What is Shot Peening and Why is it Useful?

The parts are placed on this rotating platform and the shot comes from these carefully placed nozzles to insure complete coverage of the part.

Another rotating platform with a single nozzle for shot placement.

Shot size and shot peening intensity are super important for getting the correct desired results. hardened gears require more aggressive treatment than aluminum.  Intensity is measured by the Almen method.  This uses a standardized mild street test coupon that is blasted by the shot and the deformation is measured. The shot size and velocity are adjusted to get the desired Almen range before the actual part is shot peened.

We recommend that you choose a shot peener that can work to Mil-Std-13165c.  AFCO, our shot peener typically uses hard steel shot in the range of Rockwell 55-65 C scale, mostly size 230 but sometimes size 330 to treat our parts. Usually, the Almen intensity is from .012 to .020 depending on the shot size and material.  We let them decide what is best for our application.  This is important because under-peening won’t get the job done and over-peening can actually overwork harden the surface and reduce the fatigue strength!

For us, we often do a three-step process in this exact order and in this order only.  First, we cryogenically treat the parts.  This stress relieves them, evens out the distribution of alloying elements in the part, and converts the crystal structure of ferrous parts to martensite, a harder and stronger structure.   Next, we do a rigorous shot peening and finally we WPC treat the part to further refine the grain of the stressed skin that the shot peening creates and to improve the surface friction.  We use CTP cryogenics for our cryo treatment.

We have had tremendous success shot peening and doing our triple treatment to transmission parts, and axles. Generally, as a first step, we deburr and polish the tops and sides of the teeth to remove stress risers and to reduce sharp point contact on the gear faces before we start the treatment process.


  1. For ring and pinion prep, would there be any advantage of prepping used parts that have had the surfaces worn in but not abused or would it just be better to start from zero?

    1. As a certified mecanical engineering consultant specializing in failure analysis for a few wonderful decades, I specified shotpeening for many applications with fatigue failures completely eliminated in many cases. Elimination means an “infinite fatigue life”.

      I actualy tried to do what you are asking, shot peen an industrial part that had already seen millions of cycles. And unlike a new part, the peening did not prevent an early fatigue failure.

      For those more interested in the subject, this link shows graphically what fatigue life is all about, copy & paste:


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