The requirements of forging for cooling

Cooling for forging is complex and difficult to control. The formulation of the cooling process is much more important and complex than the small parts. The key to determining the mechanical properties after forging and tempering is the nature and dispersion of the structure obtained after cooling.

In general material manuals and factory standards, the conventional mechanical properties that can be obtained after quenching and tempering or normalizing of forgings of different diameters are listed, and these are used as delivery conditions at the factory. However, there is no requirement for the state of the microstructure, which is important for evaluating properties such as high temperature creep strength and fracture resistance. In formulating the cooling process of forgings, not only should the entire section have relatively uniform conventional mechanical properties, but also the nature of the cooling structure and the structure distribution along the section.

The relationship between the cooling structure of forging and the mechanical properties after tempering has been tested and studied so far. The general conclusions are as follows:

1. If the martensite structure is obtained after cooling, it has higher strength, hardness and lower plasticity and toughness after low temperature tempering. Strength and hardness change with the increase of carbon content in steel, while ductility and toughness decrease accordingly. The strength and hardness of martensite structure decrease with the change of tempering temperature, while the plasticity and toughness change accordingly. Under normal circumstances, the alloying elements in steel can reduce the strength and hardness of steel during tempering, and greatly improve the ductility and toughness. The martensite structure is tempered at high temperature, and a uniform tempered sorbite structure can be obtained. Its strength, plasticity and toughness can be well matched, that is, high comprehensive mechanical properties can be obtained.

2. If the lower bainite structure is obtained after cooling, the mechanical properties after tempering are similar to those obtained after quenching and tempering at the same temperature, and have higher impact toughness.

3. If the upper bainite structure and pearlite structure are obtained after cooling, the strength after the same fire is low, and the plasticity is not good, that is, the comprehensive mechanical properties are poor.

4. If ferrite appears in the structure after cooling, the comprehensive mechanical properties after tempering will deteriorate significantly, mainly because the impact toughness will become lower. It can be seen that the comprehensive mechanical properties after tempering deteriorate with the coarsening of the structure obtained by cooling.

The task of the final forging heat treatment is to obtain the highest possible comprehensive mechanical properties that meet the requirements of the use of the parts through appropriate treatment processes according to the steel used.