7 th Annual Sucker Rod Pumping Workshop Renaissance Hotel Oklahoma City, Oklahoma September 27-30, 2011 The Benefit of Sucker Rod Shot Peening Norman W. Hein, Jr., P.E. Director Development, Engineering & Standardization NPS Norris/AOT Tulsa, OK 918.445.7619 nhein@norrisrods.com
Contents Abrasive Blasting Background Peening Background Shot Peening Background Process Control Conclusions & Recommendations 2
Abrasive Blasting Abrasive blasting is a process that is often used to clean the surface of a material The process usually involves compressed air and a gun that is used to shoot various types of abrasive media at the object being blasted This is a particularly effective method for removing scale, rust, paint, and minor surface flaws from metal objects Also the process can provide a clean/uncontaminated, rough surface finish for better adherence of a subsequent coating 3
Abrasive Media There is a wide variety of media available including: Aluminum Oxide (Shot) Silicon Dioxide (Sand) Iron or Steel Oxides (Grit) Glass Beads Pumice Typically media is hard, sharp and friable Also media is normally reused w/o filtering 4
Abrasive Media (cont) The selected media usually specifies the process: Shot blasting Sand blasting Grit blasting 5
Peening Peening is a manufacturing process that works the surface of a metal to improve its material properties Peening usually is done by mechanical means such as hammer blows or by blasting with shot (shot peening) Peening is normally a cold work process that: induces compressive stresses, relieves tensile stresses already present, increases hardness and strength, increases fatigue life, and may increase corrosion resistance 6
Peening (cont) Peening and cold working have been used effectively for centuries Peening changes the surface microstructure or grains by deforming a local area The deformation is very shallow; typically < 0.005 to 0.010 deep 7
Peening (cont) This is in contrast to other surface treatments such as surface/ case hardening which can be done by a variety of different (heat) treatments: Induction hardening Carburizing Nitriding Cyaniding Carbonitriding 8
Shot Peening A special form of peening is shot peening. Similarities between shot blasting and shot peening is that each process involves propelling shot media using high velocity air to strike a surface However, the media is typically small, spherical shot with a specific mix in shot size(s) 9
Shot Peening (cont) Shot peening also requires: Control over a consistent size and shape of shot media, Consistent velocity of media, Consistent volume of media striking the surface area, Consistent length of time of exposure Consistent area of coverage Careful controls yield consistent and repeatable results that are not measurable visually 10
Shot Peening (cont) Shot blast cleaning is ordinarily less controlled, therefore less expensive, and results can usually be assessed visually Shot peening should not be considered a cleaning process. Thus, care needs to occur through out the forging and heat treating processes so that excess scale is not developed that can not be effective removed by peening This is also one of the process controls that can make it work or not 11
Shot Peening (cont) Shot peening effectiveness for surface deformation can be measured using several means, both destructively and nondestructively. The most widely accepted non-destructive, commercially acceptable measurement involves the use of the Almen strip method All peening treatments need to be carefully controlled and documented like a recipe to achieve reliable, consistent, and repeatable results 12
Shot Peening Process As individual shot particles strike the rod surface, a slight, rounded depression is produced 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 13
Shot Peening Process (cont) The metal beneath this layer is not deformed, but the fibers try to restore the surface to its original shape 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 14
Shot Peening (cont) Subsequently, as other shot hits the surface, the edges of the depressions are compressed and each particle further deforms and overlaps the depressions This results in the surface area completely covered and uniform residual stress that improves rod properties 15
Main Process Control Tests Almen strip Coverage area test 16
Almen Strip Testing Almen strips are used to measure the intensity of a shot peening process Strips are controlled by an ASTM Standard An Almen strip is placed on a gauge block and processed through the machine The Almen strip is then removed and the deflection introduced from the peening process is measured The measured deflection is directly proportional to the intensity of the process 17
Almen Strip Testing (cont) 18
Coverage Area Test Defined as percent of surface area obliterated by indentations Coverage can only be determined by visual inspection of the part surface at 10-30X magnification Partial Coverage Acceptable Coverage 19
Coverage Levels 20
Why Shot Peen? The shot peening process has been proven to be beneficial to increase the fatigue life of rods The University of Tulsa performed a study on life cycle rates of AISI A-4330-M Nickel- Chromium- Molybdenum Alloy Steel both with and without special shot peening (NOR-PEENING ) The machine was designed to test 2 (0.61 m) pony rods in pure bending, from four-point loading, to failure 21
Why Shot Peen (cont) At a 89.64 ksi (618 MPa) stress amplitude condition the remaining life of the test samples increased by 129% At a 70.08 ksi (483 MPa) stress amplitude condition the remaining life of the test samples increased by 4,431% when properly NOR-PEENED 22
Tulsa University Fatigue Results 95 90 85 AVERAGE LIFE CYCLES NOR-PEENED COMPARED TO NON-SHOT PEENED FORCE (LBS.) 80 75 70 65 60 55 50 22,000,000 21,000,000 20,000,000 19,000,000 18,000,000 17,000,000 16,000,000 15,000,000 14,000,000 13,000,000 12,000,000 11,000,000 10,000,000 9,000,000 8,000,000 7,000,000 6,000,000 5,000,000 4,000,000 3,000,000 2,000,000 1,000,000 0 AVERAGE LIFE CYCLES NOR-PEENED TEST RESULTS NON-SHOT PEENED RESULTS 23
Conclusions & Recommendations Shot peening is a special surface treating process that has a long history of being effective to increase fatigue life. But, as with most special processes, it has to be done correctly, otherwise it is not effective One sucker rod manufacturer has developed special control features (Nor-Peening ) that when properly applied has shown dramatic increase in fatigue life (up to >4000 %) 24
Conclusions & Recommendations (cont) However, subsequent damage to this surface will eradicate the benefits from providing this process 25
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