in Vol. 20 - January Issue - Year 2019
Almen Strips: Performance Test, Traceability, Flatness, and improving the Mechanical Finish
Performance Test: From every lot of Almen strips, samples are selected and peened using an Ervin tester. The representative arc height data is recorded and stored for 5 years.
Peening Accessories Almen strips: Every strip is laser-measured for its flatness. The results, including lot number for traceability, are printed on the strip. An additional mass finishing process removes all burrs.
Besides fulfilling the general specifications for Almen strips, the most important requirement for the strips is the arc-height repeatability. That means that Almen strips show always the same arc height, under defined peening conditions.
Inconsistent arc height
repeatability for Almen strips
Some Almen strips users around the globe who are working within the very strict aerospace industry may have faced the following situation: They suddenly realize that the newly received Almen strips show an either generally higher or lower arc height reading.
The users in the workshop claim it is not the peening machine, operator or anything else from their side. So what can it be? The test that can easily confirm the cause comes from the Almen strips compares the strips arc height reading from an older delivery with the new delivery under the exact same peening conditions. However, the strips delivered by the supplier fulfill all the specified requirements as far as mechanical, chemical and hardness concerns. So in other words, everybody has done everything right, but the results do not satisfy. This is of course, a very unpleasant situation.
Where is the cause for the different readings?
One basically can neglect the notion that the steel producer may have delivered the wrong chemical specification of steel. The main problem, however, is the heat treatment. The different procedures within the heat treatment such as hardening, tempering, etc., are so sensitive that it is very difficult to always get exactly the same results, especially comparing different lots. Peening Accessories have improved the heat treatment process as far as possible, integrating its own audits to reduce changes in the arc height reading that resulted from areas of decarburization and inconsistent hardness level.
Insuring arc height repeatability
For years, Peening Accessories has done Ervin tests recording the arc height for every lot and collected this data. With this performance test, which is not required by external specifications, the company integrated its own internal quality standard as far as repeatability was concerned. After the performance test, a mean value for the arc height is determined and this mean value has to be within an internal tolerance determined by Peening Accessories. For most customers, this data is of no concern; however, this recording is available on request.
Traceability of Almen strips
Already in the year 2005, Peening Accessories printed a lot number on every strip. This system allows complete traceability. The German company was the first manufacturer to implement this traceability concept for every single strip.
Flatness of the Almen strips
For Almen strips, the flatness is a key value, besides the mechanical and chemical properties. The smaller the tolerance for the flatness of a strip, the better the quality and more expensive the strip. 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.
Almen strips with no burrs
Burrs on the Almen strip have the potential to influence the reading. Once the strips are placed in the Almen holder, the burr could lift the strip slightly. In order to make 100% sure that there are no burrs on the strips, Peening Accessories implemented an additional mass finishing process, which only purpose is to grind away burrs. A side effect of this additional process is a shiny surface that is free from the typical heat treatment marks.