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Shot peening can be used to recondition distorted steel conveyor belts.
Shot peening is a conservation process for flattening a deformed steel belt in which the surface of the belt is impacted by small stainless steel or carbon steel balls, called peening shot. Each ball that strikes the belt acts as a peening hammer, forming a small indentation, or dimple, on the steel belt surface.
For the indentation to be formed, the steel belt surface layer must yield in tension. [1] [ clarification needed ] The compressed grains[ clarification needed ] help to restore the surface to its original shape by producing a hemisphere[ clarification needed ] of cold-worked metal, highly stressed in compression. Overlapping indentations create a continuous layer of residual compressive stress. It is well known that cracks will not propagate in a compressively stressed zone. Since most fatigue and stress corrosion failures originate at the surface, the compressive stresses from shot peening can extend the belt’s lifespan. [2] Note that:
Press belts may become deformed and worn over time. The portable shot blasting unit is primarily used to flatten deformed press belts and prepare the belt material for reuse. The unit is designed for field use and is portable, allowing for efficient operation. All necessary equipment (excluding the carriage frame and air compressor) fits into a box with dimensions of approximately 350 × 350 × 320 mm. The combined weight of the blaster, valve, air hose, and other components is about 25 kg, with the blasting machine itself weighing 9 kg.
A pair of universal channels (38 mm × 76 mm), typically 500 mm longer than the belt width, must be provided on-site. These channels are welded together to allow the blaster to move smoothly across the belt’s surface. The total installation time, including assembling the carriage frame, generally takes only a few hours, after which the peening process can begin. [5]
An electric shut-off valve is mounted on the inlet air hose to protect the belt from over-blasting should it suddenly stop during the blasting operation. The valve solenoid must be connected (interlocked) to the press machine's power supply to be effective. For best blasting results, an air supply of 4,200 liters per minute is required at a pressure of 6 bar. [6] The unit is supplied with a flexible air hose that connects it to the local air supply. All local supply pipes should have a minimum bore diameter of 1 inch. The recommended shot blasting medium is tungsten shot (beads) with a diameter ranging from 0.2 to 0.4 mm and a hardness exceeding 40 HRC. [7]
The machine operates by drawing a quantity of tungsten shot from the bottom of the scroll case into the high-velocity nozzles. The shot is blasted onto the surface of the belt, and most of the shot bounces back into the scroll case. The air is vented through the filter socks, and any shot carried with the air is filtered out and dropped back into the scroll case.
Since the 1980s, the standard procedure for addressing the issue of deformed belts has been to turn the belt over, using what was previously the back side to form the new product side. [8] This method flattens the belt by equalizing stresses on both sides. However, over time, the belt typically reverts to its original shape, albeit in the opposite direction. As a result, it often becomes necessary to turn the belt again after approximately one year [9] .
This process is time-consuming and expensive, requiring cutting the belt, dismantling it from the press, turning it, and then reinstalling it. The reinstallation involves various belt joining operations such as welding and grinding of the joint, as well as running-in procedures [10] . Additionally, this process demands specialized equipment for handling the belt, welding jigs, and skilled personnel for joint welding. Compounding the issue, production must be halted during these operations, with stoppages lasting up to a week not being uncommon.
To address the belt cross-curvature problem, the steel belt shot peening process was introduced as a potential solution
Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys.
Rebar, known when massed as reinforcing steel or steel reinforcement, is a tension device added to concrete to form reinforced concrete and reinforced masonry structures to strengthen and aid the concrete under tension. Concrete is strong under compression, but has low tensile strength. Rebar usually consists of steel bars which significantly increase the tensile strength of the structure. Rebar surfaces feature a continuous series of ribs, lugs or indentations to promote a better bond with the concrete and reduce the risk of slippage.
In materials science, fatigue is the initiation and propagation of cracks in a material due to cyclic loading. Once a fatigue crack has initiated, it grows a small amount with each loading cycle, typically producing striations on some parts of the fracture surface. The crack will continue to grow until it reaches a critical size, which occurs when the stress intensity factor of the crack exceeds the fracture toughness of the material, producing rapid propagation and typically complete fracture of the structure.
Extrusion is a process used to create objects of a fixed cross-sectional profile by pushing material through a die of the desired cross-section. Its two main advantages over other manufacturing processes are its ability to create very complex cross-sections; and to work materials that are brittle, because the material encounters only compressive and shear stresses. It also creates excellent surface finish and gives considerable freedom of form in the design process.
An air line is a tube, or hose, that contains and carries a compressed air supply. In industrial usage, this may be used to inflate car or bicycle tyres or power tools worked by compressed air, for breathing apparatus in hazardous environments and to operate many other pneumatic systems.
Shot peening is a cold working process used to produce a compressive residual stress layer and modify the mechanical properties of metals and composites. It entails striking a surface with shot with force sufficient to create plastic deformation.
In materials science and solid mechanics, residual stresses are stresses that remain in a solid material after the original cause of the stresses has been removed. Residual stress may be desirable or undesirable. For example, laser peening imparts deep beneficial compressive residual stresses into metal components such as turbine engine fan blades, and it is used in toughened glass to allow for large, thin, crack- and scratch-resistant glass displays on smartphones. However, unintended residual stress in a designed structure may cause it to fail prematurely.
Surface finishing is a broad range of industrial processes that alter the surface of a manufactured item to achieve a certain property. Finishing processes may be employed to: improve appearance, adhesion or wettability, solderability, corrosion resistance, tarnish resistance, chemical resistance, wear resistance, hardness, modify electrical conductivity, remove burrs and other surface flaws, and control the surface friction. In limited cases some of these techniques can be used to restore original dimensions to salvage or repair an item. An unfinished surface is often called mill finish.
A foundry is a factory that produces metal castings. Metals are cast into shapes by melting them into a liquid, pouring the metal into a mold, and removing the mold material after the metal has solidified as it cools. The most common metals processed are aluminum and cast iron. However, other metals, such as bronze, brass, steel, magnesium, and zinc, are also used to produce castings in foundries. In this process, parts of desired shapes and sizes can be formed.
The Bauschinger effect refers to a property of materials where the material's stress/strain characteristics change as a result of the microscopic stress distribution of the material. For example, an increase in tensile yield strength occurs at the expense of compressive yield strength. The effect is named after German engineer Johann Bauschinger.
Laser peening (LP), or laser shock peening (LSP), is a surface engineering process used to impart beneficial residual stresses in materials. The deep, high-magnitude compressive residual stresses induced by laser peening increase the resistance of materials to surface-related failures, such as fatigue, fretting fatigue, and stress corrosion cracking. Laser shock peening can also be used to strengthen thin sections, harden surfaces, shape or straighten parts, break up hard materials, compact powdered metals and for other applications where high-pressure, short duration shock waves offer desirable processing results.
In metalworking, clinching or press-joining is a bulk sheet metal forming process aimed at joining thin metal sheets without additional components, using special tools to plastically form an interlock between two or more sheets. The process is generally performed at room temperature, but in some special cases the sheets can be pre-heated to improve the material ductility and thereby avoid the formation of cracks during the process. Clinching is characterized by a series of advantages over competitive technologies:
Oxy-fuel welding torch and oxy-fuel cutting are processes that use fuel gases and oxygen to weld or cut metals. French engineers Edmond Fouché and Charles Picard became the first to develop oxygen-acetylene welding in 1903. Pure oxygen, instead of air, is used to increase the flame temperature to allow localized melting of the workpiece material in a room environment.
Sandblasting, sometimes known as abrasive blasting, is the operation of forcibly propelling a stream of abrasive material against a surface under high pressure to smooth a rough surface, roughen a smooth surface, shape a surface or remove surface contaminants. A pressurised fluid, typically compressed air, or a centrifugal wheel is used to propel the blasting material. The first abrasive blasting process was patented by Benjamin Chew Tilghman on 18 October 1870.
Low plasticity burnishing (LPB) cold compresses metal to provide deep, stable surface residual stresses to improve damage tolerance and extend metal fatigue life; mitigating surface damage, including fretting, corrosion pitting, stress corrosion cracking (SCC), and foreign object damage (FOD). Improved fretting fatigue and stress corrosion performance has been documented, even at elevated temperatures where the compression from other metal improvement processes: low stress grinding (LSG) etc. relax. The resulting deep layer of compressive residual stress has also been shown to improve high cycle fatigue (HCF), low cycle fatigue (LCF), and stress corrosion cracking (SCC) performance.
Steel abrasives are steel particles that are used as abrasive or peening media. They are usually available in two different shapes that address different industrial applications.
Ultrasonic impact treatment (UIT) is a metallurgical processing technique, similar to work hardening, in which ultrasonic energy is applied to a metal object. This technique is part of the High Frequency Mechanical Impact (HFMI) processes. Other acronyms are also equivalent: Ultrasonic Needle Peening (UNP), Ultrasonic Peening (UP). Ultrasonic impact treatment can result in controlled residual compressive stress, grain refinement and grain size reduction. Low and high cycle fatigue are enhanced and have been documented to provide increases up to ten times greater than non-UIT specimens.
The high-frequency impact treatment or HiFIT – Method is the treatment of welded steel constructions at the weld transition to increase the fatigue strength.
A steel belt is a type of conveyor belt used in many industries such as food, chemical, wood processing, and transportation.
In metallurgy, peening is the process of working a metal's surface to improve its material properties, usually by mechanical means, such as hammer blows, by blasting with shot, focusing light, or in recent years, with water column impacts and cavitation jets. With the notable exception of laser peening, peening is normally a cold work process tending to expand the surface of the cold metal, thus inducing compressive stresses or relieving tensile stresses already present. It can also encourage strain hardening of the surface metal.
