Annealing of Steel: How's it Done?

Annealing of steel is a heat-treatment process where steel is heated above its recrystallization temperature and then allowed to cool in a regulated environment. This increases ductility while reducing hardness and relieving internal stresses, making it suitable for intricate machining and fatigue-loading applications.

This article provides a detailed guide on the steel annealing process by going through its types, advantages, limitations, and applications.

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What is Steel Annealing?

Parameter	Optimal value
Annealing temperature of steels	400 °C to 900 °C (673.12 K - 1073.15 K).
Holding time at the annealing temperature	1 hour for every 1" (25.4 mm) of thickness
Cooling rate (per hour)	15 °C to 22 °C (288.15 K - 295.15 K)

Steel workpieces can undergo various heat treatment processes such as normalizing, annealing, tempering, and surface treatment processes like anodization of steel and hot dip galvanization.

However, annealing is one of the most common metal processing treatments performed on steel workpieces to improve their machinability.

Annealing requires almost similar tools to metal melting tools, such as furnaces, tongs, crucibles, and other safety gear.

The annealing process requires heating steel in a furnace above its recrystallization temperature, maintaining the heat for a fixed time interval, and finally cooling the workpiece in a controlled atmosphere.

Annealing metals like copper, silver, and brass usually require quenching in water for cooling, whereas steel is cooled inside the furnace after turning off the heat source, similar to annealing plastics.

Heating the material alters the grain structure and reduces the number of dislocations/irregularities, which enhances ductility and reduces hardness. 

The slow cooling rate during steel annealing favors the proper development of grain structures, which results in minimal internal stresses in the workpiece.

Generally, annealing enhances the ductility of the workpiece and is often performed while hot forging steel workpieces, to enhance their ductility for forming.

Annealed steel wires are commonly used for different industrial applications due to their enhanced strength and flexibility.

Steel Annealing- The Process

The process of steel annealing is categorized into the recovery, recrystallization, and grain growth stage.

Recovery stage

In the recovery stage, the steel workpiece is heated in a furnace up to its recrystallization temperature.

Generally, steel annealing of large workpieces is performed in industrial furnaces. However, a cheaper alternative for DIY applications is to use a torch to heat the workpiece.

During this stage, the grain structure is refined to minimize dislocations (irregularities), thereby eliminating internal stresses.

At this stage, the microstructure of steel undergoes two mechanisms: annihilation and rearrangement.

The two principal mechanisms for this stage are annihilation and rearrangement.

During annihilation, the dislocations present in the grain structure, integrate and thereby reduce in number.

During the heating process, the kinetic energy of the steel workpiece increases, which increases the probability of interacting and merging the dislocations.

The recovery stage also involves the rearrangement of dislocations in such a way that the stresses interact and cancel out each other, reducing the internal energy of the material. 

Generally, a higher recrystallization temperature leads to more heat, which results in effective interaction between the dislocations, increasing the efficiency of the recovery stage.

Recrystallization stage

The second stage is recrystallization, which involves heating the steel to the annealing temperature, which is about 37.8 °C (310.9 K) above its recrystallization temperature.

Generally, the recrystallization temperature for different types of steel varies between 400 °C (673.12 K) and 900 °C (1073.15 K).

For example, in full annealing, AISI steel types 1018, 1020, and 1025 have an average annealing temperature of 877.5 °C (1150.65 K), whereas type 1030 has a temperature of around 865 °C (1138.15 K).

Similarly, AISI steel types 1060, 1070, 1080, and 1090 have an average annealing temperature of around 817.5 °C (1090.65 K), while 1040 and 1050 have an annealing temperature of around 830 °C (1103.15 K). 

The recrystallization stage also alters the grain structure to reduce any deformities present in the steel, thereby reducing its hardness while enhancing its ductility.

The recrystallization temperature of the material is dependent upon its grain size. The larger the grain size, the lower the recrystallization temperature will be.

Lower recrystallization temperature will result in reduced heat requirement to transition into the grain growth stage, reducing the overall cycle time.

Therefore, the recrystallization stage is dependent upon grain formation in the recovery stage.

Once the annealing temperature is achieved, which is evident from the orange-red color of the workpiece surface, it is maintained for a specific period.

A general principle is to maintain this temperature for approximately 1 hour for every 1" (25.4 mm) thickness of the steel workpiece.

For instance, if your steel workpiece has a thickness of 2" (50.8 mm), you must maintain the annealing temperature for at least 2 hours. 

Grain growth stage

This stage involves cooling steel inside a furnace after turning off the heat source. 

In this stage, the grain structure fully develops and reduces the internal energy by reducing the total grain boundary area.

The cooling rate is generally kept slow to facilitate the gradual development of grain structure, minimizing the brittleness of the steel.

DIY users and hobbyists can place an insulating blanket or sand over the hot workpiece to cool it down, as this method is cheaper and simpler to execute.

Generally, the optimal cooling rate for annealing steel is between 15 °C (288.15 K) per hour to 22 °C (295.15 K) per hour.

Parameters That Affect Annealing Process

The annealing temperature of the steel workpiece also depends upon its thickness, carbon composition, and geometry. 

Steels having a higher carbon composition (free-machining steel) will have a lower annealing temperature and vice versa.

Moreover, the annealing temperature increases as the thickness of the workpiece increases.

Similarly, steel stock with a greater surface area and volume will also require more heat, thereby increasing the annealing temperature.

Types of Steel Annealing

Steel annealing is classified into seven types based on the extent of workpiece heating.

Type of Annealing	Annealing temperature	Heat Maintaining Period	Cooling Rate
Complete	Above recrystallization temperature	Moderate	Moderate
Isothermal	Above recrystallization temperature	Moderate	Moderate
Incomplete	Above recrystallization temperature	Short	Slow
Diffusion	Above recrystallization temperature	Long	Slow
Spheroidizing	Above recrystallization temperature	Short	Slow
Recrystallization	Below recrystallization temperature	Moderate	Slow
Stress	Below recrystallization temperature	Moderate	Moderate

Every type of steel annealing process reduces hardness and improves ductility, but each result in specific properties being more dominant than others.

The main applications of full steel annealing and isothermal annealing include reducing hardness, obtaining a fine and uniform grain structure, eliminating internal stresses, and improving machinability.

Incomplete steel annealing mainly reduces hardness, eliminates internal stresses, and improves machinability.

Moreover, diffusion and spheroidizing steel annealing result in a uniform arrangement of grain structure.

Similarly, recrystallization and stress annealing aim to reduce internal stresses in the material.

The steel type dictates the heat maintaining period, cooling rates, and ultimately the annealing type that best suits the application.

For example, recrystallization annealing is more suitable for low-carbon steels, while complete annealing is preferable for high-carbon steels. 

It is due to the reason that higher carbon composition steels require more heat for refining the grain structure.

Advantages of Steel Annealing

Improved Mechanical Properties

Annealing increases the ductility of steel while reducing its hardness. It is beneficial in cases where a steel structure is exposed to fatigue or heavy loading. 

For example, a truss bridge comprising high-ductility annealed steel bars will successfully resist deformation due to fatigue or heavy loads. 

On the other hand, low-ductility steel bars will have a higher probability of failure and subsequent bridge collapse.

Improved Machinability

The annealing process also improves the machinability of steel, making it suitable for applications where multiple machining operations are to be performed on the steel stock.

For example, if you want to machine a complex steel component, such as a bevel gear, from steel stock, annealing the stock will facilitate a high cutting speed with low tool wear and low power consumption. 

This way, you can speed up your process and extend the tool's life.

Relieving Internal Stress

Internal stresses can cause dimensional instability, cracking, and deformations, leading to the failure of steel components during operation. 

However, eliminating them increases the overall mechanical strength of the component, lowering the probability of failure.

For example, a steel bar with internal stresses is bound to fail due to cracking. However, annealing the steel bar will result in improved mechanical properties and make it less prone to failure under stress.

Improved Magnetic Properties

Enhanced magnetic properties in steel objects can be beneficial for use in electromagnetic sensors, magnetic shielding, or electromagnets.

Limitations of Steel Annealing

High Cost

The high initial and running costs of steel annealing make it suitable for large-scale industries, with no or minimal scope for small-scale businesses and DIY users.

Although you can use a flame torch for DIY steel annealing, the energy cost of the process still remains high and the results obtained will be comparatively poor than a professional steel annealing operation.

Time Consuming

The time-consuming process of annealing can be a drawback for large-scale industries where quick cycle time is of utmost importance.

Hazardous

The steel annealing process requires handling hot workpieces and heat sources, which can cause severe injuries if proper safety protocols are not implemented.

It requires an experienced operator with good training or certification in carrying out a large-scale steel annealing process.

Coarse/Non-uniform Grain Structure

Annealing requires good process control throughout the execution of the process because even a slight variation in temperature can result in coarse-grained steel structures with non-uniform mechanical properties.

This reduces the strength of the steel workpiece and makes it prone to failure under heavy or cyclic loads.

Applications of Annealed Steel

Annealed steel holds crucial importance in different industries.

Application	Examples
Infrastructure	Building support structures, trusses, bars
Automobiles	Frames, doors, axles, bearings
Railways	Frames and supports
Aerospace	Frames and supports
Utilities	Refrigerators, HVAC equipment, boilers, turbine blades

Due to favorable mechanical properties like enhanced ductility, improved machinability, reduced hardness, minimal residual stresses, and greater strength, annealed steel has vast potential in various industries.

Automobile, railway, and aerospace applications involve exposure to fatigue and high loads. Due to this, annealed steel is best suited for use in different components associated with these industries.

Infrastructure applications also involve fatigue, excessive loadings, and high vibrations that may become seismic due to an earthquake.

Annealed steel ensures the structural integrity of infrastructure due to its high strength and durability. 

Apart from that, annealed steel is also used for utility applications like making tools and household utensils.

Final Thoughts

Annealing enhances the ductility of steel and improves its machinability, making it favorable for manufacturing operations.

Different types of annealing processes induce different mechanical properties in steel, and therefore, it is advised to identify your requirements before selecting the optimal annealing process.

Although there are various metalworking processes like tempering, normalizing, anodizing, etc., annealing steel produces the best results for applications that require extensive machining of the steel workpiece.

Apart from steel, annealing can be performed on various other metals, such as aluminum, copper, etc., to improve their properties.

Frequently asked questions (FAQ)

What is recrystallization temperature in a heat-treatment process?

Recrystallization temperature is when a material's grain structure begins to reorient itself, indicating an increase in ductility and a transition towards melting. 

Is annealed steel prone to rusting?

Yes, annealed steel is prone to rusting as annealing does not protect against rusting.

Does annealing enhance steel's cold-working ability?

Yes, annealing does enhance steel's cold-working by increasing its ductility and machinability due to changes in grain structure.