Difference Between Annealing and Normalizing

Key Difference – Annealing vs Normalizing
 

Though Annealing and normalizing are two most commonly used heat treatment methods in metallurgy which use a combination of heating and cooling operation, a distinct difference between can be noticed between the two processes, in the last cooling step. Both methods follow slightly similar procedure at the beginning of the process, but there is a notable difference in the last cooling step. The key difference between annealing and normalizing is that, in annealing, the cooling process is done in the oven while, in normalizing, it is done in the air. However, both methods are equally important, and they modify the microstructure of the material in different ways.

What is Annealing?

Annealing process  has three steps ; heating the material to a high temperature (near or above critical temperature), soaking the material at that temperature until the required material properties are achieved, and cooling the heated material at a slow rate to the room temperature inside the oven.

Annealing modifies the properties such as machinability, mechanical or electrical properties, or dimensional stability. This process softens the material. Examples of annealing materials include; copper, stainless steel and brass. There are few variations in the process depending on the properties of the resultant material. They are full annealing (conventional annealing), isothermal annealing, spheroids annealing, recrystallization annealing, and stress relief annealing.

What is Normalizing?

The heat treatment process normalizing is done by heating the material to a temperature above its critical temperature, and then the material is soaked at that temperature until the transformation occurs. Finally, the heated material is taken out from the oven and cooled to the room temperature outside the oven. This treatment enhances the grain size and improves the consistency of microstructure.

The production of large forgings such as railroad wheels and axles is an area involved in normalizing. Normalized materials are softer, but do not produce the uniform material properties of annealed materials.

What is the difference between Annealing and Normalizing?

Characteristics of Annealing and Normalizing

Procedure

The first phase in both processes are similar, but the latter part is different. In annealing, the cooling process is done in the oven. But, in normalizing it is cooled in the air.

Annealing:  

 Normalizing:

(Critical temperature: temperature at which crystalline phase change occurs)

 Properties of the material after the treatment

Annealed material	Normalized material
Low value for hardness, tensile strength, and toughness	Slightly more value for hardness, tensile strength, and toughness
Grain size distribution is more uniform	Grain size distribution is slightly less uniform
Internal stresses are least	Internal stresses are slightly more
Pearlite is coarse It usually gets resolved by the optical microscope	Pearlite is fine It usually appears unresolved with optical microscope

 Purpose

Annealing	Normalizing
To refine the crystalline structure and remove residual stressesTo increase its ductility by reducing hardness and brittleness	To get a refine grain structure before hardening.To reduce segregation in casting of forgings.To harden the steel slightly.

Cost

Annealing:  Annealing is more expensive as it uses ovens.

Normalizing: Normalizing is less expensive than annealing.

 

Image Courtesy:

“Loading anti-aircraft cases into a stress-annealing furnace to make them soft, uniform, and ductite-ready for… – NARA – 196209” by Unknown or not provided – U.S. National Archives and Records Administration. (Public Domain) via Wikimedia Commons