in Vol. 19 - September Issue - Year 2018
New Requirements and Guidance for the Shot Peening Process
Users of the peening specification AMS 2430 may have found significant changes in the revision U, published in April 2018. The first change to be found is in the title; it is no longer “Shot Peening, Automated” but simply “Shot Peening”. This is because the specification now includes requirements for manual and batch peening. Manual peening applies when the part or nozzle are manipulated manually. Batch peening is where the part’s orientation to the shot stream is not controlled and a random development of coverage is permitted; sometimes called tumble peening or barrel peening.
A second important change is that section 8 no longer includes process guidance as in the previous revisions. Section 8 provided guidance notes including recommended intensity ranges for various materials, which were used as the default values when peening some parts. Now users have another standard to refer to for this guidance.
The explanation for these changes is that there was no industry specification for manual peening nor batch peening. But these are commonly used processes and in need of a reference standard to ensure consistency of application. And the omission of the guidance notes in section 8 is because these are fully covered by the new Aerospace Recommended Practice ARP 7488.
AMS 2430 U, Shot Peening.
Changes in AMS 2430 U include a new section 1.7, which requires that an automated peening process must be used unless manual or batch peening is specifically authorised and called for in the purchasing information.
Other changes are in Tables 2A and 2B, shape requirements for non-metallic and metallic media. The tables now include the minimum magnification needed to undertake the visual inspection, as this will affect the accuracy of the tests. Also added is that some non-metallic media, AGB-15 and AZB-100 and smaller, are so small that they cannot be accurately counted and this inspection is not required.
The main body of the document is written for automated peening; appendices A & B cover the requirements for manual peening and batch-peening. These appendices detail the requirements for operator qualification and annual, documented re-qualification. The qualification includes process specific training and detailed requirements for the demonstrations of competence. Records of training and the demonstrations of competence should be generated and retained for audit purposes.
ARP 7488, Peening Design and Process Control Guidelines.
This standard details the recommended practices to be used when AMS 2430 or AMS 2432 are specified. They are the default process control criteria when the engineering authority has not made alternative requirements in their purchase order, drawing or specification requirements. The guidance is relevant to all peening processes, in the absence of other specific customer requirements.
This standard has Tables 2 and 3, which give the recommended parameters for peening Aluminium (including internal bores), Titanium, steels greater and lower than 200 ksi (1379 MPa) tensile strength. The parameters include intensity range and media size for material thicknesses above and below 0.375 inch (9.52 mm), subject to a minimum 0.090 inch thickness. These tables provide the default intensity requirement for a shot peening processor, or technical guidance for design engineers. If a prime customer document specifies parameters other than these in the ARP, the customer parameters will take precedence.
Table 1 gives residual stress depth for different intensity values used on the materials above plus Nickel alloys. This can be used when determining how much material can be removed if post-peening surface improvement is needed. It is also a guide for Stress Engineers if a peening process intensity requirement is being developed.
Table 4 gives the minimum media size to be used for three different intensity values. As the intensity requirement increases in value, it is good practice to use larger shot; otherwise, the peened surface roughness will increase and the degree of cold-working of the material also increases. This can be detrimental to the fatigue life and so a larger shot is used at a lower impact velocity, resulting in the same intensity value.
Guidance is also provided on defining the peening area and practical advice on obtaining coverage on areas which are difficult to access.
Pre-peening and post-peening topics include recommendations for cleaning, decontamination, and inspection. Also the post-peening temperature and material removal limits needed to preserve the compressive residual stress layer.
There is guidance for developing the peening process and on peening harder materials and different thicknesses.
By extending the scope of AMS 2430, there are now appropriate requirements for manual and batch peening. And ARP 7488 gives a basic document for the design and control of the shot peening process. But it must be remembered that explicit customer requirements do take priority over these general guidance notes.
Shot Peening processors and specifiers need to obtain current revisions of these documents and conduct a thorough review to ensure that their shot peening process is appropriately defined and controlled.
For questions contact
by Paul Huyton,
MFN Course Director World Wide
more information at www.mfn.li/trainers