VOL. 16 January ISSUE YEAR 2015
in Vol. 16 - January Issue - Year 2015
Fatigue life at 107 cycles for varying coverages of shot peening
Residual stress measurement showing diminishing returns for higher levels of peening coverage and the resulting increase in cold work
Shot peening is widely used to achieve a protective layer of compression for parts, with millions of parts shot peened each year in a wide range of industries. Shot peening practice as governed by specifications for media, coverage and Almen intensity has changed little for decades, while competition has grown on an international scale. The result is pressure for improved productivity, reduced media and equipment costs, faster processing, and automated control and verification.
Research at Lambda Technologies into the thermal and mechanical stability of shot-peened surfaces has led to the development of their patented Controlled Coverage Peening technology. The coverage required to achieve a depth and magnitude of compression can be far lower than what is typically produced in conventional peening. For both steels and nickel¡Vbased alloys, coverage as low as 20% can produce the same depth and magnitude of compression and fatigue benefit as 100% or even greater coverage. The benefit is a potential production rate increase of 5X with proportional media consumption and equipment cost reductions with equal or improved performance.
The amount of cold work produced during traditional shot peening is not directly controlled, but is a consequence of the extensive coverage, shot size, and peening parameters. The random nature of shot impact creates unnecessarily high cold work with areas repeatedly impacted to achieve 100% coverage. The high cold work has been found to be a potential disadvantage with no significant benefit in either residual compression or fatigue life in many applications. The high dislocation density in the deformed surface layers accelerates thermal relaxation and reduces ductility in work hardening alloys like IN718, causing loss of compression at lower working temperatures. The critical wear surface of hardened martensitic steels in gears and similar applications is work-softened by cold work from shot peening.
A lower coverage rate during peening provides major production and cost benefits. Shorter processing times save on machine wear and maintenance, and can greatly increase throughput as much as 5-fold, if coverage can be reduced to 20%. The higher throughput also proportionally lowers media consumption costs. With Controlled Coverage Peening, the compressive residual stress field is designed specifically for the part to provide optimal fatigue strength and is verified for the processing and equipment to be used. The peening process is controlled by monitoring the mass shot flow, shot velocity and/or air pressure, and peening time, which are recorded to document the processing.
Controlled Coverage Peening uses just the coverage necessary to produce the depth and magnitude of compression required for the alloy and application. The cold working can be a fraction of that produced by peening to 100% coverage. Residual stress measurement of 4340 steel, 50 HRC, shot peened with CW14 shot at an intensity of 9A shows that there is no significant difference in the depth and magnitude of compression produced for coverage ranging from 20% to over 400%. The accompanying benefit in fatigue life shows similar results. There is no practical difference in the fatigue life of high coverage shot peened pieces. In fact, a higher coverage can have a detrimental effect. Similar results have been obtained for both steels and Ni-based alloys. The benefits of low plasticity surface compression can be achieved by shot peening with greatly increased productivity.