Ex Parte Hoffman

Board of Patent Appeals and Interferences

Jun 29, 2012

09965162 (B.P.A.I. Jun. 29, 2012)

UNITED STATES PATENT AND TRADEMARK OFFICE
__________

BEFORE THE BOARD OF PATENT APPEALS
AND INTERFERENCES
__________

Ex parte STEVE E. HOFFMAN
__________

Appeal 2009-013180
Application 09/965,162
Technology Center 3700
___________

Before SCOTT R. BOALICK, CARL W. WHITEHEAD, JR., and
BRADLEY W. BAUMEISTER, Administrative Patent Judges.

BAUMEISTER, Administrative Patent Judge.

DECISION ON APPEAL
Appeal 2009-013180
Application 09/965,162

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SUMMARY
Appellant appeals under 35 U.S.C. § 134(a) from the Examiner’s
rejections of claims 1-6 and 13-19:1, 2
1. Claims 1, 2, 6, 13, 14, 18, and 19 stand rejected under 35 U.S.C. § 102
as anticipated by Vankov (US 5,802,932).
2. Claims 1, 2, 6, 13, 14, 18, and 19 stand rejected under 35 U.S.C. § 102
as anticipated by Williams (US 5,477,616).

1 As noted by Appellant, the Answer incorrectly rejects claims 20, 21, 25-27,
and 31 under 35 U.S.C. § 102 as anticipated by Vankov. (Reply Br. 2
(citing Ans. 4); see also Ans. 6-7). Claims 7-12 and 20-31 were canceled.
(Amdt. 2-5 (filed September 27, 2001); Advisory Action 2 (mailed August
16, 2005); App. Br. 4.)
2 The record is unclear as to the ground(s) under which claim 19 stands
rejected. In the respective headings of the 102 rejections over Vankov and
Williams, the Examiner states that claims 1, 2, 6, 13, 14, and 18 – but not
claim 19 – are rejected (Ans. 3, 5). However, the Examiner references claim
19 in the bodies of these rejections (e.g., Ans. 4, 5). In the Answer’s
Response to Arguments section, the Examiner references claim 19 in
relation to both Vankov (Ans. 11) and Williams (Ans. 12). However,
Appellants assert that claim 19 is rejected as anticipated by Williams, but not
by Vankov (App. Br. 9), and the Examiner agrees that Appellant’s statement
of the status of claims contained in the brief is correct (Ans. 2). Under these
circumstances, we understand claim 19 to be rejected as anticipated by both
Vankov and Williams.
With respect to the obviousness rejections, the Examiner states in the
heading of the rejections that they are based on either of Vankov or Williams
in view of Hashimoto and that “[c]laims 1, 2, 4-6, 13, 14, 1-21 22-27, and
29-31 stand rejected” (Ans. 7) and "[c]laims 3, 15, 22, and 28 stand rejected"
(Ans. 9). These statements clearly include a clerical error, rendering it
unclear whether claim 19 was intended to be included in either of them. The
bodies of the obviousness rejections make no reference to claim 19 (Ans. 7-
9). Nor does the portion of the Answer’s Response to Arguments Section
that relates to the obviousness rejections (Ans. 13-14). Under these
circumstances, we understand claim 19 to not to be rejected as obvious.
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Application 09/965,162

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3. Claims 1, 2, 4-6, 13, 14, and 16-18 stand rejected under 35 U.S.C.
§ 103 as obvious over either Vankov or Williams in view of Hashimoto
(US 5,873,770).3
4. Claims 3 and 15 stand rejected under 35 U.S.C. § 103 as obvious over
either Vankov or Williams in view of Hashimoto and Gakhar
(US 5,555,788).
We reverse the above rejections.
Pursuant to our authority under 37 C.F.R. § 41.50(b), we enter a new
ground of rejection, rejecting claims 1-6 and 13-18 under 35 U.S.C. § 112,
¶ 2, for indefiniteness.

THE INVENTION
Appellant describes the invention as a saw blade with a high precision
finish for providing a low friction surface, as well as a polishing method of
making such a saw blade. (Abstract.) According to Appellant (Spec. 10:5-
9.),
[d]uring a normal [saw blade formation] heat treating process,
microscopic fractures form within the steel blade. These
eventually become the source of blade failure by cracking.
The high speed precision polishing process used in the present
invention produces a universal stressing of the surface of the

3 The record is unclear as to whether claims 16-18 stand rejected under this
ground of obviousness. See Ans. 7-8 (initially stating that “[c]laims 1, 2, 4-
6, 13, 14, 1-21 22-27, and 29-31 stand rejected,” and subsequently
referencing claim 18 (Ans. 7), as well as claims 16 and 17 (Ans. 8); cf. App.
Br. 9 (stating that claims 16-18 are rejected as obvious). See, also, Ans. 2
(agreeing that Appellant’s statement of the status of claims contained in the
brief is correct). Under these circumstances, we treat these claims as being
included within the obviousness rejection.
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metal, similar to shot peening a part. This results in a
structurally different blade.
We understand “shot peening” to be a process that plastically compresses a
metal’s surface, thereby increasing the surface’s compressive stress and
correspondingly reducing the surface’s tensile stress.4 See Jack Champaigne
Electronics Inc., “Shot Peening Overview,” 3-4 (2001). Thus, we
understand that the invention to be reducing the tensile stress of a saw blade
surface, which stress was created during heat treatment, by polishing the
treated blade surface in a manner that increases the compressive stress.
Each of independent claims 1, 13, and 19 recites a saw blade. Claim 1
is illustrative (formatting and emphasis added):
1. An improved saw blade comprising:
a blade portion having two opposed sides which define a
blade width; and
a cutting edge formed on the blade portion, the cutting
edge having a cutting tip width;
the blade portion having a high precision surface finish
which is less than or equal to approximately 10 Ra, and
wherein the surface has reduced residual tensile stress which is
produced by a process comprising the steps of
providing a high speed centrifugal finishing
apparatus having an outer vessel and at least one inner
vessel;
placing the saw blade in the inner vessel;
adding abrasive finishing media into the inner
vessel; and

4 The tensile and compressive stresses discussed in this Decision are residual
stresses, i.e., artifacts of applied stress. Jack Champaigne Electronics Inc.,
“Shot Peening Overview,” (3) 2001.
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rotating the inner vessel at high speed relative to
the outer vessel;
the high speed rotation causing the abrasive media
to surface finish the blades.
As emphasized, the saw blade’s surface has a roughness average “less than
or equal to approximately 10 Ra.”5 In addition, the blade’s surface has a
“reduced tensile stress” imparted by centrifugal finishing.
Claim 13 is directed to a saw blade and recites the same surface
roughness, reduced tensile stress, and centrifugal finishing as claim 1.
Claim 19 is directed to a saw blade and recites the same surface roughness
and centrifugal finishing as claim 1. In lieu of reciting that the blade’s
surface has a reduced tensile stress, claim 19 recites that the surface is
formed with a compressive stress.

CLAIMS 1-6 AND 13-18
Principles of Law
35 U.S.C. § 112, ¶ 2 requires that the specification of a
patent “conclude with one or more claims particularly pointing
out and distinctly claiming the subject matter which the
applicant regards as his invention.” Because claims delineate
the patentee’s right to exclude, the patent statute requires that
the scope of the claims be sufficiently definite to inform the
public of the bounds of the protected invention, i.e., what
subject matter is covered by the exclusive rights of the patent.
Otherwise, competitors cannot avoid infringement, defeating

5 Although not set forth in Appellant’s Specification, one of ordinary skill in
the art would understand that the term “Ra” stands for the arithmetic average
surface roughness in micro-inches or micro-meters. See Hashimoto col. 4, l.
62-col. 5, l. 1. The Examiner does not dispute Appellant’s contention that
Ra, as used in the Specification, is in terms of micro-inches. (App. Br. 25;
Ans. 8:4-6.)
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the public notice function of patent claims. (“[T]he primary
purpose of the requirement is ‘to guard against unreasonable
advantages to the patentee and disadvantages to others arising
from uncertainty as to their [respective] rights.’”) The Supreme
Court has stated that “[t]he statutory requirement of
particularity and distinctness in claims is met only when [the
claims] clearly distinguish what is claimed from what went
before in the art and clearly circumscribe what is foreclosed
from future enterprise.”
Halliburton Energy Servs., Inc. v. M-I LLC, 514 F.3d 1244, 1249 (Fed. Cir.
2008) (citations omitted) (brackets in original).
[T]he patent drafter is in the best position to resolve the
ambiguity in the patent claims, and it is highly desirable that
patent examiners demand that applicants do so in appropriate
circumstances so that the patent can be amended during
prosecution rather than attempting to resolve the ambiguity in
litigation.
Id. at 1255.
Analysis
As noted above, independent claims 1 and 13 each recite a blade
surface with a “reduced tensile stress.” Claims 2-6 and 14-18, respectively,
depend from claims 1 and 13. The claims do not recite an initial amount of
tensile stress relative to which the final tensile stress is reduced. Nor do they
recite any other objective standard by which one could reasonably determine
relative to what level of tensile stress a blade surface’s stress must be
reduced. This ambiguity as to the metes and bounds of claim protection
being sought renders the claims insolubly ambiguous.
We therefore issue a new ground of rejection for claims 1-6 and 13-
18, rejecting them under 35 U.S.C. § 112, ¶ 2, as indefinite. See 37 C.F.R.
§ 41.50(b).
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As the indefiniteness of claims 1-6 and 13-18 prevents their
evaluation against the prior art, we do not sustain the Examiner’s rejections
of claims 1-6 and 13-18. See In re Steele, 305 F.2d 859, 862-63 (CCPA
1962) (holding that the Examiner and the Board were wrong in relying on
what, at best, were speculative assumptions as to the meaning of the claims
and in basing a rejection under 35 U.S.C. § 103 thereon).

CLAIM 19
As noted supra, claim 19 recites a blade surface formed with a
compressive stress. The Examiner has not addressed whether the applied
prior art teaches or suggests a blade surface formed with a compressive
stress. The Examiner only addresses whether the applied prior art teaches or
suggests reducing tensile stress (see below). Thus, we understand the
Examiner to be considering a reduction in tensile stress to be sufficient
evidence of the presence of compressive stress. Appellant does not dispute
that determination but, instead, seems to concur. (App. Br. 25:25-28; 26:23-
25; Reply Br. 8:17-19.) We accordingly evaluate claim 19 in view of the
Examiner’s determinations as to the existence of some level of tensile stress.
In the § 102 rejections respectively over Vankov and Williams (Ans.
3-6), the Examiner finds that Vankov’s electropolishing and Williams’
unspecified polishing each reduce surface roughness and, as such, inherently
reduce tensile stress of the blade surfaces. (Ans. 3:11-19; 5:13-20.) The
Examiner reasons that reducing the surface roughness, in turn, reduces a
surface’s susceptibility to various causes of embrittlement. (Ans. 10:13-
11:7.) Restated, the Examiner essentially concludes that all polishing
inherently reduces a blade surface’s susceptibility to microscopic fractures
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caused by the blade’s use and environment e.g., caused by “friction,”
“impurity of the blade,” “corrosion,” and “hydrogen embrittlement.” (Id.)
The Examiner does not provide evidence to support these conclusions.
Appellant, on the other hand, provides evidence to support his
argument that a mere reduction of surface roughness will not, in and of
itself, necessarily reduce tensile stress. (App. Br. 16:25-18:7 (citing Exh.
C); 26:11-19.)
Given the absence of convincing evidence supporting the Examiner’s
position, when weighed against the credible evidence presented by
Appellant, we conclude that Appellant has rebutted the anticipation
rejections based upon Vankov and Williams. We therefore do not sustain
the Examiner’s anticipation rejections of claim 19.

DECISION
The Examiner’s decision rejecting claims 1-6 and 13-19 is reversed.
Pursuant to our authority under 37 C.F.R. § 41.50(b), we reject claims
1-6 and 13-18 under 35 U.S.C. § 112, ¶ 2, as being indefinite.
Rule 37 C.F.R. § 41.50(b) states that “[a] new ground of rejection
pursuant to this paragraph shall not be considered final for judicial review.”
Rule 37 C.F.R. § 41.50(b) provides that Appellant, WITHIN TWO
MONTHS FROM THE DATE OF THE DECISION, must exercise one of
the following two options with respect to the new grounds of rejection to
avoid termination of the appeal as to the rejected claims:
(1) Reopen prosecution. Submit an appropriate
amendment of the claims so rejected or new evidence relating
to the claims so rejected, or both, and have the matter
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reconsidered by the examiner, in which event the proceeding
will be remanded to the examiner. . . .
(2) Request rehearing. Request that the proceeding be
reheard under § 41.52 by the Board upon the same record . . . .

REVERSED
37 C.F.R. § 41.50(b)

gvw

Notice of References Cited

Application/Control No.

09/965,162
Applicant(s)/Patent Under
Reexamination
Steve E. Hoffman
Examiner

Ghassem Alie
Art Unit

3700

Page 1 of 1

U.S. PATENT DOCUMENTS

* Document Number Country Code-Number-Kind Code Date MM-YYYY

Name

Classification

A US-

B US-

C US-

D US-

E US-

F US-

G US-

H US-

I US-

J US-

K US-

L US-

M US-
FOREIGN PATENT DOCUMENTS

* Document Number Country Code-Number-Kind Code Date MM-YYYY

Country

Name

Classification

N

O

P

Q

R

S

T
NON-PATENT DOCUMENTS
* Include as applicable: Author, Title Date, Publisher, Edition or Volume, Pertinent Pages)

U

Jack Champaigne Electronics Inc., “Shot Peening Overview,” 2001.

V

W

X

*A copy of this reference is not being furnished with this Office action. (See MPEP § 707.05(a).)
Dates in MM-YYYY format are publication dates. Classifications may be US or foreign.

U.S. Patent and Trademark Office
PTO-892 (Rev. 01-2001) Notice of References Cited Part of Paper No.
Delete Last PagelAdd A Page
H:\docs\training\SPO_08_28_2000.wpd
Shot Peening
Overview
by
Jack Champaigne
Electronics Inc
1428 W. 6TH Street
Mishawaka, In 46544
http://www.shotpeener.com
Last Revised
January 18, 2001
With special thanks to Metal Improvement Co for their technical contributions
http://www.metalimprovement.com
Shot Peening Overview
1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Shot Peening Described . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Residual Stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Load Stress Profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2. MANUFACTURING PROCESSES - EFFECT ON FATIGUE LIFE . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1 Beneficial Manufacturing Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2 Detrimental Manufacturing Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2.2 Electro Discharge Machining (EDM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3. SERVICE EFFECTS REMEDIED BY SHOT PEENING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.1 Cavitation Damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.2 Fretting and Fretting Fatigue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.3 Galling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.4 Pitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.5 Crack Arrest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.6 Stress Corrosion Cracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4. PRODUCTS BENEFITTING FROM SHOT PEENING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.1 Jet Engine Blades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.2 Connecting Rods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.3 Crankshafts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.4 Gears . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.5 Shafts and Axles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
4.6 Springs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
4.7 Torsion Bars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
5 MATERIALS APPROPRIATE FOR SHOT PENNING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
5.1 High Strength Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
5.2 Aluminum Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
5.3 Austempered Ductile Iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
5.4 Powder Metallurgy (Sintered) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
5.5 Titanium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
6 MISC. PEENING TECHNOLOGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
6.1 Peen Forming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
6.2 Contour Correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
7. STATUS OF INDUSTRY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
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1. INTRODUCTION
1.1 Shot Peening Described
Shot peening is a cold working process in which the surface of a part is bombarded with small spherical
media called shot. Each piece of shot striking the material acts as a tiny peening hammer, imparting to
the surface a small indentation or dimple. In order for the dimple to be created, the surface fibers of the
material must be yielded in tension. Below the surface, the fibers try to restore the surface to its original
shape, thereby producing below the dimple, a hemisphere of cold-worked material highly stressed in
compression. Overlapping dimples develop an even layer of metal in residual compressive stress. It is
well known that cracks will not initiate or propagate in a compressively stressed zone. Since nearly all
fatigue and stress corrosion failures originate at the surface of a part, compressive stresses induced by
shot peening provide considerable increases in part life. The maximum compressive residual stress
produced at or under the surface of a part by shot peening is at least as great as half the yield strength of
the material being peened. Many materials will also increase in surface hardness due to the cold-
working effect of shot peening.
Benefits obtained by shot peening are the result of the effect of the compressive stress and the cold
working induced. Compressive stresses are beneficial in increasing resistance to fatigue failures,
corrosion fatigue, stress by corrosion cracking, hydrogen assisted cracking, fretting, galling and erosion
caused by cavitation. Benefits obtained due to cold working include work hardening, intergranular
corrosion resistance, surface texturing, closing of porosity and testing the bond of coatings. Both
compressive stresses and cold-worked effects are used in the application of shot peening in forming
metal parts.
1.2 Residual Stresses
Residual 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.
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A typical profile of residual compressive stress as it changes with depth is shown in Fig. 1. It has four
important characteristics:
1) SS - Surface Stress, the stress measured at the surface.
2) Csmax - Maximum Compressive Stress, the maximum value of the compressive stress
induced, which normally is highest just below the surface.
3) d - Depth, the depth of the compressive stress is the point at which the compressive
stress crosses over the neutral axis and become tensile.
4) Tsmax - Maximum Tensile Stress, the maximum value of the tensile stress induced. The
offsetting tensile stress in the core of the material balances the surface layer of
compressive stress so that the part remains in equilibrium. TSmax must not be
allowed to become large enough to create early internal failures.
Fig. 1 Example of Residual Stress Profile Created by Shot Peening
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1.2.1 Magnitude
The maximum value of the residual compressive stress (Csmax) is often called the magnitude of the
residual stress induced. Variations in the shot peening process have little effect upon the magnitude of
the compressive stress induced as long as the shot used is at least as hard as or harder than the material
being peened. The magnitude of the compressive stress is primarily a function of the material itself and
has a value of at least one-half of the yield strength of that material.
Fig. 2 Residual Stress Produced by Shot Peening vs. Tensile Strength of Steel
The depth of the compressive layer is influenced by variations in peening parameters. Figure 3 shows
the relationship between depth of compressive layer and shot peening intensity for three materials: Steel
hardness Rc31, Steel hardness Rc52, and Titanium 6Al-4V. Depths for steels with other hardness can be
interpolated from the chart.
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1.3 Load Stress Profiles
1.3.1 Smooth Specimen
Figure 3 illustrates the distribution of stress in a smooth beam with no external load applied which has
been shot peened on both the upper and the lower surfaces. Since the beam is in equilibrium with no
external forces, the area under the stress distribution curve in the regions of compressive stress must be
equal to the corresponding area under the curve in the region of tensile stress. Further, the sum of the
moments of these areas must be equal to zero.
Fig. 3 Typical Residual Stress Distribution After Peening
Figure 4 shows the same beam after shot peening as in the above figure, but with an external bending
moment being applied. The resultant stress at any depth will be equal to the algebraic sum of the
residual stress and the stress due to the applied load at that depth. The resultant curve of the stress
distribution is the solid line and the bending load is shown as a dashed line.
Note that even after loading, the stress at the peened surface is greatly reduced by the compressive
stress of shot peening. This condition will help prevent initiation and, more importantly, the propagation
of surface cracks.
Fig. 4 Superposition of Applied and Residual Stress
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1.3.2 Notched Specimen
Figure 5 illustrates the distribution of stress in a beam with a notch at the surface and under bending load
(Kt=2.0). The notch dramatically concentrates and increases the effective stress experienced by the
beam at the surface (Kt=2.0 effectively doubles the surface stress).
Fig. 5 Distribution of Bending Stress in a Notched Beam
Figure 6 shows the load/stress profile of the same beam but shot peened prior to the application of the
bending load. Shot peening reduces the stress experienced at the surface of the beam by over fifty
percent - essentially negating the detrimental stress concentration effect of the notch.
Fig. 6 Load/Stress Distribution of Shot Peened Notched Beam under Bending Load
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2. MANUFACTURING PROCESSES - EFFECT ON FATIGUE LIFE
2.1 Beneficial Manufacturing Processes
Surface hardening which typically leaves the surface of the part with a residual compressive stress is a
beneficial manufacturing process. Honing, polishing and burnishing are surface enhancement processes
which remove many of the defects and stress risers which can occur during other manufacturing
operations. Surface rolling will leave the surfaced in residual compressive stress but is limited to regular
geometries while shot peening, properly controlled, produces the best results.
2.2 Detrimental Manufacturing Processes
Manufacturing processes are known to have significant effects on fatigue properties of parts. The effects
are either detrimental to fatigue properties or beneficial. Detrimental processes can include through-
hardening, grinding, abusive machining, plating and welding. These processes tend to induce residual
tensile stresses into the part, thereby lowering fatigue characteristics. Nontraditional machining, (ECM,
EDM) also results in fatigue debits. In EDM (Electro-Discharge Machining) a recast layer is produced
which can be brittle and notch sensitive and under residual tensile stress. Fatigue failures can originate
in the recast layer and propagate into the base metal. In ECM (Electro-chemical machining), the
chemical used can attack the grain boundaries leaving them in a weak, notch sensitive condition.
2.2.1 Grinding
Residual tensile stresses and surface brittleness can be caused by the generation of high surface
temperatures during severe grinding operations. It has been found that residual tensile stresses created
by grinding can approach the ultimate tensile strength of the material itself. Residual tensile stress will
dramatically reduce the fatigue or stress corrosion resistance of ground parts. Shot peening after
grinding can overcome the detrimental effect of tensile stresses induced by severe grinding, as shown in
Figure 7.
Fig. 7 Reversed Bending Fatigue of Flat Bars of Rockwell Hardness C45
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Fig. 8 Residual Stress in 4340 Steel (Hrc50) After Surface Grinding
Figure 8 shows the stress distribution created by different grinding techniques - conventional, abusive
and gentle. It is quite evident that conventional grinding and abusive grinding can generate high
magnitudes of residual tensile stress at or near the surface of the parts. This tensile stress will, of
course, dramatically affect fatigue resistance.
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2.2.2 Electro Discharge Machining (EDM)
Although the spark erosion process is essentially “force-free;” this should not be construed to mean that
the eroded components are stress free. Not all the molten metal produced during the discharge is
expelled into the working gap. That which remains resolidifies to form a hard and brittle skin on the work
surface. Accompanying thermal stresses, plastic deformation and shrinkage induce residual tensile
stresses in the work piece which, under certain conditions, have been found to approach the ultimate
tensile strength of the material near the surface.
Shot peening can be very beneficial in restoring the fatigue strength of parts that have been
electro discharge machined (see Fig 9). In this figure, the effect of shot peening on electro-chemical
machined, electro-discharge machined and electro-polished (ELP) surfaces are shown.
Fig. 9 Summary of High-Cycle Fatigue Behavior on Inconel 718, Solution-Treated and Aged (Rc44)
2.2.3 Electro-Chemical Machining (ECM)
Electro-chemical machining is the controlled dissolution of work piece material by contact with a strong
chemical reagent in the presence of an electric current. Even though ECM is considered to be a
relatively stress-free process, the reduction in endurance limit is caused by frequently observed surface
softening (rebinder effect) and surface imperfections left by preferential attack on grain boundaries.
Fortunately, a shot peening post treatment more than restores the endurance limit (see Fig. 10).
Fig. 10 Summary of High-Cycle Fatigue Response for ECM
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2.2.4 Plating
Many parts are shot peened prior to chrome and electro less nickel plating to counteract the harmful
effects of plating on the fatigue life of metal parts, which can be residual tensile stresses, hydrogen
embrittlement and brittle notch sensitive plate material at the surface. Cracks in the plating can
propagate rapidly into the base metal and lead to early failure. However, when the surface of the base
metal is compressively stressed, the cracks cannot propagate into the base metal. Figure 11 illustrates
this concept and Figure 12 clearly shows the reduction in fatigue resistance experienced on 4340 steel
due to plating and the beneficial effect of shot peening prior to the plating process.
Fig. 11 Plating Cracks Will Not Propagate into Prestressed Base
Fig. 12 Cycles to Failure 4340 Steel, 52-53Rc Rotating-Beam Fatigue
Shot peening prior to plating is recommended on cyclically loaded parts to enhance fatigue properties.
Federal specifications QQ-C-320 and MIL-C-26074A call for shot peening of steel parts that will be
chrome or electroless nickel-plated, respectively, where service requires unlimited life under dynamic
loads. Other plating processes such as cadmium can also lower fatigue strength unless they are
preceded by shot peening.
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2.2.5 Anodizing
Anodizing is another application in which shot peening improves fatigue resistance of coated materials.
Benefits similar to those shown for plating are illustrated in Figure 13 where the high-strength aluminum
base metal was shot peened prior to the hard anodize process.
Fig. 13 The Influence of Hard Anodizing and Shot Peening on the Failure Strength of Duralumin (LI)
2.2.6 Plasma
Plasma spray coatings also tend to reduce fatigue resistance. As with other coatings, shot peening is
able to recover fatigue debits due to plasma spray processes as shown in Figure 14. Again it should be
emphasized that for fatigue improvement, shot peening is performed prior to the coating processes.
Additionally, peening after coating is effective in improving the surface finish of plasma-coated parts.
Also it can be used to densify and close porosity of the plasma coating.
Fig. 14 R.R. Moore Fatigue Strength of Titanium 6AL-4V Polished vs. AS - Sprayed Coating with
Pre-spray Conditioning Grit Blast and/or Shot-Peen - METCO 64F
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2.2.7 Welding
Heat generated by the welding process often will produce tensile stresses approaching the yield strength
of the material. These harmful self-stresses in the heat-affected zone (HAZ) contribute to poor fatigue
characteristics of weldments. Improvement in fatigue resistance can be accomplished in two basic
steps:
1) Improving the weld geometry by grinding the fillet weld tow profile, since fatigue cracks
initiate from geometric discontinuities. Figures 15 and 16 show the loss in fatigue strength of
A286 and STE690 (a European alloy) caused by welding.
2 ) Inducing surface compressive residual stresses in weldments. Shot peening is the preferred
method for providing a substantial increase in fatigue resistance. For significant and repeatable
improvements, the shot peening must be closely controlled.
FIG. 15 Effect of Shot Peening on Welded A-286 Fatigue Strength
Figure 16 Effect of Shot Peening on Welded STE690 Material
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2.2.8 Weld Repair
Fatigue cracks can start in the base metal, the weld material or the heat-affected zone (HAZ). Once
detected, the recommended repair scheme is:
1) Reduce the stress concentration by avoiding stray arc strikes, by grinding the weld cracks
and by dressing the fillet.
2) Replace the weld metal.
3) Shot peen to induce beneficial compressive stress.
Recent use of the shot peening process in post-weld repair of ball mills suggests that higher intensities,
used to induce compressive mean stresses, enhance fatigue resistance.
Number of hours of operation following weld repair:
Not Shot Peened......................................7561 Hours (fatigue cracks developed)
Shot Peened..........................................35,629 Hours (no fatigue cracks)
2.2.9 Decarburization
Decarburization is defined as the loss of carbon from the surface of a ferrous alloy as a result of heating
in a medium that reacts with the carbon at the surface. It has been shown that decarburization can
reduce the fatigue strength of high-strength steels (240KSI and up) by 70% to 80% (Fig 17, and lower-
strength steels (140KSI to 150 KSI) by 45% to 55%.
It is generally accepted that decarburization is a surface phenomenon not particularly related to depth.
A depth of .003 inch decarburization can be as detrimental to fatigue strength as a depth of .030 inch.
However, the amount of decarburization can have a dramatic influence on fatigue properties of carbon
steels.
Fig. 17 Effect of Shot Peening on Decarburization
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Severe decarburization can induce significant residual tensile stresses in the surface of the part as
shown in Figure 18.
Fig. 18 The Effect of Decarburization on the Residual Stresses
Developed in Carburized and Hardened Plates.
The Carbon Content at 0.002mm Was Estimated to Be:
CURVE 1, 1%; CURVE 2, 0.64%; CURVE 3, 0.35%
Shot peening has proven to be effective in restoring most, if not all, of the fatigue strength lost due to
decarburization. Because the decarburized layer, though softer, is not easily detectable on quantities of
parts, peening can ensure the integrity of the parts in a lot if decarburization is suspected.
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3. SERVICE EFFECTS REMEDIED BY SHOT PEENING
3.1 Cavitation Damage
Cavitation damage is the result of high relative motion between a metal and a liquid. If the pressure
accompanying a high-velocity motion drops to a vapor pressure, the liquid will vaporize and form a vapor
cavity at the metal surface. At a slight increase in pressure, this bubble will collapse, causing a
concentrated liquid impact resulting in erosion and pitting of the metal surface. Once initiated, the
cavitation damage becomes progressive and cumulative. Shot peening has been shown to be effective
in retarding cavitation damage in 1020 steel and 70C brass (Fig. 19). The benefit from shot peening
seems to be associated with work hardening. The more work hardening accomplished in the material,
the greater the benefit from shot peening. Work hardening would raise the yield strength of the material
in the work-hardened area, thus making it stronger and more resistant to damage caused by the collapse
of the bubbles.
Fig. 19 Rate of Material Loss by Cavitation
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3.2 Fretting and Fretting Fatigue
Fretting can develop when the relative motion of microscopic amplitude occurs between two metal
surfaces. As the surfaces rub, fine abrasive oxides form, which contribute to the scoring of the surfaces.
Fretting gives rise to one or more forms of damage, such as fretting corrosion, fretting wear and/or
fretting fatigue. Fretting corrosion and fretting wear are any corrosion damage and wear damage,
respectively, that occur as a direct result of fretting. Surface discoloration, pitting, oxide-layer buildup,
seizing and loss of fit are all characteristics of these forms of damage. Fretting fatigue is any fatigue
damage that initiates from fretting. The most notable characteristic of this form of damage is a reduction
of fatigue strength of the parts (Figure 20 and 21).
Shot peening has proven to be successful in retarding fretting and eventually fretting fatigue by
increasing the surface hardening through cold working of the metal and providing residual compressive
stresses at the fretting surfaces. The minute pockets that are produced at the surface through shot
peening act as oil reservoirs, thus resulting in longer lubricant retention.
Fig. 20 Rotating-Bending Fatigue Curves for Untreated and Shot-Peened Specimens of
Annealed Austenitic Stainless Steel En 58A With and Without Fretting
Fig. 21 L-N Curve Clamped Up Joint 4140 Steel
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3.3 Galling
Galling is caused by strong adhesive forces whenever an imbalance of electrons exists between two
mating metal surfaces. At low stresses, minute junctions form at contacting surfaces and small
fragments of metals become detached when subsequent relative movement occurs. At higher stresses,
however, much larger junctions are formed and actual seizure may occur, stalling the equipment or
“freezing” the action of the part.
The cold-worked densified surface, generally obtained through shot peening, makes the material more
resistant to galling. In addition, the shallow indentations caused by the shot act as oil reservoirs and
provide improved lubrication throughout the pressure cycle. Also, the residual compressive stress from
shot peening retards crack growth and pit formation if only a limited amount of galling occurs.
Shot peening has worked successfully for anti-galling of A.P.I. threads, engine valve stems, and on such
materials as Inconel 718 and X750, Monel K-500, stainless steel type 300- and 400-series, 17-7PH,
Titanium 6-4 and aluminum. It has also been found that shot peening will reduce scoring on such parts
as tappet faces (Ref 45) cams and gears which are in sliding contact under high loads.
3.4 Pitting
Hydraulic wedging action is a lubricated contact that can be understood by examining two rollers, rotating
at different peripheral speeds. When the rollers are forced together radially, frictional forces parallel to
the surfaces produce tensile stress in one area of each roller surface and compressive stress in another
area (Fig. 22). These stresses and the direction of sliding and rolling on the lower speed surface tend to
open micro cracks, allowing oil to enter. After the oil is trapped, high pressure builds up as the crack
passes under the point of contact, generating high tensile stress at the crack root. This stress causes
crack propagation and eventual pitting failure.
Fig. 22 How Pitting Starts
The opposite effect occurs on the high-speed surface; therefore, pits tend to form more rapidly on the
lower-speed surface. Also, plastic deformation in the surface layer of the roller tends to close the cracks,
making pitting less likely.
Shot peening has been shown to be beneficial in retarding surface pitting and improving fatigue life of
carburized and hardened gears (Fig 23).
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Fig. 23 Comparison of Surface (Pitting) Fatigue Lives of Standard Ground and
Shot-Peened Carburized and Hardened CVM AISI9310 Steel Spur Gears, Speed, 10,000 RPM;
Lubricant, Synthetic Paraffinic Oil; Gear Temp. 350K (170 f)
Maximum Hertz Stress, 1.7X10 N/m (248,000 psi)
Figure 23 shows that shot-peened gears exhibit pitting fatigue lives 1.6 times the life of standard gears
without shot peening. Residual stress measurements and analysis indicate that the longer fatigue life is
the result of the higher compressive stress produced by the shot peening. The life for the shot-peened
gear was calculated to be 1.5 times that for the plain gear by using the measured residual stress
difference for the standard and shot-peened gears. The measured residual stress for the shot-peened
gears was much higher than that for the standard gears.
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3.5 Crack Arrest
The propagation of a crack depends upon stress conditions near its tip. Crack arrest by residual
compressive stress is based upon the following factors:
1) A crack will not propagate unless a tensile stress forces it open near the tip.
2) The crack tip will not open as long as a compressive force acts upon it.
Figure 24 shows the concept of compressive crack arrest.
a) Load stress distribution approximated by straight lines.
b) Stress distribution after shot peening.
c) Combined stress distribution obtained by adding load stresses and residual stresses.
d) Distribution under load after a crack has progressed part way into the region of tensile stress.
Dashed lines represent the original distribution of figure (c)
e) Crack progressed to the point (I) where the stress in the uncracked loaded part changed from
tension to compression. Tension now exists at that point.
f) Crack arrested by the absence of tensile stress.
Fig. 24 Crack Arrest by Compressive Self Stress
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3.6 Stress Corrosion Cracking
There are many types of corrosion, but perhaps the most catastrophic is stress corrosion cracking (SCC)
which is defined as failure by cracking under combined action of corrosion and static tensile stress, either
external (applied) or internal (residual). Cracking may be either intergranular or transgranular, depending
on the metal and corrosive medium.
Stress corrosion cracking cannot occur in an area of compressive stress.
Higher-strength alloys are generally more susceptible to SCC than lower-strength alloys. Most metals,
such as aluminum, magnesium, copper, nickel, steel and stainless steel alloys, are susceptible to SCC
given a tensile stress at or above their threshold limits and certain corrosive environments.
Fig, 25 Effect of Peening on Resistance to Stress Corrosion Cracking of 7075-T6
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4. PRODUCTS BENEFITTING FROM SHOT PEENING
4.1 Jet Engine Blades
Blades and buckets as well as discs and shafts for aircraft jet engines, stationary gas and steam turbines
and compressors are standard applications for shot peening.
Blade roots are peened to prevent fretting, galling and fatigue. Broaching of the fir tree can leave tears,
pits or scores which are stress risers. Shot peening to the depth below these surface discontinuities will
correct these detrimental effects. Many engine blade roots are shot peened as new parts and re-shot
peened upon overhaul to restore the fatigue life.
The airfoils of blades are also commonly shot peened to:
1) retard the harmful fatigue effect caused by pitting,
2) repair foreign object damage (FOD),
3) prevent stress corrosion cracking,
4) retard fretting in tie wire holes, and
5) increase fatigue life in general.
Many coating and plating processes used on airfoils tend to decrease the fatigue life. Shot peening is
used to offset these detrimental effects.
It is recommended to re-peen the blades on overhaul of the unit, as the compressive stress induced by
shot peening will recover any surface fatigue damage that may have occurred in service, thereby
preventing the initiation of propagation of fatigue cracks (Figures 26 and 27). The discs or wheels which
support the blades are also peened, especially in the blade slots and the tie-rod bores.
Fig. 26 Suppression of Fatigue Damage of
Inconel 713c Turbine Blades by Shot Peening Fig. 27 Shot Peening as a Means of Overcoming
Prior Fatigue Damage with 4340 Steel
Tested in Rotating Bending
4.2 Connecting Rods
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A rough surface in compression will resist failures better than a smooth surface in tension (Figure 28).
Note the comparison of polished verses rough forged and shot peened connecting rods in the figure
below. It indicates great savings of cost and time (Figure 29). There is usually no advantage in
producing a very fine finish prior to peening. If a better surface finish is required, it is permissible to lap,
hone or polish after shot peening so long as material removal does not exceed 10 percent of the
compressive layer.
The most critical areas of connecting rods are the fillets next to both bores, but the rods are normally
shot peened all over prior to machining the bores. Tests have shown that the peened surface protects
so well that even scratches made after peening cause no reduction in fatigue strength provided the
scratch is less than one quarter of the depth of the compressed layer.
Feed rollers, bearing races and similar parts are peened, maintaining the required surface finish as well
as increasing their resistance to galling and fatigue failures. Connecting rods made of various metal
alloys, including powder metal, have been shot peened to improve fatigue properties. In addition,
fretting and fretting fatigue failures experienced in the bores of the connecting rods have also been
prevented by shot peening.
Fig. 28 Effect of Peening and Scratches on Endurance Limit of 4340 Steel at Rc51
Fig. 29 Fatigue Durability of Fork-Type Connecting Rods
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4.3 Crankshafts
The most highly stressed area of a crankshaft is the crank pin bearing fillet (Figure 31). The high stress
point is the bottom side of the fillet when the pin is in the top dead-center position during the firing cycle.
It is common for cracks to initiate in this pin fillet and propagate through the web of the crankshaft to the
adjacent main bearing fillet, causing fatigue failure.
Figure 30 of a 6 cylinder 4340H forged crankshaft shows the unpeened part has fatigue strength of 52
KSI while the shot-peened crankshaft has the fatigue strength of 72 KSI, a 38% improvement.
Current economics emphasize recycling of large crankshafts by reconditioning which basically consists
of inspection for cracks; then blending them out by grinding to acceptable limits, even if the original
manufacturer has not cold worked the fillets initially. The fillets should be cold worked after grinding.
Controlled shot peening is the recommended means of restoring the cold-work effect and the residual
compressive stress.
All size crankshafts respond well to shot peening, from small high-speed shafts with journal bearing dia-
meters of 1”, to large slow-speed shafts with journal bearing diameters of 6” or more. Experience has
shown the process to be effective on forged steel, cast steel, nodular iron, and austempered ductile
iron.
Fig. 30 Increase in Fatigue Strength of Shot Peened Crankshaft
Fig. 31 Crankshaft and Detail
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4.4 Gears
Shot peening of gears is one of the more common application of the process. Gearing applications
range from automotive, to heavy vehicle, to marine transmissions, to axle drive gears, to small gears
used in power tools and to very large gears used in transmissions for large ships and mining equipment.
The fillets at the root of the gear are usually the areas of high stress and should be shot peened.
However, it has also been shown that the tiny indentations produced by shot peening on the face of the
gear act as very small oil reservoirs which help promote better lubrication, reduce fretting, noise,
spalling, scoring and lower operating temperature by reducing friction. Gears which have to be held to
very close tolerances may be lapped or honed after shot peening, provided the lapping or honing
operation does not remove more than 10% of the depth of the compressive stress induced.
Fig. 32 Increase in Fatigue Resistance of Spiral Bevel Gear
Gears are frequently shot peened after carburizing, as shown in the chart below. It has been found that
the life of a case hardened gear, stressed to 80,000 psi (56kg/mm2), increased from 2000,000 cycles
before shot peening to 30,000,000 cycles after shot peening. The use of special-hardness (RC55-62)
shot is recommended for peening case-hardened carburized gears in order to produce a higher
magnitude of compressive stress.
Increases in fatigue resistance of SAE8620 spiral bevel gears, carburized and hardened to RC61, are
shown in Figure 32. A residual stress profile induced by the same set of peening parameters is shown
in Figure 33.
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Figures 32 and 34 both show increases in fatigue strength of at least 30% at 10 cycles, which are typical
of results obtained by shot peening carburized gears. However, excellent results can also be achieved
on through-hardened gears ad gears with induction-hardened teeth and roots. Shot peening can also be
used as an inspection tool for “case-hardened” gears. Because of the significant difference in the
hardness between case and core, peening the side faces of the gear will show a marked difference in
appearance and make the case profile readily identifiable to verify its uniformity.
Fig. 33 Stress Profile of
Carburized Gear Tooth Root, Ground
Then Shot Peened with Special Hardness Shot.
Fig. 34 Fatigue of Carburized Gears
Lloyds of London Register of Shipping, allows an increase in tooth loading for both wear and strength of
up to 20% with the use of controlled shot-peened “gearing”. Det Norske Veritas of Norway states, “For
gears produced according to our usual demand for quality(no grinding notches, no significant surface
decarburization etc.), we add 20% to the fatigue limits stated in our rules for shot-peened parts.”
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4.5 Shafts and Axles
All sizes and many types of shafts and axles are shot peened to improve their fatigue properties. In
some instances, peening is required only in keyways, splines, fillets, shear sections or other changes in
cross section. Figures 35 and 36 show the benefits of shot peening on rear axles and notched shafts,
respectively.
Fig. 35 Fatigue Tests on Rear-axle Shafts
Fig. 36 Fatigue Tests on Notched Shafts
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In order to protect the equipment which they drive, many shafts have shear sections designed to fail
when the shaft is overloaded. Since the shear section is thinner than the balance of the shaft, it will
often be the site of early fatigue failures. By shot peening the shear section of the shaft, the life of the
shaft can be increased while retaining the same overload protection provided by the shear section.
4.6 Springs
Compression coil springs are probably the best known and most widely used parts that are shot peened.
All automobile engines contain shot-peened valve springs and have done so for forty-five years.
Springs made of wire sized as small as .005 inch (.13mm) diameter and as large as 3.0 inch (76.0mm)
have been successfully shot peened to increase fatigue resistance. Figure 37 shows the increase in
safe stress ranges of coil springs made of various metal alloys. The fatigue resistance of extension
springs and coiled torsion springs can also be increased by shot peening.
Fig. 37 Safe Stress Ranges for Coil Springs of Various Alloys
4.7 Torsion Bars
Torsion bars are an excellent application for shot peening. Figure 38 shows the increase in fatigue
resistance made possible by shot peening torsion bars.
Fig. 38 Comparison of Fatigue Curves Bending Vs. Torsion Loading
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5 MATERIALS APPROPRIATE FOR SHOT PENNING
5.1 High-Strength Steels
Increasing use has been made of steel at hardness levels above RC50 corresponding to tensile strength
above 224,000 psi (158kg/mm2) for fatigue applications, such as springs and aircraft landing gear.
Figure 39 shows the close connection between shot peening and the use of these strength levels.
Without shot peening, the greatest fatigue strength is obtained at a hardness of around RC40. At higher
hardness levels, the material loses fatigue strength due to increased notch sensitivity. With the addition
of shot peening, the fatigue strength increases proportionately in increasing hardness. At a hardness of
RC52, the shot-peened fatigue strength is 144,000 psi (101kg/mm2), more than twice the best unpeened
fatigue strength.
This chart shows that it is not sufficient to check the effect of peening at the conditions which were best
without peening. Much larger improvement may be obtained by taking full advantage of peening with
increased hardness.
The most interesting implication of this data is the permissible use of increased ultimate tensile strength
made possible by the use of peening. Normally, strength levels above 200,000 psi (140kg/mm2) without
peening are dangerous because of loss of fatigue strength. However, with the addition of peening, the
higher-strength levels can be used to withstand higher applied loads at the same time increasing fatigue
strength. Excellent examples are impact wrench hammers, anvils and sockets, and percussion drilling
tools.
Fig. 39 Comparison of Peened and Unpeened Fatigue Limits for Smooth and
Notched Specimens as a Function of Ultimate Tensile Strength of Steel
Testing has confirmed this data and added information that the fatigue strength of peened parts is not
impaired by shallow scratches which could otherwise be detrimental to unpeened high strength steel parts.
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5.2 Aluminum Alloys
The fatigue resistance of aluminum alloys, commonly used in aircraft structures, can also be improved by
the use of shot peening (see Fig. 40).
In recent years, aluminum lithium alloys have emerged, particularly in the aerospace industry. Al-Li alloys
include a small percent (normally 2-3%) of lithium which increases durability and lowers density. These
advantages result in a significant weight savings of aircraft with no loss to properties. To date, both
damage tolerant alloys (similar to 2024) and high-strength alloys (similar to 7075-T6) have been
investigated. Shot peening has shown to provide similar increases in fatigue resistance on Ai-Li alloys as
with their conventional counterparts. Figure 41 shows fatigue results with Al-Li alloy 2090-T8 compared to
the similar alloy 7075-T6. The fatigue results are essentially the same.
Fig. 40 Results of Reversed Bending Fatigue Tests on Unpeened and
Peened Specimens of 7075-T6 Aluminum Alloy
Fig. 41 Comparison of 7075-T6 Vs. Al-Li Fatigue Results
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5.3 Austempered Ductile Iron
Although known for many years, austempered ductile iron (ADI) is a material that only today is becoming
more widely used. During the past decade, advances in austempering controls have made possible
greater exploitation of the unique mechanical properties of ADI. Recent application for ADI include
crankshafts, camshafts, gears, and railroad wheels. Figure 42 shows that shot peening provides ADI gear
teeth with substantial fatigue strength increases as has been the case with the more commonly used
gearing materials. Additionally, shot peening will increase surface hardness of ADI very significantly to
promote greater wear resistance.
Figure 42 The Effect of Processing Variables on the Bending Fatigue Strength of
Austenitic-bainitic (K9805) Gear-teeth: Measured after 10 Loading Cycles.
HEAT-TREATMENT AFTER HOBBING
HOBBING AFTER HEAT-TREATMENT
SHOT-PEENING 0.35mm ALMEN A
SHOT-PEENING 0.45mm ALMEN A
SHOT-PEENING 0.55mm ALMEN A
5.4 Powder Metallurgy (Sintered)
The dynamic properties of sintered (PM) components may be enhanced by shot peening. Improvement in
fatigue life is the result of increased hardness and residual compressive stresses. Optimized peening
parameters have been shown to raise the endurance limit of sintered steel alloys by 22% and the fatigue
life by a factor of ten. This suggests that highly stressed automotive components such as gears and
connecting rods should be considered good candidates for sintered steel.
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5.5 Titanium
The low-cycle fatigue (LCF) benefit from shot peening varies in degree relative to the applied stress.
Recent data support the notch LCF benefit for dovetail slots in aerospace hardware (Fig. 43).
An evaluation of titanium compressor discs is provided for both baseline and shot-peen data. It may be
seen that the shot-peening benefit is evidenced most at the lower stress levels. Typical and minimum
(one failure per thousand) curves show an increase of .5x at the high-stress levels to 4x at the lower-
stress levels.
Fig. 43 Notched LCF Benefits to TI 8-1-1 From Shot Peening
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6 MISC. PEENING TECHNOLOGY
6.1 Peen Forming
Peen forming is a die-less forming process performed at room temperature. During the process, the
surface of the work piece is impacted upon by small, round steel shot. Every piece of shot impacting the
surface acts as a tiny peening hammer, producing elastic stretching of the upper surface and local plastic
deformation that manifests itself as a residual compressive stress. The surface force of the residual
compressive stress combined with the stretching causes the material to develop a compound, convex
curvature on the peened side (Fig. 44).
Fig. 44 Compound Curvature Result of Tri-axial Forces Induced by Shot Peening
The process is ideal for forming large panel shapes where the bend radii is reasonably large without
abrupt changes in contour. It is best suited for forming curvatures where the radii are within the metal’s
elastic range.
No dies are required for peen forming; however, for severe forming applications, stress peen fixtures are
sometimes used. Peen forming is effective on all metals, even honeycomb skins and ISO grid panels,
within certain limitations. A partial list of aircraft using peen formed wingskins is shown is Figure 45.
A6A Gruman 777 Boeing F-15 McDonnell
EA6B Gruman S3A Lockheed F5E Northrup
727 Boeing P-3 Lockheed B1B Rockwell
737 Boeing C-130 Lockheed A-10 Fairchild
747 Boeing L-1011 Lockheed
757 Boeing MD80 McDonnell CL-600 Canadair
767 Boeing MD11 McDonnell Dash 7 Dehavilland
Fig. 45 Partial List of Aircraft Using Peen-formed Wings
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Peen forming is often more effective than rolling, stretching or twisting of metal to develop the required
curvatures. Saddle-back shapes are possible and by being a die-less process, material allowance for
trimming is reduced, and costly development and manufacturing time required to fabricate hard dies is
eliminated. The process is flexible to design changes, which may occur after initial design. By only
adjusting the air pressure or wheel speed, it is possible to incorporate curvature changes. Peen forming is
accomplished at room temperature. Part size is currently limited to 15 feet wide by 110 feet long (3.0m
wide by 24.5m long), the limitation being only availability of existing equipment. Thickness range in
aluminum is 0.050 inch to 2.0 inch (1.27mm to 51.0mm); and alloy steel is 0.016 inch to 1.0 inch (.40mm
to 25.4mm).
Parts formed by peen forming exhibit increased resistance to flexural-bending fatigue. Another distinct
advantage with peen forming, unlike most other forming methods is that all surface stresses generated
are of a compressive nature. Although peen-formed pieces usually require shot peening on one side only,
the final result causes both sides to have compressive stress. These compressive stresses serve to
inhibit stress corrosion cracking and to improve fatigue resistance. Some work pieces should be shot
peened all over prior to or after peen forming to further improve fatigue and stress corrosion
characteristics. Parts which have been cold formed by other processes are often shot peened to
overcome the harmful surface tensile stresses set up by these other forming processes.
6.2 Contour Correction
Just as it is possible to create a desired curvature and shape to components by shot peening, it is also
possible to correct the shape and form of parts as well. The shot peening process avoids the unfavorable
(tensile) residual stresses produced by other straightening methods and instead produces favorable
(compressive) residual stresses. Many companies have correctively peened a great variety of parts such
as shafts, tubes, rings, gears, axles, crankshafts, jet engine discs and wing spar extrusions and forgings.
Carburized ring gears as large as 48 inches (122 cm) diameter, which were out of round as much as 0.125
inch (3.17mm) have been corrected to within 0.003 inch (.076mm).
Figure 46 is another example of corrective forming by shot peening. The carburized hardened ring gear
in this photograph is approximately 6 inches (15.24 cm) in diameter by 2 inches (5.1mc) in width. The
concentricity of this gear was between the inner circle and the large division line, corresponding to 80
microns. After the successful corrective forming operation, the ring gear became concentric within 20
microns.
FIG. 46 Contour Correction of Ring Gear
(B) Before Peen Forming (C) After Peen Forming
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7. STATUS OF INDUSTRY
A comprehensive report published in 1989 by The National Center for Manufacturing Sciences
investigated the art and practice of shot peening. This report, “High Reliability Residual Compressive
Stress Processes “(State-of-the Art-Assessment), provides an in-depth review of the peening industry and
offers several conclusions for future work. A summary of recommendations is given in the next section.
[Text from section 5.0 Recommendations]
5.0 Recommendations
Based on survey responses and review of the literature, five areas of need have been identified
which, if satisfied, would result in significant advancement in the state of the art CRS processing.
The following discussions of each of these five recommendations include (1) estimates of time and
funds necessary for effectivepursuit and (2) assessments of the relativedesirability andprobability
of success. The time and funding estimates are solely opinions of the authors. The assessments
of desirability and probability of success were derived from discussions with the NCMS TPC
members at a review meeting in June, 1989.
5.1 Development of Non-destructive Means for Crs Process Verification
This need was the most often expressed need among survey respondents. Moreover, survey
results clearly showed that, with few exceptions, neither life/endurance nor residual stress testing
is being used routinely to verify results of CRS processing. Themost probable underlying reason
is one of economics. Either the testing itself is deemed too time consuming and expensive or the
attendant destruction of the test articles is deemed an economic hardship or both. In the shot
peening area, the Almen Strip is employed nearly universally as an indirect means of process
verification. This is certainly a better situation than the absence of verification; however, onemust
recognize the inherent weaknesses of the Almen Strip approach even aside from the issue of
broad tolerances in AlmenStrips. TheAISI 1070 spring steel of the Almen Strip generally does not
display the same intensity saturation and coverage responses of the production hardware. In
general, there is no established relationship between Almen intensity and the magnitude of
mechanical property or residual stress benefit sought for the production hardware. The Almen
Strip is a non-discriminate integrator of transferred energy; i.e., different combinations of peening
parameters could yield the same Almen intensity, but would likely produce different responses in
production hardware.
Survey results also clearly showed a general lack of confidence in the consistency of CRS
processing among users. The best evidence for this is that only about one-third of users
quantitatively claimed credit for CRS process induced enhancement of hardware life. The major
reason for reluctance to claim such credit is the perception of process variability. Certainly this
perception indicates a general need for improvement of in-process control of process parameters.
Sufficient technology currently exists to accomplish good in-process control; however, the
implementation is not meaningful without some direct means of process verification.
With no intent to exclude potential application of other technologies, the authors suggest that either
ultrasonic or Barkhausen noise techniques show some promise in this area. A Barkhausen noise
technique would have the limitation of being applicable only to Ferro-magnetic materials. While
both techniques have the attribute of being non-destructive, it must be noted that bothare sensitive
to changes in residual stress along with other material characteristics; e.g., grain size and plastic
deformation. Thus, theymay lack sufficient discrimination to permit directmeasurementof residual
stress. It is quite possible, however, that they may provide ability to indicate reproducibility, or lack
thereof, in CRS processing in spite of inability to isolate effects of process-induced residual
stresses.
The cost and timing for development of a non-destructivemeans for CRSprocess verifications are
estimated to be $1 million and two years respectively.
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5.2 Development of Information for Establishing Crs Process Parameter and the
Relationship of Those Parameters to Resulting Benefits in Mechanical Properties.
This recommendationstems jointly from recurring responses of survey participants indicatingneed
for information and from the literature review. There may in fact be a wealth of published
information in this area. The literature review found well in excess of three hundred technical
publications which contained fatigue, stress corrosion and residual stress data from studies
involving CRS processing. Most of these were in the shot peening area. These references have
been compiled and tabulated in this report; however, a detailed critical review of the data was not
within the scope of this survey. As a logical first step in the process of information development,
these data should be critically reviewed to assess validity and to establish where gaps in
knowledge and data exist. From that information, plans could be drawn for experimental programs
needed to bridge at least the most significant gaps. A critical review of published data would likely
also result in an enhanced data base for improvement of existing CRS process specifications.
The cost and timing for analysis of available data are estimated to be $40 thousand and six
months; generation of additional information to bridge significant gaps would require an additional
$500 thousand and two years effort.
5.3 Development of Sensors to Enable Direct Determination and Control of PeeningMedia
Velocity
Fundamentally, shot peening and other impact treatment processes involve the transfer of kinetic
energy of a propelled media or strikes into deformation of a work piece upon impingement. To
produce this energy transfer in a controlled reproducible manner requires control of numerous
process parameters which have been identified in previous sections of this report. It has been
concluded fromsurvey responses and literature review that state of the art technologies in sensors
and microprocessor control are being implemented with success in the shot peening industry. A
notable technological gap exists, however, in the ability for direct sensing and control of media
velocity and mass flow rate. As noted in previous sections, there have been publications
describing systems based on capacitance sensors which have apparently been successful in
direct monitoring and control of media velocity andmass flow rate. Onemay argue, thus, that the
necessary development has already occurred and therefore should not be considered as amajor
need. As counterpoint to that argument, two very important facts should be noted: (1) none of the
development occurred in the U.S., and (2) the developments have not been applied on a
commercial basis. On this basis, because of the overall fundamental importance of direct media
velocity control, this area is strongly recommended for further development and applications in the
U.S.
Such development is estimated to cost $500 thousand and would likely require one year’s effort.
5.4 Development of Monitoring Technique to Determine Coverage in Peening Processes
Survey respondents clearly indicated this need in regards to peening processes. Currently, the
primary means of determining peening coverage is optically aided visual surface examination,
usually at 10x. The drawbacks of this method are (1) the requirement for an experienced observer
and (2) the subjective nature of the interpretation, particularly in the coverage range fromabout 90-
100 percent. Somepractitioners use a fluorescent paint or dyewhich is applied to a surface before
peening and allowed to dry. During peening, the dried paint is removed from the impingement of
themedia. Themajor drawback of this method is the lack of quantitative discrimination. Generally,
the fluorescent paint is completely removed significantly before 100 percent coverage is attained.
The inverse of this, however, is a definite attribute, i.e., if any fluorescent paint remains adherent
on a part after peening, one can be certain that 100 percent coverage has not been attained.
Most likely, state of the art development in the area of peening coverage determination will involve
the technologies of vision sensors and artificial intelligence. Implementation of any such
development will depend greatly on the economics since competing visual and fluorescent paint
methods currently in use are low in cost. Certainly, however, an automated coverage
determination system is likely to be economically favorable for high volume, production
environments.
Development of a coverage determination system is estimated to cost $500 thousand and would
require about one year’s effort.
5.5 Peening Intensity Determination
This area is related to the issue of CRS process verification discussed in section 5.1; however, it
applies strictly to peening processes. Survey respondents in the peening area clearly indicated
a desire for replacement of theAlmenStripwhich currently is the sole means for indicating peening
intensity. Themajor objection of theAlmenStrip is based on the time consuming iterative process
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of adjusting process parameters until the given desired Almen intensity is attained and the re-
verification required after peening has been performed. There are also technical issues of
differences in saturation response between given hardware material and the AISI 1070 steel from
which the Almen Strip is made. These things taken togethermake replacing theAlmenStrip highly
desirable. No specific suggestions for such a system were uncovered in the survey, but clearly
speed and low cost are key desirable attributes.
Such development would likely require $500 thousand to $1 million in funding and two to three
years’ investigation.
5.6 Feasibility Assessment and Ranking
Estimates of costs and timing for each of the recommended development areas have been given
in sections 5.1 through 5.5. After completion of the survey, the TPC committee met to review and
discuss results. Included in that meeting was a discussion of specific desirability and feasibility
(probability of success) of the five technical development areas. The consensus of TPC and
Metcut personnel on these points is presented on page 41. With regard to the ranking of
desirability, not that these are relative rankings among the areas identified. Developments in all five
areas were deemed highly desirable in absolute terms.
An alternate presentation of the same information is given graphically on page 42. Clearly, it is
seen that NDE Process Verification ranked highest in relative desirability, but lowest in probability
of success. The Velocity Monitor and Intensity Monitor both ranked moderately high in relative
desirability and highest in probability of success.