MFN Trainer Column
in Vol. 16 - July Issue - Year 2015
Is Almenstrip Peening Nowadays Still State Of The Art?
Shot peening has been for some time now an established life enhancement process in the manufacturing chain of dynamic highly stressed automotive components such as connecting rods, valve- and chassis springs, crankshafts etc. Also in the aero engine field, shot peening has become an indispensable process. It is possible to peen thermal and dynamic highly stressed components as e.g. compressor- and turbine blades, compressor blisks, turbine discs and shafts completely or just in the highly stressed areas. The quality and intensity of a shot peen process could be measured based on the introduced residual stresses. Unfortunately, these residual stresses can only be measured destructively in the component surface by centre-hole drilling of x-ray methods. Instead of measuring the peening effect destructively in the part surface, it is alternatively possible to control the proper peening process without destroying the part.
For this purpose, Mr. John O. Almen patented a non-destructive method in 1944 that is more than 70 years later still in use. The question that arises in our fast moving times in which we are living is, â€œIs this method which has been in use for 71 years either thoughtfully developed and the right method for this process or is this method now obsolete and should be discarded?â€ Our machines that accelerate the peening media in a controlled manner and target the millions of shots onto the component surface have been significantly refined in the last 70 years. The key components of a peening plant media wheel, media dosages -magnetic or mechanical- and pressure regulators have been developed further into totally new machines compared to earlier times. Nevertheless, for the development and verification of our peening process, we are still using the Almenstrips type A - N or C based on Mr. John O. Almen from 1944. With the help of the Almenstrips, the kinetic energy from the media stream will be converted into a shape change of a standardized spring steel strip and this shape change will be measured on an "Almengage" in thousandths of an inch or in tenths of a millimetre. Everybody who well knows the peening world knows this method; however, many are aware of the tricky aspects and inadequacies from this method: e.g. manufacturing tolerances of the Almenstrip, handling of the Almenstrip, pre-bow measurement, development and interpretation of an Almenintensity on the basis of the saturation curves, and so on. The kinetic energy from the media stream depends on the key process variables (KPV's) from the process. Some of them are:
-media hardness and size
-media flow rate
All these parameters are nowadays measureable and some of them are also controllable - that would have been considered inconceivable 70 years ago. Thereby, the shot peening process itself has been developed further but the measurement method has stood still and seems to be antiquated. The key process parameter is the media velocity while maintaining all other KPV's as stable. This is no issue with the use of machine control systems nowadays. Media velocity can be measured online even in the harsh environment of a peening cabinet. Such systems are available, which have demonstrated their technical capability in independent test series of different shot peening end users. The very good repeatability, the low measurement time required, the simplification of the Almencarrier- all these are obvious benefits of the media velocity-measurement method. After 70 years of use and recognized weak points, it needs to be asked if intensity measurement based on standard spring steel strip curvature measurement is still state of the art, because we all know the rule in our sophisticated world "Who stands still will fall behind".
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Author: Wolfgang Hennig