in Vol. 19 - November Issue - Year 2018
Traceability and Quality Control of Almen Strips
In many industries, specifications are now demanding that items used for a particular process, especially if it is a so-called critical one, can be traced back to their origin. As far as shot peening is concerned, one can find such requests for traceability for example for peening media, Almen gauges, holders and Almen strips.
However, looking at Almen strips in particular, there is so far, a loop hole in terms of traceability. While the strips are delivered with a certificate and identifiable packing, there is no clear traceability possible once the strip is used. Knowing that many shot-peening workshops use different types and/or qualities of strips, a considerable potential for mistakes and confusion exists. Since the intensity is probably one of the most important peening parameters, this can be a critical issue. Peening Accessories in Germany, which delivers a lot of strips to the aviation industry, decided already nearly a decade ago, to invest in equipment which would insure 100% traceability of a strip at any given moment. This is reached by printing a data code onto the strip. With this code, in conjunction with the certificate, no doubt can be left about the quality, type and origin of the strip.
Flatness of the Almen Strip
For Almen strips, the flatness is, besides the mechanical and chemical properties, a key value. The smaller the tolerance for the flatness, the better the quality and the more expensive the strip. Very common specifications are the SAE AMS 2432 and SAE J442. However, there is a large number of manufacturer specifications available. Since the bending of the Almen strip is measured after peening, the importance of flatness of the strip before treatment is obvious. Peening Accessories measures every single strip on both sides with a laser device and prints the values onto the strip. Since the strips are sold around the globe, the flatness is printed in both inches and millimeters.
Corrosion Protection for Strips
Because the strips are made of spring steel, corrosion protection is a demanding issue. The strips are sometimes stored in a surrounding with high humidity for a long period of time and should therefore show no sign of corrosion. Furthermore the strips are not supposed to be greasy, since that might complicate the handling or could damage the packing.
Peening Accessories has made extensive tests with different corrosion protection systems and finally found one that meets the requirements. It is a multi-function silicon-free tenside and mineral oil solution, which is abhorrent to water and avoids corrosion. Besides the corrosion protection, the strips are vacuum-packed to insure the best possible protection.