I have been involved in the contract shot peening business since 1980 in various positions including production manager of shot peening machine manufacturing, branch manager, and quality manager of subcontract peening facilities also.  During these tenures, I have dealt with a great number of shot peening specifications and have seen their constant evolution from the relatively basic and now obsolete MIL-S-13165 to AMS2430. MIL-S-13165 has formed the bedrock of just about every one of those specifications to which I refer, and in some cases near verbatim.  Since then, most Aerospace OEM shot peening specifications have, apart from their obvious links back to the MIL spec, been autonomous. That is until the advent of AMS2430 and especially since the more recent issues of this specification.

The evolution of the OEM shot peening specifications during the 80s and 90s has been somewhat Darwinian, with some choosing to keep them as basic as possible.  Others though have been written in industry-leading detail along with boundary-pushing requirements requiring the acquisition of the latest technological equipment.  I am obviously not at liberty to explain these areas in any detail; however, it would be accurate to say that the leaders have closely kept pace with the technological improvements that CNC has brought industry in terms of process control and repeatability along with other allied monitoring and measuring innovations.   During this period, specifications have taken divergent paths in terms of complexity, although none have ever truly lost their pedigree.

More recently however, there has been a revolution.  AMS2430 has become the ‘new’ MIL-S-13165.  More and more autonomous specifications are now converging on AMS2430 with very little or no extra requirements. These specifications do of course as a minimum, refer to the OEM’s own degreasing and temporary protection specifications, but the evolution of AMS2430 has been such that the OEMs seem comfortable with its contents and intent. As mentioned, technological advances in the equipment have been significant with some very exciting developments in the field of process control facilitated by advances in camera technology and Artificial Intelligence (AI) as well as data processing. These technologies can be applied to the shot flow after it has left the nozzle. This is highly significant since the upstream of the nozzle parameters controlling and affecting the shot flow characteristics as it leaves the nozzle can theoretically be checked and, in some respects, controlled.  This leads to high confidence process control and repeatability.

The reason I mention the technological advances in process control is that for them to be most efficiently employed, they need to give cost savings, both in terms of rework, consumables, and/or time. In which case, they must be permitted and controlled via the shot peening specification, since these innovative methods become an intrinsic part of the product integrity.  Hence the subject of this article.   

As discussed, AMS2430 has largely become the ‘go-to’ specification, either referred to directly by drawing or via OEM specifications; however, we already have two renditions of AMS2430 - the latter being AMS2432 specifically issued to encompass the complexities of Computer Controlled Shot Peening. If you are a NADCAP approved supplier, you will have declared which machines are automated (without computer-based controls) or computer-based whereby part programs may be stored and the equipment must have various more advanced features such as integral shot condition management systems. The introduction of new technologies that do not form part of the existing specifications will ultimately require accommodation if there is to be an all-encompassing specification.

The process is significantly affected by several variables, some obvious, and some very subtle.  Currently, the process is qualified by the Almen Strip and the resultant Saturation Curve. The process is then requalified by the requirement to achieve, within tight tolerance, the same curve.  The diverse array of upline variables that define in what manner the shot leaves the nozzle all directly affect the results achieved and the form of the saturation curve.  

The following are some of the variables that can cause a change to the process: Impingement angle of the shot, design of the nozzle, nozzle wear, blast hose diameter, length and wear, feed orifice wear, inconsistent back pressures in the pneumatic controls, and inconsistent hose runs after replacement. It is these variables that drive the development of systems that measure the shot stream for form, density and velocity after leaving the nozzle. If these are measured and controlled, then the resulting saturation curve should be highly consistent.

The industry is seeing giant leaps in these measurement technologies, which in turn are bringing exciting opportunities to automate process control even further.  The question is to what level should the specifications drive the mandatory incorporation of such technologies?  Will the diverse leaps in technological advancement prevent the harmonisation of shot peening specifications from becoming a reality?  It is the OEMs that define the path to the incorporation of new technologies.

Standards Forum

by Christian Tyroll, 

MFN Contributing Editor

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