VOL. 15 January ISSUE YEAR 2014
in Vol. 15 - January Issue - Year 2014
Peening Media Classification And Alternatives
When shot peening a component, we direct the application of energy into the surface in order to provide a compressive surface layer. Whilst we need to control several process parameters, it is the peening media (a term used for the shot) that actually contacts and changes the surface of the component. So ensuring that the media meets the specified requirements and desired performance is essential to maximize the quality, productivity and efficacy of the process. The terms "media" and "shot" are both used within the various specifications and can be considered interchangeable.
Cast steel is the most commonly used media and requirements for aerospace applications are provided by AMS 2431 and the relevant sub-specifications; 2431/1 for regular hardness (45-52 HRC) and 2431/2 for hard shot (55-62 HRC). "Non-aerospace" applications may use SAE J444 & J827; J827 gives the material properties required, and J444 gives the size requirements. With cast steel shot (and also for glass and ceramic shot) the particle size occurs as a "normal" distribution so the specifications indicate the maximum percentage of a sample which can be retained on each of a set of sieves. J444 requires four sieves for the test and AMS 2431 specifications require five sieves and permit less deviation in the particle size. The type and size of shot is identified by a product code, e.g. aerospace regular hardness shot of nominal size 0.023 inch is ASR-230, whilst hard shot of the same size is ASH-230. The "J" specifications do not differentiate hardness in the identification, so for 0.023 inch shot, these specifications simply refer to S230. This nominal size is not the average diameter of the shot particles. In AMS 2431, the size distribution tables indicate that at least 90% by weight of the media must be larger than this nominal size. A closer indication of the average size would be the next larger screen, as up to 50% of the media may be larger than this size. So for ASR-230, the average size would be nearer to 0.0278 inch as this is the next larger sieve size.
Conditioned carbon steel cut wire shot is designated AWC and then R or H for the same hardness ranges as cast steel shot. Stainless steel cut wire is AWS. The numerical reference is the diameter of the wire feedstock and so indicates the average shot diameter e.g. AWCR-28 has an average diameter of 0.028 inch. When substituting cut wire for cast steel shot the Automated Peening specification AMS 2430 indicates the use of the equivalent average diameter shot, e.g. cut wire size 28 as equivalent to S230. This is understandable but it can also be suggested that the average size of cast shot reduces in a "working mix" whilst cut wire remains close to the original average diameter. This is because cut wire shot is made from a wrought steel wire that is tougher and has less inherent defects than cast steel shot so it maintains size and shape very well. It is also a consideration that, even if the average particle diameter is the same, the cut wire shot can result in higher surface roughness readings than cast steel shot. This is because the very uniform cut wire maintains the "peaks & troughs" of the shot-peened surface whereas the distribution of different sizes in cast shot appears to level the surface profile.
The shape of the shot particles is another important consideration. Spherical particles of shot are the ideal as the uniform impact radius minimises the strain rate and shear stresses within the microstructure. Some irregular shapes can be tolerated, but sharp edges or points will give rise to stress concentrations and potential crack initiation. Internal defects in the shot, indicated by cracks or voids, are also undesirable as they may fracture on impact and damage the surface as with a sharp particle. SAE J827 and the AMS 2431 specifications give control values for the numbers of defective shot that can be accepted in a given sample size. Again, the aerospace specifications have stricter control values to minimise the risk of surface damage and incidentally provide the benefit of greater shot durability.
Procuring, maintaining and verifying the appropriate peening shot is vital to ensuring the quality of the shot peening process. AMS 2430 proposes the equivalent size of cut wire shot when used in place of cast steel but emphasizes that permission from the relevant engineering authority is still required. So an alternative size could be used if the required engineering approval has been obtained.
This short article has only explored some of the issues relating to steel peening media. AMS2431 specifications also include glass and ceramic media, which have their own benefits and limitations. Full consideration to the effects on all aspects of the process should be given when selecting the optimum peening media.
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by Paul Huyton,
MFN Course Director World Wide
more information at www.mfn.li/trainers