If you are a long-time MotoIQ reader you probably know we often prescribe shot peening for heavily stressed metal parts. You have probably read about us prescribing the shot peening treatment on everything from driveline parts to cranks and rods. If you didn’t know, shot peening entails striking a surface with shot (usually round and metallic although glass and ceramic particles can be used) with enough force to create plastic deformation. We have even included information for shot peening specs to be used on certain parts. However, recently our shot peening vendor AFCO or Abrasive Finishing Corporation advised us that our article information is out of date and needed revising.
Shot peening is used to strengthen and stress relive metal parts. The process is much like sand blasting but the object of shot peening isn’t to remove material or clean it but to create an extremely thin layer of fine-grained metal under compressive stress. Cracks have a very hard time propagating in this layer. The surface compressive stresses confer resistance to metal fatigue and stress corrosion failure. Stress corrosion failure is when corrosion starts forming in surface micro-cracks that lever the material apart, helping the cracks to spread. If done properly, shot peening will increase the fatigue strength of a part by at least 100% and 200 or 300 percent improvements are not uncommon. Now fatigue strength isn’t the same as tensile strength our ultimate strength. Shot peening will only very slightly improve those things. Fatigue strength is the number of cycles a part can experience before failure. If the rod shown above failed after 20 hours of run time, shot peening can improve its life so it can last 40 hours. It’s not going to make it into a bulletproof wonder rod that’s never going to break ever!
Shotpeening is using a round hard shot to create a compressed skin over the surface of the part a few thousands of an inch deep. The surface can be very visibly dimpled in softer metals like mild steel or aluminum or subtly in very hard gear steels, hardened or nitrided parts.
Pro Go Kart racing teams replace their chassis after every day of racing. The chassis depends on springiness and flex to help find grip. Normal people like us use their chassis until they break or bend! We shot peened Christa’s 125cc TAG kart that has been used for about 3 seasons of use. You can see the pebbled shot peening pattern on the frame.. We were hoping that this would work harden the surface of the frame and restore a better-than-new springiness to the frame.