OptimumShot Peening

Specification - IIMark Lawerenz.

lMetalllmprovement ICO•.• Inc •• IMlilwauk,ee. W'IIlmaets Ekis .•

F,ord iNew H,oUand•.New Holland, PA

Summary: Following is the second part of anarticle begun in our last issue. The first partcovered basic shot peening theory, shot peen-ing controls, and con iderarions that should gointo developing a shot peening specification.Part II covers optional peening methods and therelationship of shot peening specifications tothe drawings.

Optional Peening Methodsand Additional Considerations

Some additional peening methods and con-siderations which the reader may want to in-clude as part of any general. specification areshown below.

Strain Peening or Stress Peening - Thistechnique is applied when part are stressed inone direction only and longer fatigue life isdesired than that obtained by conventional meth-ods. The pari. is shot peened in a stressed or

Endurance limit *150

,;,- ..........,

UNPEENEO 8-12A 8 -12A+ 7 - 9A

loaded condition, and compressive tresseproduced by the peening can be as high a . thecompres ive yield tress of the material. it elf ..This technique has been used heavily i.1I nu-merous industries.

Dual lntensity Peening - Thi .rechnique alsocan be u ed when substantially longer fatiguelife is required, Research done 011 carburizedteel indicates that dual peening, which i high

intensity shot peening. followed by lower inten-silty hot peening with smaller shot, increa esthemagnitude of surface compre sive re idualstress ,I Additional testing on other materialhave confirmed thi data.2.3 (See Figs, 1 and 2.)

• Average' Fatigue Ufe - Million Cycles

F:lg.l. Dura Peening Titanium 6AI·4V Blade.)

.• 1000 KSIIat 10,000,000 cycles,

Fig. 1 - Dura. Peening Rene 95 speeimens.i

Fatigue Ufa·

25r----------------,

30 GEA.R TECHNOLOGY

7A9A l1A lIA + 9N

Plating and Salvage Methods - If machiningdi crepancies in production are sometimes al-vagedby building up an area for remachiningby the u eof chrome or other plating techniques,or if plating is used for a wear or protectivesurface •. hot peening prior to plating can beused. T:lIe hot peening will prevent crack propa-gation from microcracks in the plating to theparent metal if the part i ubjected to a cyclical

'HARD

BASE

Fig. 3 - Plating cracks will not propagate intopre- stres -ed base.

load. Cracks w.ill not propagare into layers ofcompressed stress. (See before and after shotpeening illustrations in Fig. 3 ..) In addition,significant increases in fatigue' trength closelyapproximating original unpeened surfaces areshown in Fig. 4. In some cases, shot peeningprior to plating may be required by other con-tractual agreements. Specifications, such asFederal. Specification QQ-C-320 and MIL-C-26074A. require shot peening on steel pans thatare chrome or electroless nickel plated,

An added side effect is the prevention ofhydrogen ernbrittlement by shot peening of thisparent metal prior to the plating operation.Since atomic hydrogen is extremely mobile andable (Q penetrate and interact with metal easily.the metal's ductility and ability to withstandcyclic loads is reduced. Peening has beenproven effective in retarding the migration ofhydrogen through metal.5 (See Fig. 5.)

COn/our Correction - Just a . it is po sible tocreate a de ired curvature and shape to compo-nents by hot peening, it is also possible tocorrect the shape and form. of parts. The shotpeening process avoids the unfavorable (ten-sile) residual stresses produced by other straight-ening methods and in tead produce favorable(compressive) residual stress. (See Fig. 6.)

Increasing Wear Due to Work Hardening - Inthe discussion all material considerations. con-siderable pace was given to whether a material

100.000 70

120,(1)0

\ ~V BARE· NOT PEENED I

? I Q.\Li- SHOT-PEENED, CHROMIUM -PLATED ANI BAKED

\ ~GHROMIUJ PLATEDAND BAKE

>II,.

"

I U;a.~ 80,000wIX:~(I)

60.000

40,000 281100MILLION1 MllUON10,000 10 MILUON100,000

Fig •.4 - Cycles to failure 4340 steel. 52-53 Rc rotating beam fatigue."

0.8

I AS·ANNEALEDI 'I

I I

Ji .A. ~I I I

I. & 'I,

:' 0 I,

1 0 I

:11 !II1! 0

ill 0 Ji

0.7

M'ark Lawerenzis Monager-TechnicolServices for Metallmporvement Company.MillI'{Jllkee Division,Milwaukee. WI. and aregistered professionalengineer ill the state ofWisconsin.

0.6

':E 0,.5<.J

.~ 0.4 (AIx

'~ 0.3

0.2

O.

oo 6D 1,0 180 240 300

TIME, MIN.

SHOT·PEENED (G SOP]"II I

I,

I JI:,

I

I!I I (B]

I I nJPrY i

10:-'

IP0

I0

Q

60 120 180 240 300TIME. MIN.

0.8

0.7

0.6

N

0.5.::!;u<l

0.4::I.,i...,

0.3

0.2

O.J

00

Fig. 5 - Effect of shot peening on hydrogenmigration. JB, A/CM2 hydrogen permeation rate.A) Non-treated surface. BI hot peened urface.5

Contour correction. Al

Imants E'kisis Senior MaterialsEngineer at Ford NewHolland, New Holland,PII. He hal' also bunguest lecturer all

gearing subjects atMarquelrl! Universityand is all active membera/SAE.

JANUARY/FEBRUARY 11192 31

would readily work. harden. If this factor is amajor consideration, then it should be addressedin the specification. (See Fig. 7) For materia.lswhich cannot be heat treated, but require wearresistance, shot peening should be considered.

Porosity - When this is a concern. porosityshould be reviewed as a specification option.Typica1Jy this is not utilized for the compressedstress benefits, but rather is used to compact thesurface 7 or reveal some sub-surface porosityprior to machining. It can, therefore, be utilizedas an inspection tool before machining of ques-tionable castings.

Salvage; Grinding; Before and After ShotPeening - In cases where a severe grindingoperation has developed resultant residual ten-sile stresses and surface brittleness, shot peeningof the surface after grinding should be consid-ered. Fig. 8 reveals S-N curves for a part origi-nally designed for an endurance limit with agentle grind., the resultant lowered endurancelimit after grinding, and improved endurancelimit of the severely ground surface followed byshot peening.

0.'020 'O.OJO'0.01'0 0.040 '0.050

Fig. 7 - Hardness vs, distance from surface for shot peened hastelloy"en samples."

120,'0'00 ....... -, .,... ...,.- ---, 84

70I

NE56 {,

.:><

GENTlE GRIND 42

4{!'oOO ..__ .10,000 1,0011,000

CYCLES TO FAI LU RE

10,000,000.28

100,000,000100,000

Fig. 8 • Shot peening improves endurance limits of ground parts. Re-versed bend.ingfatigue of fiat ba.rsof Rockwell hardness C45.8

·32 G EAR TEe H N 0 ~ 0 G v

Another technique that can be used, espe-cially on particularly difficult grinding opera-tions or materials, is to shot peen prior to grind-ing to prevent grinding cracks ..Sincegrinding ofcarburized gear can produce high ten ile tresses,these stresses can initiate cracks in the toothsurface. Shot peening prior to grinding wingreatly reduce this tendency. Peening here isused to prevent crack propagation from the grind-ing and not to increase bending fatigue strength.

Stress Corrosion Cracking - ~n particularlyItostile environments where a. material beingused may be affected adversely by general cor-rosion coupled with. residual or applied tensilestresses, shot peening may be a consideration.The peening will change the surface residualtensile stresses to compressive stresses, whichwill eliminate the conditions needed to promotestress corrosion cracking.

The Part Drawing'Once a sati.sfactoryin-house specification has

been established which addresses the particularneeds of a company, it is still. necessary totranslate this information to particular gears.The general specification should assist the de-sign professional regarding the necessary stepsto properly select an optimum drawing specifi-cation. The information must then be trans-ferred to the manufacturing drawing.

In specifying shot peening requirements onpart drawings, the fotlowing parameters shouldbe identified:

L Areas to be shot peened2. Areas to be masked3. 'Optional areas4. Areas where shot peening fades out5. Shot size, hardness, and material6. Locations for intensity verification and

intensity range at each location7. Coverage requirements for aU areas to be

peened, including the method used for coveragedetermi nation

8.. Applicable shot peening specification.Fig. 9 provides a theoretical example of a

gear with a suggested specification, Utilizingthe above points, the analysis of this specifica-tion is as follows:

1. Areas To Be Shot Peened - These are notedby DIM' A", and further critical area are iden-tified by "XXX." Five primary areas require theproper intensity. These are at the tooth rootf.illet, the gear pitch line. two shaft fillet transi-

tion areas, and the main shaft body .. Since onlyone peening operation is to be performed, theshot selection would indicate that the apparentgeometric limiting factor of the shot i the filletradii ofrhe gear teeth, Most likely the main shaftis being peened because the shaft may alsoexperience problems with fatigue. It is possiblethat some machining may occur onthe shaftbody after shot peening, so rather than maskthis area, peening is being allowed .. The gearpitch line area is noted becau e pittiag of thegear tooth may occur.

2. Areas To Be Masked - These are noted byDlM "B" and !DIM "C", Most likely the 0.0. ofthe gear has limitations on the potential of burr-ing at the top land, This is costly and should beavoided unless alternate ways are not available.A potentia] alternative solution may be to breakor radius all sharp edges in the areas prior topeening. This can minimize or eliminate thepotential to burr .. The thread' at the shaft. end donot require peening and must be masked asoptional. peening could damage tile e.

3, Optional Areas- Noted by DIM "D," theseare the holes in the gear body.

4. Areas Where Shot Peening Fades Out -Not applicable to this example.

S, Shot size, Hardness, and Material. - M]110 shot. inten ity 16- lOA; the MI 110 designa-tion defines a cast steel shot.

6. Location for Intensity Veriflcation andIntensity Range at Each Location - Only oneintensity is specified and is marked by "XXX."If other intensities 'Orshot size are u ed, addi-tional callouts and ymbols are required.

7. Coverage Requirements For All Areas ToBe Peened, Including tile Method Used forCov-erage Determination - 125% coverage verifiedby Peenscan."

8. Applicable Shot Peening Specification -MIL-S-13165B.

This drawing specification clearly denotes aproper shot peening requirement and shouldeasily be accomplished by the manufacturinggroup or vendor. This specification shouldreadily coincide with the company in-housepeening specification. Note, however.that thisspecification may not, and mo t likely will not,work on parts similar to this part. h is stronglyencouraged that each gear requiring shotpeening be first evaluated based on the gen-eral in-house specification prior to definite

DIM.'B' DIM. 'C'

I~XXXTYP.~,.

I.25.20

XXX !NTENSITY VER!F!CAJ!ON WCATIONS

SMT ~EfN AREA A PER M!L·S·lJI658.USING MI nOH SHOT.INJENSlTY 6-lOA.125110COVERAGE VE.RlflED BY PEENSCANMASK AREAS BAND C HOLES DAREOPTiONAL BREAXO\ITSIDECORNERS 0 018"SUORE PEEIIIING

Fig, 9 - Theoretical ex.amp~~~f~ gear with a suggested, pecificmion.

shot peening callouts being made on a manufac-turing drawing.

SummaryConfusion and some misunderstanding in

properly specifying shot peening can cause dif-ficulties in the manufacturing process. Concisein-house specifications covering consideratioasfor shot peening, coupled with accurate manu-facturing drawing callouts, can optimize the useof this effecti ve tool. With the in-house specifi-cation addres,~ing the particular needs of tbemanufacturing company's gearing requi rerncnts,and the correct specification on the manufactur-ing drawing conveying this to the vendor, shotpeening can be utilized to it fulle t advantage .•References:l. AHMED. A. and CROUCH, D. "Dual Shot Peening to

Maximize Bencficiul Residual Stresses in Carburized

Steels." ASM International Conference on Carburizing.

Processing and Performance. July. 1989.

2. "Determination of Mechanism by Which Surface

Treatment Affects the low Cycle Fatigue resistance of

Advanced Disk Materials." October, 1986. General

Electric Co.

3. CAREK. G.A. "Shot Peening Forti·6AL-4V Alloy Com-

pressor Blades." NASA Technical Paper 2711. 1987.

4. COHE • B. "Effects of Shot Peening Prior 10 Chro-

mium Plating on the Fatigue Strength of High Strength

Steel." Wright Air Development. Center. Technical Note

57-176. June, 1957.

5. "Effect of Surface Modification on the Kinetics of

Proton Discharge and Absorption :into Steel." Thesis.

1966. Ohio State University, Columbus, Ohio.

6. HO NAKER, L.P. EJ. DuPon! Co .. Inc.

7. ANDERSON. E.F. "Shot Peening 'to Reduce Porosity

in Die Casung." Modem Meta/s, 1957.

8. TARAZOV, LP. and GROVER, H.1. "Effects of Grind-

ing and Other Finishing Proces es on the fatigue Strength

of Hardened Steel." ASTM Proceedings. Vol. 50,1950.

©19R9, Societ» of Automotive Engineers, Inc. Re-printed witt: permission.

JAN U A R Y IF E B R U A R 'f , II 9 2 33