Shot peening is the oldest, most widely used surface enhancement method. Metallic or ceramic shot, ranging in size from nominally .030 inches to as large as .125 inches, impact the surface of the component, producing spherical indentations. The shot peening process forms a compressive layer using a combination of subsurface compression developed by Hertzian loading combined with lateral displacement of the surface material around each of the dimples formed. As the dimples overlap with random impacts, the entire surface is effectively elongated, driving the surface layer of deformed material into compression as its expansion is resisted and supported by the equilibrating tension in the material below.
The “media” used in the shot peening process can be metallic or ceramic. Metallic shot is often either cast steel or cut wire blasted against a carbine pate to form a nearly spherical shape. Cut wire shot can be manufactured from virtually any alloy to avoid elemental contamination. Ceramic shot is typically zirconium oxide or glass bead. The shot will wear and fracture during use, therefore the shot peening media should be constantly screened to remove broken shot and dust.
The shot peening process is defined by parameters that include the size and type of shot used, the Almen intensity achieved, and the coverage. Almen intensity is a measure of the deflection of the Almen A, C or N strip that occurs during the peening cycle, and is a measure of the elastic energy stored in the 1070 steel Almen strip by the formation of a layer of compression. Almen intensity is related to an area of shot peening under a residual stress-depth curve, and it does not uniquely define a depth or magnitude of compression. Coverage is the percent of the surface impacted by shot, and is a function of time under the shot stream. In order to ensure uniform treatment of the surface, coverage is often specified at 200% or even 400%, implying that each point on the surface was impacted at least two to four times.
Types of Shot Peening
The shot is accelerated to sufficient velocity to deform the surface on impact either by air blast or wheel machines. In wheel shot peening machines the shot is thrown from a rapidly spinning radially bladed wheel. Wheel machines are widely used for gearing, springs and other automotive applications where numerous components are shot peened without the need for detailed direction of the shot stream. Large volumes of shot are delivered by wheel machines, often in specially designed peening cabinets used for production processing in an assembly line. Air blast machines, which propel the shot in a stream of compressed air through a hardened carbide nozzle, are widely used for precision shot peening with smaller shot, as for aircraft turbine disks and blades.
“Flapper peening” is a form of controlled shot peening without free flying media. The flapper tool has captive shot embedded, rather like rivets, in rubberized fabric flaps that are attached radially to rotate around a shaft. The shaft is rotated so that the flaps successively impact the surface peening of a local area. The Flapper is used for repair or limited access work or when free shot cannot be tolerated, as in nuclear reactors or on-wing jet engine rework.
What are some common problems in the shot peening process?
By its very nature shot peening relies upon random impacts of the shot. Therefore, in order to achieve the coverage requirements, some regions will receive numerous impacts before adjacent areas are impacted at all. The result is a non-uniform and generally very highly cold-worked surface. Cold work levels range from 40% to over 100% during creation of the layer of surface compression. In the work hardening materials, such as titanium and nickel alloys, peening induced cold working can exhaust the ductility of the material, leaving a brittle surface layer. Shot peening damage in the form of “laps and folds” creates stress concentrations that reduce fatigue performance.
Shot peening is a very practical surface enhancement method, provided the components are not exposed to elevated temperature or mechanical overload. Research at Lambda has shown that the highly cold worked shot peened surface will relax more completely and much faster than a low cold worked surface at the same state of compressive stress.
Get a complete demonstration of Lambda’s patented Low Plasticity Burnishing in comparison to shot peening capabilities, by contacting us anytime through email, or during business hours by phone at (800) 883-0851.