening ona work piece, is measured by determining the resultant effect'on standard Almen test strips as shown in Figure 5.

Thomas N. Fritts

Specifications for standard Almen test strips are asfollows:

1. SAE 1070 steel CRS spring steel2. Bright or blue temper·finish3. Heat treated and tempered to 44-50 Rockwell C4. Arc height, flat ± .0015" as measured on

standard,Almen gauge

The test strip must be shot peened:while firmly locatedin a holding fixture. Design of the Almen test strip holdingfixture is shown in Figure 6.

Since the cold working of the outer layer of metal byshot peening causes a compressive stres~, the test strip is·contoured in a manner with the shot peened surface on theconvex side as shown in Figure 7.

The amount of contour in a test strip after shot peeningis most conveniently measured by a gauge developed by J. o.Almen of the Research Laboratories Division of General MotorsCorporation. His gauge is known as the "Almen Specimen Gauge"and is shown in Figure 8.

Shot peening intensity results fro~ the momentum of theshot hitting the surface and the number of hits per squareinch. This intensity is usually measured prior to shot peeningproduction parts by exposing one or more test strips in thesame manner and for the same length of time as the productionparts will be processed. 1

The shot peened test strip, after exposure, is contouredboth longitudinally and transversely. The Almen gauge is aprecision instrument having four bearing points in fixedpositions, accurately spaced. The dial· indicator plunger islocated on the unpeened surface, mid-way bet~een the fourpoints and gives the arc height reading'in thousandths of an.inch. For example, 0.15 inch is equivalent to .015 Almen Aintensity.

Recommended shot peening intensities for steel, aluminumand titanium alloys are given in Figure' 9. These values shouldbe used only as a guide. To obtain the maximum value fromshot peening, the actual part under simulated or actual condi­tions should be tested for optimum processing.

Shot peening to a high Almen intensity can cause unwantedwarpage in a production part. Figure 9 can be used as a guideto avoid part warpage.

Thomas N. Fritts

SHOT PEENING 'PROCESS VARIABLES

The effect that shot peening has on metallic surfaces isaffected by many process variables. Because of this, it isimportant to be aware of the effects of·each variable andcontrol the process to insure uniform a~d consistent shotpeening quality. Following is a list of, process variables:

1. Shot material2. Shot hardness3. Shot uniformity4. Shot shape5. Shot velocity6. Angle of impingement7. Work piece material8. Work piece hardness9. Work piece heat treatment

10. Shot peening coverage11. Almen arc height12. Post shot peening thermal treatments13. Post shot peening mechanical treatments14. Air pressure15. Distance from nozzle to work piece16. Speed and rotation of work piece17. Nozzle movement, vertical and horizontal,

as indicated by saturation curve on .Almentest strips

18. Centrifugal wheel RPM

Since there are so many processing:variables affectingthe quality of shot peening, processing standards are necessaryto insure consistent quality. Many manufacturers have devel~

oped their own process standards either:to cover the shotpeening process in their plant or to coyer specific productionparts. Many of these standards are patterned after the,following military and engineering specifications:

MIL-S-13165BMIL-S-85l

MIL-A-9954

Shot Peening of' Metal PartsSteel Grit, Cut,Wire Shot,Iron Grit, Shot.Blast Cleaningand Peening .Abrasive, Glass Beads

ARC HEIGHT EXPOSURE CURVE

Shot peening engineering requirements usually specify theshot size and type, Almen arc height, percent of coverage anaarea of part to be shot peened.

Thomas N. Fritts

At this point, it would be well to. define some terms whichfrequently become confusing when specifying shot peeningoperations.

Almen Arc Height

The amount of total indicated curv~ture on a standardtest specimen in thousandths of an inch when placed in a stan­dard Almen arc height gauge.

Visual Coverage

One hundred percent (100%) visual coverage occurs when ashot peened surface no longer retains a~y of the original sur­face which has not been struck by peening shot. Fifty percent(50%) coverage refers to that surface which retains 50% of its'orig~nal surface. A practical value of'98% coverage iscommonly used instead of 100% to avoid close inspection with amicroscope and to avoid insuring that the absolute last portionof a surface has been covered. Figure 11 shows the practicalreasoning behind the use of the 98% coverage figure.

SHOT PEENING INTENSI~

The Almen arc height is measured on an arc height expo­sure curve where, under ideal condition~*, the curve becomeshorizontal or flat. Amore practical d~finition, under lessthan ideal conditions, is the Almen arc: height value of thetest coupon when exposed for a time sufficient to be past the"knee" of the arc height exposure curve; or, the Almen archeight obtained at an exposure time, which when doubled doesnot exceed 10% in arc height value. '

If Almen test strips are exposed to a uniform blaststream of shot for progressively longer' exposure times, thecurve levels. Saturation is that point, at which the rate ofincrease of intensity with exposure tim~beings to decline.The saturation point is just to the right of the knee of thecurve and it indicates that the test strip, but not necessarilythe part, has approximately 100 percent coverage. Beyond theknee of the curve, ,a 100 percent increase of exposure timewill produce an increase of approximately 10 to 15 percent intest strip arc height. The use of this- curve is valuable in

*Ideal .conditions would include shot of' constant velocitystriking the specimen at a constant angle and identical in'size, weight, shape and hardness.

Thomas N. Fritts

evaluating and controlling a peening application. This istypical of arc height exposure used in the shot peening appli­cations in all types of metal and a multitude of partsprocessed today. In making a set-up, the test strips arecarefuliy placed to cover all areas to be shot peened and topresent the same angies to the stream to duplicate actualproduction conditions.

SHOT PEENING EQUIPMENT - AIR BLAST AND CENTRIFUGAL WHEELS

The air blast methods are divided into three differentbasic systems: Direct Pressure, Gravity and InductionSuction.

Direct Pressure

In the direct pressure system, the,shot in a pressurizedpot is fed directly into the blast cleaning equiPment hoseand discharged through a nozzle. The shot is ejected athigher speeds and with greater concentration than in thegravity method. Nozzles and lines are subjected to greaterwear and more shot is consumed.

GravityI

In the gravity fed system, a shot feed hopper is locatedabove the nozzle and shot flows by gravity to the nozzle whereair entering from a separate line mixes' with the shot to pro­pel it. A wider spray pattern results from this method andthe shot has less force than that from the direct pressuretype.

Induction Suction

In induction suction blast cleaning equiPment, a varia­tion of the gravity blast method, the medium is drawn fromthe shot collecting hopper into the nozzle by a partial vacuumcreated in the suction line leading to the nozzle by highvelocity air flow. A larger prop~rtion of air to medium makesthis method ideal for light appiication~.

Thomas N. Fritts

Air Blast System .

The most efficient air blast system for peening, in termsof cubic feet per minute of air required per pound of shotmoved, is the direct pressure method.

In the direct pressure system the peening shot must becontained in a pressure vessel so that it will drop by gravitythrough a metering orifice into the high pressure air line.When the system is continuous the pressure vessel has anupper chamber which can be vented to the atmosphere while itis filled and can be pressurized whiletthe charge is droppedinto the constantly pressurized lower chamber, which in turnfeeds continually into the high pressure air line. A cyclingmechanism controls this operation and insures a continuousflow of shot into the lower pressure vessel.

Because of their size, air nozzles,are readily aimed toreach dif£icult areas. Also, various nozzles ·can be adjustedto different angles and air pressures in one set-up for adifficult part.

Airless Blast System

Centrifugal wheels are efficient peening shot throwingdevices. They have been used for many: years for peeningautomotive springs and other parts for; which high productionwas required and for which the wheels tould be fixed inlocation.

Shot from centrifugal wheels does not vary in velocityand its control is simply a matter of dial settings.

In addition to a method for propelling the shot, a peeningmachine must provide means of collecti~g the spent shot andrecycling this shot for reuse. The shot should be passedthrough an air wash separator for the removal of extreme finesand a coarse screen for the removal of'debris and foreignobjects. A screening system should also be provided to main­tain correct shot size. A shot replenisher may be requiredon larger machines. Work handling systems may range fromsimple turntables to complex, yariable speed work cars.Indexing table systems with one or more peening stations areadaptable to small and medium sized parts and extensiveproduction. :

Thomas N. Fritts

Equipment must be automated for repeatability, tooling,masking and fixturing as required for proper positioning androtation for coverage.

Remember I there is no way to Lnspe'ct; shot peened work ..You must be sure of the peening operation--the parameters,the equipment and the process control~to be sure of theresults.

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Thomas N. Fritts

REFERENCES

(1) Almen, John, Black, Paul, "Residual Stressesand Fatigue in Metals"

(2) "Society of Automotive Engin~ers AerospaceMaterials Specifications", Ar1S 2430

(3) "American Society of Metals"" publication

(4) "SAE Standard and Recommendations, SAEManual on Shot Peening", SAE J8082

t

(5) Military Specifications:

MIL-S-13l65MIL-S-85lMIL-A-9954

(6) Society of Manufacturing Engineers,"Tool Engineers Handbook"

(7) Vacu-Blast Corporation

Aft ..... w_z% ..... 50% CWnuIaWeIolu. 1% ..... He,.t .....4~eMA u.s.sa- atlU.s. QC1u.s. Win. 90% on u.s. ~SfMe ,",--$.A.Lsan sa sa- ScrMR ~~ s- ~ ~

.0937 5 (.157) 5 (.1320) 7 (.mo) g -(.0937) 10 (.0787) 10(1 sq.in. S930 P·93

.om &(.132) 7 (.lllO), 8 (.0937) 10 (.07'm 12 (.0661) 10/1 sq. in. S7SO P·7S

.0661 7 (.ll1) 8 (.0931) 10(.0781) 12 (.0661) L4 (.0555) 12/1 sq. in. _ SO6/) P~

.0555 8 (.0937) 10 (.0787) 12 (.0661) 14 (.0555) 16 (.0409) 12/1 sq. in. S550 P·5S

.0469 10 (.0787) L2 (.0661) 14(.0555) L6 (.0469) 18 (.0394) 15/1 sq.in. S4fiO P-46

.0394 12 (.0661) 14 (.0555) 16 (.0409) 18 (.0394) 20 (.0331) 20(1 sq. in. S390 P·39

.0331 14 (.0555) 16 (.!l469) 18 (.0394) 20 (.0331) ZS (.0280) 201*sq.in. S330 p.3J

.0280 16 (.0469) L8 (.0394) 20 (.0331) 2S (.0280) 30 (.0232) 20M sq.in. ~ p.2!

.0232 18 (.0394) 20 (.0331) 25 (.0280) 30 (.023Z) 35 (.0191) 20M sq. in. S230 p.2J

.0197 20 (.0331) 2S (.028» 30 (.0232) 35 (.0191) ~ (.0165) 20M $Q. in. P·IS

.01&5 2S(.0220) 30 (.0232) 35 (.0197) ~ (.0165) 4S(.0138) 20M sq.in. S110 P·17

.0138 30 (.0232) 35 (.0197) 40 (.0165) 45 (.0138) SO (.0111) 301* sq. in. P·13

.0117 35 (.0197) 40 (.0165) 4S(.0133) SO (.0117) 80 (.0070) 40M sq. in. SIlO e-u

.0070 40 (.0165) 45 (.0138) 50 (.0117) 30 (.0070) 120 (.0049) 40fA sq. in. S 70 P·7

1. Standard "--Inc Silo« Hardn... 50 R~U C AII.rq.2. $.A.£. peeninc dalpatlon. PACeded by P _,. omitted hun SAE:SpedflQt!on in 1!M7 FI.GURE· 23. PfWTINm _inc sl\QC I. conditioned to ellminat. scale

SHOT SIZE

S-780

S-660

S-550

S-.460

5-390

5-330

S-280

5-230

_ .__ S'"'!170

S-110

5-70

0.74~ 0.745O.7~ 0.750

11--·-c I l~-3.000~.O.01S -1-1,. l-- 3.000!O.01~ -+-,-

T••• Sino N O.OJ?!o.OO1 Teststrip A O.05l;o.OO1

I . !~~I-- J.OOO!Q01S ........-t,...,..

Tttt strip C O.094~.O.OO1

PELLETS PER POUND

9,350

16,200

23,625

38,100

74,250

114,850

206,400

376,600

852,6.50

2,520,~00

9,651,,000

\ -

cations.Analysis of stock: SAE 1070, cold-rolled

spring steel edge No. 1 (on 3·in edges).Finish: Blue temper (or bright). uniformly

hardened. Heat set between flat plates underpressure for a minimum of 2 hr at 300°F±25° F. Hardness 44 to 50 Rockwell C.

Flatness: ±O.OOI in arc height (strip A);±O.0015 in (strip C); ::0.001 (strip N) asmeasured on ~ standard Almen gage No.2.

FIGURE 4

FIGURE 5

Q940[945Hoi.canters

PEEt.JED SIDE

4::?>TYPICAL REACTION ON PEENEDAL~ TEST STRIP

FIG. 7

0.37'30.377

Four 3/16hardened steel balls

I ~. 25

Ll.2 32

. 16Fig. .8 Almen gage No.2 for measuring test-strip arc height.

r.Bterial

Under •090 inchthickness

Steel under200,00' psi

Steel over200,000 psiand titanium

Aluminum alloys(Steel Shot)

Aluminum alloys(Glass beads)

.004 to .008 N

.090 to .375 inch .008 to .012 A .006 to .010 A .006 to .010 A' .008 to .012 Nthickness

over .375 inchthickness

.012 to .016 A .006 to .010 A .010 to .014 A

F!GORE 9.

.012 to . 016 N

tFIGORE 10 - PART THICRNESS VERSUS SHOr _.PEENING INTENSITY

Elamo/.Lit C,: 43'" lone cycle)

T, =2For 3 cycl..

T3=6C3=SZ"1e

THIC!\NESS OF PART

1/16"

Jl8"

Jl4"

3/8"

5/8"

3/4"

7/8" or greater

FIGURE 10

.0042A

'.OOBA

.014A

•alBA

.02lA

.007C

.oosc

.• 01oe

100r-----=======i90

~ 80U 70'":60~ 50: 40330Z 20.:; 10

o~,~Z""'!3~4~~""6~7~8...9U.'.Io.IO.,'.....' '·'.....3 '·4.....15....'.....6 '·7"""8Factor of UPOW. liMIT)

Fig. 11 Relationship of peening cover­age to exposure time. Coverages above100 percent are direct multiples of timerequired for 100 percent coverage.