Shot Peening

Shot peening is the process by which various small pieces of material, typically in the form of cast steel shot, cut wire, ceramic bead or glass bead are propelled onto and impact a material. When the media collides with the material, it creates a small dent.  The dent causes the material immediately below to push back against the indent compressing the area of impact. When large number of adjacent indents are formed on the surface, a uniform residual compressive stress layer is created. This compressive layer works to counteract tensile forces on the material, resulting in an increase in fatigue life because of delayed crack propagation. Internal compressive stresses near the surface are balanced by tensile stresses deeper into the material. However, these deeper tensile stresses are not detrimental because cracks are unlikely to spread from the interior of a component.

Or put another way, shot peening is like squeezing the surface of the material together, so when a force that pulls on the material comes along, the squeezing counteracts the pulling. Imagine a component that is continuously cyclically stressed. Over time, microcracks that are present on the surface begin to spread and if the number of cycles and loads are large enough, the cracks will spread perpendicular to the applied tensile stress. The compressive stress induced by shot peening keeps the cracks from propagating and increases the fatigue life of the component.

Application Methods

The two main ways media is shot at material are air blast and centrifugal wheels. Air blast is fairly self explanatory; the pellets are moved thru a hose via pressured air and released onto the surface. Centrifugal wheels, meanwhile, load up shot in its’ paddles and then release once up to speed similar to a shot put.  Both methods often utilize an enclosed sheet metal cabinet or even walk in container, with a closed loop system recycling the shot in both cases. Shot peening in the field, outside of a manufacturing facility can also be performed where the media is not automatically reused, however clean up and its’ associated costs typically restrict these applications to very large assemblies which are not readily moveable and other options do not exist.

Component Applications

There are many applications where it is practical to apply shot peening to extend fatigue life of components and to reduce stress corrosion cracking. Often shot peening is applied over areas of stress risers, where abrupt changes in geometry cause stress to concentrate and are a common location of failure. Examples of stress risers are holes, internal corners, grooves, and thread roots. Component application examples for shot peening include gears, springs, shafts, connecting rods, crankshafts, and turbine blades.

Peen forming

The compressive stresses imparted onto the surface of material can also be used as a forming method. For example, large panels can be shaped without expensive dies or presses due to the stretching of the outer surface which causes the panels to bend towards the shot stream. Peening forming can also be used to straighten shafts or components that are bowed.

Fatigue Life Improvement

In 1982, NASA tested the fatigue life of spur gears made from 9130 steel, case hardened to 58C and subsequently polished. One set of gears were shot peened to an intensity of 7-9A and were compared against non peened gears. Both gears were then run at 10,000rpm and at a 170°F temperature.  Residual compressive stress in the shot peened gears was increased by 40% compared to the non shot peened gears. As a result, the shot peened gears had a fatigue life improvement due to pitting resistance of 1.6x. Further, in 1991, NASA conducted a similar study on gears this time with a increase of intensity to the shot peened gears of 15-17A. This time residual compressive stressed were measured as 57% higher and fatigue life increased 2.1x  compared to the previous study.

Due to the relatively low-cost nature of shot peening and the considerable fatigue life improvement, the return on investment of shot peening can be considered to be extremely high. Amco often advises its’ customers that the value add of shot peening can rarely be duplicated especially for high cost components manufactured from expensive materials or with costly coatings. Few processes in modern manufacturing provide such considerable returns.

Specification

The common method of specifying the type of shot peening to be applied is to indicate the size of the shot , intensity with which the media impacts the material and the percent coverage. For example, the following common specification call out specifies a S110 shot size with a range of intensity between 8-10 ‘A’ and coverage of 100%: S110, 8-10A, 100% coverage

Shot Size

Shot size is defined with a S designation and the size of the shot in thousands of an inch. For example, S70 shot is approximately Ø0.007” and S280 shot is approximately Ø0.028” Common shot sizes are summarized in the following list:

• S70

• S110

• S170

• S230

• S330

• S390

• S460

• S550

• S660

• S780

If shot size is limited by part geometry as in the case of stress risers, a size of 1/4-1/2 the size of the radius being peened is considered optimal. In other words, if a shaft is limited to a 0.028” internal radius, a reasonable shot size would be S70, as 0.007/0.028 =1/4.

Intensity

Intensity is the amount of energy a system uses to propel the media onto the material. The more energy or intensity, the more material deformation. The energy of the shot is a function of the speed, angle of impact, hardness and the mass of the shot and does not depend on the amount of shot peening coverage or time spent peening, which will become important to note upon further reading.  In an air blast system, the speed of the shot depends on the amount of air pressure applied and in a centrifugal blast system speed depends on the rpm of the wheel, all else being equal.

When calling out an intensity specification, either a N, A or C scale is used, with N being the least intense and C being the most intense.  The intensity of the system is determined by peening an Almen strip, which is a thin metal strip of either 0.80mm, 1.30mm or 2.40mm thickness corresponding to the N, A or C scale, respectively.

To determine the intensity of the shot peening system, an Almen strip of suitable scale (N, A or C) is bolted onto a thick, solid metal block and shot peened with the nozzle distance and blast angle remaining consistent when compared to the application. After peening, the Almen strip is released and will bow upwards in the direction of the shot. The height of the bow is measured and recorded. It is important to note that the longer the Almen strip is exposed to the shot, the more the strip will deform or bow.  The saturation height or intensity of the system is defined by shot peening at least 4 different Almen strips and doubling the exposure time for each one, then plotting a height vs time graph. The point at which the exposure time doubles but the bow height increases exactly 10% is the defined as the intensity of the system.

In the chart above, 4 strips were peened and the bow height recorded. The intensity in this example is 10A because the intensity changed 10% (from 10 to 11 thousands) when the peening time doubled (from 20s to 40s). Although the intensity is 10A @ 20s, this is not the amount of time that is required to shot peen a component. The 20s time is the point at which the intensity increased 10% when the shot peening time doubled. Nor can the intensity be lowered to 5A by peening for 5s or the intensity increased to 11A by peening for 40s.  The intensity of the system is fixed and unless the speed, angle of impact, hardness or size of the shot changed during the peening process, it is not possible for the intensity to change.  The entire saturation curve is used to determine the intensity of the system and peening times of individual Almen strips cannot be used to satisfy an intensity requirement.

 To initially set the intensity of the system, it is necessary to expose 4 or more Almen stips as above, however to confirm intensity only the last two values need to be confirmed. ie In the example above, if the one strip measures a bow height of 10 thousands after peening for 20s and the second strip measures a height of 11 thousands after 40s, then the intensity is confirmed as 10 A.  It needs to be emphasized that using two strips is only possible as confirmation is an initial saturation curve has already been created and no other parameters in the system have changed. This is an often-misunderstood aspect of calibration. A saturation must be performed initially with at least four strips, but confirmation is possible with only two strips if the values are the same.

Measuring the intensity of a shot peening system is necessary at frequent intervals to ensure compliance to specification.  Although it is possible to measure the compressive stress of a peened part with several advanced methods, such as X-Ray diffraction and hardness profile testing, it is much easier to measure the intensity of the system before and possibly after peening to ensure compliance to specification.

Amco Manufacturing regularly performs shot peening intensity calibrations using the above methods to ensure the intensity of shot is correct on your critical components. Unfortunately, the nature of shot peening is easy to apply but difficult to measure so it is important to calibrate properly to ensure the intensity and compressive layer properties of your components are correct.

Coverage

Shot peening coverage is the density of impacts on a surface stated as a percentage of surface indented. A 10% coverage would leave 90% of the surface without peening. Coverage is important because a coverage that is too high or too low will reduce fatigue life. A common misconception is that coverage is determined from the time(s) on the saturation curve, but coverage is completely independent and has nothing to do with the saturation curve. To improve coverage in most situations, more time spent peening is required.

A coverage of 98-100% is considered totally covered and cannot be improved upon. However, shot peening specifications will sometimes be listed as 150% or 200% coverage which dictate a multiple of time that is required for 100% coverage.  Coverage is typically inspected by optical magnification (10x+) or fluorescent tracer.  

Media Maintenance

Shot peening media needs to be maintained as the size and shape of the shot may break down over a period of time. Size is determined by selecting a representative sample of media and weighing the amounts that pass through sieves of different sizes and comparing to specification. Shape inspection is performed using visual magnification of a sample of media and identifying the number of misshaped pieces of shot. If there too many pieces identified as deformed, then the media should be changed. New media is eventually required when shot breaks down past a certain point.

Glass bead

Glass bead peening is an effective media to used to decontaminate surfaces blasted with iron based shot as glass is chemically inert. It can also be an effective peening media for various non ferrous materials such as aluminum and stainless steel where the contamination of ferrous shot is undesirable.

Amco Manufacturing performs shot peening in Edmonton, Alberta utilizing an air blast shot peening system with glass bead decontamination. Amco can process parts up to Ø25.5” x 120” long which we often also conveniently machine at our facility resulting in a decreased lead time. We are able to process parts with a variety of shot and intensity requirements. Our system is calibrated and maintained as described above with a stored record of proof of calibration.  Our common peening applications are threads (particularly thread roots), internal radii and grooves of shafts, and anti galling applications.  We are able to peen interior and exterior features with custom applications available. Contact us today to discuss your shot peening questions.