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7 Types of Annealing Explained

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Key takeaways:

1. Annealing is a transformative heat treatment process tailored to alter the microstructure of metals, enhancing their workability, reducing hardness, and relieving internal stresses, with different methods like complete, isothermal, and spheroidizing annealing serving specific metallurgical needs based on the carbon content and desired properties of the steel.

2. The selection of an annealing method is strategic, aiming to achieve a balance between efficiency and desired material characteristics; for instance, isothermal annealing is chosen for high carbon steels to reduce treatment time, while diffusion annealing is used for high-quality alloy steels to homogenize composition and address segregation issues.

3. The annealing process is not one-size-fits-all; it requires careful consideration of the steel's initial microstructure and the final properties required, with techniques ranging from stress relief to recrystallization annealing, each designed to target specific outcomes like grain refinement, uniformity, and machinability in preparation for further processing.

What Is Annealing?

The process of heating a metal or alloy to a specific temperature for a given duration and then cooling it down slowly, usually with the help of the furnace, is known as annealing.

The purpose of annealing is to transform the pearlite in the steel after heating it to the austenitizing temperature.

After the annealing process, the structure of the material is close to being in a state of equilibrium.

What is annealing

Types of Annealing Process

Types of Annealing Process

1. Complete annealing

Process:

Heat the steel above Ac3 by 20 to 30 degrees Celsius, hold the temperature for a specified period of time, and then slowly cool it down (along with the furnace) to achieve a state close to equilibrium in the heat treatment process (fully austenitizing).

Complete annealing is mainly used for subeutectic steels (carbon content of 0.3 to 0.6%) such as medium carbon steel, low to medium carbon alloy steel castings, forgings, and hot-rolled profiles, and sometimes for their welds.

Low-carbon steel has low hardness and is not suitable for machining.

When hypereutectoid steel is heated above Accm to the austenitic state and annealed through slow cooling, Fe3CⅡ precipitates in a mesh pattern along the grain boundaries, significantly reducing the strength, hardness, plasticity, and toughness of the steel, which poses a potential risk to the final heat treatment.

Objective:

To achieve fine grain size, uniform structure, eliminate internal stress, reduce hardness, and improve machinability of steel.

The structure after complete annealing of hypoeutectic steel is F + P.

In order to increase efficiency in actual production, the parts are removed from the furnace for air cooling when the annealing temperature drops to around 500 degrees Celsius.

2. Isothermal annealing

Complete annealing can take a long time, especially when dealing with highly stable austenitic steel.

If the austenitized steel is cooled to a temperature slightly lower than Ar1, resulting in a transformation from austenite to pearlite, followed by cooling to room temperature, it can greatly reduce the annealing time.

This method of annealing is called Isothermal Annealing.

Process:

Heat the steel to a temperature higher than Ac3 (or Ac1). After a specified period of heat treatment, it can be cooled to a specific temperature within the pearlite range, causing the austenitic structure to transform into pearlite, followed by cooling to room temperature.

Objective:

Similar to complete annealing, with easier control of the transformation process.

Suitable for steels with a more stable austenitic structure: high carbon steels (carbon content greater than 0.6%), alloy tool steels, high alloy steels (with more than 10% alloy elements).

Isothermal annealing can also help achieve uniform organization and performance.

However, it is not suitable for large section steel parts or large batch furnace materials because it is difficult to maintain the isothermal temperature throughout the internal or batch of workpieces.

3. Incomplete annealing

The process of spherification annealing involves heating steel to a temperature between Ac1 and Ac3 (for hypoeutectic steel) or between Ac1 and Accm (for hypereutectic steel).

After holding the steel at the appropriate temperature for a set period of time, it is then slowly cooled to complete the heat treatment process.

This method of annealing is mainly used for hypereutectic steel to achieve a spherical pearlite structure, in order to reduce internal stress, lower hardness, and improve machinability. It is considered a type of incomplete annealing.

4. Spherification annealing

A heat treatment process for spheroidizing carbides in steel to obtain granular pearlite.

Process:

The steel is heated to a temperature that is 20-30℃ higher than Ac1, with a holding time of 2-4 hours. The cooling is usually done by a furnace method or isothermal at a temperature slightly below Ar1 for a long time.

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

After rolling or forging, hypereutectoid steel forms lamellar pearlite and reticulated cementite which are hard and brittle, making them difficult to cut and prone to deformations and cracks during the quenching process.

Spheroidizing annealing forms globular pearlite in which the carbides appear as spherical particles dispersed in the ferrite matrix. This structure is low in hardness and easier to machine.

Additionally, the austenite grains are less likely to coarsen during heating and have less tendency to deform and crack during cooling.

It is important to normalize eutectic steel before spheroidizing annealing if it contains reticulated cementite to ensure that the spheroidizing process is successful.

Objective:

The goal of spheroidizing annealing is to reduce hardness, improve the uniformity of the structure, and improve machinability in preparation for quenching.

There are three main methods of spheroidizing annealing:

A) One-step spheroidizing annealing process:

The steel is heated to more than 20~30℃ above Ac1 and held for the appropriate time, then cooled slowly in the furnace. This process requires the original tissue to be finely laminated pearlite without any carburized networks.

B) Isothermal spheroidizing annealing process:

The steel is heated and insulated, then cooled to a temperature slightly below Ar1 and held isothermally (usually 10~30℃ below Ar1) before being cooled slowly in the furnace to about 500℃, then taken out for air cooling. This method has the advantages of short duration, uniform spheroidization, and easy quality control.

C) Reciprocating spheroidizing annealing process.

5. Diffusion annealing (uniform annealing)

Process:

The ingots, castings, or forging billets are heated to a temperature slightly lower than the solid phase line for a prolonged period of time, then slowly cooled to eliminate unevenness in chemical composition.

Objective:

To eliminate dendritic segregation and regional segregation that occur during the solidification process, resulting in homogenization of composition and structure.

Diffusion annealing is conducted at very high temperatures, typically 100-200℃ above Ac3 or Accm, with the exact temperature depending on the severity of segregation and the type of steel. The holding time is typically 10-15 hours.

After diffusion annealing, the material must undergo complete annealing and normalizing to refine its structure. This process is applied to high-quality alloy steel and to alloy steel castings and ingots with serious segregation issues.

6. Stress Relief annealing

Process:

Heat the steel to a temperature below Ac1 (usually 500 to 650°C), hold it at that temperature, and then cool it in the furnace.

The stress-annealing temperature is lower than A1, so it does not cause any changes to the steel’s microstructure.

Objective:

To eliminate residual internal stress.

7. Recrystallization annealing

Recrystallization Annealing, also known as Intermediate Annealing, is a heat treatment process applied to metals that have undergone cold plastic deformation.

The objective of this process is to change the deformation grain into uniform and equal axial grains, which eliminates the process hardening and residual stress.

For recrystallization to occur, the metal must first undergo a certain amount of cold plastic deformation, and then it must be heated above a certain temperature known as the lowest recrystallization temperature.

The lowest recrystallization temperature for general metal materials is given below.

Trecrystallization = 0.4Tmolten

The recrystallization annealing temperature should be heated to a temperature that is 100 to 200℃ higher than the minimum recrystallization temperature (for steel, the minimum recrystallization temperature is approximately 450℃).

The annealing should be followed by proper heat preservation and a slow cooling process.

How to Choose the Annealing Method?

Selection of annealing

The following are the principles for selecting the annealing method:

  1. For hypoeutectoid steel structures, complete annealing is generally selected. If the goal is to shorten the annealing time, isothermal annealing can be used.
  2. Spheroidizing annealing is typically used for hypereutectic steel. If the requirements are not high, you can opt not to use complete annealing. Tool steel and bearing steel often use spheroidizing annealing. In some cases, spheroidizing annealing is also used for cold extruded or cold upset parts of low or medium carbon steel.
  3. To eliminate process hardening, recrystallization annealing can be used.
  4. To eliminate internal stress caused by various processing, stress annealing can be used.
  5. To improve the inhomogeneity of the structure and chemical composition of high-quality alloy steel, diffusion annealing is often used.

Purpose of Annealing

(1) To decrease the hardness of steel, increase its plasticity, and make machining and cold deformation processing easier;

(2) To uniformly distribute the chemical composition and structure of the steel, refine the grain size, and improve its performance or prepare it for quenching;

(3) To eliminate internal stress and reverse the hardening effect caused by processing, thus avoiding deformation and cracking.

Both annealing and normalizing are mainly used as a preparatory step for heat treatment.

For parts with low stress and low performance requirements, annealing and normalizing can also serve as the final heat treatment.

Annealing Oven

Types of Annealing Materials

When discussing annealing, it is essential to explore the materials that can be annealed, both metals and non-metals. This section will focus on the various materials that are commonly annealed.

Metals and Alloys

Annealing plays a significant role in the processing of various metals and their alloys. Some of the widely used annealed metals include:

  • Steel: Annealing is crucial for various types of steel, such as carbon steel, low-carbon steel, and tool steel. This process can increase steel’s ductility and facilitate easier shaping and machining.
  • Copper: Annealing copper helps enhance its ductility and relieve internal stresses. This allows it to be more effectively shaped and reduces the risk of cracks during bending.
  • Brass: Similar to copper, annealing brass enhances its ductility and workability, which is essential for manufacturing processes such as forming and machining.
  • Aluminum: This lightweight and versatile metal is annealed to improve its overall formability and create more uniform properties throughout the material.
  • Silver: Annealing is a critical step in the jewelry-making process for silver, as it softens the metal and makes it easier to work with.
  • Cast Iron: Annealing cast iron restores its ductility, making it less brittle and more suitable for applications where it needs to be machined or shaped.
  • Ferrous Metals: Annealing is beneficial for ferrous metals like steel and iron, as it helps enhance their machinability and improve their mechanical properties.

One commonly used method for annealing these materials is the use of car-bottom furnaces, which provide uniform heating and slow cooling essential for the annealing process.

Non-Metals

Annealing is also appropriate for various non-metal materials, such as:

  • Glass: Annealing glass involves heating it to a specific temperature and then gradually cooling it down. This controlled process alleviates internal stresses created during the glass-forming process.
  • Carbon: Annealing carbon materials, such as diamond and graphite, helps modify their properties to better suit specific applications. This can include modifications like improving their electrical conductivity or structural adjustment.

In conclusion, annealing is a vital process for a wide range of materials, including both metals and non-metals. By understanding the importance of annealing in different materials, we can better appreciate the role it plays in various industries.

Classification of Annealing Methods

According to the temperature used during heating, the commonly used annealing methods are categorized into:

Phase Change Recrystallization Annealing above the Critical Temperature (Ac1 or Ac3):

  • Complete annealing
  • Diffusion annealing
  • Incomplete annealing
  • Spherification annealing

Annealing below the critical temperature (Ac1 or Ac3):

  • Recrystallization annealing
  • Stress annealing

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13 thoughts on “7 Types of Annealing Explained”

  1. Thanks for your essay.
    By the way, my client had enquired for “soft-annealed” stainless steel tube TP304. What’d be the details then?

  2. Great post! The pictorial representation of the datanot only helps me understand the what is annealing but also helped me to understand at what stage it is been applied to the material and what are the result and benefits that we can obtain out of it. Great information on the post sharing with my friends to help them understand the process and its approach better. Thanks for saving the time and energy.

  3. Hello What you show in the initial Picture is that one guy manage to flex the annealed bar while the other guy don’t manage to flex the hardened bar. It gives the impression it is more difficult to flex a hardened bar , elastically. However that behaviour depends on thickness and elasticity modul and not on the yield strength. Sorry for the nurdy reflection . Olof

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