The title above is the most important question that shot peen personnel can ask themselves. Shot peening is classified as a "special process" because the results cannot be readily verified by subsequent inspection. There is no inspection technique for peening intensity, so precise determination, verification and documentation are vital in conforming to specification. Peening intensity determines the residual stress profile and influences other variables such as surface roughness. If the required residual stress profile is not achieved the fatigue life and fatigue limit (fatigue strength) of the part can be adversely affected. But in most cases the residual stress profile cannot be verified by a non-destructive process; most measurement techniques require that part material is removed to measure the residual stress at various depths. So on production parts we must accurately determine the intensity during process development and accurately verify it at intervals according to our customer’s specification. If the intensity conforms to requirements we can infer that the residual stress profile will be correct.

The initial determination of intensity requires the use of the saturation curve. The standard SAE J443 is most frequently used to specify this procedure. Some proprietary customer specifications include requirements for this procedure but they normally do not vary from those described in SAE J443. The core requirement is to determine the saturation point by using the "10% rule". This is very widely known in the shot peening industry and adopted in proprietary specifications also. Saturation occurs at the first point on the curve where doubling the exposure time results in a 10% increase in the arc height. It is proposed to amend SAE J443 to state "10%" and not the current "10% or less" which can be misleading. Beyond the saturation point any other point on the curve would have less than 10% increase for doubled exposure so the proposed amendment will reduce uncertainty in this respect.

To ensure consistency in determining the saturation point computer software can be used and this will be a recommendation in the revised SAE J443. To ensure consistency in the software results a new standard, SAE J2597 Computer Generated Saturation Curves, is proposed. This standard provides datasets of arc-height results which must produce the specified intensity value. Using this, the commercially available software packages can be certified to a common standard. Also, end-users who have developed their own programme can test its validity using the datasets.

Having determined an intensity value this must be compared to the customer’s stated requirement. The requirement is normally an intensity range, such as .006 - .010 inches, to accommodate the inherent variability in the process. One variable is the angle of impingement of the shot to the surface of the part as this has great influence on the intensity. The intensity reduces by approximately 40% if the angle changes from 90º to 45º, so it is important that the Almen strips represent the steepest and shallowest angles of impingement that will actually occur on the part to be processed. This is why SAE J443 and Nadcap checklist AC7117 requires that the Almen strips represent the faces to be peened on the part.

Having determined at the initial process development that the intensity conforms to requirements, the intensity must be verified during the subsequent processing of all production parts. SAE J443 and AMS2430 require that one strip peened at the saturation time "T" has an arc-height within the intensity range. AMS 2430 also requires that one strip must be peened for each part or at the beginning and end of a batch of parts. Some proprietary specifications have an additional requirement that the strip arc height is within a stated tolerance of the original value when the process was approved. One prime manufacturer also has a requirement that if the verification strip is not within a specified tolerance of the original, then a second strip at 2T must be peened to verify the 10% relationship between arc-heights.

As can be seen the determination of peening intensity is generally to the requirements of SAE J443. However, intensity verification during production is more variable between different customers and specifications. It is essential that shot peening personnel understand the verification requirements and procedures.

The prevention of metal fatigue is a critical safety activity for both air and surface transportation industries. Peening intensity is a key process variable which cannot be inspected by non-destructive means. Hence process planning and control is the means to achieve consistent results. Effective specification review, conforming procedures, trained and competent personnel and workshop discipline are the minimum requirements to ensure public safety. The quality management of any organization undertaking special processes must deliver these minimum requirements.

For questions contact paul@mfn.li

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