### Characteristics of the Spring Manufacturing Process
The quality of springs after heat treatment is primarily determined by their **lifespan**. From a performance perspective, it is essential to adjust the balance between **elasticity parameters** and **toughness parameters**. The performance of the spring is closely related to the **hardenability** of the spring steel.
Currently, spring manufacturing methods can be divided into three categories based on the type of steel and the process route used:
1. **Cold-rolled steel strips** and **cold-drawn steel wires** are cold-formed, then quenched and tempered or low-temperature tempered.
2. **Hot-rolled non-annealed steel** is hot-formed, then quenched and tempered. This method is commonly used in the production of **leaf springs** and **large coil springs**.
3. **Hot-rolled annealed steel** is cold-formed, followed by stress-relieving, then heated for quenching and tempering. This method is generally used for medium-sized coil springs with a wire diameter of **6-12mm**.
The heat treatment of spring steel can be classified into two categories: **heat treatment of cold-drawn steel wire** and **heat treatment of hot-rolled spring steel**.
1. **Cold-drawn steel wire** is first quenched and tempered at medium temperature to obtain a tempered troostite structure. After forming, stress-relieving tempering is performed at a temperature below **150°C**.
2. **Hot-rolled spring steel** is heat-treated after hot forming. After being heated at **830–890°C**, it undergoes oil quenching and is tempered at **400–480°C** to obtain a tempered troostite structure. If the diameter of the spring wire is too large (greater than **15mm**) or the plate is too thick (greater than **8mm**), incomplete hardening may occur, resulting in a reduction in the elastic limit and fatigue strength.
Since the **bending stress** and **rotational stress** that the spring endures during service are concentrated on the surface, the surface condition of the spring is crucial. **Oxidation and decarburization** during heat treatment must be carefully prevented. The furnace atmosphere needs to be strictly controlled during heating, and heating time should be minimized. After heat treatment, springs generally undergo **shot peening** to strengthen the surface and introduce **residual compressive stress** on the surface, thereby improving fatigue strength.
### High-Intensity Shot Peening
**High-intensity shot peening** involves blasting the spring surface with high-speed shot particles, causing **plastic deformation** in the surface layers. This process enhances the surface strength and generates surface compressive stress, improving the spring’s resistance to fatigue and stress corrosion.
When the **arc height** value is **0.15-0.60mmA**, the surface roughness of springs with a surface roughness value greater than **4μm** can be improved. This extends the **initiation period of surface fatigue cracks** and causes **multiple branching** of propagating cracks, reducing their growth rate. As a result, **bending fatigue cracks** become shorter and finer, delaying their formation and reducing the crack propagation speed, ultimately improving bending fatigue strength by **42%-56%**.
Therefore, spring components generally undergo high-intensity shot peening. It is important to note that the **technical parameters** of the shot peening process must be determined carefully. According to **JB/T10174-2000**, the “shot peening intensity should generally be greater than **0.35mmA**, and the surface coverage rate should not be less than **200%**.” Based on the influence of shot peening on the microstructure of the spring surface, a shot peening intensity of **0.35-0.43mmA** is considered optimal. Under the same conditions, the shot peening intensity for the surface area of leaf springs prone to bending fatigue fractures is only about one-third that of round bars. Thus, for optimal results, the surface coverage rate for spring shot peening should be **≥250%**.