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What is shot blasting

What is shot blasting

What is shot blasting?

Shot blasting is a process that uses compressed air as power to spray shot onto the workpiece surface and clean or strengthen the workpiece surface. The medium used for shot blasting is round particles without edges and corners, such as steel shot, iron shot, glass shot and ceramic shot. Shot blasting relies on the strong impact force of shot particles to make the workpiece surface closer, so as to make the workpiece more wear-resistant, ductile and corrosion-resistant.

Shot blasting, also known as shot blasting strengthening, is one of the effective methods to reduce fatigue and improve service life of parts. Shot blasting is to spray high-speed projectile flow onto the surface of parts, resulting in plastic deformation on the surface of parts, forming a strengthening layer with a certain thickness. High residual stress is formed in the strengthening layer. Due to the existence of compressive stress on the surface of parts, When the parts bear load, it can offset part of the stress, so as to improve the fatigue strength of the parts.

Shot blasting is a surface strengthening process widely used in factories, that is, the cold working process of bombarding the workpiece surface with shot particles and implanting residual compressive stress to improve the fatigue strength of the workpiece. It is widely used to improve the mechanical strength, wear resistance, fatigue resistance and corrosion resistance of parts.

The advantages of shot blasting are simple equipment, low cost, not limited by workpiece shape and position, and convenient operation. The disadvantages are poor working environment, low unit output and lower efficiency than shot blasting. The types of shot blasting include steel shot, cast iron shot, glass shot, ceramic shot, etc.

Types of pills

Pills

Pills are generally spherical particles without edges and corners, such as steel wire cut pills.

Cast steel shot

Its hardness is generally 40 ~ 50HRC. When processing hard metals, the hardness can be increased to 57 ~ 62HRC. Cast steel shot has good toughness and is widely used. Its service life is several times that of cast iron shot

Cast iron shot

Its hardness is 58 ~ 65HRC, brittle and easy to break. Short service life and not widely used. It is mainly used for occasions requiring high shot blasting strength

Glass pellet

The hardness is lower than the first two. It is mainly used for stainless steel, titanium, aluminum, magnesium and other materials that do not allow iron pollution. It can also be used for secondary processing after steel shot blasting to remove iron pollution and reduce the surface roughness of parts.

Ceramic pill

The chemical composition of ceramic pellets is approximately 67% ZrO2, 31% SiO2 and 2% Al2O3 inclusions, which are made by melting, atomizing, drying, rounding and screening, and the hardness is equivalent to HRC57 ~ 63. Its outstanding properties are higher density and hardness than glass. It was first used for aircraft parts strengthening in the early 1980s. Ceramic pellets have higher strength, longer service life and lower price than glass pellets. Now they have been extended to the surface strengthening of non-ferrous metals such as titanium alloy and aluminum alloy.

Therefore, there are four categories of shot: cast steel shot, cast iron shot, glass shot and ceramic shot

Special note:

Glass shot for shot blasting and glass shot for other purposes are two different concepts. The biggest feature of shot peened glass shot is that its minimum hardness is not less than 6-7 Mohs, and it has certain toughness, and the minimum rounding rate is not less than 90%. The road reflective glass shot has no requirements for hardness. Generally, ordinary glass can be used as raw material, and the rounding rate is required to be at least 75%.

The two prices are very different, but the appearance is almost the same. If ordinary glass shot is used for shot blasting, it seems that the cost is low. When shot blasting, the crushing rate is high and the workpiece with high strength is almost broken at one time. In contrast, the total cost is much higher.

Characteristics of shot blasting

  • 1) Metallic or non-metallic projectiles can be used arbitrarily to meet the different requirements of cleaning the workpiece surface;
  • 2) The cleaning is flexible, easy to clean the inner and outer surfaces of complex workpieces and the inner wall of pipe fittings, and is not limited by the site. The equipment can be placed near super large workpieces;
  • 3) The equipment has simple structure, less investment, less vulnerable parts and low maintenance cost;
  • 4) A high-power air compressor station must be equipped, which consumes more energy under the condition of the same cleaning effect;
  • 5) The cleaned surface is prone to moisture and rust regeneration;
  • 6) Low cleaning efficiency, many operators and high labor intensity.
strong shot blasting machine

▲ strong shot blasting machine

The advantages of shot blasting are simple equipment, low cost, not limited by workpiece shape and position, and convenient operation. The disadvantages are poor working environment, low unit output and lower efficiency than shot blasting. The types of shot blasting include steel shot, cast iron shot, glass shot, ceramic shot, etc.

Gear shot blasting strengthening: it is mainly to impact the surface of parts with the help of high-speed bullets to make them undergo elastic-plastic deformation, resulting in favorable changes such as residual compressive stress, work hardening and microstructure refinement, so as to improve the bending fatigue strength and contact fatigue strength of gears. It is an important way to improve the anti bite ability of gears and improve the service life of gears.

Gear shot blasting strengthening

During shot blasting, the small-size spherical steel shot strikes the surface of the workpiece to form a compressive stress. The impact of each shot will produce a certain plastic deformation of the metal, and the final surface can not completely recover to form a permanent compressive stress state.

As a surface strengthening process, shot blasting can form residual compressive stress on the surface, which is equivalent to 55% ~ 60% of the tensile strength limit of the material, and the workpiece surface is the place where cracks are easy to initiate. For carburized and quenched gears, the compressive stress can reach 1177 ~ 1725mpa, which can greatly improve the fatigue performance.

Improving surface hardness of carburized gear by gear shot blasting

Improving surface hardness of carburized gear by gear shot blasting

For example, the shot blasting equipment of FAW heat treatment branch adopts German tr5svr-1 stress shot blasting equipment. Shot blasting process: use steel shot with diameter of ¢ 0.8mm, shot blasting time of 9min and shot blasting speed of 2800r / min. The gear material is 22CrMoH steel, which is carburized, Quenched and tempered. After shot blasting, the surface structure of the gear is refined. The residual austenite content of the surface is about 10% lower than that of the workpiece without shot blasting, and the change is obvious within 0.15mm from the surface; The surface hardness of the gear after enhanced shot blasting is increased by 0.5 ~ 2hrc.

Shot blasting improves the fatigue life of gears

For example, FAW carried out the fatigue life test on the first gear of “Jiefang” automobile transmission by using the enhanced shot blasting process, which significantly improved the fatigue life of the gear. In order to improve the fatigue life of the “Jiefang” brand driving helical gear, the large arc hob is used to cut the teeth. Increasing the gear fillet can increase the service life of the driving helical gear from 208300 times to 695400 times. If enhanced shot blasting is used, the fatigue life can be increased to 2109000 times.

Classification of shot blasting

Shot blasting is divided into shot blasting and sand blasting. The surface treatment with shot blasting has great striking force and obvious cleaning effect. However, the treatment of thin plate workpiece by shot blasting is easy to deform the workpiece, and the steel shot strikes the workpiece surface (whether shot blasting or shot blasting) to deform the metal substrate. Because Fe3O4 and Fe2O3 are not plastic, they are stripped after crushing, and the oil film is deformed together with the substrate, shot blasting and shot blasting cannot completely remove the oil stain on the workpiece with oil stain. Among the existing workpiece surface treatment methods, sand blasting has a good cleaning effect.

Sand blasting is suitable for cleaning the workpiece surface with high requirements. However, the general sand blasting equipment in China is mostly composed of original heavy sand conveying machinery such as hinged dragon, scraper and bucket elevator. Users need to build a deep pit and make a waterproof layer to install machinery. The construction cost is high, the maintenance workload and maintenance cost are great, and a large amount of silicon dust generated in the sand blasting process cannot be removed, which seriously affects the health of operators and pollutes the environment.

Shot blasting is divided into general shot blasting and stress shot blasting. During general treatment, when the steel plate is in a free state, the inside of the steel plate is hit with high-speed steel shot to produce preloading stress on its surface. To reduce the tensile stress on the steel plate surface and increase the service life. Stress shot blasting is the pre bending of the steel plate under a certain force, and then shot blasting.

Application of shot blasting

Shot blasting is used to remove oxide scale, rust, molding sand and old paint film on medium and large metal products with a thickness of not less than 2mm or not required to maintain accurate size and contour, as well as castings and forgings. It is a cleaning method before surface coating (plating). It is widely used in large shipyards, heavy machinery factories, automobile factories, etc.

Shot blasting is a cold treatment process, which is widely used to improve the fatigue resistance of metal parts in long-term service under high stress conditions, such as aircraft engine compressor blades, fuselage structural parts, automobile transmission system parts and so on.

Shot blasting strengthening is to spray the medium called steel shot at a high speed and continuously in a fully controlled state and hammer it to the surface of the part, so as to produce a residual compressive stress layer on the surface. Because when each steel shot hits the metal part, it is like a miniature rod hammering the surface, hammering out small indentation or depression. In order to form a depression, the metal surface must be stretched. Under the surface layer, the compressed grains try to restore the surface to its original shape, resulting in a hemisphere under the action of high compressive force. Numerous depressions overlap to form a uniform residual compressive stress layer. Finally, under the protection of compressive stress layer, the fatigue strength of parts is greatly improved and the safe working life is prolonged.

Difference from shot blasting and sand blasting

Difference between shot peening and shot blasting

Shot peening uses high-pressure air or compressed air as power, and shot blasting is generally a high-speed rotating flywheel to shoot out the steel sand at high speed. Shot blasting has high efficiency, but there will be dead corners, while shot blasting is more flexible, but the power consumption is large.

Although the two processes have different spraying power and methods, they both aim at high-speed impact on workpieces, and their effects are basically the same. In comparison, shot peening is relatively fine and easy to control accuracy, but the efficiency is not as high as shot blasting. It is suitable for small workpieces with complex shapes. Shot blasting is more economical and practical, easy to control efficiency and cost, and can control the particle size of shot to control the spraying effect, However, there will be dead corners, which is suitable for batch processing of workpieces with single shape and surface.

The selection of the two processes mainly depends on the shape of the workpiece and the machining efficiency.

Difference from sand blasting

Both shot blasting and sand blasting use high-pressure air or compressed air as power to blow it out at high speed and impact the workpiece surface to achieve cleaning effect, but the effect is also different due to different media. After sand blasting, the dirt on the workpiece surface is removed and the surface area is greatly increased, thus increasing the bonding strength between the workpiece and the coating.

The surface of the workpiece after sand blasting is of the natural color of metal, but because the surface is rough and the light is refracted, it has no metal luster and is a darkened surface.

After shot blasting, the dirt on the workpiece surface is removed, and the workpiece surface is slightly damaged, which is not easy to be damaged. The surface area has increased. Because the workpiece surface is not damaged in the machining process, the excess energy generated during machining will lead to the surface strengthening of the workpiece matrix.

The surface of the workpiece after shot blasting is also the natural color of metal, but because the surface is spherical and the light is partially refracted, the workpiece is processed into matte effect.

Quality evaluation of shot blasting

Cleaning quality level

a. Most thorough cleaning level (SA3)

The cleaned steel surface is completely silver gray, with a certain surface roughness to improve the adhesion of the coating.

b. Very thorough cleaning level (SA2.5)

The cleaned steel surface shall be free of grease, dirt, scale, rust, corrosion products, oxides and other impurities. Shadows and color differences due to incomplete cleaning are allowed, but at least 95% of the surface per square inch shall reach the level of the most thorough cleaning, and only slight shadows and color differences shall appear in the rest.

c. More thorough cleaning level

The cleaned steel surface shall be free of grease, dirt, rust scale and other impurities. The oxide scale, rust and old paint shall be removed. Light shadows and color differences due to incomplete removal of rust and oxide scale are allowed, and the area shall not exceed 33% per square inch; If pitting corrosion has occurred on the steel surface, a small amount of rust and old paint are allowed at the depth of the corrosion point.

d. Non thorough cleaning level

After the surface is completely cleaned, grease, dirt, loose oxide skin and loose paint skin are removed. Oxide skin, rust, paint and coating that are firmly bonded to the substrate and cannot be removed with a very sharp blade are allowed to remain on the surface after cleaning. A large number of evenly distributed metal spots appear on the surface.

Surface roughness

Surface roughness and surface cleanliness are produced at the same time. Determining the appropriate surface roughness is as important as determining the correct cleanliness requirements.

Effect of surface roughness

  • 1) The actual bonding area between the coating and the workpiece surface is increased, which is conducive to improving the bonding force of the coating;
  • 2) The coating will produce great internal stress during curing. The existence of roughness can effectively eliminate the stress concentration in the coating and prevent the coating from cracking;
  • 3) The existence of surface roughness can support the quality of some coatings and help to eliminate sagging, especially for vertically coated surfaces.

The factors affecting roughness are as follows:

  • 1) Abrasive particle size, hardness and particle shape;
  • 2) Hardness of workpiece material;
  • 3) Pressure and stability of compressed air;
  • 4) The distance between the nozzle and the workpiece surface and the included angle between the nozzle and the workpiece surface.

Several problems related to surface roughness:

  • 1) The cleaning time is almost independent of the surface roughness;
  • 2) The angle between the nozzle and the surface will affect the surface roughness, but the change is not obvious between 45 degrees and 90 degrees;
  • 3) Cleaning the difficult surface with large particle abrasive can improve the work efficiency, but the surface roughness will be high. The research shows that the roughness value caused by abrasive with particle size greater than 1.2mm is high. The surface with high roughness can be cleaned again with small grain abrasive to reduce the roughness to the specified requirements.

Shot blasting produces greater compressive stress on the surface than sand blasting.

Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)

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