SPECIALIST ENGINEERING FEATURE
SHOT PEENING AND C.A.S.E.
Residual stress and surface roughness can radically affect a components performance. However, until recently engineers have only designated roughness constraints on drawings, with little heed to residual stress. This is now changing as industries become aware of the significance of these issues and both are now designated on drawings and critical applications.
CONTROLLED SHOT-PEENING Controlled Shot-Peening is the cold working of a surface with spherical media, impinging at a pre-determined velocity under controlled conditions. The surface yields, but it is restrained by the substrate inducing a uniform residual compressive stress resulting in increased fatigue strength.
C.A.S.E.TM
TM
SUPERFINISHING OF SURFACES TO OPTIMISE ROUGHNESS AND STRESS
SUPERFINISHING
This process is beneficial in situations where metal to metal contact occurs and where high temperature, fluid flow and optimum performance are required. It is especially beneficial as a post controlled shot peening treatment, the combination of the two processes improving fatigue performance and increased running efficiency.
This is a technique of surface refinement in a controlled gentle manner to give a negative Rsk with very low Ra and Rv. It works by preferentially removing surface asperities whilst retaining the beneficial negative valleys.
Recent work on case carburised components in EN36, ground to simulate gear flanks produced the following results:
As Ground
Across Lay
Ra
0.083 Rt
1.079 Sm
23.931 HSC
155 Slope 0.1o
Note: All dimensions in microns.
Ra = Arithmetic Mean of the departures of the roughness profile from the mean line.
Rt = Maximum peak to valley height of profile over assessment length.
Sm = Mean Spacing between profile peaks at mean line.
HSC = The High Spot Count is the number of complete profile peaks projecting above the mean line.
Slope = Slope of profile throughout the assessment length.
98
www.windenergynetwork.co.uk
Along Lay
0.084 0.619
185.243 22
0.06o Shot Peened
Across Lay
0.233 1.990 80.44 52
0.23o
Along Lay
0.248 1.933 81.448 49
0.23o
SP +
C.A.S.E.TM Superfinishing
Across Lay
0.044 0.277 80.413 48
0.06o
Along Lay
0.043 0.389 64.965 55
0.07o CONCLUSION
Controlling the residual stresses and surface roughness will significantly improve the performance of a component. These improvements are achieved by introducing a repeatable magnitude and depth of compressive stress by controlled shot peening and then controlled
C.A.S.E.TM Superfinishing of the surface, to remove the asperities likely to puncture the lubricant film and cause premature failure.
Metal Improvement Company, LLC
www.cwst.co.uk
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