Shot peening service
Shot peening is a surface strengthening process widely used in factories. It is a cold processing process that uses pellets to hit the workpiece surface and implant residual compressive stress to improve the fatigue strength of the workpiece. It is widely used to improve mechanical strength, wear resistance, fatigue resistance and corrosion resistance of parts.
Shot peening is also call Shot blasting is similar to sand blasting, but the abrasive used is different. The abrasive used in shot peening is steel shot or glass shot instead of sandblasting abrasive. Shot peening creates compressive stress on the part without the contamination of silicon-containing dust. It is mainly used to generate compressive stress on parts to improve their fatigue strength and stress corrosion resistance, and has a corrective effect on distorted thin parts, and the surface of shot peening is softer and softer than the surface of sandblasting. Shot peening is sometimes used to form large thin-walled aluminum parts.
Shot peening can be cast iron shot, cast steel shot or glass shot, ceramic shot. Cast iron shot has high hardness, but is brittle and fragile, and is mainly used in places where shot peening strength is required. Cast steel shot has good toughness, its life is several times longer than that of cast iron shot, and it is widely used. Glass shot and ceramic shot have the lowest hardness and are mainly used for stainless steel, titanium, aluminum or other parts where iron contamination is to be avoided. Sometimes after shot peening with cast steel shot, glass shot and ceramic shot are shot again to remove iron pollution or reduce surface roughness.
Shot peening, also known as shot peening, is one of the effective ways to reduce fatigue of parts and improve life. Shot peening is to inject high-speed projectile flow onto the surface of the part to plastically deform the surface of the part to form a strengthening layer of a certain thickness. A high residual stress is formed in the strengthening layer. Due to the existence of compressive stress on the surface of the part, part of the stress can be offset when the part is under load, thereby improving the fatigue strength of the part.
Shot peening is used to remove scale, rust, molding sand and old paint films on medium and large metal products with a thickness of not less than 2mm or which do not require accurate dimensions and contours, as well as castings and forgings. It is a cleaning method before surface coating (plating). Widely used in large shipyards, heavy machinery factories, automobile factories, etc.
Shot peening is to spray countless small circular media called steel shot at high speed and continuously under a fully controlled state, and beat it to the surface of the part, thereby generating a residual compressive stress layer on the surface. Because when each steel shot hits the metal part, it is like a miniature rod hitting the surface, making small indentations or dents. In order to form the depression, the metal surface must be stretched. Below the surface, the compressed grains try to restore the surface to its original shape, creating a hemisphere that is highly compressive. Numerous depressions overlap to form a uniform residual compressive stress layer. Ultimately, the part is protected by a compressive stress layer, which greatly improves the fatigue resistance and prolongs the safe working life.
Main classification of shot peening:
Shot peening is further divided into shot peening and sandblasting. Surface treatment with shot peening has a strong impact and obvious cleaning effect. However, the treatment of thin-plate workpieces by shot peening is easy to deform the workpiece, and the steel shot hits the surface of the workpiece (whether shot blasting or shot peening) to deform the metal substrate. Since Fe3O4 and Fe2O3 have no plasticity, they peel off after being broken, and the oil film and the The substrate is deformed together, so for workpieces with oil stains, shot blasting and shot peening cannot completely remove the oil stains. Among the existing workpiece surface treatment methods, sandblasting is the best cleaning effect.
Sandblasting is suitable for cleaning the workpiece surface with high requirements. However, most of the general sandblasting equipment in our country is composed of primitive and bulky sand conveying machinery such as auger, scraper, bucket elevator and so on. The user needs to build a deep pit and make a waterproof layer to install the machinery. The construction cost is high, the maintenance workload and maintenance cost are huge, and a large amount of silica dust generated during the sandblasting process cannot be removed, which seriously affects the health of the operators and pollutes the environment. .
Shot peening is divided into general shot peening and stress shot peening. In general treatment, when the steel plate is in a free state, hit the inside of the steel plate with high-speed steel shot to generate pre-compression stress on the surface. In order to reduce the tensile stress on the surface of the steel plate during work and increase the service life. Stress shot peening is to pre-bend the steel plate under a certain force, and then carry out shot peening.
There are 4 categories of shot (abrasive): cast steel shot, cast iron shot, glass shot, ceramic shot:
1, 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, and its service life is several times that of cast iron shot.
2, cast iron shot
Its hardness is 58~65HRC, brittle and easy to break. Short life, not widely used. Mainly used for occasions requiring high shot peening intensity.
3, glass pellets
The hardness is lower than the previous two, mainly used for stainless steel, titanium, aluminum, magnesium and other materials that do not allow iron pollution, and can also be used for the second processing after steel shot peening to remove iron pollution and reduce iron pollution. The surface roughness of the part.
4, ceramic pellets
The chemical composition of ceramic pellets is roughly 67% ZrO2, 31% SiO2 and 2% Al2O3-based inclusions, which are made by melting, atomizing, drying, rounding, and sieving. The hardness is equivalent to HRC57~ 63. Its outstanding properties are higher density and higher hardness than glass. It was first used in aircraft parts strengthening in the early 1980s. Ceramic pellets have higher strength, longer service life and lower price than glass pellets, and have been extended to surface strengthening of non-ferrous metals such as titanium alloys and aluminum alloys.
Shot blasting equipments:
1. Metal or non-metal projectiles can be used arbitrarily to meet the different requirements for cleaning the surface of the workpiece;
2. The cleaning flexibility is large, it is easy to clean the inner and outer surfaces of complex workpieces and the inner wall of pipe fittings, and it is not limited by the site, and the equipment can be placed near the super-large workpiece;
3. The equipment structure is relatively simple, the investment of the whole machine is less, the wearing parts are less, and the maintenance cost is low;
4. It must be equipped with a high-power air compressor station. Under the conditions of the same cleaning effect, the energy consumption is relatively large;
5. The cleaning surface is prone to moisture and easy to regenerate rust;
6. Low cleaning efficiency, many operators and high labor intensity
Difference from sandblasting:
Sand blasting vs shot blasting
Shot peening and sandblasting both use high-pressure air or compressed air as power, and blow it out at high speed to impact the surface of the workpiece to achieve the cleaning effect, but the effect is different depending on the selected medium. After sandblasting, the dirt on the surface of the workpiece is removed, and the surface area is greatly increased, thereby increasing the bonding strength of the workpiece and the coating/plating layer.
The surface of the workpiece after sandblasting is metallic, but because the surface is rough, the light is refracted, so there is no metallic luster, and it is a dark surface.
After shot peening, the dirt on the surface of the workpiece is removed, and the surface of the workpiece is very small and not easily damaged. The surface area has increased. Since the surface of the workpiece is not damaged during processing, the excess energy generated during processing will lead to surface strengthening of the workpiece matrix.
The surface of the workpiece that has been shot peened is also metallic, but because the surface is spherical, the light is partially refracted, so the workpiece is processed into a matte effect.
Cleaning quality level
a. The most thorough cleaning level (Sa3)
The surface of the cleaned steel is completely uniform 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 is free of grease, dirt, scale, rust, corrosion products, oxides and other impurities. Shadows and color differences due to incomplete cleaning are permitted, but at least 95% per square inch The above surface reaches the most thorough cleaning level, and the rest only has mild shadows and color differences;
c, more thorough cleaning level
There are no grease, dirt, rust and other impurities on the cleaned steel surface, and the oxide scale, rust and old paint are removed, and slight shadows and color differences due to incomplete removal of rust and oxide scale are allowed. No more than 33% per square inch; if pitting corrosion has occurred on the steel surface, a small amount of rust and old paint is allowed in the depth of the pit;
d. Non-complete cleaning level
The surface is thoroughly cleaned to remove grease, dirt, loose scale and loose paint, and scale, rust, paint and coatings that are firmly bonded to the substrate and cannot be removed with a very sharp spatula are allowed to remain after cleaning on the surface. A large number of evenly distributed specks of metal appear on the surface. [3]
Surface roughness
Surface roughness and surface cleanliness occur simultaneously, and determining the proper surface roughness is just as important as determining the correct cleanliness requirements.
The role of surface roughness
1) Increase the actual bonding area between the coating and the surface of the workpiece, which is beneficial to improve the bonding force of the coating;
2) The coating will generate a lot of internal stress during the curing process, and 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 a part of the paint, which is beneficial to eliminate the phenomenon of sagging, especially for vertically painted surfaces.
The factors that affect roughness are as follows:
1) The particle size, hardness and particle shape of the abrasive;
2) The hardness of the material of the workpiece itself;
3) Pressure and stability of compressed air;
4) The distance between the nozzle and the workpiece surface and the angle between the nozzle and the workpiece surface.
Several issues related to surface roughness:
1) The length of cleaning time has almost nothing to do with the surface roughness;
2) The angle between the nozzle and the surface will affect the surface roughness, but the change is not very obvious between 45 degrees and 90 degrees;
3) Cleaning hard-to-clean surfaces with large-grained abrasives can improve work efficiency, but the surface roughness will be high. Studies have shown that abrasives with a particle size greater than 1.2mm cause high roughness values. Re-cleaning the surface with high roughness with small-grained abrasive can reduce the roughness to the specified requirements.
Examples of Shot peening service
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