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Surface plastic deformation processes like shot peening (SP) have been extensively used by engineering industries to introduce compressive stresses at the surface to achieve superior mechanical properties (especially fatigue and wear resistance). A new family of surface severe plastic deformation processes (S2PD) using high-energy balls, widely known as the Surface Mechanical Attrition Treatment (SMAT), is gaining prominence. The investigation addresses the analysis of residual stresses in SMAT-exposed pure iron and SA 516 Grade 70 pressure vessel steel components by Magnetic Barkhausen emission (MBE) technique which is a potential nondestructive evaluation tool. The stresses corresponding to optimized band filter frequencies in MBE technique exhibit good correlation with the X-ray-diffraction based residual stress measurement data. The stresses correlate at a filter frequency of 100-250 kHz in the case of pure iron and 100-300 kHz in the case of SA 516 Grade 70 pressure vessel steel. Depth profiling by chemical etching showed that the nature of stresses changed from compressive to tensile. Microhardness measurements revealed an exponential decrease of hardness with depth.
Date of Publication: Dec. 2012