Shot peening of workhardening materials, particularly after prior surfacedeformation caused by turning, grinding, etc., canproduce a pronounced "hook" in the form of a rapidincrease in compression just beneath the samplesurface. Typical subsurface residual stress gradientsare evident at the surface of the residual stress profilesshown for various methods of processing Inconel 718in Figs. 4, 5 and 6.Fig. 4 Residual Stress and Cold Work DistributionsProduced by Abrasive Cutting of Inconel 718050100150200250300350-1000-800-600-400-2000200400600INCONEL 718SHOT PEENED110 STEEL SHOT, 6-8A, 100%RESIDUAL STRESS DISTRIBUTIONRESIDUAL STRESS MPaDEPTH, MICRONS0501001502002503003500510152025COLD WORK %PERCENT COLD WORK DISTRIBUTIONDEPTH, MICRONSFig. 5 Residual Stress and Cold Work DistributionsProduced by Shot Peening (6-8A) Iconel 718The rate of attenuation of the x-ray beam can bedetermined by calculating the linear absorptioncoefficient from the density and composition of thealloy. If XRD measurements are made at fineincrements of depth by electropolishing, the trueresidual stress distribution can be calculated from theapparent distribution(7). Failure to make the correctioncan lead to errors as high as 300 MPa and can evenchange the sign of the surface results. Non-destructivesurface XRD stress measurements cannot be correctedand must, therefore, be used with caution.050100150200250300350-1000-800-600-400-2000200400600INCONEL 718ABRASIVELY CUTRESIDUAL STRESS DISTRIBUTIONRESIDUAL STRESS MPaDEPTH, MICRONS0501001502002503003500510152025COLD WORK %COLD WORK DISTRIBUTIONDEPTH, MICRONSLambda Technologies▪[email protected]Lambda Technologies▪[email protected]Ph: (513) 561-0883 Toll Free/US: (800) 883-0851
X-Ray Diffraction Characterization of Residual Stresses Produced by Shot PeeningPage -6-050100150200250300350-1000-800-600-400-2000200400600INCONEL 178SHOT PEENED110 STEEL SHOT, 5-7C, 400%RESIDUAL STRESS DISTRIBUTIONRESIDUAL STRESS MPaDEPTH, MICRONS050100 150 200 250 300 3500510152025COLD WORK %PERCENT COLD WORK DISTRIBUTIONDEPTH, MICRONSFig. 6 Residual Stress and Cold Work DistributionsProduced by Shot Peening (5-7C) Inconel 718.Effects of Prior ProcessingWhen employing residual stress measurement tomonitor shot peening, it is important to realize that theresidual stress distribution after shot peening willdepend not only on the peening parameters used, butalso on the prior processing of the material as well. Fig. 7 shows the near-surface residual stressdistributions produced by shot peening carburized 8620steel to 22A intensity with 230H steel shot for 200%coverage. The stress distributions are shownimmediately beneath the surface for areas on the samesample on the original surface, with a decarburizedsurface layer, and after electropolishing to remove thedecarburized layer. A reduction in surface residualstress is evident in the decarburized area, even thoughthe two areas were identically shot peened. Thepresence of the decarburized layer is evident in the(211) peak width distribution shown at the bottom ofFig. 7. Without subsurface residual stressmeasurement, the anomalous results would likely beattributed to the shot peening process rather than theprior heat treating.
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- Spring '18
- Dr. maarten bakker