The amount of cold work produced during shot peening is not directly controlled, but is a consequence of the extensive coverage, shot size, and peening parameters. The high cold work is often a disadvantage because the dislocation density produced in the deformed layers accelerates thermal relaxation and develops yield strength gradients, making the surface subject to both thermal and mechanical relaxation in service.
Research at Lambda into the thermal and mechanical stability of peened surfaces has led to the development of the patented optimized coverage peening technology. Lambda found that the coverage required to achieve a depth and magnitude of compression could be far lower than that typically used in production peening. For both steels and nickel–based alloys, coverage as low as 20% can produce the same depth and magnitude of compression as 100% or greater coverage. Two immediate benefits are achieved: (1) Reduced cold work during peening and (2) Reduced peening time and cost.
Optimized Coverage Peening uses just the coverage required to produce the necessary depth and magnitude of compression. The cold working can be a fraction of that produced by peening over 100% coverage. The data for 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%. Similar results have been obtained for both steels and Ni-based alloys. The reduced cold work provides greater thermal stability. Effectively, low plasticity surface compression can be achieved by shot peening. Alternately, peening can be performed to produce a given controlled amount of cold work, if work hardening of the surface is desired. Optimized peening with 20% coverage produces the same depth and magnitude of beneficial residual compression as conventional 100% coverage.
Depth and magnitude of compression from shot peening 4340, 50 HCR steel with 3% to 400% coverage, showing no improvement over 20%.