in Vol. 19 - September Issue - Year 2018
Improving Production Efficiency through Shot Peening Optimization
Fatigue Testing Results of Varying Shot Peening Coverage in 4340 Steel
Residual Stress Distribution of Varying Shot Peening Coverage in 7075-T6 Aluminum
Residual Stress Distribution of Varying Shot Peening Coverage in Inconel 718
Peening Coverage Curve Representing Coverage Progression from 0-100% for 4340 Samples
The concept of manipulating the coverage of shot peening media is not new. Over 15 years ago, in 2002, Lambda Technologies began studying the effects of varying coverage. Kirk and Hollyoak published their own extensive research1 in 2005, and others have explored the area as well, in both empirical and analytical studies.23
Lambda Technologies sought to optimize coverage because of the results of studies undertaken in their surface integrity laboratory, Lambda Research, Inc. Shot-peened parts submitted for residual stress testing often exhibited micro-cracks, folds, and other damage, particularly around sharp features like edges, bolt holes, and corners. This damage served as crack initiation sites, reducing the final fatigue performance and the potential life of the part. It was evident that the damage was caused by the random repeated impacts by peening media to achieve full "100%" coverage. Was it really necessary to peen to such an extent that there would be surface damage? Paul Prevéy and Dr. John Cammett initiated an investigation of the effect of peening coverage on both the residual stress distributions produced and the resulting fatigue performance.4
Because of the random nature of shot impact, 100% coverage can only be approached asymptomatically, and actual "full" coverage is generally 95%-98%. The 200% to 400% coverage commonly used in peening specifications resulted in many repeated impacts as expected, but still left other areas untouched. In addition to the damage resulting from repeated impacts, coverage at 200%+ took twice as long or more, with twice as much media, but produced no further benefit in fatigue performance. Higher coverage negatively impacts production rates, increases media consumption, and increases wear on the peening equipment—directly impacting the bottom line with increased costs.
Traditionally, specifications require coverage of 100% or more under the assumption that every point on the surface must be impacted to impart uniform residual compression. Prevéy and Cammett, studying coverages much less than those used in common industry practice, found that beneficial residual compression can extend as far as five times the impact dimple radius, depending on the alloy. This has been confirmed in Kirk and Hollyoak’s observations and numerous finite element solutions. The regions between impacts are actually in compression rather than tension. Because surface compression is supported by subsurface equilibrating tension, surface fatigue initiation is suppressed even at lower coverage. Because coverage slowly increases asymptotically with peening time, 80% coverage can be achieved in about 1/5 of the peening time needed for 100% coverage. It was found that far less than 100% coverage was necessary to achieve the same fatigue life improvement, but a large savings of peening time. For example, in 4340 steel, 80% coverage produces essentially the same depth and magnitude of compression as 100% or greater coverage, in 20% of the time. Peening to slightly lower coverage greatly improves production times and reduces media usage, yielding huge savings in production and manufacturing settings, with reduced surface damage and no loss of beneficial compression or fatigue life.
It is important to note that the optimum coverage differs with the material, and that coverage can be found and optimized. While 4340 steel yielded the best results at 20% of the full coverage peening time, 7075-T6 aluminum performed best around 60%, and Inconel 718 performed best at 40%. Peening parameters are efficiently developed for each intended material and media application to minimize peening time, media consumption, and processing cost while achieving optimum part performance. Beneficial residual compression, cold work, finish, phase transformations, fatigue, and even corrosion performance are verified in accredited surface integrity laboratories. Lambda Technologies received a patent5 for the peening optimization method in 2007, offered today as a service through their laboratory.
The most important aspect of this research is the potential benefit in high production environments, like the automotive industry, where millions of parts require shot peening annually. Optimizing coverage limits the occurrence of repeated impacts, improves thermal stability, reduces surface damage, and media consumption, and yields improvement in production rates up to a factor of five. Shot peening optimization is cost-effective and can be applied to any component specification. The savings in cycle time and peening media alone virtually demands process optimization.
Further reading on the effects of shot peening coverage adjustments can be found in the footnotes of this article.
1Kirk and Hollyoak, "Relationship Between Coverage and Surface Residual Stress." ICSP9, 2005, p273.
2Ludian and Wagner, "Coverage Effects in Shot Peening of AL 2024-T4."ICSP9, 2005, p269.
3Ilaneza and Belzunce, "Optimal Shot Peening Treatments to Maximize the Fatigue Life of Quenched and Tempered Steels." Journal of Materials Engineering & Performance, Vol 24(7), 2015, p2806.
4Prevey and Cammett, "The Effect of Shot Peening Coverage on Residual Stress, Cold Work and Fatigue in a Ni-Cr-Mo Low Alloy Steel." ICSP8, 2002, p295.
5US Patent No 7,159,425
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Cincinnati, OH 45227, USA