What is Annealing and How Does it Work?

Annealing is a heat treatment process that can alter a material’s physical properties, and sometimes its chemical properties, typically by increasing ductility and reducing hardness, making it easier to machine or work.

Overview of Annealing

Annealing is a heat treatment process that involves heating a metal to a suitable temperature, holding it for a certain period, and then slowly cooling it (typically by furnace cooling) to room temperature. The essence of annealing is to heat the steel for full austenitization, followed by a slow cooling process that facilitates the pearlitic transformation, resulting in a near-equilibrium microstructure.

Function of Annealing

Annealing is performed to achieve several critical objectives:

• Grain refinement and microstructure homogenization: It refines the grain structure, homogenizes the internal microstructure, and reduces chemical segregation, preparing the material structurally for subsequent heat treatment processes.
• Stress relief: It eliminates residual internal stresses within the workpiece, preventing potential cracking or excessive deformation that those stresses could cause.
• Hardness reduction and plasticity improvement: It reduces the material’s hardness and enhances its plasticity, which is crucial for enhancing subsequent cold working, cold forming, and machining operations.

The annealing process is generally divided into 3 main stages:

1. Recovery stage: In the recovery stage, a furnace or other type of heating device is used to raise the material’s temperature to a point where its internal stresses are released.
2. Recrystallization stage: In the recrystallization stage, the material is heated above its recrystallization temperature but kept below its melting temperature. This causes new, stress-free grains to form.
3. Grain growth stage: As the new grains form, they continue to develop fully. This growth is controlled by allowing the material to cool at a specified rate.

The completion of 3 three stages results in enhanced ductility and reduced hardness of the material. Sometimes, subsequent processes may follow annealing to further modify the material’s mechanical properties.

Types of Annealing

Annealing methods are broadly categorized based on the heating temperature relative to the critical temperatures (Ac1 or Ac3):

Phase transformation recrystallization annealing (Above Ac1 or Ac3) includes full annealing, diffusion annealing, incomplete annealing, and spheroidizing annealing.

Annealing below the critical temperature (Below Ac1 or Ac3) includes recrystallization annealing and stress-relief annealing.

Full Annealing

The process involves heating steel to 20–30°C above the Ac3 line, soaking for a period, and then cooling slowly (furnace cooling) to obtain a near-equilibrium microstructure (achieving full austenitization).

• Application: Primarily used for hypoeutectoid steels (0.3% to 0.6% Carbon content), typically medium-carbon steels and low-carbon alloy steel castings, forgings, hot-rolled shapes, and sometimes weldments.
• Purpose: To refine the grain structure, homogenize the microstructure, eliminate internal stresses, reduce hardness, and improve the steel’s machinability. The resulting structure in hypoeutectoid steel is Ferrite + Pearlite (F+P).
• Note: In production, to increase productivity, the parts are often removed from the furnace and air-cooled once they reach around 500°C.

Isothermal Annealing

Full Annealing can be time-consuming, especially for alloy steels with relatively stable supercooled austenite. Isothermal annealing significantly shortens the time by rapidly cooling the austenitized steel to a temperature slightly below the Ar1, holding it isothermally to transform Austenite (A) into Pearlite (P), and then air-cooling to room temperature.

It is suitable for steels with stable microstructures, such as high-carbon steels (C > 0.6%), alloy tool steels, and high-alloy steels (total alloy content > 10%). It helps achieve a uniform microstructure and consistent properties. It is not ideal for large cross-section steel parts or large furnace loads, as achieving a uniform isothermal temperature throughout the load can be challenging.

Incomplete Annealing

The steel is heated to a temperature between Ac1 and Ac3 (for hypoeutectoid steel) or between Ac1 and Acm (for hypereutectoid steel), soaked, and then slowly cooled to obtain a near-equilibrium microstructure.

It is used for hypereutectoid steels to obtain a spheroidal pearlite structure, which helps eliminate internal stresses, reduce hardness, and improve machinability. Spheroidizing Annealing is a form of Incomplete Annealing.

Spheroidizing Annealing

A specific heat treatment process designed to spheroidize the carbides in steel, resulting in a granular pearlite (spheroidal carbide) structure. The material is heated to 20–30°C above Ac1, soaked for an appropriate time (typically 2–4 hours), and usually cooled slowly in the furnace or held isothermally for a long period about 20°C below Ar1.

It is mainly used for eutectoid and hypereutectoid steels, such as carbon tool steels, alloy tool steels, and bearing steels.

The post-forging or post-rolling structure of hypereutectoid steel—lamellar pearlite with cementite networks—is hard, brittle, difficult to CNC machine, and prone to deformation/cracking during final quenching. Spheroidizing annealing yields spheroidal pearlite, where the fine, globular cementite particles are dispersed in the ferrite matrix. This structure has lower hardness, better machinability, and, crucially, prevents austenite grain coarsening during subsequent quenching, thus reducing the tendency for deformation and cracking.

If a cementite network is present in hypereutectoid steel, it must be eliminated by normalizing before spheroidizing annealing can proceed effectively.

Diffusion Annealing

Involves heating ingots, castings, or large forgings to a high temperature, slightly below the solidus line, soaking for an extended period, and then slowly cooling to eliminate chemical non-uniformity (dendritic and regional segregation).

Heating temperature is very high, typically 100–200°C above Ac3 or Accm, depending on the degree of segregation and steel grade. Soaking time is usually 10–15 hours.

To eliminate segregation generated during the solidification of ingots, homogenize the chemical composition and microstructure.

Diffusion annealing is usually followed by Full Annealing or normalizing to refine the coarse grain structure.

It is mainly applied to high-grade alloy steels and alloy steel castings or ingots with severe segregation issues.

Stress-Relief Annealing

Stress-relief annealing involves heating the steel part to a specific temperature below Ac1 (typically 500–650°C), holding it at that temperature (soaking), and then cooling it slowly (furnace cooling). Because the temperature utilized for stress-relief annealing is below the A1critical line, the process does not induce any microstructural changes. It is to eliminate residual internal stresses within the component.

Recrystallization Annealing

Heating a cold-worked metal to a temperature above its recrystallization temperature for an appropriate time. This process causes the deformed grains to transform into uniform, equiaxed grains, thereby eliminating work hardening and residual stresses.

The heating temperature should be 100–200°C above the lowest recrystallization temperature (for steel, around 450°C). The minimum recrystallization temperature is generally Tc = 0.4*Tm (absolute temperature).

Considerations for Choosing Annealing

The selection of the appropriate annealing method generally follows these principles:

• For steels with hypoeutectoid microstructures, full annealing is generally selected. Isothermal annealing may be chosen to shorten the annealing time.
• Hypereutectoid steel generally selects spheroidizing annealing. If the requirements are not high, incomplete annealing can be selected. Tool steels and bearing steels commonly select spheroidizing annealing. Spheroidizing annealing is sometimes also used for cold-extruded and cold-headed parts made of low-carbon or medium-carbon steel.
• Recrystallization annealing can be selected to eliminate work hardening.
• Stress-relief annealing can be selected to eliminate internal stresses caused by various manufacturing processes. Diffusion annealing is often selected for large castings of some high-grade, quality alloy steels to improve the non-uniformity of microstructure and chemical composition.