Problems in Production Peening

Problems in the shot peening of production parts have been corrected by a variety of solutions. A small forging, as shown in Fig. 11(a), contained two holes with thin-wall sections. The inner bearing surfaces had to be held to a tolerance of 0.013 mm (0.0005 in.). In shot peening a hole with a heavy walled section, the size of the hole is reduced by the peening action. With the thin-wall section here, the size of the hole was not reduced, but the hole became oval shaped. The solution to this problem required the establishment of new dimensions before shot peening that permitted a light honing operation after shot peening to bring the hole size to within dimensional requirements.

Fig. 11 Production parts that presented problems in shot peening

In another instance, a flat ring, as shown in Fig. 11(b), failed in fatigue either by cracking from the inside diameter to the outside diameter or by flaking. Shot peening eliminated the fatigue failures but caused the parts to dish as well as to warp. These parts were required to retain flatness to within a tolerance of 0.013 mm (0.0005 in.). After various intensities were tried, the distortion problem was solved by peening one side of the ring at a higher intensity than that used on the other side, depending on warpage direction.

In shot peening the fir-tree serrations of type 410 stainless steel compressor blades used in a jet aircraft engine, as shown in Fig. 12, it was determined that a maximum improvement of 26% in the fatigue life of serrated blade roots could be obtained only if a closely controlled peening procedure was followed. Certain variations from this procedure actually proved harmful to blade life. An alteration in peening intensity had a marked effect on fatigue characteristics. Variations in blast intensity that could be obtained by changing nozzle size, aspirator size, or the distance from the nozzle to the work are given in Table 4.

Table 4 Peening intensity as affected by nozzle and aspirator sizes and distance from nozzle to work

Test

Nozzle size

Aspirator size

Almen intensity, A strip, 0.025 mm (0.001 in.),

No.(a)

at given distance from nozzle to work,

mm (in.)

mm

in.

mm

in.

200 (8)

180 (7)

150 (5)

120 (5)

100 (4)

1

9.5

3

4

5

5.5

4

5

5.5

5

8

32

2

9.5

3

5.5

7

4

5.5

5

5.5

5

8

32

3

13

1

4

5

3

2.5

3

3

3.5

2

32

4

13

1

5.5

7

5

6

7

7

7

2

32

5

16

5

4

5

7

6

5.5

7

7-8

8

32

6

16

5

7

9

6

6.5

7

7

7.5

8

32

Fig. 12 Relation of nozzle angle, angle of load face, and resulting angle of impingement in peening root serrations of compressor blades. Angle of impingement, 83°

The procedure adopted consisted of peening the root serrations with S70 steel shot at an intensity of 0.007 to 0.008 A at an impingement angle of 83°. The intensity indicated was measured at an impingement angle of 90°. The true intensity on various portions of the root serrations was considerably less, depending on impingement angle; however, maximum intensity was obtained at the critical root radius, using the 83° impingement angle calculated to have this effect. The relation between impingement angle and intensity is shown in Fig. 13.

Fig. 13 Peening intensity as a function of angle of impingement

Peening was performed in a gravity-feed, continuous-conveyor production cabinet, using a 16 mm (- in.) bore diameter

nozzle, a 4 mm (352 aspirator, 635 kPa (92 psi) air line pressure, and a shot flow of 3.4 kg/min (7.5 lb/min). The distance from the nozzle to the work was set at 100 mm (4 in.). The cabinet conveyor moved at a fixed speed that exposed the work to the blast for a period of 5 s. The peening operation required two passes under the nozzle, one for each side of the blade root. The airfoil sections of the blade were protected from the blast by a sheet rubber covering.

0 0

Post a comment