Vol. 14
May Issue
Year 2013

Standards Forum

in Vol. 14 - May Issue - Year 2013
Computer Monitored Peening

Paul Huyton

A revised Aerospace Material Specification AMS 2432D Shot Peening, Computer Monitored has been published by the SAE. The specification has been revised by the Surface Enhancement Committee operating as part of the Aerospace Materials Engineering Committee (AMEC). This was a scheduled five-year revision that was well timed to follow the complete and comprehensive review of AMS 2430 Shot Peening, Automatic which was published at revision ¡§S¡¨ in 2012. Many of the improvements in AMS 2430S have been incorporated in AMS 2432D.
Computer monitored shot peening is an enhancement of automated peening because the key process parameters are continually monitored by computer and compared to defined limits of variation. If the variation exceeds the limits the machine must automatically shut down within one second and indicate which parameter is out of tolerance and the time when the violation occurred. Table 1 of AMS 2432D defines these key parameters and the process tolerance shut-down limits. The key parameters include shot flow, air pressure, wheel speed and process time. Also included in the table are the relative position and movement of the part with respect to the nozzle or wheel. These parameters must be calibrated at least once every twelve months and the shut-down function of the machine must also be tested. In accordance with standard quality assurance provisions, records of the calibration and tests should be maintained as evidence of compliance for quality audits.
Paragraph 3.7.2 additionally requires that Almen strip tests are conducted at the saturation intensity time to ensure that the required peening intensity range is not exceeded when operating at the limits of the process tolerance. The process tolerances will determined by the following two tests; test 1: The maximum permitted pressure combined with the lowest shot flow will result in the maximum arc-height value; test 2: The lowest permitted pressure combined with the maximum shot flow will give the lowest arc-height. Both of these arc-heights must be within the required intensity range for the parts to be peened. If this is not the case, then the shut-down limits must be reduced until the intensity range is not exceeded. Processors are required to conduct these development tests and this should be documented so that it is available during audit.
Nadcap, the global programme for Special Process quality assurance, differentiates between the computerized and automated techniques and has checklists AC7117/1, Computer Controlled and AC7117/2, Automated. The use of the appropriate checklist is dictated by the requirements for the parts and not the capability of the processor¡¦s peening machine. If the parts require processing to a specification such as AMS 2432D, then the AC7117/1, Computer Controlled checklist shall be used. If the parts require an automated process such as defined by AMS 2430S, then the AC7117/2, Automated checklist shall be used even if the supplier has a machine with computer monitoring.
AMS 2432D now utilizes many of the same requirements as the Automated specification and the following must be in accordance with AMS 2430S:

AMS 2432D paragraph Topic
3.1 Peening media: new and in-process media Sub-size, shaded or masked strips
3.2.9 Media size inspection equipment
3.2.10 Media shape inspection equipment
3.3 Pre-peening Preparation
3.7.1 Process Development
3.10 Tolerances
3.11 Test Methods
4.1 Inspection and Process Control
4.3.5 Training Requirements
4.3.6 Re-testing and corrective action

Table shows the requirements common to AMS 2430S and AMS 2432D

Other requirements in AMS 2432D are based on AMS 2430S, but some are enhanced to provide the greater control that is expected in computer-monitored peening. One enhanced requirement is for the Almen test strips. The pre-bow or flatness tolerance is reduced from .001 to .0005 inch (0.025 to 0.013 mm). The hardness range for the strips is 73.0 to 74.4 HRA for ¡§N¡¨ strips and 45 to 48 HRC for ¡§A¡¨ strips, tolerances which are narrower than in the automated specification. Also, the Almen gauge must have the end stops which are optional in SAE J442. These enhanced requirements provide greater accuracy of the arc-height measurement when performing intensity tests.
AMS 2432D and similar specifications for computer-controlled peening are used for the most critical parts such as some rotating parts in aircraft engines. It is also used when the parts are used at the limit of their material properties and the benefits of shot peening must be consistent. By reducing process variation and ensuring that every part is monitored during shot peening, the safe performance of the parts is enhanced and, in some cases, inspection intervals can be extended.
By revising and harmonizing these two specifications, the quality control of shot peening processes is improved whilst reducing complexity and technical conflicts. The AMEC SE committee is working to add specifications for manual peening and batch tumble peening. This will provide a suite of harmonized specifications suitable for all types of conventional shot peening.

For questions contact paul@mfn.li

Author: Paul Huyton
Standards Forum
by Paul Huyton,
MFN Course Director World Wide
more information at www.mfn.li/trainers