Shot blasting heat treatment of spring

Shot blasting is commonly called shot peening or shot peening. It is one of the effective methods to improve the fatigue life of mechanical parts and is widely used in spring manufacturing.
The principle of shot blasting is shown in Figure 1. Shot blasting is to spray the surface of the spring with a high-speed projectile stream to plastically deform the surface of the spring and form a surface strengthening layer with a certain thickness (Figure 1-a). From the point of view of the stress state, a higher residual compressive stress is formed in the strengthened layer (Figure 2-b). Due to the existence of residual compressive stress on the surface of the material, when the spring is subjected to a variable load, it can offset part of the maximum tensile stress under the action of the variable load, thereby improving the fatigue strength of the spring. From the organizational structure, a very high density of dislocations is formed in the strengthening layer (Figure 1-c). Under the subsequent variable stress, temperature, or both, the dislocations are gradually arranged regularly to form a polygon, that is, strengthening More tiny sub-grains (substructures) are gradually formed in the layer (Figure 1-d). The result of cold work hardening of this surface layer also has the effect of increasing fatigue strength.


a) The spring is in the process of shot blasting b) The stress distribution in the surface plastic deformation layer (strengthening layer) after strengthening

c) Organization (and dislocation) structure d) Changes in the organization (and dislocation) structure under the action of subsequent alternating loads (or temperature)

In addition, shot blasting can also remove defects (small defects, decarburization) on the spring surface, reduce stress concentration, etc., thereby eliminating or reducing fatigue sources.

1. Projectile The effect of the projectile is related to the material, hardness, size, shape, speed and density of the projectile, as well as the exposure duration and collision angle. There are three kinds of projectiles, namely, cast iron shot, cast steel shot, and cutting wire shot.

Cast iron shots are cheap, because they are brittle and easy to break, resulting in sharp edges and corners, causing scars on the polished surface, which becomes a stress concentration point and a source of fatigue damage. Therefore, it is less used in spring shot blasting.

The price of cast steel shot is higher than that of cast iron shot, but it is not easy to be broken and its service life is higher than that of cast iron shot.

Cutting steel wire shot is more expensive and has a long service life. The new steel wire shot is an angular cylinder, but in use, the corners will be ground and rounded to become a sphere with a diameter similar to that of the original steel wire. Because the edges and corners of the cutting wire shot will scratch the surface of the spring, which affects the impact of shot blasting. The effect of improving fatigue performance, so when using new steel wire shot, you should throw unqualified parts or waste springs for several hours (after the edges and corners are removed and rounded), and then add the normal springs in the shot blasting machine.

The diameter of the projectile is generally in the range of 0.5-1.5mm. The manufacturer can choose the appropriate projectile size according to the specific conditions of the spring.

In addition to the above three types of projectiles, there are also glass pellets, but they are rarely used in the spring manufacturing industry. When the diameter of the spring wire is less than 2mm, and the deformation after shot blasting is large, and the spring size tolerance requirements cannot be met, glass shots can be used for shot blasting. The diameter of the glass shots is less than 0.5mm. Glass pellets are generally used for strengthening the surface of small springs in military and aviation industries.

2. Shot blasting machine There are two commonly used shot blasting machines (also called shot blasting machines), air spray type and centrifugal type. The air spray type uses compressed air to eject the projectiles. It needs compressed air and consumes a lot of kinetic energy. Because the compressed air contains water vapor, the projectiles will stick together after the machine is stopped, which will easily cause the shot blasting device and the projectile to rust. Therefore, the air-jet shot blasting machine is rarely used in spring shot blasting; the advantage of the centrifugal shot blasting machine is that it consumes less power and has a relatively stable speed. When the coil spring is shot blasting, it must be turned or rotated continuously to make the spring spray evenly and densely. The spiral spring is widely used for shot blasting by centrifugal shot blasting machine. The guide plate around the runner of the centrifugal shot blasting machine is easily abraded.

Centrifugal shot blasting machines are commonly used in three types: drum type (Figure 2-a), crawler type (Figure 2-b) and rotary raceway type (Figure 2-c).


a) Drum type b) Crawler type c) Rotary raceway type

1 a drum 2, 6, 7 a shot blasting head 3, 5, 8 a workpiece

4 a crawler 9 a revolving race 10 a conveyor belt

3. Shot blasting effect inspection The effect of shot blasting is affected by many factors, such as the shape and size of the projectile, the material of the projectile, the speed and density of the projectile, and the projecting time. However, in the actual processing, it is impossible to measure the above factors one by one, but the inspection method of arc height and coverage rate is adopted.