Shot
Peening Success Stories for Metal Finishing
Shot
peening has proved its effectiveness in extending the
service life and enhancing the performance of metal
components by protecting them against fatigue, fretting
fatigue, stress corrosion cracking and a variety of
other failure mechanisms. Following are some of these
success stories for metal finishing applications:
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Al 7050-T7651 High Strength Aluminum
Fatigue
specimens were prepared from high strength Al 7050-T7651.
All four sides of the center test portion were shot
peened. Fatigue tests were conducted under a four-point
reversed bending mode (R = -1). The S-N curve of the
shot peened versus non-shot peened alloy is shown in
the accompanying table. It was found that shot peening
improved the fatigue endurance limit by approximately
33%. Even in a regime where the stress ratio is between
the yield strength and the endurance limit, the fatigue
strength increased by a factor of 2.5 to almost 4.
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Anodized Aluminum Rings
Aluminum
(AlZnMgCu 0.5) rings with external teeth were tested
for comparison purposes with anodizing and shot peening.
The rings had an outside diameter of ~ 24" (612 mm)
and a tensile strength of ~ 71 ksi (490 MPa). The (hard)
anodizing layer was ~ 0.0008" (0.02 mm) thick. Bending
fatigue tests were conducted to find the load to cause
a 10% failure probability at one million cycles. The
table shows the results.
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High Density Powder Metal Gears
Tooth root bending fatigue studies were performed using pulsator tests to compare a reference wrought gear steel to a 7.5 g/cm3 powdered metal gear. Both gears were 3.5 mm module consisting of 25 teeth and case hardened to 60 HRC. The wrought gear was a 16MnCr5 steel and the powdered metal gear was Fe-3.5Mo alloy content.
In the accompanying graph the powdered metal gear results are depicted with the blue curves. The endurance limit improved ~ 35% with the addition of shot peening. The endurance limit improved from ~ 95 ksi (650 MPa) to ~ 128 ksi (880 MPa). The endurance limit of the shot peened powdered metal compares very closely with the non-peened 16MnCr5 material. Due to the significant cost savings of powdered metal, the shot peened powder metal gear may be a suitable replacement to the more expensive wrought steel gear. Shot peening was performed at 0.013" A (0.32 mm A) intensity for all samples.
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Sulfide Stress Cracking
Hydrogen
sulfide (H2S) is commonly encountered in
sour gas wells. Certain metal alloys when exposed to
H2S will experience a significant decrease
in fatigue strength. The test results shown in the table
illustrate the response of precipitation hardened 17-4
stainless steel exposed to H2S with and without
shot peening.
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