Peening

Hearst Mining building (stone, left) with expansion (shot peened aluminum alloy, right).

Peening is the process of working a metal's surface to improve its material properties, usually by mechanical means, such as hammer blows, by blasting with shot (shot peening), or blasts of light beams with laser peening. Peening is normally a cold work process (laser peening being a notable exception). It tends to expand the surface of the cold metal, thereby inducing compressive stresses or relieving tensile stresses already present. Peening can also encourage strain hardening of the surface metal.

Residual stress

Plastic deformation from peening induces a residual compressive stress in a peened surface, along with tensile stress in the interior. This stress state resembles the one seen in toughened glass, and is useful for similar reasons.

Surface compressive stresses confer resistance to metal fatigue and to some forms of corrosion, since cracks will not grow in a compressive environment. The benefit comes at the expense of higher tensile stresses deeper in the part. However, the fatigue properties of the part will be improved, since the stresses are normally significantly higher at the surface in part due to surface imperfections and damage.

Work hardening

Cold work also serves to harden the material's surface. This makes cracks less likely to form at the surface and provides resistance to abrasion. When a metal undergoes strain hardening its yield strength increases but its ductility decreases. Strain hardening actually increases the number of dislocations in the crystal lattice of the material. When a material has a great number of dislocations, plastic deformation is hindered, and the material will continue to behave in an elastic way well beyond the elastic yield stress of the non-strain hardened material.

Residual Strain / Stretching

Plastic deformation from peening can be useful in stretching the surface of an object.

One common use of this peening (stretching) process can be seen in the auto repair and auto custom fabrication industries where manual or machine assisted peening is used to stretch thin sheet metal to create curved surfaces. The manual method uses a hand held peening hammer and is a form of planishing. There are also machine assisted methods that use a version of a power hammer to peen the sheet metal.

Another use of the peening process is to flatten sheet metal and is specifically used as a primary technique to flatten steel belts used in industrial conveying and pressing operations. In this process a steel belt that has a cross curvature can be flattened by peening the concave surface to stretch it and thereby removing the cross-curvature by equalizing the surface length across the belt between the previously concave and convex surfaces. The peening of steel belts is usually achieved by a shot peening process, using specialized equipment and special steel belt peening shot.

When peening is used to induce residual stress or work harden an object, care needs to be taken with thin parts not to stretch the workpiece. Where stretching is unavoidable then allowances may need to be made in the part design or process application.

Use with welding

Hand peening may also be performed after welding to help relieve the tensile stresses that develop on cooling in the welded metal (as well as the surrounding base metal). The level of reduction in tensile stress is minimal and only occurs on or near to the weld surface. Other methods, like heat spots (if applicable), help reduce residual tensile stresses. Peening will induce a higher hardness into the weld and this is something that should be avoided. For this reason, peening is not normally accepted by the majority of codes, standards or specifications (ex. ASME B31.3 para 328.5.1 (d) location changes when new codes are published). Any peening that is carried out on a weld should have been carried out on the weld procedure qualification test piece.

The welding procedure qualification test piece replicates all of the essential variables that will be used in production welding. If the weld is peened during the qualification of a welding procedure, the subsequent mechanical testing of the procedure qualification test piece will demonstrate the mechanical properties of the weld. These mechanical properties must, as a minimum, match the mechanical properties of the materials that have been welded together. If they do not, the procedure has failed and the welding procedure is not acceptable for use in production welding.

History

The first published article about peening was written in Germany in 1929, and was specifically about shot peening. The first patent for shot peening was also taken out in Germany in 1934, but was never commercially implemented. Independently in 1930, a few engineers at Buick noticed that "shot blasting" (as it was originally termed) made springs resistant to fatigue. This process was then adopted by the automotive industry. Zimmerli first published a report in 1940. John Almen did more research, and during World War 2 introduced it to the aircraft industry.^[1]

By 1950, peening became an accepted process and was being included in engineering literature. In the same year, peen forming was invented to form the wing skin of the Super Constellation aircraft.^[1]

In the early 1970s peening experienced a major innovation when researchers such as Allan Clauer at Battelle labs in Columbus, Ohio applied high intensity laser beams onto metal components to achieve deep compressive residual stresses, which they patented as Laser Shock Peening, and became known as laser peening in the late 1990s, when it was first applied to gas fired turbine engine fan blades for the U.S. Air Force.

See also

• Almen strip
• Ball-peen hammer
• Case hardening
• Heat treatment
• Peen plating
• High Frequency Impact Treatment
• Ultrasonic impact treatment
• Laser peening

References