Exploration • Drilling • Field Services


Controlled shot peening of welded structures


Ben Hayes outlines the benefits of the surface processing technique Controlled shot peening.


between overhaul periods extended must be a consideration in today’s cost conscious market. Tis is particularly relevant to the onshore and


T


offshore oil and gas industry where components are exposed to harsh environments and operating conditions 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 Te 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.


he major problem of premature failures is not necessarily the cost of repairing the items, but the loss of operating time. Any technique where premature failure can be prevented and the times


Te 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. Te 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).


Te 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


Fig. 1. Shot peening influence on applied stress.


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