If you are performing shot peening, the main parameter you need to rely on to measure intensity is most probably the Almen intensity.

This process established by John O. Almen back in 1942 and later patented in 1944 is the undisputed reference for the characterisation of peening processes and to date, no other method is available to rapidly quantify the force of impact of a media stream on the surface of a part...

The purpose of this column is not to review the technical aspects nor judge the pros and cons of any alternative method, but rather explain what the specifications AMS 2430 and AMS 2432 permit in terms of process validation during production.

The AMS 2430 primarily calls for the J443 in terms of Almen verification process, which is the most standard procedure every processor will follow when it comes to define their machine’s settings.

The Almen process with its saturation curves and 10% rule is often complicated at first glance.  Fortunately, the use of target arc heights is easier to understand for intensity verification and process tracking.  

At the end of Chapter 3.7.1.1 of the AMS 2430, the following phrase mentions that: “An alternative intensity verification method for production part peening shall only be used when approved by the cognizant engineering organization”. I believe that the first thing that comes into mind of every peening expert when reading this phrase is the use of the “residual stresses measurement method” as an alternative of the Almen method. 

The residual stresses method is a correlation method between the Almen intensity and the residual stresses found on the superficial layers of the parts rather than an   alternative method to measure impact force of the media.  Once this correlation has been established in a study prior to the production, this is a very fine method to verify intensity during production, although it is a destructive method and the part measured is useless for production.

An interesting quote in the AMS 2432 (that a lot of the time calls for the AMS2430) is the following: “An alternative intensity verification method, such as use of a machine verification tool or media velocity sensor, for production part peening may be used only if approved by the cognizant engineering organization”. Meaning that Almen intensity can be correlated with any other technology capable of measuring with enough accuracy a third parameter that can be linked to it.  Some companies have investigated the use of media velocity sensors.  These sensors correlate well to Almen intensity but are difficult to calibrate and give measurements at the exit of the nozzle instead of on the surface of the part where it is most important.

New technologies could open the door to a new standard for defining the peening intensity. For reminder, we use the Almen process for lack of better method, because no other alternative currently exists.  

Another subject covered by these AMS specifications is the coverage, by referring the J2277 to determine the peening coverage on the part. This second parameter is also complicated to master, as there is no precise tool or method to control the coverage of the surface impacted. Only visual inspections and charts are in the standard, but a reliable technology has yet to be developed in order to avoid the human factor. Fluorescent tracers and replication paste, when they can be used, are a good help to avoid reflections and optical illusions. 

Shot-peening machines have evolved tremendously in the last 20 years and now control the peening process better than ever before by using a robot to move the nozzle, proportional valves to control the pressure, and flow valves for the media.  However, the AMS 2430 and 2432 are showing room for improvement in the quality control or process verification for the intensity and coverage. 

There is a need for reducing tact time and consumables in the industry altogether while improving accuracy, to make the process easier to use, greener and more efficient. The peening industry has great machines and great standards, but the next challenge seems to be the improvement of the process verification methods.

Standards Forum

by Charlie Clouet, 

MFN Contributing Editor

more information at www.mfn.li/trainers