21
2019
-
05
The “Annealing” Process Among the “Four Heat Treatments”
Annealing process
The heat treatment process in which a metal or alloy is heated to an appropriate temperature, held at that temperature for a specified period, and then slowly cooled—typically by allowing it to cool along with the furnace—is called annealing.
The essence of annealing is to heat steel to the austenitizing temperature and then allow pearlitic transformation to occur. The microstructure obtained after annealing is close to the equilibrium microstructure.
The purpose of annealing:
(1) Reduce the hardness of steel, increase its ductility, and facilitate machining and cold forming processes;
(2) Homogenize the chemical composition and microstructure of steel, refine grain size, improve steel properties, or prepare the microstructure for quenching;
(3) Eliminate internal stresses and work hardening to prevent deformation and cracking.
Annealing method
1. Complete annealing
Process: Heat the steel to 20–30°C above Ac3, hold it at that temperature for a certain period, and then cool it slowly (in the furnace) to obtain a heat treatment process that yields a microstructure close to equilibrium (complete austenitization). In actual production, to improve productivity, the steel is annealed and cooled to around 500°C before being removed from the furnace and air-cooled.
Objective: To refine grain size, homogenize the microstructure, relieve internal stresses, reduce hardness, and improve the machinability of steel. After complete annealing, the microstructure of hypoeutectoid steel consists of ferrite (F) and pearlite (P).
Application: Complete annealing is primarily used for hypoeutectoid steels (with carbon content ranging from 0.3% to 0.6%), typically medium-carbon steels as well as castings, forgings, and hot-rolled sections made of low- and medium-carbon alloy steels; it is also sometimes applied to welded joints of these materials.
2. Incomplete Annealing
Process: A heat treatment process in which steel is heated to the temperature range of Ac1–Ac3 (for hypoeutectoid steels) or Ac1–Accm (for hypereutectoid steels), held at that temperature for a specified period, and then slowly cooled to obtain a microstructure close to equilibrium.
Application: Primarily used for obtaining a spheroidal pearlite structure in hypereutectoid steels to relieve internal stresses, reduce hardness, and improve machinability.
3. Isothermal Annealing
Process: A heat treatment process in which steel is heated to a temperature above Ac3 (or Ac1), held at that temperature for an appropriate duration, then rapidly cooled to a specific temperature within the pearlite region and held isothermally to allow austenite to transform into pearlite, followed by air cooling to room temperature.
Objective: Similar to full annealing, the transformation is easier to control.
Applications: Suitable for relatively stable steels, including high-carbon steel (with carbon content >0.6%), alloy tool steel, and high-alloy steel (with a total alloy element content exceeding 10%). Isothermal annealing also helps achieve uniform microstructure and properties. However, it is not suitable for steel parts with large cross-sections or for large batches of workpieces, as isothermal annealing makes it difficult to ensure that both the interior of individual parts and all parts in a batch reach the same isothermal temperature.
4. Spheroidizing Annealing
Process: A heat treatment process used to spheroidize carbides in steel and obtain a granular pearlite structure. The steel is heated to a temperature 20–30°C above Ac1; the holding time should not be too long—typically 2–4 hours is ideal. The cooling method usually involves furnace cooling or, alternatively, prolonged isothermal holding at a temperature about 20°C below Ar1.
Objective: To reduce hardness, homogenize the microstructure, and improve machinability as a preparatory step for quenching.
Applications: Primarily used for eutectoid steels and hypereutectoid steels, such as carbon tool steels, alloy tool steels, bearing steels, and others. Spheroidizing annealing produces spheroidal pearlite, in which the cementite forms fine, spherical particles that are uniformly dispersed throughout the ferrite matrix. Compared to lamellar pearlite, spheroidal pearlite not only has lower hardness, making it easier to machine, but also exhibits smaller austenite grain growth during quenching heating and shows reduced tendency toward deformation and cracking during cooling.
5. Diffusion Annealing (Homogenizing Annealing)
Process: A heat treatment process in which steel ingots, castings, or forged billets are heated to a temperature slightly below the solidus line, held at that temperature for an extended period, and then slowly cooled to eliminate compositional non-uniformities.
Objective: To eliminate dendritic segregation and regional segregation that occur during the solidification of ingots, thereby achieving uniformity in composition and microstructure.
Application: This process is applicable to certain high-quality alloy steels as well as castings and ingots of alloy steels with severe segregation. The heating temperature for diffusion annealing is relatively high—typically 100–200°C above the Ac3 or Accm temperatures. The specific temperature depends on the degree of segregation and the steel grade. The holding time usually ranges from 10 to 15 hours. After diffusion annealing, complete annealing and normalizing treatments are required to refine the microstructure.
6. Stress-relief annealing
Process: Heat the steel part to a temperature below Ac1 (typically 500–650℃), hold it at that temperature for a specified period, and then allow it to cool slowly in the furnace.
The stress-relief annealing temperature is below the A1 temperature; therefore, stress-relief annealing does not cause microstructural changes.
Objective: To eliminate residual internal stresses.
Applications: Primarily used to eliminate residual stresses in castings, forgings, welded components, hot-rolled parts, and cold-drawn parts. If these stresses are left unaddressed, they may cause deformation or cracking in steel components after a certain period of time or during subsequent machining operations.
Exploring Heat Treatment Manufacturers: The Appeal and Applications of Their Products
Learn about the high-quality products offered by heat treatment manufacturers and their critical applications across various industries.
2025-09-30
Unveiling the Working Principles Behind Heat Treatment Manufacturers
Learn how heat treatment manufacturers operate, understand their working principles, and master the secrets to enhancing material performance.
2025-09-23
Frequently Asked Questions in the Heat Treatment Industry
Learn about common issues faced by heat treatment manufacturers to help you better select and utilize heat treatment services.
2025-09-16
Points to Consider When Selecting a Heat Treatment Manufacturer
Understand the key considerations when selecting a heat treatment manufacturer to ensure high-quality heat treatment services.
2025-09-09
The Diverse Applications of Heat Treatment Manufacturers in Modern Industry
Explore the application scenarios of heat treatment manufacturers across various industries and learn how they enhance product quality and performance.
2025-09-02