Annealing vs Tempering: Two Different Processes

Annealing and tempering are the most commonly used heat treatment processes in various industries like automobile, aerospace, construction, etc.

Although both treatments involve heating the metal workpiece to alter its properties, there are significant differences in their process, outcome, and applications.

The difference between annealing and tempering lies in the temperature at which the material is heated during the process. Annealing involves heating metal above its recrystallization temperature to enhance its ductility and machinability, whereas tempering involves heating the workpiece below its recrystallization temperature to reduce brittleness.

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Difference Between Annealing and Tempering

The primary purpose of annealing is to improve the ductility and machinability of a workpiece, whereas tempering aims to reduce the hardness and brittleness of a material.

As a result, both treatments are important metal processing techniques to alter the mechanical properties of metals, making them suitable for different applications.

Apart from this fundamental difference, several other aspects distinguish these two heat treatments. 

Parameter	Annealing	Tempering
Heating	Above recrystallization temperature	Below recrystallization temperature
Cooling process	In still air or via quenching	In still air
Ductility	Relatively higher	Relatively lower
Hardness	Relatively lower	Relatively higher
Machinability	Higher	Lower
Strength	High	High
Internal stresses	Low	Moderate
Change in color	Negligible	Noticeable
Applicable materials	Different metals and metal alloys like steels, aluminum, brass, copper	Mainly steels
Applications	Mechanical components, electrical components, and household items	Construction industry, industrial machinery, and automotive components

Based on Process Parameters

Annealing and tempering are heat treatment processes where the workpiece is heated to a specific temperature, maintained at that temperature for a particular period, and then allowed to cool under different conditions.

The heat preservation period in both heat treatments depends upon the material composition.

In the case of annealing, ferrous materials are generally allowed to cool gradually inside the furnace with the heat source turned off, whereas metals like copper and brass are cooled rapidly by quenching in water.

On the other hand, tempering involves cooling the workpiece by exposing it to ambient air or quenching it in water or oil. 

Based on Desired Material Properties

Both processes alter the grain structure by realigning the dislocations/irregularities in the microscopic arrangement.

However, the differences in the process parameters result in different grain structures of the workpieces, which leads to differences in the properties of the workpieces.

Although both these techniques increase ductility, strength, and machinability, annealing results in a comparatively more ductile and machinable workpiece.

The annealed workpiece has greater strength and reduced hardness, making it ideal for various applications that involve machining metal workpieces.

Whereas tempering aims to reduce the workpiece's brittleness while enhancing its ductility, machinability, and strength.

Furthermore, annealing comprehensively eliminates internal stresses from the workpiece, while tempered workpieces can have internal stresses that enhance their hardness.

Tempering also results in discoloring of the workpiece surface, whereas this effect is negligible in annealing.

Apart from these two process, to achieve a refined grain structure and improved workpeice hardness, you can also opt for normalizing your metal workpieces.

Based on Applicable Materials

Both these techniques apply to a wide range of metals and certain non-metals like glass and plastics, but annealing is best suited for steel, aluminum, copper, and brass. 

Non-ferrous metals can not undergo tempering, while steel workpieces are most favorable for the tempering process.

If a non-ferrous material undergoes tempering, it may embrittle or harden, making it more vulnerable to fracture.

Based on Applications

As annealed materials are more ductile and easier to machine, they are used for making household items like kettles and utensils, knives, electrical components like wires, and mechanical components like gears and bearings.

In contrast, tempered workpieces possess higher strength and relatively higher hardness, making them ideal for high-loading and fatigue applications like automotive components, industrial machines, buildings, and bridges.

Factors that Determine the Selection of Appropriate Process

A proper selection criterion is crucial before selecting a heat treatment process for your application.

The first and foremost aspect should be the purpose of the heat treatment process.

Annealing aims to improve ductility and machinability, while tempering is intended to reduce brittleness and hardness.

These two aims may seem similar, but the resulting material properties differ in many aspects.

This fundamental aspect falls under the four key factors that govern their selection.

Type of Workpiece Material

Each heat treatment process suits a specific group of materials, which means that your workpiece material is crucial in selecting the best-suited process.

Tempering is recommended for ferrous material stock that is to be subjected to high fatigue and high-load situations.

On the other hand, annealing can be performed on almost any metal that requires enhanced ductility.

Generally, annealing is performed on parts that are cast or forged, such as annealing of cast aluminum.

Required Application

The material properties resulting from each heat treatment process dictate its suitable applications.

For instance, tempering will be the right option for treating steel beams required in building construction, as it will reduce their brittleness while improving their toughness and strength.

This will ensure the structural integrity of the building as the constituent beams will be more durable after tempering.

Similarly, an automotive component subjected to fatigue and high loading conditions should undergo tempering for improved strength, toughness, and lower hardness.

On the contrary, a brass stock, that is to be machined for a complex medical implant, should first undergo annealing to enhance its machinability without any adverse effect on its mechanical properties.

An application requiring machining a steel block into a spur gear will ideally require the stock to undergo annealing, as it will enhance its ductility and machinability while improving its strength. 

The resulting spur gear will be accurate in shape and perform optimally under high or fatigue loading.

Cost of the Process

Cost is also a deciding factor in selecting the appropriate heat treatment technique.

Although both processes involve high costs, making them unfeasible for DIY users, tempering is comparatively cheaper.

The equipment required in both these techniques is similar, but the heating requirement in annealing is higher as a workpiece needs heating beyond the recrystallization temperature, which leads to higher fuel costs.

Therefore, tempering should be your go-to process if enhanced ductility and machinability are to be achieved under a budget.

Therefore, if a user needs a simple heat treatment for a workpiece, the tempering process will be preferable in terms of cost.

Time Taken to Complete the Process

Both techniques are time-consuming, but even a slight difference in times can affect productivity, especially if a bulk manufacturing industry is in question.

The annealing process takes longer than tempering as the stock requires more heating, whereas tempering heats the object below the recrystallization temperature, reducing the cycle time.

For instance, a bolt manufacturing factory would be better suited to temper its stock so that the bolts are durable and produced in larger quantities in shorter times. 

Furthermore, tempering would be the better choice if a DIY user or hobbyist wants to treat a workpiece using a hand torch, as it will save the user's time and effort.

Annealed Steel vs Tempered Steel

Annealed Steel	Tempered Steel
More ductile	Less ductile
Negligible internal stresses	Moderate internal stresses
Lower strength	Higher strength
Higher machinability	Lower machinability
Relatively lower hardness	Relatively higher hardness
Negligible discoloring	Discoloring of the workpiece
Enhanced conductivity	Slightly improved conductivity

Annealed and tempered steel differ in various aspects. 

Annealed steel is more ductile and machinable due to its enhanced grain structure resulting from dislocations during the annealing process, which renders it suitable for applications requiring cold working or machining.

Tempered steel has higher strength and toughness, with some degree of hardness, which makes it suitable for applications requiring regular heavy or cyclic loading.

Furthermore, tempered steel has low compressive stresses on its outer surface and tensile stresses on its inner side, resulting in enhanced hardness of the workpiece.

Conversely, annealed steel has negligible residual stresses, which leads to relatively lower hardness. 

There is no discoloring on the surface of annealed steel, meaning that the original color is maintained.

On the contrary, tempering leads to the formation of colored shades on the material surface, varying from gray to blue hues.

These different colors show the varying degrees of mechanical properties developed in the material after the process.

Although both tempering and annealing improve electrical conductivity, annealed steel offers comparatively greater conductivity than tempered steel.

Final Thoughts

Both annealing and tempering alter the mechanical properties of the material, making them favorable for significant industrial applications.

While annealing is ideal for applications requiring enhanced machinability and high ductility, tempering is recommended for applications that require enhanced machinability with improved hardness.

Selecting the appropriate heat-treatment process based on your application will ensure improved functionality.

Frequently Asked Questions (FAQ)

What is recrystallization temperature in a heat treatment process?

Recrystallization temperature is the temperature at which a material’s grain structure begins to reorient itself, indicating an increase in ductility and a transition towards melting. 

Does tempered or annealed steel rust?

Yes, tempered and annealed steel can rust as these heat-treatment processes do not induce any corrosion-resistant properties.

How does tempered glass differ from annealed glass?

The main difference between tempered glass and annealed glass can be observed in the way they break when subjected to load. Tempered glass generally shatters into small pieces, whereas annealed glass breaks with long shards, which can be dangerous.