VOL. 8 September ISSUE YEAR 2007
MFN Trainer Column
in Vol. 8 - September Issue - Year 2007
Peening - Art Or Science?
MFN Trainer Kumar Balan
This column is a regular feature and is written by one of our MFN trainers or the Editorial Office. Readers are invited to send comments or questions to firstname.lastname@example.org. For more information about the trainers, see our website www.mfn.li/trainers.
I welcome the opportunity to join a group of very knowledgeable people as an MFN Trainer. The technique of shot peening carries its own significance in the niche field of blast techniques. However, it is commonly seen that users of blast equipment tend to utilize blast cleaning equipment to also achieve peening results. As an esteemed colleague of mine explains the difference rather eloquently – cleaning is an art, peening is a science!
Though there are other methods to evaluate cleaning quality, most commonly, cleaning results are purely visual. A part is either clean or it isn’t. If a particular anchor profile or etch is desired on a part, the roughness becomes a parameter to check using a profilometer. In comparison, peening is more tangible and defined. The shot peener is required to achieve a particular Almen intensity, saturate the part and demonstrate 100% or higher coverage. This intensity directly translates to a particular value of compressive stress that the component designer has designated as a ‘requirement’. In most cases, the size of blast media i.e. steel shot, conditioned cut wire, glass bead etc. is also dictated by the designer.
The end-users of these components, the majority of them in the automotive and aerospace industries, are also stringent about compliance to specifications that they have stipulated for the peening process. Compliance to such specifications is paramount to being qualified as a vendor to such end-users.
Therefore, there is more ‘science’ to shot peening and no room for subjectivity as in the ‘art’ of blast cleaning.
Science has taught us that there is more than one path to arrive at the solution. Sure, there is an optimum way and that is what users strive to identify when assessing their shot peening application. Centrifugal blast wheels are generally more productive than using compressed air to propel peening media through a blast nozzle or multiple nozzles. Blast nozzles are ideal when specific targets/areas on the part are required to be processed. They are also utilized when the requirement is to peen with non-ferrous media such as glass bead and ceramic. Blast nozzles are more adaptable to automation (nozzle manipulation) than centrifugal blast wheels. However, blast nozzles require compressed air, availability of which may not be easy in an existing production environment.
Blast wheels are ideal and most likely the only solution when peening parts within a prescribed time constraint. Such instances occur in high-production environments such as in automotive when peening connecting rods, gears, leaf and coil springs etc. In the aerospace industry, peening OD of landing gears and peen forming of wing skins are also applications where blast wheels are preferred over nozzles.
Whether with compressed air or blast wheels, shot peening requires monitoring and closed feedback loops for critical process parameters such as media velocity and media flow. Except for media velocity, which is controlled by varying the wheel speed in a wheelblast machine, and varying compressed air pressure in a nozzle machine, other parameters are monitored and controlled using common components in both types of media propulsion systems.
The ‘science’ of peening rests in all such parameters and close adherence will result in a final product that performs to or above expectations.
Thank you for taking the time to peruse my thoughts. I look forward to sharing and learning through my interaction with the global shot peening community!
Author: Kumar Balan