The peening industry widely relies on the shot peening method, which consists of either a machine using air pressure to shoot the media on the part or a centrifugal wheel turning at a given RPM that “slings” the media on the surface of the part.
Shot peening has the advantage to be the most ancient method, using the same principle as sandblasting, and with a very small modification on the nozzle, the sandblasting unit turns into a shot peening device (this is a very quick shortcut to sum things up obviously).
Over time, some new technologies were needed to solve different constraints such as energy consumption, machine footprint or accessibility.
The flapper peening technique has answered some of these problems by using a Kevlar flap on which some tungsten carbide bearing balls are glued; the speed at which the flap is rotating gives the momentum to the balls that transfer their kinetic energy onto the surface of the part.
There are various peening technologies that the SAE AMS specifications are framing (AMS2430 for the shot peening, AMS2590 for the flapper peening…) and all of those papers precisely describe what the key parameters are to pay attention to and the technique to properly perform the peening according to the chosen technology.
One of the latest AMS released, the AMS2545 defines the pneumatic-controlled needle peening.
While the needle peening is sometimes confused with the needle hammering (we are talking semantics here), those two technologies are still much closer to each other while their applications are strictly different.
The needle hammering aims to “peen” (or “hammer”) the welds toe area in order to modify its profile while introducing compressive stresses all at once. It uses a large 100Cr6 hardened needle in most applications.
The needle peening, is much more of a “peening” technology per se, the tungsten carbide needles are way smaller and the radius tip mimics the shot peening media commonly used to achieve a similar surface finish. The applications the pneumatic needle peening under the AMS2545 is referring to are mostly aiming for aero engines, power generation turbines, airframes or any other critical component where the other peening technologies show their limits of accessibility or practicability.
It goes without saying that the parameters to master are different than the common peening technologies (let’s say shot peening and flapper peening, which are the most commonly used nowadays) but the requirements it has to match are strictly the same. The intensity is measured thanks to the Almen strip (or any approved alternate method), so the surface finish will be measurable the same way that shot peening is and the peening exposure will be determined by the time (note that for this parameter, some shot peening machine specifications will count the number of passes instead of an exposure time).
Talking about the Almen strips, note that the AMS2545 appendix shows the Almen block that shall be used with the pneumatic-controlled needle peening. To the difference of the flapper peening, both the magnetic block and the standard 4-screws-block used for the shot peening are acceptable, making it very convenient and easier to use than the magnetic block that needs to take into account the magnetic correction to plot the Almen saturation curve.
This is made possible thanks to the needles that are able to cover more surface on the Almen than the flapper peening, which will leave some un-peened area close to the head of the screws resulting in a heterogeneously peened zone where the intensity may vary.
Another interesting feature of the needle peening is the capability of doing a shaded Almen strip quite easily.
This is not directly related to the AMS2545, but the J443 “Procedure for Using Standard Shot Peening Almen Test Strips” explains the method of doing shaded test strips.
The needle peening allows the operator to perform low intensities (N scale), which usually take much longer than other A or C scales to perform. By using the Almen shaded method, one can save a lot of time during the intensity verification process.
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by Christian Tyroll,
MFN Contributing Editor
more information at www.mfn.li/trainers