This project aims to develop surface engineering processes that will improve the dynamic performance of coil springs to a level that is unattainable with current technology. Improved dynamic performance will enable the design of lighter springs, capable of operating at higher speeds, and with greater safety margins, thus affording improved reliability.
1. a 10% improvement in fatigue stress limit has been achieved by optimum shot peening of oil tempered chrome vanadium steel, and the stainless steel spring steels grades, 302HLS and 17/7PH providing that the raw material is sufficiently clean such that inclusions do not initiate fatigue failure (ie. contains no inclusions larger than 15 microns).
2. The oil tempered silicon chromium steel shows potential for the highest fatigue performance of all the spring materials in the shot peened condition, but only the index 3 1/2 springs consistently achieved a very high performance.
3. Electropolishing after shot peening is effective in raising the fatigue limit of springs, made from each material, to a level unattainable prior to the start of this project.
4. Ion implantation proved effective in improving the fatigue performance of shot peened and electropolished chrome vanadium springs, and enabled the highest performance to be achieved fro this grade of spring steel. This process is also, by far, the most expensive tested in this project.
5. PVD coatings of one micron thickness on shot peened and electropolished springs, made from low alloy steels, gave fatigue performance levels that were equal to, or better than, that due to electropolishing alone on shot peened springs, but not quite so good as the performance due to ion implantation. PVD coatings are expensive, but not so expensive as ion implantation.
6. Hardcor (R) treatment is an effective surface engineering technique for improving the fatigue performance of stainless steel springs, especially after electropolishing, but at the slight expense of some corrosion resistance.
7. Nitrocarburising has potential as a surface treatment for low alloy steel springs that are too small to peen.
The effect of surface engineering processes such as shot peening, physical vapour deposition, ion implantation and carbo-nitriding, and combinations thereof, will be accurately measured by undertaking a very large number of spring fatigue tests.
In order to verify that the results of these laboratory fatigue tests are reproduced in service, engine tests and component testing, incorporating redesigned springs with the newly developed surface engineering processes, will be undertaken by two of Europe's leading manufacturing companies.
The result of this project will be disseminated to a large number of the SME springmakers upon whom European manufacturing industry is dependent if it is to remain competitive in world markets.
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