Shot peening

Shot peening may be considered as the most versatile of aU cold-working treatments, being applicable to virtually all metals and shapes. It consists on 

Steel used in the gear industry [62]. However, some problems may arise from residual stresses being self-equilibrating. Beyond residual compressive stresses there must be tensile stresses, as shown in Figs. 3.51 and 3.53. Considering that compressive stresses develop within 200-300 pm from the surface, according to 

Type A medium intensity applications on casts and wires  

Type N low intensity applications on glass or ceramics and non-ferrous metals Type C high intensity applications (steels) 

The depth within compressive residual stresses dissolves depends on the Almen intensity and the hardness of the material, as shown in Fig. 3.66 [64] and, therefore, on the ultimate strength of the material, as shown in Fig. 3.67 [65]. 

Today s technology is not able to decode all the detailed shot peening parameters and its d5mamic effects, but it does provide abundant 

ASME ANSI. 2002. Surface texture, surface roughness, waviness, and lay. ASME ANSI B46.1-2002. New York ASME ANSI. 

Standard test method for evaluating the microstructure of graphite in iron castings. ASTM A247-06. West Conshohocken, PA ASTM International. 

Standard specification for precipitation-hardening nickel alloy bars, forgings, and forging stock for high-temperature service. ASTM B637-06el. West Conshohocken, PA ASTM International 

Interacting cracks and large-scale pitting in the polished sample [23]. With kind permission of Elsevier 

Observe in Fig. 7.73 that the static-load capacity of the polished reference samples is 4 kN at a 50 % fracture probability, whereas the cyclic-load capacity of the SP silicon nitride samples is 10.4 kN. The SP treatment increased the static-load capacity to more than 16 kN. Thus, the static load capacity is about 4 times higher. The capacity of the testing device was not high enough to introduce any cracks into the SP surfaces. From the no failure results of the static load up to 

The above results indicate the importance of introducing compressive-residual stress by the SP of ceramics-which is a new development of the last decade. Even in brittle materials, like most ceramics, microplastic deformation is introduced 

Materials Sintering parameters Average grain size (pm) Relative density (%TD) Young s modulus (GPa) Vickers hardness 

A maximum compressive-residual stress of 300 MPa is observed on the surface of the SP specimen, decreasing in the depth direction. In the case of the SP + CH specimen, the residual stress also decreased, but still exhibited a compressive-residual stress of 200 MPa. However, the induction of compressive-residual stress was accompanied by simultaneous CH. SP, in combination with CH, is a useful technique for improving the strength of ceramics, as may be seen in Fig. 7.82, which compares plots of the fracture probability against the crack-initiation load. 

Ruglic, Flame Hardening, Heat Treating, Vol 4, ASM Handbook, ASM International, 1991, p 260-267 

Budinski, Surface Hardening by Flame and Induction, Chapter 5, Surface Engineering for Wear Resistance, Prentice Hall, 1988, p 120-137 

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Fig. 9. Effect on fatigue strength of the plastic deformation of a carburized steel surface by shot peening (B) as compared to nitriding (A) and heat treating...

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. 

Contain tensile residual stresses which are usually as large as the yield stress. Weld liable to fatigue even when applied stress cycle is wholly compressive. Reduce residual stresses by stress relieving, hammering or shot peening. 

Fig. 5-4 Cell currents between a coated specimen (5 = 300 cm ) and uncoated steel electrode (5 =1.2 cm ) in NaCl solutions at 25°C. Left shot-peened steel sheet, 150 pva EP-tar. Right hot-dipped galvanized steel sheet, 150 jim EP-tar.

Fig. S-6 Relation between the density of cathodic blisters and potential shot-peened steel sheet without primer ( ) and with about 40 /tm primer coat (Zn ethyl silicate + polyvinyl butyral) ( ) top coat 500 /tm EP-tar ...

Fig. 5-7 Effect of potential on the composition of the blister liquid, shot-peened pipe with 300 to 500 jim EP-tar, artificial seawater, 1300 days at 25°C.

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

Further evidence of the relative resistance of maraging steel is reproduced in Fig. 3.33 from Reference 24. Maraging steel is shown to be superior to a die steel and low alloy steel (both unidentified) in bent beam tests stressed at 75% of the yield strength in distilled water. Also shown is the beneficial effect in smooth surface tests of cold rolling. Shot peening has a similar beneficial effect . 

Shot peening is a beneficial surface treatment since it puts the surface into a state of compression and generally obscures the grain structure. Subsequent painting of the peened surface is often useful. If pitting occurs then cracking can be expected in susceptible material when the attack penetrates the depth of the compressed surface layer. 

Shot peening results in considerable increase of the operating and service life of constructions affected by dynamic loads. 

The easiest way to produce a mechanical effect consists in using a brush that is actually brought into intimate contact of the substrate. Other techniques have been recently proposed such as laser flash [10] and shot-peening with argon or ice microballs [11] but they are still not mature enough to be developed in this chapter. 

As Fig. 4.1-42 c indicates, the geometries of the components must cope with the necessary accessibility for the shot-peening tool. 

Noyan, I.C. and Cohen, J.B., X-ray diffraction study of the residual stress-strain distributions in shot-peened two-phases brass, Mater. Sci. Eng. 75, 1983, 179. 

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. 

Restricting the degree of the movement shot peening offers the double advantage of inducing residual surface compression stress to retard corrosion and roughening the surface to increase friction.91... 

Application of surface treatments such as shot peening, nitriding of steels and sandblasting of the surface of metal or others producing constraints of compression are beneficial.1... 

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

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