What Are the Microscopic Changes in Austenitizing during Heat Treatment?

Normalizing Process

Normalizing is a metal heat treatment process that involves heating the steel parts above the critical temperature (the temperature at which complete austenitization occurs) by 30~50℃, maintaining them at the temperature for an appropriate time, and then removing them from the furnace for cooling in air, water spray, mist, or blowing.

Purpose

(1) Refine grain size and homogenize carbide distribution;

(2) Remove internal stress of the material;

(3) Increase the hardness of the material.

Material: Hypoeutectoid cast iron

Condition: Normalized

Microstructure: Graphite brown, martensite light yellow, pearlite green and dark yellow, spheroidized cementite light brown

Material: A-299

Condition: Normalized

Microstructure: Brown ferrite yellow, blue, and white pearlite brown

Material: 13MnNiMoNb

Condition: Normalized

Microstructure: Light brown pearlite grayish brown, ferrite red, yellow, blue

Material: 45 steel

Condition: Normalized

Microstructure: Light blue ferrite, pearlite with various colors

Advantages

(1) Normalizing cooling rate is slightly faster than annealing cooling rate, resulting in smaller spacing between pearlite layers. Normalized microstructure is finer than annealed microstructure, resulting in higher hardness and strength;

(2) Normalizing cooling outside the furnace does not require equipment, resulting in higher productivity.

Application

Only suitable for carbon steel and low to medium alloy steel, not suitable for high alloy steel. This is because the austenite in high alloy steel is very stable and will form martensite structure when cooled in air.

Specific Applications

(1) For low carbon steel and low alloy steel, normalizing can increase their hardness to improve machinability;

(2) For medium carbon steel, normalizing can replace quenching and tempering, prepare the microstructure for high-frequency quenching, and reduce deformation and processing costs;

(3) For high carbon steel, normalizing can eliminate the network of carbide structure, facilitating spheroidizing annealing;

(4) For large steel forgings or steel castings with drastic section changes, normalizing can be used instead of quenching to reduce deformation and cracking tendencies, or to prepare the microstructure for quenching;

(5) For quenched and tempered steel parts, normalizing can eliminate the overheating effect, allowing re-quenching;

(6) Used for cast iron to increase the amount of pear