Fig. 1. Abrasive shot blast media

Not all similarly sounding processes are created equal and a good example of this is the dif-ference between shot blasting and shot/laser peening. Let's learn more.

Fig. 2. The shot peening process

Shot Blasting

Shot blasting or blast cleaning is a process in which an abrasive material (Fig. 1) is accelerated through a pressurized nozzle or centrifugal wheel and directed at the surface of a part to clean or otherwise prepare the part surface for further treatment. The media used will vary as a function of the type of cleaning process and includes sand; steel shot; cut wire shot; chilled iron; garnet, a sharp hard abrasive which is used to prepare surfaces on non-ferrous metals; olivine, a soft abrasive material for use on decorative stone or non-ferrous metals; and glass beads used to polish rather than remove surface coatings on soft metals and plastics.

Shot blasting can be used on castings, forgings, and stampings to produce a uniform surface texture and for descaling, deburring, and deflashing. A certain degree of fatigue resistance is imparted to the material due to compressive stresses produced at the surface although the effects are non-uniform and thus shot blasting is not considered a controlled process. Shot blasting is used in a wide variety of industries including automotive, marine, mining, and medical applications.

Fig. 3. Typical shot peening curve

Shot Peening

By contrast, shot peening is a cold working process, which uses the mass and velocity of a shot stream to produce residual compressive stress at the surface of the part. Each piece of shot striking the metal surface acts as a tiny ball peen hammer, imparting a small indentation, or dimple on the surface. At the time of shot impact, the metal surface yields in tension due to localized stretching that occurs, while the near surface layer is left in a residual compressive state due to the material's attempt to restore the surface to its original shape (Fig. 2).

Media used for shot peening consist of small spheres of cast steel, cut wire formed into nearly round shape (both carbon and stainless steel), ceramic, and glass material.

A variety of different metallic based materials are currently shot peened including: high strength steels, carburized and decarburized steels, cast and austempered ductile iron, and nonferrous alloys of aluminum, titanium, and magnesium. Powder metal parts also benefit from shot peening. Common application uses for the technology include, for example, gears, connecting rods, crankshafts, compression springs, torsion and anti-sway bars and metal implants.

Benefits of shot peening include improvements in: fatigue life (torsional, axial, bending and thermally induced fatigue), fretting, pitting, galling, and corrosion failures (stress corrosion cracking, exfoliation and intergranular corrosion). It has been demonstrated that most cracks will not initiate or grow in a highly compressively stressed zone. Since nearly all fatigue and stress corrosion failures originate at or near the surface of a part due to tensile stress, the compressive stresses induced by shot peening (Fig. 3) provide increased part life. The magnitude of the compressive stress produced by shot peening is at least as great as half the tensile strength of the material being peened.

Shot peening is a precisely controlled process relying on careful selection and control of media, intensity, coverage and equipment. Peening media must remain predominately round and uniform in diameter to avoid surface damage upon impact and to maintain a uniform compressive stress layer. Damaged or mixed size media must be removed and replaced. Intensity (or shot stream energy) is measured using Almen strips, a spring steel peened on one side only whose height is measured as an indication of induced residual stress. Complete coverage control is critical and must be at least 100% of the surface area.

Dual shot peening can also be performed to further enhance fatigue performance at increased cost. Dual peening is a secondary peening application that results in improvement of the compressive stress at the outmost surface thus imparting additional crack resistance.

Fig. 4. Laser peening process (illustration courtesy of Metal Improvement, Inc.)

Laser Peening

An emerging technology in the peening industry uses the energy of a laser pulse to impart the residual compressive stress into a material's surface (Fig. 4). The primary benefits of laser peening are a residual compressive layer that is 4-5 times deeper than traditional shot peening with minimal cold work of the surface which can be a detriment when shot peening at high energy levels.

Laser peening generates high-pressure plasma that creates a high intensity pressure wave in a localized area. The pressure wave is capable of driving the compressive stress much deeper into the material. Some failure modes such as foreign object dam-age (FOD) and fretting fatigue respond significantly better to a deeper residual compressive layer. This can be best illustrated by looking at a comparison of the compressive stress depth achieved by a typical shot peen versus a typical laser peen for Inconel 718 (Fig. 5).

Fig. 5. Comparison of shot and laser peening (Inconel 718)
While both peening processes offer a high magnitude of compressive stress at the surface, the shot peen compressive layer is mostly dissipated by 0.010" deep. Should this peened surface incur a scratch 0.010" deep, there would be little protection preventing further potential growth of the flaw. With laser peening there is approximately 100 KSI of residual compressive stress at 0.010" deep. This compressive stress would attempt to arrest potential growth of the flaw. IH