Benefits of Shot Peening
What Peening Does
Peening causes the material in the surface zone to yield by literally shaking the metal grains into a more relaxed state. As a result, the surface layer counteracts tensile stress and effectively prevents cracking.
Why Peening Is Needed
Machining and forming metal parts creates stress concentrations internally (burns, scratches, welds, etc.) and can cause other additional defects.
These problems may be responsible for premature fatigue failure, porosity, deteriorated strength and corrosion, but can be prevented with peening.
Since fatigue cracks generally begin at surface imperfections, a compressively stressed peened skin is highly effective in preventing crack formation and growth.
In recent years, gears have been shot peened after carburizing and have been made of steel at a hardness level above Rockwell 60RC for fatigue and shock applications. The figures below show the close connection between peened versus unpeened gears.