This article provides comprehensive information about shot peening equipment.
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Shot peening equipment refers to specialized machinery designed to project highly concentrated peening media forcefully onto a surface. This process alters or prepares a part for various applications. Shot peening utilizes multiple methods, including centrifugal wheel, ultrasonic, wet, and laser techniques, although some methods don't involve traditional peening media.
The peening media, known as shot, consists of diverse materials, which vary depending on the equipment type and the material undergoing treatment. Common media include steel, glass, and ceramic. The choice of peening media primarily depends on its characteristics and properties. By impacting the surface, the media induces compressive stresses that create small indentations, compressing the underlying material of the part.
Shot peening is a cold working process used to modify the mechanical properties of components by generating compressive stress on their surfaces. This technique boosts component strength while optimizing its stress profile. It extends component lifespan by increasing resistance to fatigue, corrosion, cavitation erosion, and cracking. The required level of compressive stress varies based on the intensity and scope of the peening media applied.
There are various types of shot peening equipment available:
Shot peeners use round media in place abrasives that are used in sandblasters. The types of media widely vary from natural less aggressive types to plastics, glass beads, steel shot, steel grit, aluminum oxide, and silicon carbide. Media sizes are measured in mesh sizes, which are the openings in the mesh used to screen media or in microns (µ), the actual size of the particles.
As the peening media impacts the surface of a component, it tends to break down into smaller fragments, which reduces its aggressiveness and force. Consequently, different sizes of media may be utilized or added during the peening process. To achieve optimal results, users must monitor the process and replenish the media as necessary to ensure consistency.
While sharp blast abrasives cut or abrade a surface, rounded shot media causes deformation of the surface. The metal underneath the impacting shot or bead is stretched and compressed, resulting in the formation of small craters, dimples, or indentations.
Despite their similarities, shot peening and sand blasting have distinct purposes. Sand blasting is used to remove material from the surface of a component by employing very fine fragments and pieces. It is typically used to prepare a surface for additional finishing processes, such as coating or painting. Although shot peening can remove some material, such as the tops of peaks created by machining, it is not primarily a material removal process. Additionally, sand blasting generates significantly more metal dust compared to shot peening.
Shot peening equipment employs pneumatic or air pressure, or centrifugal wheels, to project abrasives or media onto the surface of a component. Its primary goal is to enhance the fatigue strength of materials and has been effectively used for many years. Unlike plating, hardening, or welding, shot peening does not diminish the fatigue strength of metals.
Shot peening equipment encompasses various techniques, each defined by the type of energy used to propel the media and generate an impact or shock wave:
Conventional or mechanical shot peening methods include air, wet, and wheel shot peening, which are among the most widely used techniques.
Air or Dry Shot Peening Equipment – Air or dry shot peening equipment employs compressed air to propel the shot peening media. These pneumatic or compressed air shot peening systems are categorized into two types: suction and direct pressure.
Suction or Siphon Shot Peening Equipment – Suction shot peeners or shot peening cabinets operate using the Venturi effect, which introduces the abrasive into a high-pressure stream of air or water. Venturi devices narrow the fluid stream to create a pressure drop or vacuum. This process draws the shot peening media into the air or water stream at the point of constriction.
Venturi devices find application across various sectors. They are used to create vacuums for mechanical clamping and work in conjunction with ejectors and eductors to move fluids, powders, or solids in chemical processing environments.
Suction shot peening cabinets or portable siphon units are generally more affordable than pressure shot peening systems. Budget-friendly shot peeners are often of the suction type. They don’t require a pressure vessel and use only about half the amount of pressurized air needed by pressure shot peening cabinets. However, they need higher air pressure to keep the media flowing effectively.
Suction shot peening cabinets are less aggressive and require more time to achieve thorough cleaning. They are commonly used for smaller production runs, maintenance tasks, and remote field applications.
The reduced aggressiveness of suction or siphon shot peeners means that cleaning or stripping parts takes more time. On the plus side, this gentler approach reduces wear on the components inside a suction shot peener, often resulting in longer-lasting equipment and lower maintenance expenses.
Small, hand-held shot peeners may have a cup positioned above the gun to enable gravity feeding into the gun’s venturi point. Gravity shot peeners are a type of suction shot peener, as they still utilize the venturi siphon effect.
Pressure or direct pressure abrasive shot peening equipment use 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 project shot peening media at significantly higher flow rates or speeds than suction shot peening systems. The impact or kinetic energy (K) of the media is calculated as K = ½ mv², where m represents the mass of the media and v is the velocity. Increasing the flow or velocity twofold results in a fourfold increase in impact energy and efficiency of shot peening.
Due to the higher speeds of shot peening media, direct pressure systems can clean parts up to four times faster than suction shot peening machines. Direct pressure cabinets offer more precise control over pressure levels, allowing for more accurate cleaning and surface modification.
Direct pressure shot peening equipment can handle and propel heavy media like steel shot, cut wire shot, and steel grit more effectively compared to suction systems. Suction peeners often struggle with heavier media. Increasing the mass of the shot peening media results in a proportional increase in impact energy. Denser or larger media are more efficient for cold working or deformation than lighter, smaller media.
A larger diameter or wider nozzle, or using multiple nozzles, enhances the shot peening of parts with greater mass and energy. To accommodate the higher mass, air compressors with increased flow capacity (CFM) are needed to power the shot peening system.
The increased speeds, capacity to manage heavy steel media, more aggressive operation, and enhanced control have led to the widespread use of direct pressure shot peening equipment in high-volume production and automated settings. The elevated media speeds allow pressure shot peener guns to function effectively at greater stand-off distances. These systems use two to three times more compressed air (CFM) than suction shot peening equipment.
Maintaining pressure vessels is crucial for operator safety, as any failure can pose significant risks and cause equipment damage. All pressure vessels used in shot peening are constructed to meet the standards set by the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel (BPV) Codes.
In wet abrasive shot peening, water is used instead of air to propel the shot peening media. This method significantly reduces dust generation by more than 90%, which is crucial when dealing with parts that contain hazardous materials or heavy metals. Additionally, water shot peening allows for enhanced surface treatment through the addition of water-based chemicals. These additives can help break down stubborn films or grease and include anti-rust agents to protect the components until coatings are applied.
Despite being termed dustless, wet abrasive shot peeners still produce a minimal amount of dust during the peening process.
Using water in shot peening helps keep components cooler, reducing the risk of warping or distortion in thin sections. It also minimizes static discharge and the risk of ignition from explosive dusts like aluminum and titanium. Moreover, wet shot peening consumes up to 50% less media and provides a cushioning effect that extends the life of the media.
Wet shot peening systems often incorporate closed-loop media recyclers, oil separators, demisters, and filtration systems. These systems convert dust into clean, disposable sludge. Parts do not require pre-degreasing, as the process integrates cleaning with shot peening. Detergents can be mixed with water to loosen and dissolve oils or grease, enhancing the cleaning efficiency.
To prevent rusting of wet steel after the shot peening process, rust-inhibiting agents can be added to the water. Without such agents, the metal must be dried, oiled, or painted immediately after peening to avoid rust.
Shot peening water can become a breeding ground for bacteria and microbes, leading to slime and unpleasant odors. To combat this, antimicrobial agents should be added to the water, similar to other metalworking fluids and grinding coolants.
Dry shot peening systems can be adapted to perform wet shot peening by retrofitting with slurry shot peening setups, also known as air abrasive water shot peeners. This adaptation involves adding a water injection nozzle or a water ring (halo) to an air shot peening gun, which introduces water into the media stream. This method reduces dust generation by 50% to 85%, though it is less effective in controlling dust compared to systems with dedicated water induction or injection nozzles.
Slurry shot peeners are highly versatile, capable of performing dry shot peening, wet shot peening, rinsing, and drying parts. However, they present a challenge due to the muddy residue they create, which requires thorough cleanup. Additionally, handling and transporting slurry shot peening equipment can be cumbersome due to the heavy water hose involved.
Wet venturi shot peeners function similarly to dry suction shot peeners, but they utilize a venturi vacuum generated by compressed air to draw in a mixture of abrasive and water. Often referred to as modified shot peeners by some manufacturers, these systems effectively reduce dust. However, they necessitate high shot peening pressures for optimal performance, which leads to substantial consumption of both water and shot peening media, and offers limited control over flow rates.
In vapor shot peening systems, the abrasive and shot peening media are pre-mixed within a pressurized vessel. These machines, also known as mist shot peeners, dustless vapor shot peeners, or dust-free shot peeners, offer superior dust suppression, reducing dust by up to 95%.
The abrasive slurry moves through a shot peening hose to the nozzle. Additional compressed air can be introduced to adjust the aggressiveness of the wet shot peening. This controlled air pressure produces a mist of wet shot. With separate control for air pressure and media consumption, vapor shot peeners use significantly less water and media compared to venturi and slurry shot peeners, offering a broad pressure range and finer control over the wet shot peening process.
Centrifugal wheel shot peening systems operate by feeding media into the center of a rotating turbine impeller. Blades or paddles on the centrifugal wheel accelerate and project the abrasive media outward. Adjustments around these spinning blades help shape the media stream and control the shot peening pattern, including the size and position of the "hot spot." The width of the shot peening pattern can vary from a few inches to several feet, depending on the dimensions of the wheel, its speed, and the distance between the wheel and the part being processed.
The hot spot on the peened surface indicates where the media flow is most concentrated. This area becomes noticeably warm and shows the operator the center of the shot peening impact zone. Hot spots represent the most intense part of the shot peening stream, with the highest density of impacts. If the parts are not warm to the touch, the operator can adjust the pattern to reposition the hot spot.
Centrifugal wheel shot peeners are particularly effective for handling heavy shot media, such as cut wire steel shot, stainless steel shot, and cast steel shot. Besides peening, round steel shot is also useful for cleaning castings and descaling forgings, which are typically achieved through blasting processes.
The characteristics of the media, including its shape and hardness, or even the decision to use media at all, are key factors in controlling the shot peening process. Spherical media such as steel, stainless steel, glass beads, or ceramic balls apply compressive residual surface stresses to the component. Steel and stainless steel media types include cast shot and conditioned cut wire. While cut wire and steel grit have sharp edges that are effective for abrasive blast cleaning, they are less suitable for peening.
Although most shot peening equipment relies on centrifugal wheels or pressure systems, alternative technologies such as lasers and ultrasonic energy are also used. Since J. Almen's introduction of projectile-based shot peening in the 1930s, less aggressive methods have emerged that do not utilize traditional media. These newer techniques leverage advancements in sound and light to achieve effective peening results.
Laser peening systems use a tape, water, or coating layer to create a pressure or shock wave on the surface. This pressure wave introduces compressive residual stresses to a depth of up to 0.47 inches (12 mm) with minimal cold working, compared to the typical 0.2 inches (6 mm) depth achieved by traditional shot peening. This greater depth results in significant strength improvements and enhances galling and spalling resistance of alloys.
Laser peening allows for precise and selective control without the need for masking. It produces minimal cold work and is advantageous at elevated temperatures. As a media-free process, laser peening can handle complex geometries and sharp edges, offering consistent, high-quality results with digital control and no wear on media.
Vibratory and ultrasonic methods can also be employed to selectively shot peen the surfaces of components. A small chamber is constructed around the area of the part needing peening, with an ultrasonic transducer integrated into the chamber’s side or base. The peening media on the transducer behaves like a liquid in an ultrasonic cleaner. The transducer generates ultrasonic waves, causing the media to create rapid impacts on the part and the chamber walls.
Ultrasonic shot peening is effective for treating surfaces within cavities or holes that might be difficult to reach with traditional impact cleaning methods or peening lances.
Ultrasonic energy can be utilized to alter crystal structures or to develop a surface layer with nanoscale crystals through a process known as ultrasonic nanocrystal surface modification (UNSM). Additionally, researchers are exploring the integration of laser technology with ultrasonic methods, such as laser-assisted ultrasonic nanocrystal surface modification (LA-UNSM).
Ultrasonic Needle Peening (UNP) - Ultrasonic needle treatment (UNT) or impact treatment (UIT) involves repeatedly driving a pin or needle into a surface at ultrasonic frequencies ranging from 10 to 60 kHz. This method allows for highly targeted peening of specific areas on a part without the need for masking. It is particularly effective on welds or stress concentrations resulting from excess material. Although no peening media is used, the carbide needle will need periodic replacement.
Cavitation Peening - Cavitation peening, also known as water jet peening or ultrahigh-pressure water peening, operates at extremely high pressures of 20,000 psi or more. The high-pressure stream is directed through a cavitation nozzle plate to create vaporized water or cavitation bubbles intentionally. When these bubbles collapse upon a metal surface, they produce a pressure shock wave akin to that generated in laser peening. This shock wave can exceed 145 ksi in pressure, inducing compressive residual stresses in the metal.
As there is no media involved, water jet peening can access crevices, corners, and complex shapes. Unlike shot peening and laser peening, cavitation peening is a cooler process with no heat generated.
Pin or Dot Peen Marking - Pin marking, also known as dot peening, involves impacting a small pin onto a surface to create dimples that form text, graphics, codes, and other identifying marks. The pin or stylus used in dot peening is typically made from tungsten carbide or diamond and is driven pneumatically or electrically. This process is highly controlled and precise.
Unlike shot peening, dot peening is not a surface engineering technique but a method for marking parts. It offers an advantage over scribing or cutting methods, which can introduce tensile stresses. Dot peening generates marks with very low or compressive stress. It is particularly favored for marking flight-critical aerospace components due to its controllability and the favorable stress profile it produces.
Rotary Flap Peening - Rotary flap peening employs carbide shot that is bonded to polymer fabric flaps. These flaps are mounted onto a mandrel or wheel, which is then attached to a rotary power tool. The tool spins the flaps at high speeds, allowing the operator to flap peen the metal surface effectively.
Flap peening is advantageous for targeting specific areas of a surface without the need for masking. It is particularly effective for peening the inner diameters of holes by rotating the flaps at higher speeds.
Ball Peen Hammer Peening - For many years, ball peen hammers were commonly used for peening before shot peening equipment became available. Although the term "ball peen hammer peening" is used, any type of hammer can be employed to manually peen a part's surface. Traditionally, blacksmiths and metalworkers have used small hammers to peen forgings and castings, thereby hardening and strengthening their surfaces.
Often, welds are hammered to clean and enhance the surface, but manual hammering is generally considered uncontrolled and is discouraged by manufacturing standards and codes. Research is ongoing into robotic hammer peening (RHP), although no commercial systems are currently available.
Hammering is occasionally used to correct bent or distorted parts. However, it can lead to significant musculoskeletal strain on the operator. Shot peening offers a safer alternative for straightening bent parts without causing harm to the worker.
Shot Peen Forming - Shot peen forming machines are employed to straighten or bend parts, and are particularly effective for creating large radius curves in components such as wings, ships, storage tanks, or process vessels.
Wheel shot peening systems are suitable for forming large, less complex parts like wings, while air peening is used for parts with intricate features, as the nozzle can direct media into tight spaces. Ultrasonic and laser peen forming processes are also capable of creating curved shapes in parts.
Parts can be peen-formed either as received, prestrained, or clamped to a fixture for preshaping. The radius of curvature must stay within the elastic range of the metal, and abrupt changes in curvature are not permissible.
Strain Peening - Strain peening involves applying a preloading or prestressing force to a part, inducing surface tensile stress up to its elastic limit before engaging in shot peening. During the shot peening process, the surface yields plastically as the shots create dimples, stretching the metal beyond its yield point. This combination of tensile preloading and impact loading results in higher levels of compressive residual stress within the part.
Dual Peening and Multi-Peening - Techniques such as dual peening, tri-peening, and multi-peening enhance fatigue strength more effectively than conventional single-pass peening. These methods involve peening the metal surface two, three, or multiple times, respectively, using progressively finer media and reduced intensities. Multiple passes with decreasing media sizes help to cold work and refine peaks that were not adequately addressed in previous passes.
Shot peening equipment consists of shot peening equipment or abrasive shot peening packages preconfigured with the required components or be modular. A shot peening system can be specifically configured for an 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 comes in various sizes and configurations tailored to specific applications. Factors such as the dimensions of the parts being treated, the intended function, and the environment (laboratory, workshop, production line, or field/remote locations) influence the choice and design of the equipment.
Benchtop Micro Shot Peening – Micro shot peening, or fine particle peening, utilizes nozzles with diameters ranging from 0.018 to 0.125 inches. Some of these nozzles are crafted from sapphire or single crystal alumina for increased durability.
Micro shot peening systems can be benchtop models or floor-mounted units. Handheld pencil shot peeners are another type of micro shot peening equipment, typically featuring a small gravity-fed media cup.
While conventional shot peening uses steel shot media with diameters of 600 to 800 microns, micro shot peening involves media sizes between 10 and 200 microns.
To achieve similar impact or kinetic energy (K = ½ mv²) with finer media, the velocity must be higher. A specialized technique known as WPC treatment propels micro-shot at speeds up to 450 miles/hour (200 meters/second), compared to the 150 miles/hour (70 meters/second) of conventional shot peening. This high-speed impact not only mechanically deforms but also heats the surface, effectively microforging it. WPC stands for “wonder process craft” or wide peening and cleaning.
The microforging action can also be used for peen plating, where another metal or solid lubricant is mechanically deposited onto the surface. This method offers unique surface properties that traditional plating or thin film deposition cannot achieve. Solid lubricants such as molybdenum disulfide can be embedded into surfaces. Peen plating, also known as impact plating, mechanical plating, particle impact plating (PIP), cementation plating, dry plating, and mechanical deposition, avoids hydrogen embrittlement that can occur with wet plating of hardened high carbon steels with Rockwell C hardness greater than 40.
Micro peening creates small dimples with a higher density per unit area, resulting in a lower Ra or surface roughness average compared to conventional shot media. This microscale peening is highly effective in reducing friction.
Manual Shot Peening Cabinets or Cabinet Shot Peeners – Manual shot peening cabinets are equipped with two holes fitted with rubber gloves, allowing operators to handle parts and control the shot peening nozzle at the proper distance to generate and move a hot spot across the surface for cleaning or etching.
These cabinets feature a window and internal lighting to enable operators to view the parts and equipment while protecting themselves from shot peening debris. Some models also blow air across the window to prevent dust accumulation and maintain clear visibility.
In shot peening cabinets, spent media falls through the steel or fiberglass grate and into a collection hopper. From there, the used media is typically conveyed to a media separator for recovery. Economy shot peening cabinets may skip this separation step and directly reuse the spent media. Without proper separation, dust and finer fragments can be reused, potentially reducing efficiency and causing damage to filters.
Ideally, both the media and the dust generated during shot peening should be contained within the cabinet and its filtration system. Over time, wear and tear on the seals of the cabinet can lead to leaks. A common issue is leakage around the doors, as these are not always tightly sealed along their entire perimeter.
A new patent-pending technology addresses this problem with “leak or warp” proof cabinet doors, featuring double panels and two rigid steel channels. These doors have an enhanced closing mechanism that clamps shut along the edges, ensuring a positive seal with a knife-edge design each time they close.
Shot peening cabinets can be equipped with various door configurations, including front, top, and side openings. Some cabinets have multiple doors, allowing for the simultaneous loading of a dirty part and removal of a cleaned part. The doors may swing open, lift up, or slide to accommodate different operational needs.
Pass-through cabinets are specifically designed for processing glass sheets or metal plates. These systems feature a narrow gap with special brush or rubber flap seals to prevent abrasive leakage. Parts are pushed into the gap and transported through the cabinet on rollers.
Manual shot peening cabinets are well-suited for use in machine shops, garages, body shops, light production runs, part touch-ups, prototyping, and custom projects.
Production shot peening equipment is designed to meet higher quality standards and withstand the rigors of industrial environments. These systems are configured to handle large volumes of parts or oversized components such as castings, forgings, extrusions, or structural shapes.
Production shot peening setups often include multiple shot peening guns, each positioned to target specific areas on a part. For instance, a system with 12 guns will have a high rate of compressed air and shot peening media consumption.
Automated vs. Manual – Production shot peening systems can either be manually operated or automated. Manual systems may be adequate for varying sizes, shapes, and production volumes, but automated systems are preferred for higher production quantities or stringent quality requirements.
Shot peening processes can be semi-automatic, fully automatic, or turnkey. In a semi-automatic system, operators manually load or position parts on a table or hanger. The shot peening chamber door is then closed, and the parts are rotated and peened with pre-set or programmed parameters. After the initial cycle, the operator may need to flip or reposition the part to ensure complete coverage of shadowed areas. In contrast, turnkey or fully automated systems handle part loading, handling, and manipulation, as well as shot peening parameters, automatically.
Automating part handling and gun manipulation enhances precision, allowing for controlled gun standoff and travel speed across the part surfaces. This results in more consistent shot peening and eliminates the need for touch-ups. The higher upfront cost of automation can be offset by reduced labor costs, increased throughput, fewer rejects and rework, and improved quality.
Fixed Station and Robotic Shot Peeners – In fixed station and robotic shot peening systems, the shot peening guns are mounted on robotic arms. Parts are either manually or automatically loaded into the machine. The robotic arm then maneuvers the gun to scan and peen the required areas.
Robotic shot peeners are commonly used in the aerospace and automotive industries for treating 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, such as tumble and table shot peeners, process groups of parts simultaneously. Pass-through systems, on the other hand, allow parts to move through an opening equipped with brush or rubber flap seals to prevent abrasive leakage.
Inline and Continuous Shot Peening Systems – For high-volume and continuous production, shot peening machines can be integrated directly into production lines. These systems are used to peen alloy strips, plates, or sheets, enhancing fatigue strength for demanding applications. Inline and continuous systems can handle various metal forms, including sheet, strip, plate, bar, rod, wire, wire rope, and structural steel (e.g., I-beams, channels, angles).
Large-scale inline shot peening systems are designed for production lines where materials pass by shot peening guns or wheels. These systems often feature automatic or remotely operated guns.
For continuous or semicontinuous stock materials, shot peening nozzles or wheels are positioned above and below the material. The system continuously peens the steel or stainless steel sheet as it moves along the production line via rollers.
Shot Peening Rooms – Shot peening rooms are designed for parts that are too large for shot peening cabinets, table shot peeners, or hanger shot peeners. These rooms are spacious enough to accommodate both an operator and a vehicle or material handler. Used shot peening media falls through the grating on the floor of the shot peening room and is conveyed mechanically or pneumatically to the reclaimer or separator.
Operators in shot peening rooms wear protective gear, including a shot peening suit with a hood, gloves, respirators or air supply, and hearing protection. The shot peening process is carried out using either a portable shot peening pot or a shot peening machine integrated into the shot peening room.
Media Separators and Dust Collectors – In most production shot peening rooms and systems, media separators are used to recover or reclaim used shot peening media. The recovered media is then returned to the shot peening media pot or bin. Dust and disintegrated media are directed to a dust collector and filtration system, which removes the dust for proper disposal.
Tumble Shot Peeners – Tumble shot peeners use a tumbling basket or continuous rubber belt to rotate parts during shot peening. Parts must be compatible with tumbling action, as delicate components with thin fins, protrusions, or intricate shapes may be damaged or become jammed. For such parts, hanger shot peeners or wire mesh belt peeners are preferred.
The basket should be adequately loaded to avoid excessive wear on the belt or basket. However, overloading is not recommended, as it may prevent the parts from receiving adequate shot peening and can potentially damage the equipment.
Table Shot Peeners – Table shot peening systems are used for cleaning heavy castings and forgings. The parts are mounted on a rotary table inside a shot peening chamber. With the chamber doors closed, the table rotates while the part is shot peened.
The underside of the part resting on the table is not exposed to the shot peening media, requiring the part to be flipped to clean the bottom surface.
Hanger Shot Peening Systems – In hanger shot peening systems, parts are suspended from hooks. This setup ensures that virtually all surfaces of the part are exposed to the shot peening stream, allowing for comprehensive treatment.
Wire Mesh Belt Shot Peeners – Wire mesh belts, made of wear-resistant manganese steel, convey parts past a stream of shot peening media. It is important not to run the belt without parts. When only a few parts need treatment, the belt should be loaded with dummy parts or scrap to minimize wear.
Monorail Shot Peening System – Monorail shot peening equipment features an overhead rail. Parts hanging from the monorail enter the shot peening machine through doors or pass-through openings. After shot peening, the parts exit the machine where they are removed from the monorail.
Roller Conveyor Shot Peening System – Roller conveyors are used for shot peening heavier metal stock such as billets, thick plates, and structural members (e.g., I-beams).
Portable Shot Peeners and Shot Peening Pots – Mobile and portable shot peening equipment is used for treating large surfaces or revitalizing parts in the field. Smaller portable shot peeners typically include a shot peening pot, air hoses, shot peening hoses, shot peening guns, and an air compressor. These components are lightweight and designed for easy transport to the site.
Internal Shot Peeners – Specialized tools or shot peening lances are designed for peening the inner diameter of holes, tubes, and hollow shafts. These tools feature collars to center the shot peening nozzle. A tungsten carbide deflecting tip directs the shot peening media against the inner wall of the pipe. Some internal shot peening units are equipped with rotating heads and multiple nozzles angled toward the interior surface of the pipe for effective treatment.
The construction of shot peening equipment involves parts such as cabinets, pressure vessels, hoses, guns, and nozzles, which are created using sheet metal fabrication, casting, welding, mechanical fastening, machining, and specialized processes. Shot peening cabinets and shot peening rooms typically start as fabricated metal boxes. These are made by cutting, bending, and forming steel sheets, plates, and structural steel into sides, legs, and doors to assemble the basic structure.
As the assembly progresses, shot peening guns, windows, glove ports, doors, turntables, grating or screens, gun or part holders, pneumatic valves, foot pedals, lights, hoses, and reclamation devices are added. This transformation converts the basic metal box into a highly effective industrial tool.
Shot peening cabinet parts can be either welded or fastened together. Fastening allows for easier removal of parts for cleaning, repair, and replacement. Welded shot peening cabinets, on the other hand, tend to be more airtight, reducing leakage of shot peening media and dust into the shop. However, replacement of worn cabinet sides or bottoms is more challenging in welded cabinets.
The abrasive shot peening stream wears down the bottom and sides of the cabinet over time. Additionally, seals and windows on the cabinet will age, wear, and eventually require replacement.
Shot peening cabinets and rooms designed for dry or air shot peening are usually crafted from steel and finished with powder coatings, zinc galvanizing, or industrial paint. Wet shot peening systems, on the other hand, use more corrosion-resistant materials, like stainless steel, to withstand moisture.
For certain dry shot peening tasks, such as those involving surgical tools and medical implants, stainless steel is chosen to prevent iron contamination on the treated surfaces.
When shot peening stainless steel components, stainless steel shot or alternative non-metallic media are typically used. Employing steel components or steel shot can result in metal transfer to the stainless steel, which might affect its passivation layer and lead to rust formation.
To extend the lifespan of shot peening machinery, wear-resistant steel liners or plates are installed in critical areas. These liners are made from durable alloys like manganese steel or nickel-chromium white cast iron, including Ni-Hard alloys.
Shot peening equipment experiences wear and tear due to the harsh action of the shot peening media. Components are subject to gradual degradation as media circulates through or over them. While some media types, such as steel shot, rounded cut wire shot, ceramic beads, and glass beads, can be reused multiple times through the equipment, they still contribute to wear.
As with any industrial machinery, maintaining shot peening equipment requires routine inspections to ensure proper functioning. Regular checks are crucial for abrasive shot peeners, wheel shot peeners, and other peening systems to monitor component wear. Changes in the internal diameter of nozzles or alterations in the geometry of throwing blades can significantly impact the efficiency of the equipment.
Some of the most durable materials include boron carbide, alumina, pure tungsten carbide, and silicon carbide ceramics.
Depending on the shot peening media, cemented tungsten carbide and SiAlON nozzles 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, costs 3 times as much as cemented WC but lasts 3 to 25 times longer than cemented WC or sialon. Nozzles made of boron carbide do not have the toughness and impact resistance of cemented tungsten carbide. Binderless tungsten carbide (WC) nozzles have double the life boron carbide nozzles.
In terms of wear resistance alone, boron carbide and binderless tungsten carbide can last up to seven times longer than cemented tungsten carbide. However, if a boron carbide or silicon nitride nozzle is struck against a part, grate, or cabinet wall, it is more prone to cracking compared to a cemented tungsten carbide nozzle.
The lifespan of a nozzle is influenced by the type of shot peening media it processes. Hard ceramic beads tend to cause more rapid wear on nozzles compared to spherical cast steel shot.
The fundamental operation of shot peening equipment is straightforward: it involves directing shot media at high velocity onto a component's surface to either remove material or induce plastic deformation. This continuous impact results in indentations, creating a compressed stressed layer. Various methods, including compressed air, pressurized water, ultrasonic energy, or centrifugal blast wheels, are used to propel the media at the part.
The shot or spherical media impacts the surface, causing plastic deformation and creating dents or dimples. This deformation introduces cold work and dislocations within the material, enhancing hardness and inducing compressive residual stress. This type of stress is the opposite of tensile residual stress, which is a positive stress that can reduce fatigue strength. Manufacturing processes such as welding, casting, machining, grinding, and heat treatments often generate tensile residual stresses, which shot peening equipment helps mitigate.
The impacts from the media flatten high points on rough surfaces, while on smoother surfaces, they create valleys, dimples, and peaks. Textured surfaces resulting from shot peening are often engineered to have lower friction compared to mirror-polished surfaces.
The degree of cold working or residual stress produced by shot peening is regulated by its “intensity” and “coverage.” Intensity is the key parameter for controlling the process and is assessed using an Almen strip and an Almen gauge. The intensity of the blast stream causes deformation in an Almen strip, which helps gauge the amount of compressive residual stress applied to the component. Factors such as shot size, hardness, density, and velocity influence the intensity of the shot peening. To ensure adequate compressive stress, the shot media must be as hard as the parts being peened; harder media typically induces higher compressive stress.
Coverage refers to the proportion of a surface that has been marked by the peening process, with full coverage being defined as 98% of the surface showing indentations. It is assessed using an Almen strip, which evaluates the degree of surface indentation by measuring the angles of the conical blast stream. Coverage measures the percentage of the part's surface area that has been impacted by the shot media. Effective shot peening is closely tied to achieving the right coverage, as both insufficient and excessive coverage can negatively affect the surface, fatigue strength, and longevity of a component. The Society of Automotive Engineers (SAE) in J2277_202301 provides guidelines on achieving proper shot peening coverage.
Shot peening equipment is employed to enhance and relieve stress in metal components, creating a fine-grained surface layer that helps to prevent cracks and other forms of damage. By inducing surface compressive stress, materials gain increased resistance to fatigue and corrosion. This enhancement in fatigue strength extends the number of cycles a part can endure before failure, making shot peening an indispensable process for producing durable and long-lasting products.
Peening and surface engineering offer a range of surface modifications, including:
Shot peening is particularly effective at creating dimpled textures on part surfaces, which can modify frictional properties and help retain lubricants.
Spherical or rounded media, such as cast shot, glass beads, or ceramic spheres, tends to produce a smoother surface profile compared to angular or crushed media.
Improving surface finish or reducing profile roughness can significantly enhance fretting fatigue strength, potentially increasing it by 20% to 200%. Initially, larger diameter or heavier cast shot is used to introduce a deep residual stress layer. Subsequently, the surface is peened with smaller spheres or microbeads to achieve a finer finish.
For parts with a rough surface finish, like those that are as-cast or as-forged, peening can moderately improve the surface texture. Conversely, if a part has been ground or machined to a smooth or low Ra finish, shot peening may result in a rougher surface. Peening can also help eliminate tool marks from machining or grinding.
Surface engineering involves altering the surface of a component to impart specific characteristics, thereby enhancing or enabling its performance in particular applications.
Shot peening involves impacting the surface of a part with spherical media made of steel, stainless steel, glass, or ceramics. This process strain-hardens the material and introduces compressive residual stresses on the surface. Typically, cast steel shot with a Rockwell C hardness of 40 to 55 is used.
The application of compressive residual stress through shot peening can enhance the fatigue strength of components by 30% to 500%. This increased strength and resistance to stress-corrosion cracking is crucial for various applications, including fasteners, gears, axles, dies, molds, shafts, springs, aircraft landing gear, and other rotating and structural components.
Additionally, the residual stresses from shot peening can be utilized to peen form and straighten shafts to precise tolerances. The process also increases surface hardness and wear resistance, helps close surface porosity, and can detect and remove hidden sub-surface corrosion, particularly around fasteners.
Shot peening can create textured surfaces by producing patterns of dimples or depressions. This dimpled texture enhances the ability of surfaces to retain lubricants, grease, inks, or other fluids. Additionally, peened textures can modify the gripping and frictional properties of surfaces.
Shot peening equipment is utilized for enhancing surface textures and applying impact forces. These machines are designed to refine surfaces with high precision, improving their performance. The process involves the rapid propulsion of media at high speeds, which significantly transforms surfaces and is applicable across various industrial sectors.
Aerospace – In the aerospace industry, shot peening is essential for inducing compressive residual stresses and minimizing distortions in structural components such as landing gear, turbine disks, jet engine blades, aircraft wheels, spars, ribs, chords, and stringers.
Components like wings, winglets, stabilizers, and other aerodynamic surfaces undergo peen forming to achieve the desired curvature.
Given its critical role in enhancing fatigue resistance in aerospace components, shot peening is governed by numerous aerospace, military, and industrial regulations that ensure its effectiveness.
Automotive OEM – Numerous engine components undergo shot peening to enhance their material characteristics and prolong their lifespan. Parts such as camshafts, coil springs, connecting rods, crankshafts, engine components, and leaf springs benefit from this treatment to boost their durability.
The 1930s saw the first industrial application of shot peening with automotive leaf springs. Springs that have been peened can endure up to a million cycles, compared to just 250,000 cycles for those that haven't been treated.
Foundries and Forges – In the manufacturing of metal castings and forgings used in high-stress rotating machinery, shot peening can enhance their resistance to fatigue, improving the overall performance and longevity of these components.
Machine Shops / Manufacturing – Shot peening enhances the fatigue resistance and durability of machined or ground mechanical components and tools. This process is beneficial for improving the performance of various items, including:
Medical / Dental – Techniques such as dimpling, micro-dimpling, and shot peening are employed to enhance the functionality of medical devices and dental restorations. Implants for hips, shoulders, bones, and joints are subjected to shot peening to increase their fatigue resistance and overall longevity.
Welding, Brazing & Soldering – Post-joining shot peening enhances the characteristics of the weld and the heat-affected zone (HAZ) by introducing compressive residual stresses. This can counteract the tensile stresses that often arise during the welding process. It is crucial to ensure that the welding process does not embrittle the weld and HAZ, as this could lead to cracking during peening.
The primary aim of shot peening equipment is to enhance the longevity of components by applying a compressive layer to their surfaces. This process has become integral to contemporary manufacturing due to the growing need for more durable and longer-lasting parts. The advantages of shot peening are extensive, offering various protective benefits and improvements in product performance.
By applying compressive stresses, shot peening can boost a surface's fatigue strength by 30% to 500% and extend the life of parts by as much as ten times.
While shot peening equipment offers numerous benefits, selecting the right equipment is not always a straightforward task. Like other industrial machinery, careful consideration is required to ensure the equipment meets the specific needs of its intended application.
The initial step in acquiring shot peening equipment is to identify a reputable and experienced supplier who can offer expert advice and support. Manufacturers typically provide guidelines and assistance, particularly for those purchasing such equipment for the first time.
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).
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