Normalization
Normalizing Heat Treatment is a process that involves heating metal materials to above 900°C and then cooling them in air, which results in a fine grain structure. This treatment can eliminate internal stresses in the material, thereby enhancing its strength and toughness.
The fundamental principle of normalizing is to control the heating and cooling processes of the material to adjust the metal structure and phase transformations, forming a structure with good mechanical properties and stability. The main steps include:
- Heating: The metal material is heated to just above 900°C. At this temperature range, the phases within the material undergo phase transformations, resulting in a uniform single-phase structure.
- Soaking: The material is held at high temperature for a period to ensure it fully resides in the single-phase structure. This allows for grain refinement, which improves strength and toughness.
- Cooling: The material is rapidly cooled from high temperature to room temperature to form fine grains. The heat generated during the cooling process is quickly transferred to the surrounding air, accelerating the cooling rate. The faster the cooling, the finer the resulting grains.
Methods of Normalizing
There are several methods for normalizing heat treatment, including complete normalizing and isothermal normalizing:
- Complete Normalizing: The metal material is heated to above the eutectoid temperature and held for a certain time before being air-cooled to room temperature. This method is suitable for low carbon steels and alloy steels, allowing for microstructure refinement, elimination of internal stresses, improved toughness and ductility, while also reducing hardness and strength.
- Isothermal Normalizing: The metal material is heated to above the eutectoid temperature and then held at a certain temperature for a period to stabilize the structure before air-cooling to room temperature. This method is suitable for certain special materials, such as some low carbon steels and stainless steels, and allows for control over the soaking temperature and time to stabilize the structure, enhancing strength and toughness.
The main difference between these two normalizing heat treatment methods lies in the soaking time and cooling method, which can be selected based on the material characteristics and heat treatment requirements.
