In metallurgy, peening is the process of working a

metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...

's surface to improve its material properties, usually by mechanical means, such as hammer blows, by blasting with shot (

shot peening Shot peening is a cold working process used to produce a compressive residual stress layer and modify the mechanical properties of metals and composites. It entails striking a surface with shot (round metallic, glass, or ceramic particles) with ...

), or focusing light (

laser peening Laser peening (LP), or laser shock peening (LSP), is a surface engineering process used to impart beneficial residual stresses in materials. The deep, high-magnitude compressive residual stresses induced by laser peening increase the resistance o ...

). Peening is normally a

cold work Cold is the presence of low temperature, especially in the atmosphere. In common usage, cold is often a subjective perception. A lower bound to temperature is absolute zero, defined as 0.00K on the Kelvin scale, an absolute thermodynamic ...

process, with laser peening being a notable exception. It tends to expand the surface of the cold metal, thereby inducing

compressive stress In long, slender structural elements — such as columns or truss bars — an increase of compressive force ''F'' leads to structural failure due to buckling at lower stress than the compressive strength. Compressive stress has stress units (fo ...

es or relieving

tensile stress In continuum mechanics, stress is a physical quantity. It is a quantity that describes the magnitude of forces that cause deformation. Stress is defined as ''force per unit area''. When an object is pulled apart by a force it will cause elonga ...

es already present. Peening can also encourage

strain hardening In materials science, work hardening, also known as strain hardening, is the strengthening of a metal or polymer by plastic deformation. Work hardening may be desirable, undesirable, or inconsequential, depending on the context. This strength ...

of the surface metal.

Residual stress

Plastic deformation In engineering, deformation refers to the change in size or shape of an object. ''Displacements'' are the ''absolute'' change in position of a point on the object. Deflection is the relative change in external displacements on an object. Strain ...

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 Tempered or toughened glass is a type of safety glass processed by controlled thermal or chemical treatments to increase its strength compared with normal glass. Tempering puts the outer surfaces into compression and the interior into tens ...

, and is useful for similar reasons. Surface compressive stresses confer resistance to metal fatigue and to some forms of

corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...

, 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 working 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 In materials science and engineering, the yield point is the point on a stress-strain curve that indicates the limit of elastic behavior and the beginning of plastic behavior. Below the yield point, a material will deform elastically and ...

increases but its

ductility Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile str ...

decreases. Strain hardening actually increases the number of

dislocation In materials science, a dislocation or Taylor's dislocation is a linear crystallographic defect or irregularity within a crystal structure that contains an abrupt change in the arrangement of atoms. The movement of dislocations allow atoms to s ...

s 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

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 Planishing (from the Latin ''planus'', "flat") is a metalworking technique that involves finishing the surface by finely shaping and smoothing sheet metal. Process This is done by hammering with a planishing panel hammer or slapper file against a ...

. There are also machine assisted methods that use a version of a

power hammer Power hammers are mechanical forging hammers that use an electrical power source or steam to raise the hammer preparatory to striking, and accelerate it onto the work being hammered. They are also called open die power forging hammers. They ha ...

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 shot peening of steel belts is usually achieved by using specialized equipment and special 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 work-piece. 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 Welding is a fabrication process that joins materials, usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool, causing fusion. Welding is distinct from lower temperature techniques such as bra ...

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. 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.

Sharpening blades

Scythe A scythe ( ) is an agricultural hand tool for mowing grass or harvesting crops. It is historically used to cut down or reap edible grains, before the process of threshing. The scythe has been largely replaced by horse-drawn and then tractor mac ...

and

sickle A sickle, bagging hook, reaping-hook or grasshook is a single-handed agricultural tool designed with variously curved blades and typically used for harvesting, or reaping, grain crops or cutting Succulent plant, succulent forage chiefly for feed ...

blades have traditionally been sharpened by occasional peening followed by frequent honing in the field during use. A blade can be sharpened by reforming the malleable steel to create an edge profile that can then be honed. Nicks and cuts to the blade edge can be worked out of the blade by peening and a new edge profile then formed for honing. Peening jig anvil

Blades can be free-peened using various designs of peening anvils, or worked on a peening jig. A peening jig may have interchangeable caps that set different angles: a coarse angle can be set first about back from the edge, and a fine angle is then set on the edge, leaving an edge that lends itself to being easily honed. The blade can then be honed using progressively finer honing stones until it is ready for use.

History

During construction of Barker Dam hydroelectric facilities in

Colorado Colorado (, other variants) is a state in the Mountain states, Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the wes ...

between 1908 and 1910, workers discovered that striking welds while they were still warm improved their strength. 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 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.. 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 The Lockheed L-1049 Super Constellation is an American aircraft, a member of the Lockheed Constellation aircraft line. The L-1049 was Lockheed's response to the successful Douglas DC-6 airliner, first flying in 1950. The aircraft was also produc ...

aircraft. 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

in the late 1990s, when it was first applied to gas-fired turbine engine fan blades for the U.S. Air Force.

References

{{Authority control Metalworking Shot peening