Normalizing is a heat treatment in which the steel is heated to 30 °C - 50 °C above Ac3 (hypoeutectoid steel) and Acm (hypereutectoid steel), and after a period of heat preservation, it is cooled to room temperature in air or in forced air method. Normalizing has a faster cooling rate than annealing, so the normalized structure is finer than the annealed structure, and its strength and hardness are also higher than that of the annealed structure. Due to the short production cycle and high equipment utilization of normalizing, normalizing is widely used in various steel castings.
The purpose of normalizing is divided into the following three categories:
Normalizing as the final heat treatment
For metal castings with low strength requirements, normalizing can be used as the final heat treatment. Normalizing can refine the grains, homogenize the structure, reduce the ferrite content in the hypoeutectoid steel, increase and refine the pearlite content, thereby improving the strength, hardness and toughness of the steel.
Normalizing as a pre-heat treatment
For steel castings with larger sections, normalizing before quenching or quenching and tempering (quenching and high temperature tempering) can eliminate Widmanstatten structure and banded structure, and obtain a fine and uniform structure. For the network cementite present in carbon steels and alloy tool steels with a carbon content greater than 0.77%, normalizing can reduce the content of secondary cementite and prevent it from forming a continuous network, preparing the organization for spheroidizing annealing.
Improve cutting performance
Normalizing can improve the cutting performance of low carbon steel. The hardness of low carbon steel castings is too low after annealing, and it is easy to stick to the knife during cutting, resulting in excessive surface roughness. Through normalizing heat treatment, the hardness of low carbon steel castings can be increased to 140 HBW - 190 HBW, which is close to the optimal cutting hardness, thereby improving the cutting performance.