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Vol. 8
May Issue
Year 2007
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MFN Trainer Column


in Vol. 8 - May Issue - Year 2007
The Degree Of Coverage



MFN Trainer Markus Halder

This column is a regular feature and is written by one of our MFN trainers or the Editorial Office. Readers are invited to send comments or questions to info@mfn.li. For more information about the trainers, see our website www.mfn.li/trainers.

Last year the Metal Finishing News presented the first edition of the book “Shot Peening - A Dynamic Application and Its Future”. After this impressive collaboration with specialists of many countries I would like to thank the MFN for the opportunity of participating in this great project with the chapter “Coverage” and to give a short insight with this article.
Shot peening is a method to improve the properties of near surface areas of mechanical components. It is employed to treat vibration stressed work pieces like springs, turbine blades, and shafts, as well as components stressed by wear and impacts such as gears which are shot peened. In shot peening mostly spherical steel balls are accelerated by different methods and directed onto the work piece to be treated. The impact of the balls causes plastic deformations both on the surface of the work piece and in the balls. In this way the shot peened material is strengthened. Moreover, residual compressive stress is created in a thin layer on the surface, which impedes the growth of tracks. These changes in the surface properties depend on a large number of influence quantities.
For the most part shot peened elements are highly stressed components of an assembly. In many cases in technical operation border layers are the most highly stressed areas of components and are mainly responsible for failures. One of the most important requirements to meet these high demands is to obtain the degree of coverage determined in the specifications. Quality variations are not acceptable and may cause premature failures of the components.
The notion of ‘coverage’ is based on the research work of the US company Wheelabrator Corporation in Mishawaka, Indiana. The coverage is practically the most important ‘measurable’ variable of the shot peening process – but there is no measuring instrument with a scale that indicates the degree of coverage degree. It is defined as the ratio of the area covered by hits and the complete surface treated by shot peening expressed in percentage, whereby parts of the surface with double hits are not taken into account.
A degree of coverage which may still just be assessed visually is a coverage of 98%. The corresponding duration of the shot peening process after which this coverage degree is obtained is frequently called shot peening time t98%. The required duration of the treatment is the product of the shot peening time t98% and a security factor s (e.g. 2 times t98%), which makes sure that the work piece has a coverage at least of 100%, and on the other hand is not overpeened by an excessive duration of the treatment. This factor is determined as a shot peening parameter, and is indicated on the work diagram as a specification. This indication must be adhered to under any circumstances. In related production methods such as form peening a coverage degree of 100% is not required. Here deformations of different strength may be controlled by different coverage degrees.
The coverage degree is one of the most important values of the shot peening process. Quality is defined by the observance of the required specifications, i.e. it is obligatory to control and check the shot peening process and to document the results. The control of the coverage degree may be very subjective, especially around 100% and in areas of different degrees and intensities. The assessment is particularly difficult in the case of complicated geometries of the components, if duo-processes with different peening media are required, or if very hard surfaces are to be treated. Nevertheless, a precise adjustment is crucial to ensure a controlled and successful process.

Best Regards

Markus Halder
markus@mfn.li




Author: Markus Halder