Annealing and normalizing are both heat treatment methods where steel is heated to an appropriate temperature, held for a certain period, and then cooled at a slow rate. The main difference is that annealing is usually done by furnace cooling, while normalizing is done by air cooling.
1.1 Annealing
Annealing is a heat treatment process in which steel is heated to a specific temperature, held for a certain period, and then slowly cooled.
(1) Purpose of Annealing
Reduce hardness and improve the machinability of the workpiece;
Eliminate residual stress to prevent deformation and cracking of the workpiece;
Refine the grain structure and improve the organization to enhance the mechanical properties of steel and prepare the structure for final heat treatment.
(2) Types and Applications of Annealing
Depending on the chemical composition of the steel and the purpose of annealing, it can be categorized into full annealing, isothermal annealing, spheroidizing annealing, homogenizing annealing, recrystallization annealing, and stress relief annealing.
Full Annealing
Full annealing involves heating the steel to a completely austenitized temperature (Ac3 + 30℃~50℃), holding it for a period, and then furnace cooling it down to below 550℃ before air cooling, to achieve a nearly equilibrium structure.
Full annealing is mainly used for heat treatment of castings, forgings, hot-rolled sections, and welded structures of hypoeutectoid steel. It is not suitable for hypereutectoid steel as heating hypereutectoid steel above the Accm temperature for complete austenitization may lead to the precipitation of network-like secondary cementite along austenite grain boundaries during the slow cooling process, reducing the strength and toughness of the steel.
Isothermal Annealing
Isothermal annealing involves heating the steel to above Ac3 (or Ac1), holding for a certain period, and then cooling rapidly to a temperature below Ar1 for isothermal treatment to transform austenite into a pearlite-type structure, followed by slow cooling.
The purpose of isothermal annealing is the same as that of full annealing, but it can control the desired structure and properties by adjusting the isothermal temperature. Isothermal annealing is generally used for the annealing of alloy steel with stable austenite. Compared to full annealing, isothermal annealing significantly reduces the annealing time and achieves better results.
Spheroidizing Annealing
Spheroidizing annealing is a heat treatment process where eutectoid or hypereutectoid steel is heated above Ac1 by 20℃~30℃, held for a certain period, and then furnace cooled to below 550℃ before air cooling to form granular carbides.
This process is mainly used for eutectoid or hypereutectoid carbon and alloy steels. These steels often contain coarse lamellar pearlite and secondary cementite after hot rolling or forging, which reduces their machinability and may cause deformation and cracking during quenching. Spheroidizing annealing can transform the lamellar cementite and network-like secondary cementite in pearlite into spherical carbides. This structure, where spherical carbides are uniformly distributed on the ferrite matrix, is called spheroidized pearlite.
For steel with severe network secondary cementite, a normalizing process may be performed first to break up the network before spheroidizing annealing.
Homogenizing Annealing
Also known as diffusion annealing, homogenizing annealing heats the ingot, casting, or forging to 100℃15 hours, and then slowly cools to achieve a uniform structure. Although this method consumes more energy and is costly, it is mainly used for high-quality alloy steel ingots, castings, or forgings to allow elements in the steel to diffuse sufficiently for homogenization. After homogenizing annealing, the grain size may become coarse, so full annealing or normalizing is needed to refine the grains.
Stress Relief Annealing
Also known as low-temperature annealing, this process heats the workpiece to 100℃~200℃ below Ac1 (usually between 500℃ and 600℃), holds for a certain period, and then cools slowly in the furnace. Since the heating temperature is below point A1, no phase change occurs during the stress relief annealing process.
Stress relief annealing is primarily used to eliminate residual stress in castings, forgings, welded parts, cold-stamped parts, and machined components to stabilize the workpiece dimensions, reduce deformation, and prevent cracking due to stress during subsequent machining or use.
1.2 Normalizing
Normalizing involves heating the steel to 30℃~50℃ above Ac3 (or Accm) for complete austenitization, followed by air cooling to obtain a finer pearlite structure. When the carbon content is below 0.6%, the normalized structure is ferrite + sorbite; when the carbon content exceeds 0.6%, it is sorbite.
The main difference between normalizing and annealing is that the cooling rate in normalizing is faster, resulting in a finer structure, higher strength, and hardness. The process is simple, and the production cycle is shorter. Normalizing is mainly applied in the following cases:
Improving the Machinability of Low-Carbon Steel and Low-Carbon Alloy Steel
The structure after normalizing is fine pearlite, which improves hardness and reduces the "stickiness" during machining, thus lowering the surface roughness of the workpiece.
Eliminating Network Cementite
Normalizing can remove severe network cementite in hypereutectoid steel or the surface layer of carburized parts.
Pre-Heat Treatment for Medium Carbon Steel Parts
Normalizing can eliminate coarse grain structures and residual stress, preparing the structure for final heat treatment.
Final Heat Treatment for Ordinary Structural Parts
For certain large or complex parts where quenching may cause cracking, normalizing may be used instead of quenching and tempering as the final heat treatment.