Shot Peening

Sucker Rod Shot PeenerWe understand that many people believe “shot blasting” or “shot cleaning” is the same thing as “shot peening.”  However, there is a technical and material difference between the two processes.  These processes are very different and are applied to achieve different results.

The only similarity between shot blasting and shot peening is that each process involves propelling media at a high velocity to strike a surface.  Shot peening, however, requires control over a consistent size and shape of media, a consistent velocity of media, a consistent volume of media striking the surface area, and for a consistent length of time.  Careful controls yield consistent and repeatable results that are not measurable visually.

Blast cleaning is ordinarily less controlled, therefore less expensive, and results can usually be assessed visually.

Shot peening can be measured using several means, both destructively and non-destructively.  The most widely accepted non-destructive, commercially acceptable measurement involves the use of the Almen method, which is the method used at TRC.  All treatments are carefully controlled and documented like a recipe for reliable, consistent, and repeatable results.

TRC shot peens the rods as compared to all other rod service companies that only shot clean their rods.  This process is the cornerstone difference between TRC and our counterparts.  They reclaim your used sucker rods while we recondition your used sucker rods.  Our reconditioning process will reduce the residual stress built up in your rod and will therefore extend the life of your rod by 50% to 100%.

The Shot Peening Process

Shot peening of sucker rods is a cold working process in which the surface is bombarded with small spherical media called shot.  As each individual shot particle strikes the rod surface, it produces a slight rounded depression.  Plastic flow and radial stretching of the surface metal occur at the instant of contact and the edges of the depression rise slightly above the original surface.  The residual stress layer usually extends to about .005” to .010” below the surface.

The metal beneath this layer is not deformed, but the fibers try to restore the surface to its original shape, and a reaction-induced tensile stress develops to achieve equilibrium.  The magnitude of the compressive stress varies, but it will normally have a value of at least one-half the yield strength.

  

Benefits

Benefits obtained by shot peening are the result of the effect of the compressive stress and the cold working induced. Compressive stresses are beneficial in increasing resistance to fatigue failures, corrosion fatigue, stress corrosion cracking, and hydrogen assisted cracking (Hydrogen Embrittlement).

Fatigue Failures

Shot peening is effective in reducing sucker rod fatigue failures caused by cyclic loading. These failures usually originate in surface areas under tension load, and cracks propagate from a surface defect or other stress riser.

Shot peening prevents such failures by inducing an even, compressive stress layer in the surface of the sucker rod, permitting an increase in service life or the allowable stress level. As a shot peened sucker rod is loaded, its critical surface area will not develop tensile stresses until the peen induced compressive stresses are first overcome.

The American Petroleum Institute states, "We consider shot peening, when properly controlled and applied, a satisfactory method of cold working a metal surface to improve fatigue life."

Stress Corrosion Cracking

Stress corrosion cracking is defined as failure by cracking under combined action of corrosion and static tensile stress, either external (applied) or internal (residual). It is a complex interaction of sustained tensile stress at a surface and corrosive attack that can result in brittle failure. Stress corrosion cracking cannot occur in an area of compressive stress. The compressive stresses induced by shot peening can effectively overcome the surface tensile stresses that cause stress corrosion.

Hydrogen Embrittlement

Atomic hydrogen is extremely mobile and able to easily penetrate and interact with metal, greatly reducing ductility and its ability to withstand cyclic loads. Shot peening has been shown to be effective in retarding the migration of hydrogen through metal.

Surface Integrity

Shot peening improves the surface integrity of the sucker rod, because no matter how carefully it is manufactured, it still contains minute surface imperfections. These may be localized areas of tensile stress, or pits, scratches, and other surface imperfections. As peening cold-works the rod surface, it blends small surface imperfections and effectively eliminates them as stress concentration points.

TRC has patent pending for a device used in shot peening a sucker rod.