The difference between annealing and tempering comes down to how it is treated. Annealing involves heating steel to a specified temperature and then cooling at a very slow and controlled rate, whereas tempering involves heating the metal to a precise temperature below the critical point, and is often done in air, vacuum or inert atmospheres.
What is Tempering?
Tempering, also referred to as drawing, is a heat treatment process in which the components are heated and held to a set temperature below the critical point for a certain duration. The components are then cooled to room temperature in still air.
Like other heat treatment processes such as annealing and normalising, the tempering process alters the metal’s undesirable mechanical properties to be more in line with the proposed application.
Tempering affects the entire component’s mechanical properties from the surface to the core. But partial tempering is also possible in induction plants.
Tempered metals are useful in applications that need a certain level of flexibility from their components.
This heat treatment process may also be used to reduce the hardness of recently welded components. The high localised temperature from the welding process can lead to high hardness in heat-affected zones. Tempering can help us alleviate these high-hardness sections.
In theory, tempering can be carried out on a wide range of metals but it is generally associated with carbon steel as few other metals react to this heat treatment method in the same manner as steel.
The Tempering Process
Like other heat treatment processes, the tempering process occurs in three stages. These stages are:
- Heating
- Dwelling
- Cooling
Heating
In this stage, we heat the metal to a set temperature between room temperature and the lower critical temperature. This temperature is our tempering temperature.
The heating to the exact temperature should happen at a controlled rate because if the metal is heated too quickly, it can lead to cracking. The suitable temperature varies depending on the type of steel and the desired change in properties.
For example, tool steels are tempered at around 200-300°C, spring steels at 300-400° and structural steels at 450-650°C.
Typically, the metal is heated in a furnace (gas, electrical or induction) in the presence of an inert gas or a vacuum to prevent oxidation. But certain steels are tempered in salt baths or even in the presence of air.
The chosen atmosphere also affects the surface of the components.
Dwelling
Once the metal has achieved the desired temperature below the critical point, it must be held at that temperature for a predetermined duration.
The duration depends on the type of steel, component cross-sections, charge size and the required mechanical properties.
Depending on the tempering temperature and dwell time, the mechanical properties of the hardened steel change.
The ductility, impact strength and toughness increase with higher temperatures and dwell time. The ultimate tensile strength, however, will reduce with rising temperatures.
The effect on hardness depends on the share of different phases such as martensite, retained austenite and graphite nodules.
As the time in the oven is increased, the martensitic phase reduces and retained austenite increases. As the austenitic phase is relatively softer, the entire component’s hardness reduces.
Cooling
The cooling stage is just as important as the first two. In the cooling process, the component is cooled, usually in the presence of air, in a predetermined manner.
The cooling rate and method used depends on various factors. For tempering, cooling usually takes place in still air.
What is Annealing?
Annealing is a heat treatment of metals or alloys that restores some of the material’s original physical properties. In particular, it increases ductility and decreases hardness.
Annealing is done by raising the temperature of the metal to above its recrystallization temperature but below its melting point. This higher temperature gives enough energy to the metal to allow migration within the metal’s microstructures.
This in turn results in dislocations being rectified, and internal stresses being relieved. Once the metal is cooled down again, its ductility will have been restored.
Annealing is understood to have been discovered as a heat treatment in the Middle Ages, around the 12th century. The word likely originated from the Middle English term “anelen”, meaning to set on fire or to bake.
It was in Europe, through advances in blacksmithing, that craftsmen discovered that annealing (heating to above a certain temperature and then allowing the metal to cool) changed the properties of iron and steel.
Annealing is important because it significantly reduces the risk of metal cracking from work hardening, and counteracts the work hardening that occurs during forming of a metal component.
Annealing allows the original physical properties of a metal to be restored after the component has been formed, removing grain deformations or internal stresses that would have been introduced during forming.
Annealing Process
The annealing process involves three main steps: recovery stage, recrystallization stage, and grain growth stage. Each has a unique function and must be tailored to the material being annealed.
1. Recovery Stage
The recovery phase is the initial step in the annealing process. During this phase, the material is heated below its recrystallization point.
The internal stresses and distortions introduced during prior processes, such as cold working, begin alleviating at this temperature.
This phase is crucial as it helps reduce residual stresses and restore some of the material’s original properties, improving its overall stability and performance.
2. Recrystallization Stage
The second step is the recrystallization stage. In this phase, the material is heated above its recrystallization temperature, causing new grains to form without any pre-existing stresses.
The high temperature allows atoms to move more freely in the crystal lattice, enabling them to form new, stress-free grains. The recrystallization temperature varies depending on the material, but it is typically around 50% of the melting point (in Kelvin) for most metals.
3. Grain Growth Stage
The final step in the annealing process is the grain growth stage. After recrystallization, the material is kept at a high temperature, allowing the new grains to grow.
This growth continues until the material is cooled, which should be done slowly to prevent new stresses from forming.
The slow cooling process allows atoms to rearrange themselves in the metal’s crystal lattice into a more stable and lower-energy state.
The result is a more ductile and less hard material. Annealing metal also minimizes deformities and increases toughness by relieving internal stresses.
The annealing process requires precise control of temperature and time at each step. The exact temperature and duration depend on the material being annealed and the desired properties.
For example, steel is typically annealed at temperatures ranging from 700 to 900 degrees Celsius, while aluminum is annealed at much lower temperatures, around 200 to 400 degrees Celsius.
Annealing Vs Tempering
The difference between annealing and tempering comes down to how it is treated. Annealing involves heating steel to a specified temperature and then cooling at a very slow and controlled rate, whereas tempering involves heating the metal to a precise temperature below the critical point, and is often done in air, vacuum or inert atmospheres.
While both processes are heat treatments, they follow different rules to produce different results for different purposes.
Tempered steel is used in applications where strength, toughness and elasticity are paramount. This includes large-scale construction work, industrial machinery and automotive drive trains. Tempering makes these applications possible and reduce any associated danger.
Annealing produces softer metals that can be used for products that do not need to endure substantial stresses. This includes many household items and other everyday products.
Differences Between Annealed and Tempered Steel
The processes for both annealing and tempering are somewhat similar; both involve the heating and cooling of steel, even though the timings and temperatures differ.
The reasons behind utilising these processes on steel are comparable; however, the intended applications and uses of the final products vary.
Purpose of Annealed Steel
The primary purpose of annealing steel is to achieve a good balance between hardness and ductility. When steel is hardened, it can lose its ductility; the harder the steel, the more brittle it will be.
Annealing helps the crystallisation structure of steel realign in a less brittle way while still keeping its strength. This process makes the steel much more durable, as it can be bent and absorb impact without cracking or breaking.
As well as this, annealing can also help to reduce the internal stresses caused by most hardening processes. For example, steel can often contain internal stresses in the crystal lattice structure when heated and formed, making the material more likely to break.
Annealing reorders, the grains in the crystal structure, allowing them to realign in a way that removes the pre-existing stresses naturally.
Purpose of Tempered Steel
The purpose of tempering steel is to achieve a similar effect to that of annealing steel; as previously mentioned, hardened steel is often more brittle than is useable.
Hardening processes are needed to make the material strong enough for particular uses, but it can cause stress in the structure that results in brittleness.
Therefore, tempering reduces tensions in the steel, allowing for the optimum balance between hardness and strength.
Tempered steel is less brittle, with increased ductility and machinability. It is also much easier to weld without the internal stresses in the lattice structure.
Application of Annealed Steel
Steel is annealed in order to achieve a stronger and more ductile final product. These qualities make annealed steels a prominent material used across the manufacturing industries.
As well as this, annealed steels can be used in tool-making and machines as they are ductile enough to be moulded into shape but strong enough to resist wear and tear that comes with use.
Cold-rolled sheet steel and ground flat stock are often annealed as the process of cold rolling can make the steel too hard and therefore brittle.
By annealing the steel, we restore the material’s ductility, allowing for bending, cutting, and punching without the risk of cracking the steel.
Application of Tempered Steel
The overall purpose of tempering steel is to make it more wear-resistant, increase ductility, and make it less brittle. Tempering steel also improves the material’s weldability, which is helpful in many industries.
Common uses for tempered steel include bridge construction, gears and mechanical parts, deflector plates, and storage tanks.
Tempering is also an important process in the knife-making industry. The high ductility and wear resistance make tempered steel ideal for knives and swords. In addition, traditional Japanese katanas were created using tempering!