Peening processes

The peening process introduces a compressive stress in the peened surface layer by constraint of surrounding material. 

The effectiveness of these processes to mitigate SCC of Type 304 reactor internal material have been demonstrated by laboratory testing. Compressive residual stress of several hundred MPa to a depth of 300 1000 im is obtained. SCC susceptibility can be significantly reduced or eliminated by peening processes... 

The surface residual stress introduced by the shot peening process will not have any effect on the stress amplitude, but only on the mean stress that becomes... 

Recently, a combined pack aluminizing and ball peening process was developed by the authors group (collaborative research at USTB and UOA) to produce nanostmctured aluminide intermetallic coatings on carbon steel, stainless steel and/or Ni-based superalloys at a relatively low operating temperature (440-600°C) and in a short time (15-120 min) (see Fig. 7.3) (Zhan et al, 2006a,... 

Zhan Z., He Y, Wang D. and Gao W. (2006d), Nanocrystalline iron-aluminum coatings assisted by ball peening processes , Int J. Nanosci., 5, 775-80. 

Example case history 1. A gas turbine s compressor rotor developed blade tensile stresses while in operation that would quickly lead to failure. The OEM specified that a glass-bead peening process needed to be conducted all over the surface of the... 

Mechanical Plating. Impact or peen plating is a mechanical process whereby the metal powder is compacted and welded to parts by mechanical energy. This process is limited to relatively small parts of no more than about one kilogram. The parts are placed ia a specially desigaed barrel... 

The surface may gain a very high (eg, 1000 Vickers) hardness from this process. Surface deformation also produces a desired high compressive residual stress. Figure 9 illustrates the improvement in fatigue properties of a carburized surface that has been peened (18). 

Newer high velocity thermal spray coating processes produce coatings in compression rather than tension because of the shot peening effect of the supersonic particles on impact. This has permitted coating as thick as 12,500 p.m without delamination as compared to older processes limited to 1,250 p.m. The reduced residence time of particles at temperature minimises decomposition of carbides present in conventional d-c plasma. This improves wear and hardness (qv) properties. 

Post-processing, e.g. machining (for dimensional restoration) or shot peening (remove residual stresses), can be an added source of variability. 

Determination of residual stress of a failed component is one of the most important steps in failure analysis. The determination of residual stress is useful when failed components experience stress concentration, overload, distortion or the formation of cracks in the absence of applied loads, subjected to corrosive environments as in stress corrosion, mechanical or thermal fatigue due to cyclic loading, or when faults in processing such as shot peening, grinding, milling and improper heat treatment such as stress relief, induction hardening, thermal strains, exposure temperature are involved. 

Some practical cases are determination of residual stress in steel springs, the effect of mechanical loading on stress relaxation of machined and shot-peened nickel-base alloys,65 determination of residual stress level in turbine engine disks as they accumulate engine cycles,65 66 effect of manufacturing processes on residual stress, measurement of stress gradients in mechanical, electronic and structural components, effect of heat treatment on residual stress in steel coil springs, effect of variable heat treatment temperature on residual stress in iron alloys, measurement of stress in multiphase materials and composites and stress measurements at locations of stress concentrations. 

Shot peening and other mechanical processes that create compressive residual stresses at the surface are recommended. 

On the other hand, plastic deformation by grinding or shot peening produces maCrostresses that are accurately measurable by x-rays, as shown by excellent agreement between x-ray and dissection measurements. Figure 16-11 shows an example. Deformation by these processes appears to be multiaxial, rather than uniaxial, and pseudo-macrostress is accordingly absent. 

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