Controlling the Shot Peening Process
Controlled shot peening is different than most manufacturing processes in that there is no nondestructive method to confirm that it has been performed to the proper specification. Techniques such as X-Ray Diffraction require that a part be sacrificed to generate a full compressive depth profile analysis. To ensure peening specifications are being met for production lots, the following process controls must be maintained:
CWST currently meets or exceeds the most stringent quality standards requested by its industrial, automotive and aerospace customers. Based on local industry requirements, our facilities maintain quality system compliance or registration to ISO9001:2000, TS-16949:2002 and/or AS9100. Further, CWST facilities that support the aerospace community participate in Nadcap’s rigorous accreditation program.
Figure 11-1 demonstrates acceptable and unacceptable media shapes. Peening media must be predominantly round. When media breaks down from usage, the broken media must be removed to prevent surface damage upon impact.
Shot peening intensity is the measure of the energy of the shot stream. It is one of the essential means of ensuring process repeatability. The energy of the shot stream is directly related to the compressive stress that is imparted into a part. Intensity can be increased by using larger media and/or increasing the velocity of the shot stream. Other variables to consider are the impingement angle and peening media. Intensity is measured using Almen strips. An Almen strip consists of a strip of SAE1070 spring steel that is peened on one side only. The residual compressive stress from the peening will cause the Almen strip to bend or arc convexly towards the peened side (Figure 11-2). The Almen strip arc height is a function of the energy of the shot stream and is very repeatable.
Complete coverage of a shot peened surface is crucial in performing high quality shot peening. Coverage is the measure of original surface area that has been obliterated by shot peening dimples. Coverage should never be less than 100% as fatigue and stress corrosion cracks can develop in the non-peened area that is not encased in residual compressive stress. The adjacent pictures demonstrate complete and incomplete coverage. (Figures 11-3A and 11-3B). If coverage is specified as greater than 100% (i.e. 150%, 200%) this means that the processing time to achieve 100% has been increased by that factor. A coverage of 200% time would have twice the shot peening exposure time as 100% coverage.
PEENSCAN (Coverage Verification) – Determination of shot peening coverage can be fairly easy when softer materials have been peened because the dimples are quite visible. A 10-power (10x) magnifying glass is more than adequate for these conditions. In many applications determination of coverage is more difficult. Internal bores, tight radii, extremely hard materials and large surface areas present additional challenges in determining coverage. Complete Shot Peening
CWST has developed the PEENSCAN process using DYESCAN fluorescent tracer dyes for this reason. PEENSCAN is ideal for measuring uniformity and extent of coverage for difficult conditions. The whitish-green dye is not visible under normal lighting conditions and must be viewed under a UV (black) light.
The coating can be applied by dipping, brushing or spraying the part under analysis. As the coated surface is impacted with peening media, the impacts remove the fluorescent, elastic coating at a rate proportional to the actual coverage rate. When the part is viewed again under a black light non-uniform coverage is visibly evident. The shot peening process parameters can then be adjusted until the PEENSCAN procedure verifies complete obliteration of the area of concern.
4. Automated Shot Peening Equipment
Throughout the world, CWST service centers are equipped with similar types of automated shot peening equipment. When required, this network allows for efficient, economic and reliable transfer or duplication of shot peen
processing from one location to another.
CWST also offers Computer Monitored Shot Peening (CMSP), which utilizes additional process controls and records data during the production shot peening of each part. For components designed to incorporate shot peening for product life enhancement, customers should request adherence to the computer monitored process specification AMS-2432.
CWST has developed CMSP equipment that has the capability to monitor, control and document the following parameters of the peening process:
- Air pressure and shot flow at each nozzle
- Wheel speed and shot flow of each wheel
- Part rotation and/or translation
- Nozzle reciprocation
- Cycle time
These parameters are continuously monitored and compared to acceptable limits programmed into the computer. If an unacceptable deviation is found, the machine will automatically shut down within one second and report the nature and extent of deviation. The machine will not restart processing until machine parameters have been corrected.