The strength and service life of components exposed to high dynamic stresses can be significantly improved by shot peening. In order to be able to quickly develop tailor made shot peening solutions for our customers, Rösler recently invested in a lab equipped with x-ray diffractometer costing approximately 250,000.00 Euro.
Shot peening is a special shot blasting process during which spherical blast media is thrown at the surface of metallic work pieces at high speeds and under highly controlled operating conditions. The impact energy of the “pellets” “cold forms” the upper layers of the metal similar to hammering and forging processes. The metal is alternately stretched and compacted causing a compressive stress in the upper surface layers. This offsets any inherent tensile stress as well as external tensile or compressive loads acting on the work piece. It increases fatigue strength and significantly reduces the risk of stress cracks. Depending on the components and the loads they are exposed to, shot peening can increase their fatigue life by up to 1,300%.
X-ray diffractometry expedites the development of peening processes
Starting with the customer specifications, Rösler, with a wealth of experience and knowhow, develops the required process and equipment parameters through extensive test trials and their careful documentation. Important process parameters are:
- Material, nature and shape of the work pieces
- Material and size of the blast media
- Throwing speed and impact angle of the blast media on the work piece surface and
- Blast media quantity and coverage (number of “hits” within a certain time period)
To ensure that the peening process is perfectly adapted to the material and the shot peening objectives, the compressive stress values achieved during the processing trials must be carefully measured and documented. X-ray diffractometry has proven to be a highly accurate and reliable method to make such compressive stress measurements. The upper work piece layers are exposed to x-rays hitting the crystalline and partially crystalline structure of the metal at a preset angle. The majority of the x-rays are passing through the crystals. However, a special detecting device registers the small portion of x-rays that are deflected or reflected by the crystals. Based on the known material characteristics this x-ray diffraction in combination with the Bragg equation allows measuring the compressive stress values. Multiple measurements with different angle settings ensure that the compressive stress readings are highly accurate.
Investing in a lab with an x-ray diffractometer now gives Rösler the ability to make these measurements in-house without having to send the results of processing trials to external labs for evaluation. This helps significantly speed up the development of customer specific shot peening processes. In addition, it allows a faster reaction to the challenges posed by the development of new metal alloys. Rösler can now also offer x-ray diffraction measurements for different types of work pieces as a service, thus allowing the review of complete shot peening processes, even those run in other manufacturers’ peening equipment.
Monitoring of the shot peening process with Almen tests
A simple method for controlling the parameters of shot peening processes is the Almen test. Standardized test strips made from tool steel are mounted onto a special steel block and exposed to the blast stream over different time periods at a constant blast intensity which causes the strips to bend. The maximum arc height of the Almen strips provides a measurement of the degree of peening saturation, which is entered into a saturation curve. The saturation point is reached when a doubling of the blasting time increases the arc height by no more than 10%.