What is shot peening
Shot peening is a process that uses high-speed sand shots and iron shots to hit the surface of the workpiece to improve part of the mechanical properties of the parts and surface state changes.
Shot peening can be used to improve the mechanical strength, wear resistance, fatigue resistance and corrosion resistance of parts.
It can also be used for surface matting, descaling and eliminating residual stresses in casting, forging and welding etc.
Shot blasting to remove old paint and rust on the surface of steel wheels
Types of shot peening
Shot peening is divided into shot peening and shot blasting.
Surface treatment with shot blasting has a great impact force and obvious cleaning effect.
However, the treatment of thin plate workpieces by shot peening can easily deform the workpiece, and the steel shot hits the surface of the workpiece (no matter shot blasting or shot peening) to deform the metal substrate.
Because Fe3O4 and Fe2O3 have no plasticity, they peel off after being broken.
The oil film is deformed together with the substrate, so the oil cannot be completely removed by shot blasting or air blasting for oil-contaminated workpieces.
Among the existing surface treatment methods for workpieces, the best cleaning effect is sandblasting.
Shot blasting is suitable for cleaning the workpiece surface with higher requirements.
However, the current general sandblasting equipment in China is mostly composed of primitive and heavy sand conveying machinery such as hinge dragon, scraper, bucket and elevator etc.
Users need to construct a deep pit and make a waterproof layer to install the machinery, which has higher construction cost, larger maintenance workload and higher maintenance cost.
Moreover, a large amount of silica dust produced 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 processing, the steel plate is in a free state, and high-speed steel shot is used to strike the inside of the steel plate to generate pre-compression stress on the surface, which 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 perform shot peening.
Types of shot peening equipment
There are two main types of shot peening equipment:
1. Mechanical centrifugal shot penning machine
Suitable for parts that require high shot peening strength, small variety, large batch, simple shape and large size.
2. Pneumatic shot blasting machine (compressed air type)
Suitable for parts that require moderate shot peening strength, complex shapes and small sizes.
Introduction of the projectile
1. Cast steel shot
The 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, which is brittle and easy to break with a short lifespan, which is not widely used.
It is mainly used on occasions where the high intensity of shot peening is required.
3. Glass bead
The hardness is lower than the former two, which is mainly used for titanium, aluminum, magnesium and other materials that do not allow iron pollution.
It can also be used for the second processing after steel shot peening to remove iron pollution and reduce the surface roughness of parts.
4. Ceramic bead
The chemical composition of the ceramic bead is roughly 67% ZrO2, 31% SiO2 and 2% Al2O3 as the main inclusions, which are made by melting, atomizing, drying, rounding, and sieving.
The hardness is equivalent to HRC57~ 63.
Its outstanding performance is higher density and higher hardness than glass.
It was first used to strengthen aircraft parts in the early 1980s.
Ceramic beads have higher strength, longer life than glass beads with lower prices.
They have now been extended to the surface strengthening of non-ferrous metals such as titanium alloys and aluminum alloys.
Introduction to the nozzles of blasting machines
1. Straight barrel nozzle
Straight barrel nozzle has a simple structure, its internal structure only has two parts: contraction and flat section.
2. Venturi nozzle
The venturi nozzle is divided into three parts: the contracting section, straight section and diffusion section, which is more difficult to make.
3. Double venturi nozzle
Double venturi nozzles have a front and back, with a spacing between the two, and several small holes around the spacing.
4. Square hole nozzle
A nozzle with square inlet and outlet ends has been developed abroad.
Various tests show that this nozzle is more efficient and more economical than the venturi nozzle.
Parameters affecting shot peening quality
Shot peening intensity
The process parameters that affect the strength of shot peening mainly include: projectile diameter, projectile velocity, projectile flow rate and projectile time etc.
Influencing factors of coverage rate: hardness of parts material, projectile diameter, spray angle and distance and projectile time etc.
Influencing factors of roughness: the strength and hardness of the part material, the diameter of the projectile, the spray angle and speed, and the original surface roughness of the part.
Characteristics of parts after shot peening
The size of the residual compressive stress on the sprayed surface of the part and the depth of the compressive stress layer depends on the properties of the sprayed material and the peening strength.
The higher the strength and hardness of the material, the greater the compressive stress and the shallower the depth of the compressive stress layer.
The higher the shot peening strength, the greater the depth of the compressive stress layer.
Changes in the material organization of the blasted surface layer
The roughness of the blasted surface: The roughness of the blasted surface becomes worse with the increase of shot peening strength, the decrease of surface hardness and the decrease of shot size.
Size increase: The metal on the sprayed surface is extruded to form tiny metal wave crests, so the size increases.
Application range of shot peening
Shot peening can prevent bending fatigue of automobile parts;
Shot peening can improve the bending fatigue properties of the tooth profile root;
Shot peening can improve the fatigue resistance and safe service lifespan of the crankshaft;
Shot peening can increase the strength and rigidity of the connecting rod;
The powerful shot peening process can improve the bending fatigue strength and contact fatigue strength of gear teeth, which is an important way to improve gear anti-seizure ability and increase gear life;
Surface shot peening is an effective surface strengthening process to improve the stress corrosion resistance and hydrogen embrittlement fracture of fasteners.
After shot peening, the reliability and durability of the workpiece can be significantly improved.
Surface shot peening achieves the purpose of improving fatigue performance (including stress corrosion performance) by changing the structure of the material, which can improve the quality of fasteners.
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