VOL. 12 January ISSUE YEAR 2011
in Vol. 12 - January Issue - Year 2011
A Decade of Development
The SAE specifications J442, Test Strip, Holder and Gage for Shot Peening and J443, Procedures for Using Standard Shot Peening Test Strip, are cornerstones of the shot peening process. Virtually all proprietary customer specifications invoke J442 or J443, either directly or indirectly via other specifications e.g. AMS2430 Automated Peening. The Nadcap audit criteria AC7117 also include J442 and J443 as checklist requirements. With the tenth anniversary of the MFN journal approaching, I am reviewing the developments of these important specifications over the last ten years.
This specification has been revised three times in the last ten years, the first of these revisions being published in December 2001. This revision included a minor change to the maximum thickness of Almen strips to the same value as in AMS-S-13165, formerly MIL-S-13165C. This specification had been in use for many years but both the AMS and MIL versions of this specification are now obsolete in favour of AMS2430. However, parameters and procedures established under MIL/AMS-S-13165 are still authorized on legacy parts by a "grandfather" clause in AMS2430.
The May 2006 revision of J442 contained many small changes for Almen strips and Almen gauges, but two of these changes were significant from a compliance viewpoint. Paragraph 6.3 quantified flat-spot wear on the support balls to a maximum of 1.0 mm, whereas previously any visual signs of wear were not permitted. And paragraph 6.4 gave a ± .001 mm flatness requirement on the zero block. This was later decided to be excessively accurate in relation to the requirements for intensity measurement and was relaxed to ± .005 mm in the November 2008 revision of J442.
This specification has seen two revisions published in the last decade. The January 2003 revision was undertaken to assist the shot peening industry with the separation of intensity measurement and part coverage inspection. So procedures regarding part coverage were deleted from SAE J443 and placed in a new recommended practice, SAE J2277, Shot Peening Coverage.
June 2010 saw a complete revision of J443 and three of the changes are appropriate to note here.
Saturation is defined as the point at which doubling the exposure time leads to a 10% increase in the arc height. In the previous revision of J443 figure 1 had indicated "10% OR LESS" increase in arc height would identify the saturation point and hence the intensity value. The "OR LESS" would permit any position on the curve with exposure time equal to or greater than the saturation time "T" to indicate the intensity value. Removing this ambiguity gives clearer instructions to accurately determine the intensity of a shot peen process.
A second important change was the additional requirement that intensity verification values are within ± .038 mm (.0015 inches) of the original saturation curve value. Previously, the arc height of a strip peened to saturation had to be within the intensity requirement for the part e.g. .006 to .010 inches, but the verification arc height was not required to be within a specified tolerance of the saturation curve. An additional requirement such as this is to be found in some prime aerospace OEM specifications and provides a higher confidence level that the saturation curve has not altered significantly from the initial qualified set-up.
Another new concept is explained in paragraph 7.3 of J443 June 2010. In cases where an intensity verification tool with multiple strip positions is used it is accepted that the saturation time will vary between the strips depending on the shot coverage experienced at each position. To simplify the intensity verification procedure a single verification exposure time may be selected. The time selected should be the longest saturation time for the group of strips or other value agreed with the customer. The arc height value on each of the corresponding original saturation curves at this same selected time is called a "target arc height". The verification arc height readings must then repeat the value achieved in the corresponding original saturation curve ± 0.038 mm (± 0.0015 in). The verification arc height values do not have to be within the intensity requirement for the part since the single verification time at a given location may be substantially less than or greater than T. The purpose of the verification Almen strip is to confirm that the arc height response at a particular location is consistent. The fact that an Almen strip exhibits similar curvature at a similar exposure time is deemed to be sufficient evidence that the saturation curve has not deviated significantly from the original qualified set-up.
The revision history of these SAE specifications is evidence of continual improvement in shot peening standards. The MFN team are proud to have contributed to this decade of development by educational activities and the dissemination of best practice in shot peening.
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by Paul Huyton,
MFN Course Director World Wide
more information at www.mfn.li/trainers