What is Annealing: Complete Annealed Heat Treatment Guide

Understanding Heat Treatment

Let us begin this discussion on a more general note. Annealing is essentially a heat treatment process, which is a broader field in metal production covering a range of processes.

In simple words, heat treatment is a controlled process that changes the physical and sometimes chemical properties of a material. Doing this is important in many cases because metals, fresh out of a foundry, generally have internal stresses and non-uniform microstructures due to cold working during their fabrication process. This renders them difficult to cut, hard, and non-isotropic. Therefore, heat treatments like the annealing process are essential to remove these undesirable material properties.

To heat treat a material, engineers pass it through a sequence of heating and cooling operations for specific time periods. Commonly, material properties like ductility, strength, and hardness improve as a result of these processes.

What is Annealing?

A comprehensive annealing definition – Annealing is a heat treatment process that involves heating a metal to a specific temperature, holding it at that temperature for a period, and then slowly cooling it back to room temperature. As a result, the metal undergoes a reduction in hardness and internal stresses, while improvements in ductility and workability make it more machinable.

Generally, annealing metal prepares it for further processing by ensuring it meets specific functional requirements.

How Does the Annealing Process Work?

The annealing process, among other heat treatment processes, is relatively simpler. It comprises three major steps, each of which requires professional process control and application of engineering knowledge. Let’s look at these three steps in detail.

1. Recovery

The annealing process begins with the stage of ‘Recovery’. In this process, engineers put the material in a furnace and heat it to its recrystallization temperature. At this temperature point, the annealed metal undergoes the removal of linear defects (dislocations) in its crystal lattice structure. In other words, this results in relieving internal stress and considerable softening of the metal.

In a typical recovery process, the material stays at this elevated temperature for a specified period (up to a few hours) to allow ample time for the dislocations to disappear.

2. Recrystallization

The second stage is the ‘Recrystallization’ phase, which is the core process of achieving the correct annealing metallurgy. During this process, the furnace heats the metal beyond its recrystallization temperature, to a temperature between the recrystallization temperature and the melting point.

At this temperature, the metal forms new grain structures without melting. These new, strain-free grains grow and replace the deformed grains of the metal, which are the main cause of internal stress and stored energy in the lattice structure. The new grains, due to fewer dislocations, are more uniform and isotropic. This leads to a more homogenous material with better ductility and reduced hardness.

3. Grain Growth

The final stage in the annealing process is ‘grain growth’. After recrystallization, maintaining the metal at high temperatures causes the grains to grow further. Larger grains enhance the toughness of the material by being capable of absorbing more energy. However, they also reduce material strength since larger grains have fewer grain boundaries in between them, and these boundaries are essential to resist the motion of dislocations (strains).

Precise control over the process in this stage helps achieve a balance between toughness and strength. Steering off the heat treatment guidelines even a bit can cost time and money to fix.

After the grain growth stage ends, the furnace is turned off, and the metal is allowed to cool down to room temperature without being removed from the furnace. This ensures a slow cooling rate free of any abrupt changes in the material’s thermal state.

The three stages of annealing are all about altering the thermal state of the material to

• remove prior dislocations,
• form new grains,
• grow new grains.

Why is Annealing Performed?

There are several reasons to anneal materials. These include improvement of material properties, workability, and overall functional value.

Generally, engineers use a range of mechanical testing techniques to measure material properties and determine the need and specifications for annealing processes.

Increase Ductility

Ductility is a measure of how easy it is to deform a material under stress. While it may seem to be an undesirable quality as we typically want fewer deformations, it is the core goal of the annealing process.

The reason is that higher ductility makes a metal easier to process. Ductile metals, as opposed to brittle metals, are better suited for operations like bending, rolling, or drawing without the risk of failures like cracking.

Reduce Hardness

Reducing hardness is among the core definitions of the annealing meaning. This also sounds counterintuitive, considering hard metals are generally favorable. However, they are also difficult to machine and shape.

These are major problems in manufacturing environments. Hard materials consume a lot more resources than relatively soft materials as they take more time to machine, require specialized tooling, and have higher tool wear.

Relieve Internal Stresses

Internal stresses develop in metals during manufacturing processes like casting, welding, or machining. These stresses can cause material failures by cracking or distortion. The annealing process relieves these internal stresses by removing the distortions in the microstructure to sort out. This redistributes the grains and reduces stress concentrations inside the material, making it more uniform and resistant to failures.

Enhance Toughness

Toughness is an important material property, defining how much it can plastically deform without fracturing. The grain structure refinement during annealing enhances the material’s toughness by promoting a more uniform grain size and reducing stress-causing impurities in the microstructure.

Improve Microstructural Homogeneity

Metals have complex microstructures that are sensitive to impurities and dislocations within the lattice structure. These irregularities can cause the material to have low ductility, exhibit non-isotropic behavior, and have stress concentrations.

The annealing process, by allowing unrestricted grain growth, removes such irregularities. In other words, it makes the metal’s microstructure more homogenous, improving its isotropic qualities and material properties.

Types of Annealing

The annealing process is very versatile and has various types. These annealing types differ in how the metal is processed through different stages but with the common goal of improving its material properties. Generally, the percentage carbon content in alloy steels determines which annealing process and temperature is suitable, as the figure below shows.

Let us dive into the common annealing process types.

Full Annealing

Full annealing is just another word for the annealing process described above, but is often also cited as a subtype of annealing. The annealed metal is heated up to a high enough temperature to allow readjustments of dislocations and inconsistencies.

Then, it is held at this temperature for a period for recrystallization to occur. Finally, in the grain growth stage, the new, uniform grains grow to larger sizes with defined grain boundaries.

Process Annealing

Process annealing, or subcritical annealing, is mainly for low-carbon steels. The metal heats up to a temperature slightly below its lower critical temperature (~727°C for steel), internal relieving stresses as with normal annealing but without causing a major phase change.

It is like an intermittent annealing process as the temperatures are not as high. Primarily, its goal is to restore ductility and toughness after light working (hammering).

Spheroidize Annealing

Spheroidize annealing is an annealing process that produces spheroidal or globular carbide forms in carbon steels. This enhances the material’s machinability significantly.

The annealing steel is heated to a temperature a little below its critical temperature range and held there for some time. Alternatively, engineers may also use heating and cooling cycles. This heat treatment promotes the formation of spheroidal carbides.

Generally, spheroidizing annealing is quite useful for tool & die applications.

Isothermal Annealing

Isothermal annealing is a specialized kind of annealing process that focuses on achieving maximum microstructural homogeneity with minimal distortions. Isothermal annealing is the same as full annealing up to the cooling stage.

Instead of letting the metal cool down in one go, engineers stop the cooling cycle at an intermediate point and hold the material at that point for a period. This allows the material to achieve an isothermal temperature state (same temperature throughout the body), promoting uniformity in the resulting material properties.

Which Materials Can Be Annealed?

Annealing is a versatile heat treatment process that is compatible with a wide range of materials. This includes various types of metals and other material types.

For each of these categories, the main goals of the annealing process are the same: enhance material properties for better workability and homogeneity.

Below is a list of common annealing materials:

• Steel: Annealing steel is often a necessary functional requirement to reduce hardness and enhance ductility and toughness.
• Cast Iron: Annealing softens the cast iron and makes machining easier.
• Copper: Copper annealing improves ductility, formability, and electrical conductivity, preparing it for applications like wire forming.
• Gold: Jewelers commonly anneal gold, and even silver, to make them softer and more convenient to work with for jewelry making.
• Glass: Annealing glass helps soften it for glass-forming processes. It also relieves internal stresses, enhancing its durability and resistance to breaking.

Annealing vs. Other Heat Treatment Processes

The annealing process is highly versatile, but not the only heat treatment process common in industrial fabrication. In this section, we present a comparison of annealing with some other heat treatment procedures.

• Annealing vs. Tempering: Tempering is done after hardening processes. It reduces hardness and brittleness and increases toughness.
• Annealing vs. Normalization: In normalization, the cooling cycle takes place in the air instead of the furnace, leading to a finer grain structure and higher strength.
• Annealing vs. Quenching: The cooling cycle in quenching is very rapid, almost instantaneous. This freezes the microstructure in its brittle and hard microstructural state instead of letting grains grow as with annealing.
• Annealing vs. Precipitation Hardening: Precipitation hardening improves the strength and hardness of metal alloys by heating them to a moderate temperature and then cooling them at a rate that promotes the formation of small, uniform particles (precipitates) in the metal matrix. The particles block dislocation movements, leading to higher strength and hardness.

Common Applications for Annealing

The annealing process is truly universal among heat treatment techniques. Below is a comprehensive list of common annealing applications for different materials.