Cn 1 1 Cl

where C1 is the percentage of coverage (decimal) after 1 cycle, Cn is the percentage of coverage (decimal) after n cycles, and n is the number of cycles.

This relationship indicates that coverage approaches 100% as a limit. Accurate measurements above 98% coverage are difficult to obtain, but a measurement at a lower degree of coverage serves as a means of determining the exposure time or equivalent time required to obtain any desired coverage. Because accurate measurement can be made up to 98%

coverage, this value is arbitrarily chosen to represent full coverage or saturation. Peening at less than saturation is ineffective because of the amount of unpeened surface. Beyond this value, the coverage is expressed as a multiple of the exposure time require to produce saturation. For example, 1.5 coverage represents a condition in which the specimen or workpiece has been exposed to the blast 1.5 times the exposure required to obtain saturation. Figure 1 shows the relationship between exposure time and coverage and indicates that after a measurement of a low percentage of coverage has been established, the correct exposure time for any percentage of coverage can be readily determined.

Fig. 1 Area coverage as a function of exposure time in shot peening

Measurement of Coverage. Direct methods for measuring coverage include visual methods and the Straub method. One of the indirect methods is the Valentine method, which involves layer removal.

Visual methods, although not quantitative, are almost universally used. The simplest of these consists of visual inspection, with or without the aid of optical (10x) magnification of the surface of the peened part. This method may be supplemented by a series of reference photographs illustrating various percentages of coverage.

Another visual method consists of preparing a transparent plastic replica of the peened surface and comparing it, by means of photographic projection, with reference replicas having various percentages of coverage.

The Straub method consists of exposing a polished surface to the shot stream, projecting the surface at a magnification of 50 diameters on the ground glass of a metallographic camera, tracing the images of the indented areas on translucent paper, and measuring the total area and the indented area with a planimeter. Percentage of coverage is expressed as the ratio of indented area to total area multiplied by 100. About 15 min is required to make one measurement.

The Peenscan method is offered in lieu of visual inspection in MIL-S-13165 and consists of painting a part before peening with a dye sensitive to ultraviolet light, shot peening the part, inspecting the part under the ultraviolet light for any missed areas, shot peening the part, and reinspecting the part under ultraviolet light. Complete removal of the dye indicates 100% coverage of the part.

The Valentine method consists of making a duplicate of the part from low-carbon steel, peening the part, annealing it for several hours to promote recrystallization and grain growth, and relating peening coverage to the amount and continuity of grain growth by metallographic examination of cross-sectional areas.

Because of the difficulty in quantitatively measuring coverage by these methods, percent coverage is usually estimated from the curve of Almen arc height against the duration of shot exposure. The Almen test is described in the section "Peening Test Strips, Holder, and Gage" in this article. The graph in Fig. 2 shows the relation between shot coverage and doubling exposure time. A change in arc height of 10% or less indicates saturation peening.

f 2i

Exposure Hme

Fig. 2 Relation of measuring coverage to peening time. Coverage is considered full at time t, if doubling exposure to time 2t results in change in arc height less than 10%.

Peening intensity is governed by the velocity, hardness, size, and weight of the shot pellets, and by the angle at which the stream of shot impinges against the surface of the workpiece. Intensity is expressed as the arc height of an Almen test strip at or at more than saturation coverage. Arc height is the measure of the curvature of a test strip that has been peened on one side only. At or above saturation of the Almen strip, arc height is a measure of the effectiveness of the peening operation on a specific part. The Almen test is the primary standard of quality control and should be used at regular intervals, often on a day-to-day basis, and in the same location in the peening setup. Used correctly, a lower arc height indicates a reduction in peening intensity caused by a reduction in wheel speed or air pressure, excessive breakdown of shot, or other operational faults, such as undersized shot in the machine or clogged feed valves.

Selection of Intensity. The lowest peening intensity capable of producing the desired compressive stress is the most efficient and least costly, because the peening process can be achieved with the minimum shot size in the minimum exposure time. Conversely, an intensity may be considered excessive if, as with very thin parts, a condition is produced in which the tensile stresses of the core material outweigh the beneficial compressive stresses induced at the surface. Figure 3 presents data that indicate the relation of peening intensity to cross-sectional thickness.

Fig. 3 Relation of peening intensity to cross-sectional thickness of parts peened

The depth of compressed layer to be produced by peening is a factor in selecting peening intensity. For instance, a heavy steel component with a partially decarburized skin requires a peening intensity high enough to induce a compressive stress beneath the decarburized layer. The relation between peening intensity and depth of compressed layer, for steel hardened to 31 and 52 HRC, is shown in Fig. 4.

Fig. 4 Relation of depth of compressed layer to peening intensity for steel of two different hardnesses

Types and Sizes of Media

Media used for peening can be iron, steel, or glass shot, or cut steel or stainless steel wire. Metallic shot is designated by numbers according to size. Shot numbers, as standardized by MIL-S-13165, range from S70 to S930. The shot number is approximately the same as the nominal diameter of the individual pellets in ten thousandths of an inch. Standard size specifications for cast iron and steel shot are given in Table 1.

Table 1 Cast shot numbers and screening tolerances

Numbers in parentheses measured in inches

Table 1 Cast shot numbers and screening tolerances

Numbers in parentheses measured in inches

Peening shot size No.

All pass U.S. screen size

Maximum 2% on U.S. screen

Maximum 50% on U.S. screen

Cumulative minimum 90% on U.S. screen

Maximum 8% on U.S. screen

Maximum number of deformed shot acceptable

930

5 (0.157)

6 (0.1320)

7 (0.1110)

8 (0.0937)

10 (0.0787)

5(a)

780

6 (0.132)

7 (0.1110)

8 (0.0937)

10 (0.0787)

12 (0.0661)

5(a)

660

7 (0.111)

8 (0.0937)

10 (0.0787)

12 (0.0661)

14 (0.0555)

12(a)

550

8 (0.0937)

10 (0.0787)

12 (0.0661)

14 (0.0555)

16 (0.0469)

12(a)

460

10 (0.0787)

12 (0.0661)

14 (0.0555)

16 (0.0469)

18 (0.0394)

15(a)

390

12 (0.0661)

14 (0.0555)

16 (0.0469)

18 (0.0394)

20 (0.0331)

20(a)

330

14 (0.0555)

16 (0.0469)

18 (0.0394)

20 (0.0331)

25 (0.0280)

20(b)

280

16 (0.0469)

18 (0.0394)

20 (0.0331)

25 (0.0280)

30 (0.0232)

20(b)

230

18 (0.0394)

20 (0.0331)

25 (0.0280)

30 (0.0232)

35 (0.0197)

20(b)

190

20 (0.0331)

25 (0.0280)

30 (0.0232)

35 (0.0197)

40 (0.0165)

20(b)

170

25 (0.0280)

30 (0.0232)

35 (0.0197)

40 (0.0165)

45 (0.0138)

20(b)

130

30 (0.0232)

35 (0.0197)

40 (0.0165)

45 (0.0138)

50 (0.0117)

30(c)

110

35 (0.0197)

40 (0.0165)

45 (0.0138)

50 (0.0117)

80 (0.0070)

40(c)

70

40 (0.0165)

45 (0.0138)

50 (0.0117)

80 (0.0070)

120 (0.0049)

40(c)

Per area, m. square.

Per area, 4 m. square

Glass shot, used primarily for peening nonferrous material, is available in a wider range of basic diameters. Hardness of glass shot is equivalent to 46 to 50 HRC. For further information on glass beads, see the article "Mechanical Cleaning Systems" in this Volume. Table 2 shows the effect of shot size and peening intensity on fatigue life.

Table 2 Effect of shot peening on fatigue strength of aluminum alloys and carbon and low-alloy steels

Metal tested

Type of specimen

Stress cycle

Surface condition as-received

Aluminum alloys

2014-T6

Plain, 38 mm (1.5 in.) diam

Reversed bending

Smooth turned(a)

2024-T4

Plain, 38 mm (1.5 in.) diam

Reversed bending

Turned(a)

7079-T6

Plain, 38 mm (1.5 in.) diam

Reversed bending

Turned(a)

7075-T6

Reversed bending

Turned(a)

Carbon and low-alloy steels

5160 spring steel(c)

Flat leaf, 38 mm (1.5 in.) wide, 4.88 mm (0.192 in.) thick

Unidirectional bending

Machined before heat treatment(d)

1045 steel (165 HB)

Plain (R.R. Moore)

Rotating bending

Machined

1045 steel (285 HB)

Plain (R.R. Moore)

Rotating bending

Machined

9260 steel (526 HB)

Plain (R.R. Moore)

Rotating bending

Machined

Ingot iron (121 HB)

Plain (R.R. Moore)

Rotating bending

Machined

4340 steel (277 HB)

Plain (R.R. Moore)

Rotating bending

Machined

4118 steel (60 HRC)

Single gear tooth

Unidirectional bending

Machined

8620 steel (58 HRC)

Single gear tooth

Unidirectional bending

Machined

S-11 steel(n)

Grooved, 7 mm (0.3 in.) D(o)

Rotating bending

Machined

0.54% C steel(q)

Plain, 8 mm (0.315 in.) diam

Rotating bending

Decarburized

Plain, 5.99 mm (0.236 in.) diam

Reversed torsion

Decarburized

10.0 mm (0.394 in.) diam bars:

Smooth

Rotating bending

Polished

Round-notched(r)

Rotating bending

Machined

V-notched(s)

Rotating bending

Machined(t)

Music wire(u)

Coil spring

Not reversed

4340 steel(w)

14 mm (0.560 in.) diam(x)

Reversed torsion

Smooth turned

4340 steel(z)

6.4 mm (0.250 in.) diam(x)

Rotating bending

Highly polished

4340 steel(})

6.4 mm (0.250 in.) diam(x)

Rotating bending

Highly polished

Metal tested

Peening conditions

Fatigue strength

Strength gain by peening, %

Ref

Shot

Intensity

As-received

Polished

rec'd

Over polished

Type

Size No.

0.025 mm

0.001 in.

MPa

1 ksi

MPa

1 ksi

7 MPa

1 ksi

Aluminum alloys

2G14-T6

Cast steel

7G

G.15

6 A

215

31(b)

26G

38(b)

23

23G

G.76

3G A

215

31(b)

26G

38(b)

23

550

0.33

13 A

215

31®

260

38(b)

23

2024-T4

Cast steel

230

0.25

10 A

180

26®

240

35(b)

34

7079-T6

Cast steel

230

0.25

10 A

195

28®

250

36.5®

30

7075-T6

MIL-5

0.15

6 A

220

32®

275

40(b)

25

(~)

Carbon and low-alloy steels

5160

spring steel®

Chilled iron

230®

0.15

6 C(f)

880

128®

1340

194®

51

(!)

230®

0.15

6 C

880

128®

1215

176®

37

230®

0.15

6 C(h)

880

128®

970

141®

10

1045 steel (165 HB)

Chilled iron

(i)

(j)

(j)

275

40®

305

43.8®

10

(?)

1045 steel (285 HB)

Chilled iron

(i)

(j)

(j)

560

81®

515

75(k)

-7

(?)

9260 steel (526 HB)

Chilled iron

(i)

(j)(i)

(j)(i)

750

109®

730

106®

-2

(?)

Ingot iron (121 HB)

Chilled iron

(i)

(j)(i)

(j)(i)

185

27®

185

27(k)

0.7

(?)

4340 steel (277 HB)

Chilled iron

(i)

(j)

(j)

455

66®

540

78(k)

18

(?)

4118 steel (60 HRC)

Cast steel

110

A

16,200

2,350(m)

20,900

3,025(m)

29

(>)

8620 steel (58 HRC)

Cast steel

230

0.41

16 A

86,185

12,500(m)

105,150

5,250(m)

22

(>)

S-11 steel(n)

280

(p)

(p)

260

38

420

61

62

(<)

0.54% C steel(q)

Chilled iron

460

0.48

19 A

310

45

475

69

54

(+)

Chilled iron

460

0.48

19 A

225

33

325

47

43

Chilled iron

460

0.48

19 A

585

85

600

87

3

(+)

Chilled iron

460

0.48

19 A

285

43

395

57

33

Chilled iron

460

0.48

19 A

185

27

325

47

73

■ (u) wire

110

825

120(v)

1310

190(v)

58

(++)

4340 steel(w)

Cast steel

170

0.20

8 A

275

40(g)

515

75(g)(y)

87

(*)

4340 steel(z)

0.25

10 A

570

83

270

39({)

675

98(|)

150

(**)

4340 steel(})

0.25

10 A

725

105

380

55({)

710

103(|)

87

(**)

(b) Values at 1,000,000 cycles.

(c) Oil quenched and tempered at 370 °C (700 °F), hardness is 46 to 50 HRC.

(e) Shot peened only on side subjected to tension in fatigue test.

(f) Peened under a strain +0.60, 1240 MPa (180 ksi).

(g) Fatigue limit based on 5,000,000 cycles.

(h) Peened under zero strain.

(i) Equal parts of 170 and 280.

(k) Fatigue limit based on 10,000,000 cycles.

(l) Stress relieved at 205 °C (400 °F) for 20 min after peening.

(m) Fatigue limit in kilograms (pounds) load based on 5,000,000 cycles.

(n) 3% nickel-chromium steel, oil quenched from 830 °C (1525 °F), tempered at 600 °C (1110 °F), tensile strength 930 MPa (135 ksi).

(q) Tensile strength, 1410 MPa (205 ksi).

(r) Notch depth 1.0 mm (0.040 in.), radius 0.051 mm (0.002 in.).

(t) Root of notch not hit by shot.

(v) For 400,000-cycle life.

(w) Tensile strength 1860 MPa (270 ksi).

(x) Chromium plated.

(y) Same value obtained for peened and chromium plated; not peened and plated is less than 275 MPa (40 ksi).

(z) Tensile strength 1520 MPa (220 ksi).

({) Fatigue limit for chromium plated and baked.

(|) Fatigue limit for peened, chromium plated, and baked.

(~) Fatigue Strength of 7075-T6 Aluminum Alloys When Peened with Steel Shot or Glass Beads, Potters Industries PII-I-74, 1974.

(!) R.L. Mattson and J.G. Roberts, The Effect of Residual Stresses Induced by Strain-Peening upon Fatigue Strength, Internal Stresses and Fatigue in Metals; Elsevier, Amsterdam, 1958.

(?) J.M. Lessells and W.M. Murray, Proc. ASTM, 41, 659 (1941).

(>) J.A. Halgren and D.J. Wulpi, Trans. SAE, 65, 452 (1957).

(<) W.J. Harris, Metallic Fatigue, Pergamon, 1961.

(+) S. Takeuchi and M. Honma, Effect of Shot Peening on Fatigue Strength of Metals, Reports of the Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendai, Japan, 1959.

(*) "Effect of Chromium Plate on Torsion Fatigue Life of Shot Peened 4340 Steel," Douglas Aircraft Co. Report No. MP 20.005 (Sept. 13, 1960); available through SAE.

(**) B. Cohen, "Effect of Shot Peening Prior to Chromium Plate on the Fatigue Strength of High Strength Steel," WADC Technical Note 57-178, U.S. Air Force, June 1957

Cast steel shot is made by blasting a stream of molten steel with water and forming globules that rapidly solidify into nearly spherical pellets. This process is also called atomizing. The pellets are screened for sizing, reheated for hardening, quenched, and tempered to the desired hardness. According to SAE Recommended Practice J827, "Cast Steel Shot," 90% of the hardness measurements made on the representative sample should fall within the range equivalent to 40 to 50 HRC. To maximize the peened effect, shot should always be at least as hard as the workpiece. For hard metals, special hard-cast steel shot, 57 to 62 HRC, should be used.

Cast steel shot is the most widely used peening medium. With suitable heat treatment, it has a useful life many times that of cast iron shot. Its improved impact and fatigue properties markedly lower the rate of shot breakage, increase peening quality, and extend the life of components of peening machines.

Cast iron shot or chilled iron is brittle, with an as-cast hardness of 58 to 65 HRC. It breaks down rapidly; however, its inherently high hardness yields higher peening intensities for a given shot size, in comparison to softer materials. A high rate of shot breakage complicates the control of peening quality and increases the cost of equipment maintenance and cost of shot, because broken shot must be eliminated for best results.

Glass beads are used for peening stainless steel, titanium, aluminum, magnesium, and other metals that might be contaminated by iron or steel shot. They are also used for peening thin sections. Relatively low Almen A shot peening intensities, seldom exceeding 0.15 to 0.25 mm (0.006 to 0.010 in.), are used. Glass beads can be used in either wet or dry peening processes.

Cut wire is generally manufactured from carbon steel or stainless steel. The cut wire is mechanically manufactured and as a result is more uniform than cast steel or iron shot in size distribution. For peening applications, it is "conditioned" or blasted into a spherical shape. This conditioning process also hardens the cut wire media. For information about sizing and related subjects, see the article "Mechanical Cleaning Systems" in this Volume.

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