This article provides comprehensive information about shot peening as well as shot peeners or shot peening equipment. You will learn how shot peening equipment is made and the materials used for construction, as well as applications, advantages, and drawbacks.
Read further to answer questions like:
- How does shot peening work?
- Why should you use shot peening instead of other stress or surface treatments?
- What can shot peeners or shot peening equipment do? How can you optimize your manufacturing operations with shot peening equipment?
- How are shot peeners made? What are the materials of construction?
- What types and sizes of shot peeners are available from leading manufacturers?
- What types of peening media are used?
- How do I select shot peening equipment when ordering or submitting an RFQ?
- Can I dispose of, reuse, and recycle used shot and peening media?
- Does the shot media wear out equipment components?
- And much more...
I. What is Shot Peening Equipment?
Shot peening equipment includes systems or machinery and components for projecting shot peening media against a part’s surface to modify the surface or surface engineer the part.
Shot peening equipment includes a wide variety of individual types, such as:
- industrial shot peening machines
- laser ablation or laser peeners
- ultrasonic needle peening tools
- flap peeners
- peen markers
- shot peeners
- dry shot peening cabinets
- wheel shot peening machinery
- micro shot peeners
- shot peening rooms
- shot peening cabinets
- air shot peener
- airless shot peener
- portable shot peeners
- wet shot peener
- micro dimpling equipment
- peen formers
- peening machines
II. How Shot Peeners Work
The short answer to “How do shot peeners work?”:
- Metal shot and other shot peening media are driven or propelled at a part using compressed air, pressurized water, ultrasonic energy, or a centrifugal blast wheel.
- Spherical or rounded media is required such as cast shot, ceramic beads, glass bead, and rounded or conditioned cut wire shot.
- The impacting shot or spherical media plastically deforms or stretches the metal producing dents or dimples at the impact sites.
The plastic deformation introduces cold work and dislocations into the material increasing hardness and inducing compressive residual stress.
Compressive residual stress is negative, while tensile residual stress is positive. Fatigue strength lowering tensile residual stresses can be generated during manufacturing processes such as welding, casting, machining, grinding, and heat treatment. Shot peening can help solve this problem.
- The impacts can hammer down high points on the roughness profile, but they can also generate valleys, dimples, and peaks on smooth surfaces.
Specific surface textures can be engineered with lower friction than mirror-polished surfaces.
The level of cold work or residual stress generation is controlled by “intensity” and “coverage” of the shot peening process.
Intensity - Intensity is a major parameter used to control the shot peening process. Intensity is measured using an Almen strip and an Almen gauge. The intensity of the blast stream is measured by the amount of deformation or bending occurring in an Almen strip during the shot peening process. The compressive residual stress induced in the strip causes the strip to bend.
Shot size, shot hardness, shot density, and velocity adjustments control shot peening intensity. Increasing shot size, shot hardness, shot density, and velocity increases intensity.
The shot or peening media must be at least as hard as the parts being shot peened. Higher hardness media will induce greater magnitudes of compressive stress.
Coverage - Coverage is another important parameter in shot peening. Coverage measures the percent surface area of the part that has been shot peened or dimpled by shot media. In high-quality shot peening of critical parts, the coverage must be 100%.
III. Shot Peening Equipment Types and Technologies
Shot Peening Equipment Technology
If you want a deeper answer to, “How do shot peeners work?”, then you will need a better understanding of the various types of peening processes and the technology empowering shot peening equipment.
Shot peeners utilize rounded media in place of the sharp, abrasive media used in sandblasters.
While sharp blast abrasives cut or abrade a surface, rounded shot media deforms a surface. The metal under the impacting shot or bead is stretched and compressed forming a small crater, dimple, or dent.
While not a metal removal process, small amounts of metal can be removed with peening such as the higher peaks from machining or grinding. Peening generates less metal dust compared to abrasive blasting.
The technology employed to energize or propel the shot peening media is an important aspect characterizing shot peening equipment types. Shot peening machines use either pneumatic or air pressure, centrifugal wheels, ultrasonic energy to project abrasives or media.
Peening processes can encompass a wide range of techniques depending on the energy employed to project the media or create an impact or shock wave:
- Air Shot Peening
- Cavitation Peening or Water Jet Peening
- Centrifugal Wheel Shot Peening
- Hammer Peening
- Laser Ablation Peening
- Peen Marking
- Powder Impact Plating (PIP)
- Rotary Flap Peening
- Shot Peen Forming
- Ultrasonic Needle Peening (UNP) or Ultrasonic Impact Treatment (UIT)
- Ultrasonic Shot Peening
- Water or Wet Shot Peening
Convention or mechanical shot peening processes include air, wet, and wheel shot peening. These are the most common prevalent processes in industry today.
The pneumatic or air shot peening machinery employed to peen, or modify surfaces include:
Air or Dry Shot Peening Equipment – Air shot peening or dry shot peening equipment utilized compressed air to move the shot peening media. Pneumatic or compressed air shot peening systems can be divided into two types, suction, and direct pressure.
Suction or Siphon Shot Peening Equipment – Suction shot peeners or shot peening cabinets employ the venturi siphon effect to such abrasive into a pressurized stream of fluid, air, or water.
Venturi devices utilize a constriction in a moving stream of fluid to generate a pressure differential or vacuum. Shot peening media is sucked into the air or water stream at the constriction.
Venturi devices are found in many industries. Venturi generators are used to create a vacuum for mechanical holding applications. Ejectors and eductors move fluids, powder, or solids in chemical process industries.
Suction hot peening cabinets or portable siphon shot peening pots cost less than pressure shot peening systems. Economy shot peeners tend to be suction shot peeners. They do not require a pressure vessel and consume only half of the pressurized air compared to pressure shot peening cabinets. However, suction shot peeners require higher air pressure levels to maintain media flow.
Suction shot peening cabinets are less aggressive and take longer to impact clean. Suction shot peeners are commonly used in short-run or light production, maintenance, and remote field applications.
The less aggressive nature of suction or siphon shot peeners increases the time to strip or clean parts. On the upside, the lower aggressiveness reduces the wear rate of the parts within a suction shot peener and tends to last longer, reducing shot peening equipment maintenance costs.
Small handheld shot peeners can have a cup above the shot peening gun to gravity feed into a shot peening gun’s venturi point. Gravity shot peeners are a type of suction shot peener because gravity-fed shot peening guns still make use of the venturi siphon effect.
Have you ever wondered why direct pressure shot peeners are found more often in industrial applications than suction shot peeners?
Direct Pressure or Pressure Shot Peening Equipment – Pressure or direct pressure abrasive shot peening equipment makes use of a pressure vessel to energize the abrasive media. A pop-up or metering valve on the pressure vessel is opened to release pressurized fluid and shot peening media into a shot peening hose. The pressurized media travels through the shot peening hose to the direct pressure cabinet and shot peening gun.
Direct pressure cabinet shot peening machines expel the shot peening media at much higher flow rates or speeds compared to suction shot peening equipment. Impact or kinetic energy (K) of the shot peening media is K = ½ mv2 where m = shot peening media mass and v = velocity. Doubling the flow or velocity quadruples the impact energy and shot peening efficiency.
The higher shot peening media speeds enable direct pressure systems to impact clean parts up to 4 times faster than suction shot peening machines. Pressure levels are more adjustable with direct pressure shot peening cabinets, so cleaning and surface modification can be more precisely controlled.
Direct pressure shot peening equipment can lift and propel heavy media such as steel shot, cut wire shot, and steel grit more easily than suction equipment. Suction shot peeners struggle with heavy media. Doubling the shot peening media mass doubles the impact energy. Heavier shot peening media (denser or larger) is more efficient at cold working or deforming compared to lighter, lower density, or smaller media.
A higher density or larger diameter as well wider diameter nozzle or multiple nozzles will shot peen parts with more mass and higher energy impacts. Air compressors with greater flow capacity (CFM) are required to drive the higher mass through the shot peening system.
The higher speeds, ability to handle heavy steel media, more aggressive nature, and better control of direct pressure shot peening equipment make pressure shot peening equipment more widely employed in high volume production and automated shot peening applications. The higher shot peening media speeds enable pressure shot peeners guns to operate at greater stand-off distances from the part.
Pressure shot peeners consume two to three times the volume (CFM) of compressed air compared to suction shot peening equipment.
Pressure vessel maintenance and safety are a concern. The failure of a pressure vessel could harm operators and damage equipment. Shot peening system pressure vessels need to be manufactured to American Society Mechanical Engineers (ASME) Boiler and Pressure Vessel (BPV) Codes.
Wet Abrasive Shot Peening Equipment – Water can replace air as the fluid used to propel the shot peening media. Wet abrasive shot peening or water shot peening can reduce the dust generated during shot peening by over 90%, which can be important when stripping or cleaning a part containing heavy metals and hazardous materials.
An aspect I find interesting about water shot peening equipment is the ability to provide additional surface modification with water additives. Chemicals can be added to break down hard to remove films or grease. Anti-rust agents can be added, so the parts will be protected until coatings are applied.
While some suppliers refer to wet abrasive shot peeners as dustless shot peening equipment, no shot peening system suppresses 100% of the dust generated.
Water shot peening can keep the parts cooler compared to dry shot peening, which can reduce warping or distortion of thin sections. If the dust generated during shot peening is explosive such as aluminum or titanium dust, then wet shot peening is advantageous because static discharges and ignition are suppressed.
Water shot peening can consume up to 50% less shot peening media compared to dry shot peening. The water provides a cushioning and lubricant effect, which increases the life of shot peening media. Wet shot peening systems can employ closed-loop media can recyclers, oil separators, demisters, and filtration systems. Dust is converted to clean, disposable sludge waste.
In dry shot peening, the dust must be removed from parts with an air blow gun or washing. In factory settings, dry shot peeners required dust collectors.
In dry shot peening, parts typically require degreasing. Parts do not have to be degreased before wet shot peening. Wet shot peening combines washing and dedusting with the shot peening step.
Detergents can be added to the water to loosen deposits, dissolve oils or greases and accelerate the cleaning process.
Rust inhibiting agents can be added to the water to prevent rusting of wet steel after the shot peening process is complete. If rust suppressors are not added to the water, then the steel should be dried, oiled, or painted immediately after shot peening to prevent rust formation.
Bacteria and microbes can grow in the shot peening water forming slime films and releasing odors. The shot peening water like other metalworking fluids and grinding coolants require the addition of antimicrobial agents.
There are several types of water shot peening machines.
Slurry Shot Peening Equipment – Dry shot peening equipment can be retrofitted to provide wet shot peening. Slurry shot peening systems are also known as air abrasive water shot peeners. A water injection nozzle or water ring (halo) is attached to an air shot peening gun, which introduces water into the shot peening media stream. They suppress 50% to 85% of the dust generated.
The water ring or halo nozzles provide less dust control than the water induction or injection nozzles.
Slurry shot peeners have a high degree of versatility because they can dry shot peening, wet shot peening, rinse, and dry parts. One drawback of slurry shot peeners is cleaning up the muddy mess generated in field applications. Slurry shot peening equipment is tiresome for operators to carry and handle because a heavy water hose is attached.
The water shot peening technology that I personally find the most interesting is vapor shot peening.
Wet Venturi Shot Peeners – Wet venturi shot peeners are like dry suction shot peeners except a compressed air-generated venturi vacuum sucks in an abrasive-water mixture. Some manufacturers call these systems modified shot peeners. While venturi shot peeners successfully suppress dust, they require high shot peening pressure to operate effectively resulting in high consumption rates of water and shot peening media. Flow control is limited.
Vapor Shot Peening Equipment – In vapor shot peening machines, the abrasive and shot peening media are premixed in a pressurized vessel. Vapor shot peeners are also known as mist shot peeners or dustless vapor shot peeners, and dust-free shot peeners. Vapor shot peening systems provide the highest level of dust suppression, up to 95%.
The abrasive slurry travels through a shot peening hose to the slurry shot peening nozzle. Additional compressed air can be added to regulate the aggressiveness of the wet shot peening process. The regulating compressed air generates a mist of wet shot. The regulating air pressure and the shot peening media consumption rate can be controlled independently. Vapor shot peeners consume much less water and media compared to venturi and slurry shot peeners.
Vapor shot peeners have a wide operating pressure range and finer control of the wet shot peening process compared to other wet shot peeners.
Wheel Shot Peening Equipment -Another major type of shot peening equipment is based on centrifugal wheel shot peening technology. Shot peening media is fed into the axis of a spinning turbine wheel impeller. A series of throwing blades or paddles on the shot peening wheel accelerate and fling the shot or abrasive media onto the parts.
Adjustments to the control cage around spinning blades can direct the stream of media to control the shot peening pattern as well as the size and location of the “hot spot”. The shot peening pattern can be several inches to feet in width depending on the wheel dimensions, wheel speed, and distance between the wheel and part surfaces.
The hot spot is a region of the shot peening surface, which becomes hot to the touch. The hot spot tells the operator where the center of the shot peening flow is located. The hot spot is the most aggressive portion of the shot peening media stream. In other words, the hot spot has the highest density of impacts per unit area. If the parts are not hot to the touch, then the operator adjusts the shot peening pattern to move the hot spot onto the parts.
Wheel shot peeners are ideal for moving heavy shot peening media like condition cut wire steel shot, stainless steel shot, and cast steel shot. In addition to peening, round steel shot is excellent at cleaning castings, and descaling forgings, but we usually consider that a blast cleaning process.
The shape and hardness properties of the media, or whether any media is used at all, is another parameter controlling the shot peening process. Spherical steel, stainless steel, glass beads, or ceramic ball media impart compressive residual surface stresses on a part.
Steel and stainless steel media include cast shot and conditioned cut wire. As-cut wire and steel grit have sharp edges. This makes them useful for abrasive blast cleaning, but not peening.
What I find really intriguing about shot peening equipment is the ability to alter the strength of parts by orders of magnitude or up to 5 times or improve life up to 10 times!
Alternative and Emerging Peening Processes
While the majority of shot peeners or shot peening machines employ centrifugal wheels or pressure shot peening, there are some alternative peening technologies. Newer and emerging peening technologies utilize lasers and ultrasonic energy.
Laser Peening - Laser peening machines ablate a tape, water, or coating “tamping” layer to induce a pressure or shock wave into the surface. The pressure wave induces compressive residual stresses deep into the surface 0.47 inches (12 mm)] with minimal cold working. Shot peening typically induces compressive residual stress to depths of only 0.2 inches (6 mm). This thinner depth is sufficient to make large improvements in strength. Laser peening also appears to improve the galling and spalling resistance of alloys.
Laser peening can be highly selective without masking because the laser can be controlled and scanned precisely. Laser peening imparts minimal coldwork, so the benefits of laser peening are retained at higher temperatures.
Laser peening is a media-less peening process, so parts with more complex shapes and sharp corners can be peened. Since laser peening is digitally controlled and has no media to wear out, the process should provide more consistent, quality results.
Ultrasonic Shot Peening - Vibratory and ultrasonic energy can also be used to selectively shot peen surfaces of parts. A miniature chamber is made around a part or region of the part requiring peening. An ultrasonic transducer is built into the side or bottom of the chamber. Shot Peening media laying on the ultrasonic transducer acts like a liquid in an ultrasonic cleaner. The ultrasonic transducer energizes the beads or shot causing rapid peening impacts on the part and chamber walls.
Ultrasonic shot peeners can peen surfaces within cavities or holes, which might be shadowed during conventional impact cleaning processes or require a peening lance.
Ultrasonic energy can be used to modify crystal structure or produce a surface layer with nanoscale crystals. The process is called ultrasonic nanocrystal surface modification (UNSM). Researchers are also experimenting with combinations of lasing and ultrasonic technology such as laser‐assisted ultrasonic nanocrystal surface modification (LA‐UNSM).
Ultrasonic Needle Peening (UNP) - Ultrasonic needle treatment (UNT) or impact treatment (UIT) repeatedly drives a pin or needle into a surface at ultrasonic frequencies of 10 to 60 kHz. Ultrasonic needle peening can be highly selectively on the region peened on a part without the need for masking. Welds or the stress concentration formed in welds with excess material can be needle peened to improve strength characteristics. No peening media is required, but the carbide needle will require replaced over time.
Cavitation Peening - Cavitation peening, water jet peening, or ultrahigh-pressure water peening machines operate at ultra-high pressure of 20,000 psi or higher. The high-pressure stream flows through a cavitation nozzle plate to intentionally create vaporized water or cavitation bubbles. Cavitation bubbles collapse on a metal surface generating a pressure shock wave similar to the shock wave in laser peening. The shock pressure can be over 145 ksi. The high-pressure shock wave creates compressive residual stresses in the metal.
Since no media is involved water jet peening can peen into crevices, corners, and complex geometries. Cavitation peening is a cool process, no heat is generated as in shot peening and laser peening.
Pin or Dot Peen Marking - Pin marking or dot peening impacts a small pin into a surface to create dimples of text, graphics, codes, and other marks for identifying products. The dot peen pin or stylus is usually made of tungsten carbide or diamond, which is pneumatically or electrically driven. Dot peening is a highly controlled process.
Dot peening is not a true surface engineering process like shot peening, rather a part marking method. Dot peening has an advantage over other marking processes using a scribe or cutting action, which can impart tensile stresses. Dot peening marks with very low stress or compressive stress. Flight critical aerospace components are almost always identified with dot peening because the process is very controllable and generates a preferred stress profile.
Rotary Flap Peening - Rotary flap peening uses carbide shot adhesively bonded to polymer fabric flaps. The flaps are loaded onto a mandrel or wheel. The flap peen mandrel or wheel is mounted on a rotary power tool. The rotary power tool spins the flaps at high speeds and the operator flap peens a metal surface.
Flap peening is useful for peening selected areas of a surface without masking. Inner diameters of holes can be peened by rotating the peen flaps at higher speeds.
Hammer Peening - Ball peen hammers, air or pneumatic hammers, boilermakers hammers, or any hammer can be used to manually peen the surface of a part. Long ago, blacksmiths and metal workers would peen forgings and castings with small hammers to harden and strengthen surfaces.
Welds are often hammered to clean and improve the surface of the weld. However, manual hammering is not a controlled process, so its use is discouraged in manufacturing standards and codes. Researchers are developing a robotic hammer peening (RHP) process, but no commercial systems are available yet.
Hammering is sometimes used to straighten bent or distorted parts. The power hammer operator can incur a lot of musculoskeletal damage. Shot peening can be used to straighten bent parts without harming the worker.
Shot Peen Forming - Shot peen forming machines are used to straighten or bend parts. Peen forming is most useful for generating large radius curves for wings, ships, storage tanks, or process vessels.
Wheel shot peening systems are used for forming large parts without high complexity such as a wing. For highly complex parts, air peening is utilized because the peening nozzle can direct media into a part’s nooks and crannies. Ultrasonic peen forming and laser peen forming processes can also form curvatures in parts.
Parts can be peen formed as received, prestrained, or clamped to a fixture for preshaping. The radius of curvature has to be within the metal’s elastic range. The part cannot have abrupt changes in curvature.
Strain Peening - In strain peening, the part is preloaded or prestressed with a surface tensile stress up to its elastic limit before shot peening. During shot peening, the surface plastically yields as the shots create dimple stretching the metal beyond its yield point. The combination of tensile preloading and impact loading creates a higher maximum of compressive residual stress value in the part.
Dual Peening and Multi-Peening - Dual peening, tri-peening, and multi-peening provide even greater improvement to fatigue strength compared to conventional single-shot peen processing. Dual peening, tri-peening, and multi-peening shot peen a metal surface twice, thrice, or multiple times, respectively, with progressively finer shot media and lower intensities. Multiple peening passes with smaller and smaller media tend to cold work the peaks missed by previous passes.
Shot Peening Equipment Types – By Size and Application
Shot peening equipment can consist of complete shot peening machines or abrasive shot peening packages preconfigured with the required components. Shot peening equipment can be modular. You can configure a shot peening system specifically for your application by selecting various parts from a supplier’s catalog. Parts include shot peening guns, wear-resistant nozzles, pressure vessels, valves, deadman handles, shot peening cabinets, shot peening rooms, and shot peening hoses.
Shot peening equipment can vary greatly in size and configuration depending on the specific application, the size of the parts being shot peened, the end-use or function, and setting (lab/shop, production line, or field/remote sites) where they are used.
Shop and Laboratory Shot Peening Equipment
Benchtop Micro Shot Peening – Micro shot peening or fine particle peening machines have small 0.018-to-0.125-inch diameter nozzles. Some nozzles are made of sapphire or single crystal alumina for enhanced wear resistance.
Micro shot peening cabinets are benchtop or floor mounted. Handheld pencil shot peeners are a type of micro shot peener. They usually have a small gravity-fed media cup.
In conventional shot peening, steel shot media diameters of 600 to 800 microns. Media sizes ranging from 10 to 200 microns are used to micro-shot peen surfaces.
To generate a similar impact or kinetic energy (K = ½ mv²) with fine shot media, the velocity needs to be increased. A specialized version of micro shot peening called WPC treatment propels blasts of micro-shot at speeds of 450 miles/hour (200 meters/sec). In conventional shot peening, the steel shot travels at 150 miles/hour (70 meters/second). The high-speed impacts mechanically deform as well as heat the surface. The high-velocity fine particle peening microforges the surface. WPC is an acronym for “wonder process craft” or wide peening and cleaning.
The forging action can be used to mechanically plate another metal or solid lubricant onto a surface in a peen plating process. Peen plating can provide unique surface compositions unattainable by conventional plating or thin film deposition.
Solid lubricants like molybdenum disulfide can be embedded into a surface. is also known as impact plating, mechanical plating, particle impact plating (PIP), cementation plating, dry plating, and mechanical deposition.
Peen plating's main advantage is the avoidance of hydrogen embrittlement, which occurs during wet plating hardened high carbon steels with hardnesses greater than Rockwell C 40.
Micro peening produces dimples with a much smaller diameter at a higher density per unit area The microscale of fine particle peening maintains a low Ra or surface roughness average compared to conventional shot media. As a result, the micro dimples can be highly effective in reducing friction.
Manual Shot Peening Cabinets or Cabinet Shot Peeners – Manual shot peening cabinets have two holes with rubber shot peening glove protruding into the cabinet, which allows an operator to manipulate parts and hold the shot peening nozzle at the appropriate stand-off distance to generate a hot spot and then move the hot spot across the surface to clean or etch a surface.
The shot peening cabinets have a window and internal lighting, so the operator can see the parts and guns without getting shot peening in the face. Some shot peening cabinets shot peening air across the window to maintain visibility by preventing dust build-up.
Spent shot peening media drops through the cabinets steel or fiberglass grate and into a collection hopper. The used shot peening media was conveyed to a media separator to recover the abrasive. Economy shot peening cabinets might skip the separator step and directly reuse the spent media. Without separation, dust and finer broken media are reused, which can reduce efficiency and damage filters.
Ideally, the media and dust generated during shot peening parts are contained within the cabinet and filtration system. The shot peening cabinet develops leaks as seals wear and tear over time. Leaking around doors is a common problem because cabinet doors are not always clamped tight around the whole periphery.
One leading manufacturer, Titan Abrasive Systems, has developed a patent-pending technology for “leak or warp” proof shot peening cabinet doors. The doors have double panels and two rigid, steel channels. The doors also have a better closing mechanism to better clamp the door shut along the edge. A knife-edge on the door forms a positive seal every time the door closed.
Shot peening cabinets can have front, top and side opening doors. Some cabinets have multiple doors. Double doored cabinets allow shuttling in of a dirty part, while a cleaned part is removed. Shot peening cabinet doors can swing open, swing up, or slide open.
Pass-through cabinets are available for etching glass sheets or panes and metal plates. Pass-through shot peening systems come up with a narrow gap with special brush or rubber flap seals to prevent abrasive leaks. The sheet or part is pushed into the gap and passed through the cabinet on rollers.
Manual shot peening cabinets are ideal for machine shops, garages, body shops, light production or short runs, touch-up of production parts, prototyping, and custom work.
Production shot peening Equipment
Production shot peening machines are built to a higher level of quality and durability to handle the wear and tear found in industrial settings. Production shot peening systems are designed for handling larger production run volumes of parts or larger-sized casting, forgings, extrusions, or structural shapes.
They can have multiple shot peening guns with each prepositioned to shot peening specific areas on the part. A shot peening system with 12 guns will have exceedingly high consumption rates of compressed air and shot peening media.
Automated versus Manual – Production shot peening systems can be manually operated or automated. If your parts vary greatly in size, shape, and production volume, then a manual system for loading or unloading parts might be sufficient. shot peening system automation is advisable when production quantities are larger or when quality is an issue.
shot peening processes can be semi-automatic, fully automatic, or turnkey. In a semi-automatic system, operators might manually load or fixture parts on a table and hanger. Then, the shot peening chamber door is shut, and the parts rotated and shot peened with preset or programmed parameters. At the end of the first cycle, the operator might have to flip or reposition the part to complete the shot peening cleaning of surfaces shadowed during the first cycle. In a turnkey or fully automated shot peening system, the part loading, handling, and manipulation, as well as the shot peening parameters, are programmed or controlled automatically.
Part handling, as well as gun manipulation, can be automated. Automation can more precisely control gun standoff and travel speed back and forth across the part surfaces. This can result in more consistent shot peening with few or no missed spots requiring touch-ups. The additional cost of automation can be offset by reduced labor cost, higher throughput, few rejects and rework, and increased quality.
Fixed Station and Robotic Shot Peeners – In fixed station and robotic shot peening systems, the shot peening gun or guns are mounted onto the end of a robot arm. The parts are load manually or automatically into the shot peening machine and then the robotic gun nozzle scans and shot peens the areas requiring surface treatment.
Robotic shot peeners are commonly found in aerospace and automotive industries for shot peening delicate and complex components such as turbine blades, pump impellers, engine parts, and valve components.
Batch and Pass-through shot peening Chambers – Batch production systems include tumble shot peeners and table shot peeners. In these systems, batches of parts are placed in the shot peening machine and processed. In pass-through shot peening systems, parts are pushed through an opening with special brush or rubber flap seals to prevent abrasive leaks.
Inline and Continuous Shot Peening Systems – In very high-volume and continuous production applications, shot peening machines can be integrated into production lines or placed in-line. Alloy strip, plate, or sheet in a primary or secondary metal mill can be continuously shot peened to provide a product with higher fatigue strength for demanding applications. Inline and continuous shot peening systems can be used for peening metal stock (sheet, strip, plate, bar, rod), wire, wire rope, and structural steel (I-beams, channels, angle).
Large in-line shot peening systems are available for production lines where parts or materials are passed in front of shot peening guns or shot peening wheels. Production shot peening systems have automatic guns or remotely operated shot peening guns.
For shot peening steel or metal sheet or other continuous or semicontinuous stock materials, shot peening nozzles or shot peening wheels are mounted above and below the stock material. The shot peening system continuously peens the steel or stainless sheet as rollers move it along the mill production line.
Shot Peening Rooms – shot peening rooms can clean part parts too large for shot peening cabinets, table shot peeners and hanger shot peeners. Shot peening rooms are large enough to accommodate an operator and sometimes even vehicles or material handlers.
Used shot peening media drops through the grating on the shot peening room floor. The used media is mechanically or pneumatically conveyed to the reclaimer or separator.
Shot peening operators don a complete shot peening suit including a shot peening hot peening hood, shot peening gloves, respirators or air supply, and hearing protection. The surface shot peening is accomplished with a portable shot peening pot or shot peening machine integrated into the shot peening room.
Media Separators and Dust Collectors – Most production shot peening rooms and shot peening systems have media separators to allow recovery or reclamation of used shot peening media. Recovered media and returned to the shot peening media pot or bin. Dust and disintegrated media are sent to a dust collector and filtration system, which removes the dust for disposal.
Production Shot Peening Systems with Integrated Part or Material Handling
Tumble Shot Peeners - Tumble shot peeners have a tumbling basket or continuous rubber tumble belt to tumble parts while being shot peened. The part must be suited for the tumbling action. Certain parts with thin fins, protrusions, or complex geometries can be damaged during tumbling or get jammed together. A hanger shot peener or wire mesh belt shot peener for those parts.
The basket should be fully loaded to protect the belt or basket from excessive wear. Overloading the tumble belt or tumble basket is not advised because the surface or part might not see the shot peening media. Overload can also damage the tumble belts or baskets.
Table Shot Peeners – Table shot peening systems are used with shot peening clean heavy castings and forgings. The parts are mounted on a rotary table inside a shot peening chamber. The chamber doors are closed and the part shot peened while the table rotates inside the chamber.
The bottom of the part sitting on the table is not exposed to shot peening media. The part has to be flipped over to clean the underside of the part.
Hanger Shot Peening Systems – In hanger shot peening systems, the parts hang down from hooks. Virtually all of the part is exposed to the shot peening stream or multiple shot peening streams, so almost every surface can be peened.
Wire Mesh Belt Shot Peeners – Wire mesh belts have a wear-resistant manganese steel mesh belt to convey parts past a stream of flowing shot peening media. The belt should not be shot peened without parts. If only a few parts need a surface treatment, then the belt should be loaded with dummy parts or scrap to reduce belt wear.
Monorail Shot Peening System – Monorail shot peening equipment has an overhead rail. Parts hanging from the monorail enter the shot peening machine through doors or a pass-through opening. The parts are shot peened and then they exit the other end of the machine where the peened parts are removed from the monorail.
Roller Conveyor Shot Peening System – Roller conveyors allow heavier metal stock such as billets, thick plates, and structural members (I-beams) to be shot peening cleaned.
Field and Special Purpose Shot Peening Systems
Portable Shot Peeners and Shot Peening Pots – Mobile and portable shot peening equipment is used in shot peening large surfaces or revitalizing part in the field. Smaller portable shot peeners consist of portable shot peening pots, air hoses, shot peening hoses, shot peening guns, and air compressors. Portable shot peeners can be carried to the site.
Internal Shot Peeners – Specialized tools or shot peening lances are available for shot peening the inner diameter of holes, tubes, and hollow shafts. The tools have collars to center the shot peening nozzle. The tungsten carbide deflecting tip directs the shot peening against the inner wall of the pipe.
Spin shot peening units have rotating heads with several nozzles directed at an angle to the pipe surface.
III. How is Shot Peening Equipment Constructed?
Shot peening equipment is constructed from a wide variety of parts cabinets, pressure vessels, hoses, guns, nozzles. These individual components are made using sheet metal fabrication, casting, welding, mechanical fastening, machining, and specialized processes.
Shot peening cabinets and shot peening rooms essentially start as fabricated metal boxes. They are typically made by cutting, bending, and forming steel sheets, plates, and structural steel into sides, legs, and doors needed to form a box.
With the addition of shot peening guns, windows, glove ports, doors, turntables, grating or screen, gun or part holders, pneumatic valves, foot pedals, lights, hoses, and reclamation devices, the box is transformed into a powerful tool of industry, a shot peening cabinet or shot peening room.
shot peening cabinets parts can be welded or fastened together. Fastening allows the parts to be more easily removed for cleaning, repair, and replacement. Welded shot peening cabinets tend to be more airtight with less leakage of shot peening media and dust into the shop, but replacement of worn cabinet sides or bottoms is difficult.
The abrasive shot peening stream wears the bottom and sides of the cabinet over time. The seals and windows on the cabinet will also age, wear and require replacement.
What materials are used to build shot peening cabinets and rooms?
Shot peening cabinets and shot peening rooms for dry or air shot peening are made of steel with powder coatings, zinc galvanization, or industrial paint. Wet shot peening cabinets are made of more corrosion-resistant materials such as stainless steel.
In certain dry shot peening applications such as surgical instruments and medical implants, stainless steel construction materials might be used to avoid iron contamination of a surface.
A system for shot peening stainless steel parts would typically use stainless steel shot or a nonmetal peening media. Steel components or steel shot impacting stainless steel parts can transfer metal to the stainless surface, which can alter passivation and lead to rust on the surface.
Wear-resistant steel liners or wear plates are strategically placed within shot peening chambers to reduce wear of the shot peening machine. Wear-resistant alloys include manganese steels such as Manganal and nickel and chromium white cast irons such as Ni-Hard alloys.
What are the parts and consumables in a shot peener?
Shot peening equipment tends to be self-destructive due to the aggressive shot peening media. shot peener parts are consumable and will wear out over time as media flow over or through these parts.
Of course, the shot peening media or abrasive grits are consumable as well. Some types of shot peening media like steel shot, rounded cut wire shot, ceramic beads, and glass beads can be recycled through the shot peener a hundred times or more.
The parts in abrasive shot peeners, wheel shot peeners, and shot peeners need to be regularly inspected for wear.
When the nozzle's inner diameter changes or the throwing blades change geometry, the efficiency of the shot peening process can be compromised.
shot peeners parts include:
- Shot peening guns
- Shot peening nozzles
- Shot peening wheel parts - wheel blades, cages, and impellers
- Wear plates
- Pressure regulators
- Shot peeners valves – Air inlet valves, abrasive metering valves, shut-off valves, media mixing valves, deadman valves, and pop-up valves
- Shot peening cabinet windows
- Shot peening cabinet grating
- Shot peening hose
- Shot peening room floor grating
- Deadman controls, handles, and valves
- Foot pedals
- Dust collectors filters
- Breathing air filters
- Media separator screen and parts
Shot peening nozzles are made of extremely wear-resistant materials including:
- Ceramic, aluminum oxide, or alumina (Al2O3)
- Binderless tungsten carbide, Pure WC (ROCTEC®, Cerbide™)
- Boron carbide (B4C) (Norbide®)
- Cemented tungsten carbide, WC with cobalt binder
- SiAlON or silicon aluminum oxynitride
- Silicon Nitride
- Zirconium oxide or zirconia, (Zr02) or zirconia-alumina
Boron carbide, alumina, pure WC, and silicon carbide ceramics are amongst the most wear-resistant materials.
Depending on the shot peening media, cemented tungsten carbide and SiAlON nozzles will last 10 to 20 times longer than ceramic or alumina nozzles. Boron carbide is the hardness and the most wear-resistant of the nozzle materials.
Boron carbide typically costs 3 times as much as cemented WC, but boron carbide lasts 3 to 25 times longer than cemented WC or sialon. They do not have the toughness and impact resistance of cemented tungsten carbide. Binderless tungsten carbide (WC) nozzles have double the life boron carbide nozzles.
Based solely on wear resistance boron carbide and binderless WC last up to seven-time longer than cemented WC. If you bang a boron carbide or silicon nitride nozzle into a part, grate, or cabinet wall, then the nozzle is more likely to crack compared to a cement tungsten carbide nozzle.
Nozzle service life will depend on the media shot peening passing through the nozzle as well. Hard ceramic beads will wear nozzles faster than spherical cast steel shot.
In addition to consumables and wear components, a variety of shot peening accessories and ancillary equipment can improve the shot peening process:
- Air Blowguns
- Shot peening hose back pressure tester
- Shot peening nozzle wear gage
- Shot peening water additives - Passivates, Rust Inhibitors, and Antimicrobial agents
- Dust suppressants
- Industrial vacuums
- Masking caps and shields
- Material handling equipment
- Media separators reclaimers and recyclers
- Moisture traps, water separators, air dryers
- Shot peening masking tapes, films, and materials
shot peening and shot peening processes also require shot peening personal protective equipment (PPE) for operators:
- Shot peening masks
- Shot peening hoods
- Shot peening suits
- Shot peening respirators
- Breathing air supply filter or system
- Shot peening gloves or shot peening cabinet gloves
- Hearing protection
IV. How is Shot Peening Used?
What Can Shot Peening Equipment Do?
Shot peening End-Uses or Surface Modification Operations
Peening and surface engineering can provide surface modifications such as:
- Cold or work hardening, which increase surface hardness and strength
- Compressive residual stress generation, which increases fatigue strength up to 500X
- Increased resistance to fretting and stress corrosion cracking
- Surface smoothing on a rough surface to surface roughening of smooth surfaces.
- Dimpling, which creates reservoirs or pockets for lubricants that lower friction
- Powder impact plating where solid lubricants and metal powder can be mechanically plated onto a surface
- Marking, peen marking systems can produce controlled dimples of text for part marking
Surface Engineering - Surface Dimpling or Texturing – Shot peening is highly effective at producing dimpling or dimpled textures on the surface of parts to alter frictional characteristics and retain lubricants.
Surface Engineering - Surface Finish Refinement (Smoothing)
Spherical or rounded media (cast shot, glass bead, ceramic spheres) produces a smoother or lower Ra profile compared to angular or crushed media.
Refining the surface finish or reducing the surface profile can increase fretting fatigue strength from 20% to 200%. A larger diameter or heavier cast shot can be used first to impart a deep residual stress layer. Next, the surface is peened with small spheres or microbeads to refine the surface finish.
If the part has a rough (high roughness average, Ra) surface finish like an as-cast or as-forged surface, then peening can modestly refine the surface finish. If the part has been ground or machined to a smooth or low Ra finish, then shot peening will result in a rougher surface finish. Shot peening can remove tool marks from machining or grinding.
Surface Engineering - Residual Compressive Stress Generation
Surface engineering entails modifications to the surface of a part to provide unique characteristics enabling or enhancing performance in specific applications.
Peening impacts the surface with spherical steel, stainless steel, glass, or ceramic media, which strain hardens and imparts compressive residual surface stresses on a part. The common media used is cast steel shot with a Rockwell C hardness of 40 to 55.
The compressive residual stress from shot peening can increase the fatigue strength of parts by 30% to 500%. Increasing fatigue strength and resistance to stress-corrosion cracking is important for fastened components, gears, axles, dies, molds, shafts, springs, aircraft landing gear, and other rotating and structural parts.
The residual stress generated during shot peening can be used to peen form and peen straighten shafts back into tolerance. Shot peening also hardens the surface of parts, which increases hardness and wear resistance. Shot peening can also close surface porosity. Shot peening is used to find and remove spots of hidden sub-surface corrosion in parts and around fasteners.
Shot peening can texturize a surface or produce specific patterns of dimpling or depressions. A dimpled texture can better retain lubricants, grease, inks, or other fluids. Peened textures can also alter the gripping and frictional characteristics of surfaces.
V. Industrial Applications of Shot Peening
Aerospace – Shot peening is used to induce compressive residual stresses and reduce distortions in aircraft structural components such as landing gear, turbine disks, jet engine blades, aircraft wheels, spars, ribs, chords, and stringers.
Aerospace parts such as wings, winglets, stabilizers, rubbers, and other airfoils are peen formed to develop curvature.
Since shot peening is a critically important process in attaining high fatigue strength in aerospace parts, there are many aerospace, military, and industrial standards controlling the shot peening process, such as:
- Society of Automotive & Aerospace Engineers - SAE J2441 - covers the engineering requirements for peening surfaces of parts by impingement of metallic shot, glass beads, or ceramic shot.
- Airbus - ABP 1-2028
- SAE Aerospace Material Specification - AMS-S-13165, AMS 2430, and AMS 2432
- Boeing Aircraft Corporation - BAC-5730
- Military - MIL-S-13165C, MIL-P 81985(AS), and MIL-STD-852
- Rolls Royce - RPS-428
- General Electric - P11TF3
- Pratt & Whitney Aircraft - PWA-6
Automotive OEM – Many engine components are shot peened to improve material properties and extend life. Automotive parts benefiting from shot peening to increase like include camshafts, coil springs, connecting rods, crankshafts, engine parts, and leaf springs.
Shot peening of automotive leaf springs in the 1930s marked the first commercial use of the process in industry. Peened springs can live for a million cycles while unpeened springs only last 250,000 cycles.
Foundries and Forges – Metal casting and forgings are often utilized in demanding, rotating machines prone to fatigue failure. Shot peening can improve the fatigue properties of castings and forgings.
Machine Shops / Manufacturing – Shot peening can increase the fatigue strength and life consistent of machined or ground mechanical components and tools such as:
- Precision shafts shafts, gears, pinions, and structural parts
- Reamers, drills, taps, end mills, cutting tools, broaches, hobs,
- Dies, molds, punches, and forming tools
- Dimensional gages
Medical / Dental – Dimpling, micro-dimpling, and shot peening can improve the performance of medical devices and dental restorations. Hip, shoulder, dental and other bone, and joint implants are shot peened to improve fatigue life.
Welding, Brazing & Soldering – Shot peening is applied after joining to improve the properties of the weld and heat-affected zone (HAZ) metal. The compressive residual stress can overcome the tensile residual stress developed during welding. The weld and HAZ metal must not be embrittled by the welding process or peening could cause cracking.
VI. Benefits and Drawbacks of Shot Peening Equipment
Sand Shot Peening Advantages
Shot peening excels imparting residual compressive stresses into a surface to improve fatigue strength by 30% to 500%, and part life by up to 10x.
Shot peening also inhibits or impedes:
- Distortion from applied tensile, bending, and torsional stress - Shot peened parts with compressive stresses are strengthened and can resist mechanical forces, which might elastically deform an unpeened part.
- Fretting - Fretting wear occurs when fastened metal joints with high loads experience micromotions or micro sliding. Cold welding followed by detachment produces wear particles in the fastened joint. The wear particles oxidize or corrode, which contributes to increased wear and surface damage. Fretting wear and fretting corrosion result in reduced fatigue strength. The surface hardening and texture generated by shot peening reduces fretting and fretting fatigue.
- Hydrogen embrittlement (HE) - Hydrogen embrittlement occurs when hardened steels are electroplated or pickled in an acid bath. Hydrogen diffuses into the part reducing the steel’s toughness and ductility. Peening can reduce hydrogen pickup. Laser peening to reduce hydrogen pickup had been patented. Particle impact plating can plate in a dry process with zero hydrogen pickup.
- Stress corrosion cracking - Stress corrosion cracking is localized corrosion accelerated by applied tensile stress at a crack tip and anodic or galvanic potential differences.
- Shot peening creates dimples on the surface of a part, which improves the adhesion of lubricants and reduces friction.
- Finer sizes of shot peening media can be used to shot peen inside holes, crevices, and the intricate details of a part.
- Shot peening can handle round or concave as well as convex curved surfaces. Shot peen forming also can straighten parts or produce desired curves in large parts because the process induces stress in the part.
- Shot peening is highly versatile because shot peening machines are available for extremely large parts to exceedingly small parts.
- When properly performed, shot peening does not impart any surface damage or burning to a metal part.
- A wide variety of shot and shot peening media are available with different hardness values, shapes, and media sizes, which allows the shot peening process to be precisely tuned and optimized for different materials and applications.
- Micro-shot peening can mechanically plate a surface with a protective film to enhance lubricity or corrosion resistance.
- With the proper shot peening media, surface changes can improve material properties and part performance. Shot peening can improve fatigue strength, corrosion resistance as well as corrosion fatigue, and fretting fatigue properties.
- Depending on the shot peening media used, shot peening can be environmentally friendly and non-toxic.
- Shot peening can be automated or robotically operated to increase efficiency and quality. Shot peening can be easier to automate compared to part cleaning and finishing with grinding wheels, rotary files, and abrasive flap wheels.
Shot peening can be more cost-effective when compared to other methods because:
- Larger surfaces can be rapidly shot peened.
- Conventional shot peening is more flexible and less labor-intensive than alternative methods to impart residual compressive stress. Alternative surface enhancements methods include several mechanical cold-working processes such as rolling, burnishing, or ball burnishing. Stress-relieving heat treatment can eliminate tensile stress, but they often distort parts.
- The process can be automated
- Shot peening equipment, media, and consumables are relatively inexpensive.
- Typically, shot peening media can be reclaimed, separated and reused several times, and then recycled. Certain shot peening media types can be reused hundreds of times.
Drawbacks of shot peening
- Shot peening and shot peening equipment generate high decibel noise and dust.
- Proper collection, handling, and disposal of the media and dust are required as well. Wet or water shot peening systems reduce the dust problem.
- The shot peening process puts a lot of wears out internal components of shot peening equipment. Media and consumable wear parts must be gaged to maintain consistency and then repaired or replaced at appropriate intervals to assure quality and safety. The cost of media and consumables should be factored into your shot peening equipment selection process.
Shot peeners or shot peening operators can be injured during shot peening. The high-pressure abrasive stream can harm skin and eyes. Media and dust can be inhaled or ingested. Inhalation can cause lung disease, breathing disorders, and other health problems.
Dust collectors and filters must be used and maintained to prevent operators and other workers from dust hazards. In shot peening rooms and remote field locations, shot peening operators should wear shot peening suits, shot peening hoods or shot peening helmets, respirators, shot peening cabinet gloves, and other personal protective equipment (PPE).
- Shot peening can generate heat during the abrasion process, which can warp thin parts. Wet shot peening keeps parts cool during shot peening.
- Shot peening media can get lodged into crevices on a part and can be difficult to remove.
VII. Selecting and Ordering shot peening Equipment
Answering these questions will help you in selecting the right shot peening system for your surface treatment application.
Start with the part size, shape, and materials as well as the annual production volumes when considering the type of shot peening equipment to select.
- What’s your production volume (parts per year), the size of the parts, or the surface area being shot peened?
- What level of automation and materials handling is appropriate for your production volumes and parts?
- What is the part alloy requiring surface treatment?
- Will the material generate hazardous dust requiring containment?
Next, understand your cleaning and surface treatment requirements.
What standards apply to my application?
ISO, ASTM, SAE, ASME, or AMS?
- Where are the surfaces located – in a shop, garage, factory floor, shipyard, oilfield, or highway?
- If possible, request a trial at a supplier’s facility or at one of their customer’s sites to evaluate different shot peening processes and shot peening media.
- Verify the shot peening process parameters with an additional test or trial.
What are the operating costs of the shot peening equipment?
Estimate the annual operating and consumable costs such as compressed air, water, and electrical power consumption.
- What are the consumable costs such as shot peening media, wear part replacements, and system maintenance costs?
What are the media choice options to generate the required surface engineering (residual stress)?
- Do you need a system designed for specialized media?
- OR, Is a general-purpose blasting and shot peening equipment capable of handling a variety of media types for a range of end-uses? Many machines are capable of abrasive blasting and shot peening.
Examine the different shot peening media options choices in terms of total cost-benefit. hile shot peening media cost is one factor, consider shot peening media efficiency, durability, and life.
- A larger size or heavier shot peening media can be faster and more efficient in cold working a surface, but the media must be able to reach and peen inside corners.
- Shot peening media that can be recovered and reused for hundreds of cycles can have a lower annual media cost compared to a lower-cost shot peening media with a short life or capable of only a few reuse cycles.
- Denser and larger-diameter media such as metal shot and metal grit is fast but requires high flow (CFM) pressure shot peeners or wheel shot peeners.
- What are the labor costs and training requirements? How many operators are required to run the shot peening system? Is special safety and automation system training required?
- Submit a quote for the shot peening equipment along with any additional questions to clarify training as well as annual estimated operating, maintenance, and consumable costs.
- Today’s shot peening equipment suppliers provide a vast range of product variations manufactured with high-quality materials and methods.
- Shot peening equipment suppliers are constantly upgrading equipment with new technology innovations such as laser peening and ultrasonic peening.
- Shot peening experts at the leading suppliers know how to select the specific shot peening and peening systems, media types, media recovery equipment, and material handlers for a broad range of industry applications.
- Shot peening manufacturing experts are willing to work with customers on the development of new applications that would benefit from shot peening and peening technology.
- The outlook for increased use of shot peening and shot peening equipment is extremely promising considering the broad range of capabilities that modern equipment OEMs can provide to their customers as well as the benefits to the environment.