The Purpose and Types of Annealing in Forging and Forged Parts

Annealing is a heat treatment process in which a forged part is heated to an appropriate temperature, held at that temperature for a certain period of time, and then slowly cooled.

Annealing is a heat treatment process in which a forged part is heated to an appropriate temperature, held at that temperature for a certain period of time, and then slowly cooled.
The essence of annealing is to heat steel to austenitize it and then induce a pearlitic transformation. After annealing, the microstructure typically consists of ferrite plus lamellar pearlite for hypoeutectoid steels, and granular pearlite for eutectoid or hypereutectoid steels. In short, the annealed microstructure is one that closely approximates an equilibrium state.
The main purposes of annealing are as follows: to reduce the hardness of steel and increase its ductility, thereby facilitating machining and cold forming processes; to refine the grain structure and eliminate structural defects caused by casting, forging, and welding; to homogenize the microstructure and chemical composition of the steel, thereby improving its mechanical properties or preparing it for subsequent heat treatments; and to relieve internal stresses within the steel, thus preventing deformation and cracking.
The commonly used annealing methods include full annealing, spheroidizing annealing, stress-relief annealing, recrystallization annealing, and homogenizing annealing, among others.
1. Complete Annealing: Also known as recrystallization annealing, complete annealing is a heat treatment process in which a forged part is heated to achieve complete austenitization and then slowly cooled, resulting in a microstructure that closely approximates the equilibrium state.
Complete annealing is primarily used for hypoeutectoid steels—typically medium-carbon steels as well as low- and medium-carbon alloy structural steels—in forgings, castings, and hot-rolled sections; it is sometimes also applied to welded components made from these materials. Complete annealing is not suitable for hypereutectoid steels, because fully annealing such steels requires heating them above the critical temperature. During slow cooling, cementite will precipitate along the austenite grain boundaries in a network-like pattern, significantly increasing the material’s brittleness and leaving hidden defects that could compromise subsequent heat treatments.
2. Spheroidizing Annealing: An annealing process carried out to spheroidize carbides in forged parts. In this process, the steel is heated to a temperature 20–30°C above AC1, held at that temperature for a specified period, and then slowly cooled, resulting in a microstructure in which spherical or granular carbides are uniformly distributed throughout a ferritic matrix.
Spheroidizing annealing is primarily suitable for eutectoid steels and hypereutectoid steels, such as carbon tool steels, alloy tool steels, and bearing steels. After rolling or forging and air cooling, these steels develop a microstructure consisting of lamellar pearlite and network carbides. This microstructure is hard and brittle, making machining extremely difficult and increasing the likelihood of deformation and cracking during subsequent quenching processes. In contrast, spheroidizing annealing produces a microstructure characterized by spherical pearlite, in which the carbides appear as spherical particles uniformly dispersed throughout the ferrite matrix. Compared to lamellar pearlite, this microstructure exhibits lower hardness, facilitating easier machining. Moreover, during austenitization heating, the austenite grains are less prone to significant grain growth, and upon cooling, the forged parts show reduced tendencies toward deformation and cracking. Additionally, for certain hypoeutectoid steels that require improved cold plastic deformation (such as in stamping or cold heading operations), spheroidizing annealing can also be employed in some cases.
Since spheroidizing annealing involves heating only slightly above the AC1 temperature, the austenitization process is “incomplete”—only lamellar pearlite transforms into austenite, with only a small amount of excess carbides dissolving. Therefore, it is impossible for this process to eliminate network carbides. If hypereutectoid steels contain network carbides, normalizing must be carried out prior to spheroidizing annealing to remove these carbides; only then can the spheroidizing annealing proceed smoothly.
3. Stress-relief annealing: An annealing process conducted to eliminate internal stresses caused by plastic deformation, machining, or welding in forgings, as well as residual stresses present in castings.
The internal stresses present in forgings are highly detrimental. If not promptly relieved, these stresses can cause deformation during machining and service, thereby compromising the precision of the forgings. Moreover, when combined with external loads, internal stresses can also lead to unexpected material fractures. Therefore, after forging, casting, welding, and machining, forgings should undergo stress-relief annealing to eliminate the internal stresses generated during processing.
The heating temperature for stress-relief annealing is slightly below the phase-transition temperature; therefore, no microstructural transformation occurs throughout the entire process. Internal stresses are primarily relieved during the holding and slow-cooling stages of the forging. To ensure more thorough elimination of internal stresses in the forging, the heating rate should be carefully controlled—typically starting with a low-temperature entry into the furnace and then heating at a rate of about 100°C/h to the specified temperature. The heating temperature for welded components should be slightly higher than 600°C. The holding time depends on the specific situation but usually ranges from 2 to 8 hours. For castings undergoing stress-relief annealing, the upper limit of the holding time is recommended. The cooling rate should be controlled within 20–50°C/h, and the casting should not be removed from the furnace until it has cooled down to below 300°C.
4. Recrystallization Annealing: Recrystallization annealing, also known as intermediate annealing, involves heating a forging that has undergone cold plastic deformation to a temperature above the recrystallization temperature and holding it for an appropriate duration. During this process, the crystallographic defects generated during cold plastic deformation are largely eliminated through recrystallization, allowing the formation of a new, uniform, equiaxed grain structure. This annealing process serves to relieve work hardening effects and residual stresses.
Recrystallization annealing takes advantage of the recrystallization phenomenon that occurs when a material, after undergoing cold plastic deformation, is heated. This process transforms elongated, flattened, or fractured grains into uniform equiaxed grains, thereby eliminating work hardening and restoring the material's plasticity, thus facilitating further deformation and processing.
5. Homogenizing Annealing: Also known as diffusion annealing, this annealing process is primarily aimed at reducing the degree of chemical composition and microstructural inhomogeneity in forgings. It involves heating the material to a high temperature and holding it at that temperature for an extended period before allowing it to cool slowly.
The heating temperature for homogenizing annealing is typically selected 100–200°C below the steel’s melting point, usually ranging from 1050 to 1150°C. The holding time is generally 10–15 hours, ensuring sufficient diffusion and achieving the goal of eliminating or reducing chemical composition or microstructural inhomogeneities. Due to the high heating temperature and long holding time involved in homogenizing annealing, the grain size will inevitably become coarse. Therefore, it is essential to subsequently carry out either a full annealing or normalizing treatment to refine the microstructure once again.