Why are sucker rods shot peened?

Sucker rods work under cyclic tensile loading for millions of cycles downhole, and that is the mechanism that drives them to fatigue failure. Cracks initiate at the surface, where tensile stresses are highest. Shot peening introduces a layer of residual compression at that surface: as the rod works, the applied tension must first overcome that compression before producing effective tensile stress, which delays crack initiation and increases fatigue life.

In corrosive wells the benefit is twofold: the compressive layer also helps against corrosion fatigue and stress corrosion cracking (SCC), because these mechanisms need surface tensile stress to propagate, and compression counteracts it.

This is why shot peening is a standard treatment in the manufacture of steel sucker rods (for example 4140, API 11B), applied to the rod body, where the failure mode concentrates.

The process may be single or double depending on the demand; for the machines and process detail, see the sucker rod shot peening page and the introduction to the shot peening process.

CYM designs and builds shot peening machines for sucker rods, including in-line double-process configurations.