Ben Hayes outlines the benefits of the surface processing technique controlled shot peening
Controlled shot peening is a technique applied to many machined or fabricated structures to reduce the incidence of fatigue, corrosion fatigue, stress corrosion cracking, fretting or fretting fatigue.
The benefit is gained by the removal of any surface residual tensile stresses, which can be considerable, and substituting residual compressive stresses whose magnitude can be 80 per cent of the materials yield strength and depth up to 2mm, depending on the parameters of shot peening selected. The induced residual compressive stress will be beneficial in reducing the mean stress at the structures surface, hence the advantages gained beyond stress relieving a structure. Welded structures can and have gained significantly by the application of this technique.
Controlled shot peening is a surface processing technique impinging the surface of a metal with spherical media to yield that surface in tension. The core of the material resists the stretching of the outer layers, resulting in a near surface residual compressive stress, whose maximum value is approximately equal to 80 per cent of the yield strength of the base material, but in compression (Fig. 1).
The media used to conduct shot peening can be steel, stainless steel, ceramic or glass and vary in size from 50 micron to 3mm in diameter with projecting velocities varying from gravity to 200 metres per second. A range of process controls have been developed over the years to ensure that the depth and magnitude of stress is repeatable when adopted as a production technique. This is particularly critical when applied to large welded structures as the surface/weld profile can influence the benefits gained. The media selected will be a function of the size of the structure, the geometry at the toe of the weld and the surface roughness generated by the weld lapping process. A dressed weld will remove these geometrical considerations.
The traditionally accepted problem of welding stresses at the heat affected zone has been tackled by a variety of means and their success varies. Thermal or mechanical stress relieving has an effect on the residual tensile stresses, although some may still remain, but less effect on the applied stresses.
Controlled shot peening can be applied to a welded area with benefit. However a greater benefit can be achieved by peening a dressed weld, because the stress concentration areas have been reduced and impurities close to the weld surface have been removed.
Controlled shot peening can be applied to many structures on site. The industries presently using this technique include the chemical, oil and gas, highway and construction, power generation, aerospace and other large transport industries.
The process controls mentioned earlier are applied in all of these situations. Media is taken to site and examined initially and continuously to ensure the desired profile of residual compressive stress is maintained. Should the volume of site work demand it, grading equipment is taken to the site and media recycled for size and shape consistency. The intensity of peening will be governed by the equipment taken on site to impart the necessary level of intensity required on welded structures. Offshore welded structures provided adequate scaffolding and access is available, provides no problem.Ben Hayes is with Metal Improvement Company, part of Curtis-Wright Surface Technologies, Newbury, Berkshire, UK.. www.cwst.co.uk
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Ben Hayes is with Metal Improvement Company, a business unit of Curtis-Wright Surface Technologies, Newbury, Berkshire, UK. www.cwst.co.uk