Post-Forging Heat Treatment Processes: Effects of Annealing and Normalizing on Forging Properties

Post-Forging Heat Treatment Processes: Effects of Annealing and Normalizing on Forging Properties

After forging, forgings tend to have residual internal stress and uneven microstructure. Annealing or normalizing is required to optimize their properties, laying a foundation for subsequent processing or application. Although both processes consist of core steps of heating, heat preservation, and cooling, their parameter controls differ significantly, and their effects on forging properties also have distinct focuses.

The annealing process is characterized by "low-temperature slow cooling". Generally, forgings are heated to 20-30°C above Ac3 (for hypoeutectoid steels) or Ac1 (for hypereutectoid steels). After a period of heat preservation, they are slowly cooled along with the furnace. This process can effectively eliminate residual internal stress in forgings: during slow cooling, atoms diffuse slowly, and stress is gradually released, preventing deformation or cracking during subsequent processing. Meanwhile, annealing can refine grain structure, transforming the coarse grains formed due to high temperatures during forging into a uniform and fine microstructure. This reduces the hardness of forgings (e.g., the hardness of 45 steel after annealing is approximately 180-220 HB), improves ductility and toughness, and makes them easier to machine. In addition, after spheroidizing annealing, hypereutectoid steels can convert network cementite into spherical cementite, improving the machinability of the material and the quality of subsequent quenching. This makes it suitable as a pre-treatment process for high-carbon tool steel forgings.

The normalizing process, by contrast, emphasizes "high-temperature fast cooling". The heating temperature is similar to that of annealing, but after heat preservation, the forgings are cooled in air (with a faster cooling rate than annealing). The relatively fast cooling rate inhibits grain growth, enabling forgings to obtain a finer pearlite microstructure. Their hardness is slightly higher than that of annealed parts (e.g., the hardness of 45 steel after normalizing is approximately 200-250 HB), with improved strength and wear resistance while maintaining a certain level of toughness. Normalizing can homogenize the microstructure of forgings, making it particularly suitable for medium and low-carbon steel forgings. It eliminates microstructural segregation after forging and provides a uniform original microstructure for subsequent quenching and tempering processes. Furthermore, for structural parts that do not bear high loads, normalizing can be used as the final heat treatment process, directly meeting the strength requirements for application and simplifying the production process.

The selection of processes should be based on the forging material and application requirements: when it is necessary to reduce hardness for easier processing or eliminate stress, annealing is preferred; when it is required to improve strength, homogenize microstructure, or use it as the final heat treatment for medium and low-carbon steels, normalizing is more suitable. Rational application of these two processes can maximize the performance of forging materials and ensure product reliability.