Annealing can be divided into three phases.

Phase 1: Heating

The first phase is the heating step. During this step, the workpiece is heated to the annealing temperature and then heated all the way through. In this phase, it is important to observe the specific heating rate for the respective material.

Phase 2: Dwell

The second phase is called the dwell phase. The component is held constant at the annealing temperature, allowing temperature equalisation throughout the entire workpiece. In addition, the desired physical and chemical processes can now be balanced. The necessary dwell time depends on the type of material, the shape of the component, and the position in the annealing furnace.

Phase 3: Cooling

The process concludes with the cooling. The component is now cooled down to the ambient temperature. The cooling rate can play an important role in this phase as well. This ends the annealing process.

Annealing colours for the annealing of steel

More complex thermal treatments

The three annealing phases may have to be subdivided further especially when high quality requirements are imposed on a material. For some types of material, the treatment is subdivided into nine stages. These subdivisions are specified in annealing specifications/annealing programmes. However, the term "annealing programme" is also used for several consecutive annealing processes applied to different products or to the same workpiece.

Distinction by annealing method

Annealing is categorised into the following methods:

• Normal annealing
• Soft annealing
• Annealing to spherical cementite
• stress-free annealing
• Coarse grain annealing
• Recrystallisation annealing
• Diffusion annealing or homogenisation annealing
• Low hydrogen annealing or hydrogen effusion annealing

The normalising of steels is also referred to as normal annealing. The goal of this process is to evenly distribute a fine-grained crystallite microstructure across the entire workpiece. The necessary temperature depends on the carbon content. Low-carbon metals can be treated by normal annealing up to 950 °C. Steels with a high carbon content, on the other hand, are normally annealed just below 800 °C.

The temperatures used during soft annealing range between 650 °C and 800 °C. In this temperature range, the precipitation of cementite and pearlite is reduced. The steel's hardness and strength are both reduced, and its formability is increased.

The process of ASC annealing, short for annealing on spherical cementite, is comparable to soft annealing. The focus here is on attaining a high degree of incorporated carbide, in order to enable cold forming at room temperature. This is achieved by cycle annealing and slow cooling.

The objective of stress-free annealing is to eliminate residual stress in the steel, without affecting its other properties. This is achieved at temperatures between 480 °C and 680 °C.

As the name suggests, coarse grain annealing increases the size of the crystallites, resulting in reduced strength and toughness - and creating ideal conditions for metal-cutting machining.

During recrystallisation annealing, the material is heated to a temperature just above the individual recrystallisation temperature. This temperature depends on the degree of deformation and the melting temperature of the material. It generally lies between 550 °C and 700 °C. The aim of this annealing method is to return crystallite microstructures previously altered by cold forming to their original state.

Another method is diffusion annealing, also called homogenisation annealing. At temperatures between 1,050 °C and 1,300 °C, foreign atoms are incorporated evenly into the metal lattice of the workpiece being treated. This process can take up to two days.

Finally, there is hydrogen effusion annealing, or low hydrogen annealing. At temperatures between 200 °C and 300 °C, an effusion process takes place. This involves the release of hydrogen atoms which previously embrittled the material.

The advantages at a glance

In summary, various annealing methods can yield the following benefits:

• Enhanced mechanical properties
• A microstructure better suited for cold forming
• Reduction of stresses
• Improved preparation for cutting and non-cutting machining
• Restoration of the initial state

The correct temperature 

Temperature range during soft annealing

The advantages at a glance

Negative aspects

Stress-free annealing is used to reduce intrinsic tensions in metallic microstructures. The process prevents the intrinsic stresses in the metal from combining with load stresses that can occur through further processing or use of the workpiece. This protects the workpiece from warping and breakage.

Processes that can lead to intrinsic stresses in the metal include:

• Metal-cutting machining such as turning or milling
• Welding
• Cold forming such as deep drawing or bending
• Uneven cooling caused by, for example, prevention of contraction during casting
• Microstructural transformation

If executed properly, stress-free annealing can generally reduce stresses by up to 90%. Stress-free annealing can take place in a convection furnace.

In order to protect the workpiece surface from oxidation, stress-free annealing is carried out in a protective gas furnace. For applications that impose stringent requirements on the component surface, treatment in a vacuum furnace may also be considered.

Individual phases

Stress-free annealing is essentially divided into three phases: warming up, dwelling, and cooling. During the warm-up time, the material is heated. This warm-up phase is followed by a dwell time that lasts four to six hours at a constant temperature.

The annealing temperature and the annealing time play essential roles in the success of the process. Finally, during the cooling time, the workpiece is slowly cooled in the air or in the furnace, in order to prevent stresses and cracking from reoccurring.

Both the warm-up phase and the cooling phase should be carried out in a slow and controlled manner in accordance with the thermal conductivity of the material. This ensures a low temperature gradient between the surface and the core of the workpiece. If performed too fast, the warm-up or cooling phase can lead to new stresses. This is of particular significance for workpieces with large differences in shape and wall thickness.

The advantages at a glance

The advantages offered by stress-free annealing include:

• Reduction of stresses by more than 90%
• Improved machinability of the material and thus extended service life for tools
• Further processing possible with minimum warping
• Risk of cracking is minimised
• No increased hardness on the component surface
• Greater precision
• Short processing times thanks to low machining allowance

Stress-free annealing is useful after the rough machining of workpieces. It is, for example, suitable for removing stresses from welded structures.

Moreover, this process is a good preparatory measure for the fine machining of workpieces. For example, components having low dimensional tolerances that are to be treated by nitrocarburising must be as tension-free as possible.

The correct annealing temperature is essential for achieving the desired result. It is dependent, among other factors, on the type of material and the pre-treatment. Quenched and tempered steels , for example, are annealed at least 30 °C below the last tempering temperature. Hardened cast irons cannot be annealed at all, because the annealing temperature would trigger a tempering effect.

Untempered castings are suitable for annealing. The correct temperature depends on the alloy composition. For non-alloy grades of cast iron, the temperature is 500 °C to 550 °C, for low-alloy grades, 550 °C to 600 °C, and for high-alloy grades, 600 °C to 650 °C.

Fine grain steels are annealed at temperatures below 580 °C, because they will otherwise experience coarsening of the microstructure. Stainless steels are usually treated using solution annealing.

Copper and brass components can also be treated using stress-free annealing. Depending on the composition of the alloy, the annealing temperature for brass components ranges between 250 °C and 500 °C, and for copper workpieces between 150 °C and 275 °C.

Precipitation hardening takes advantage of the fact that the solubility of some alloy constituents falls as the temperature decreases. The desired outcome requires the completion of three steps: solution annealing, quenching, followed by ageing (during which the actual precipitation takes place).

Solution annealing (diffusion annealing, homogenising)

In order for the subsequent precipitation to succeed, all necessary elements must first be present as dissolved elements. Solution annealing is used for this purpose. During this step, the correct temperature is of key importance. It must be high enough to minimise the presence of coarse particles. However, it must also not be too high, because otherwise structural components will melt, rendering further processing impossible.

This process may only take a few minutes, but can also last several hours. This depends on the size of the component, the fineness or coarseness of the microstructure, the alloy type, and the processing of the semi-finished product (e.g. forged, pressed, etc.).

INFO: Dispersoids
The so-called dispersoids are precipitated as early as solution annealing. These particles impede the movements of the grain boundary and thus control recrystallisation. Due to their size and low concentration in the material, they only cause a negligible increase in strength.

The next step in the process is quenching in a suitable medium. Suitable media are water or oil, and also gas or compressed air. The quenching medium depends on the material. Quenching prevents diffusion, and causes the mixed crystal to assume a metastable, oversaturated, and single-phase state.

Ageing

The actual precipitation/ageing takes place in the final step of the process. The temperature determines the duration and the type of the precipitation. Factors such as nucleation and precipitate maturation can be adjusted.

The correct temperature for precipitation depends primarily on the material alloy. For aluminium alloys and maraging steel, the temperatures range between 150 °C and 190 °C and between 450 °C and 500 °C respectively.

While the diffusion or precipitation is accelerated by the temperature increase, the oversaturated, single-phase mixed crystal is transformed into a two-phase alloy.

The first phase is the matrix – it is coherent in terms of volume, and usually makes up the larger portion. The newly formed second phase is the precipitate– a homogeneous microstructure of many small precipitates that can be adjusted specifically.

Normal annealing is intended to compensate negative effects to the microstructure caused, for example, by casting or welding. During rolling, normal annealing can even be carried out while the procedure is in progress. This is referred to as normalising rolling.

Normal annealing is generally a process with good reproducibility. It consists of three steps: heating, dwell time, and cooling.

The heating is carried out up to above the GSK line. During the process, the material approaches its hardness temperature between 800 °C and 920 °C. At this temperature, the larger ferrite grains are transformed into smaller austenitic grains.

Heating is followed by the dwell time, to achieve a uniform transformation of the microstructure. The dwell time depends on the size and shape of the component. It usually ranges between one and eight hours.

The workpiece is afterwards cooled in the air or in gas. During this phase, ferrite grains form again, but with a finer grain size. The objective of normal annealing has thus been achieved – a homogeneous microstructure.

In order to protect the material from decarburising and oxidising, normal annealing can be performed in a protective gas atmosphere.

INFO: Formula for calculating the dwell time

Dwell time (in minutes) = 60 + maximum workpiece diameter (in mm)

Changes in microstructure during normal annealing

The role of temperature

For energy-related reasons, steel microstructures are predisposed to form a single large grain. In the course of this, coarse grain formation takes place. High temperatures promote the necessary diffusion processes. This is why it is advisable to choose a normal annealing temperature that is not too high. To avoid coarse grain formation, it should generally not exceed the GSK line by more than 30 °C.

For supra-eutectoid steels, the process should produce a pearlite-cementite microstructure. To this end, temperatures just above the upper transformation point A1 are selected.

For sub-eutectoid steels with a carbon content of less than 0.8%, the objective of normal annealing is to create a ferritic-pearlitic microstructure. To achieve this, annealing temperatures between 30 °C and 50 °C above the upper transformation point A3 should be selected.

Temperature range during normal annealing

The advantages at a glance

Normal annealed workpieces benefit from the following advantages:

• Elimination of unwanted bainite or carbide components in the microstructure
• Formation of a fine-grained, homogeneous microstructure
• Enhanced mechanical properties
• Improved workability and machinability

To achieve grain refinement, normal annealing is often performed after steel processing procedures such as casting, forging, or welding. It is therefore suitable, for example, for forged parts, welded structures, and cast steel parts.

The materials listed below are particularly suitable for normal annealing:

• Low-alloy steels
• Various copper alloys
• Aluminium and its alloys
• Titanium

Non-transforming steels, such as ferritic and austenitic stainless steels, are not suitable for normal annealing. These types of steel do not meet the basic prerequisite of a α-γ-α transformation.

We are delighted that you wish to commission normal annealing from us. In order to quote you an offer as soon as possible, we require the following information in advance:

• Material number
• Other types of processing

Refer to our interactive location overview to find out which Härtha sites offer normal annealing.

The advantages at a glance

The objective of intermediate annealing is to optimise the crystalline structure of the material and thus improve its mechanical properties. Depending on the material and application, intermediate annealing can also help to increase corrosion resistance and resistance to high temperatures.

Intermediate annealing can be used directly during the production process of a workpiece, for example after case hardening. For this, after carburising or grain refining, the workpiece is heated below the lower transformation point for an extended period, and then slowly cooled.

The process is also used to extend the service life of previously used components, for example moulded steel components in die casting moulds. The temperature changes resulting from use have created stresses in the components, and these are now relieved.

In quenched and tempered steel parts , intermediate annealing corresponds, in principle, to a tempering treatment. The selected temperature is between 30 °C and 50 °C lower than the tempering temperature during production. The dwell time is between two and four hours. This type of intermediate annealing can be repeated after the component was used for a further period.

The advantages at a glance

Intermediate annealing offers various advantages for workpieces in production, and for components already in use:

• Reduced stresses
• Improved formability
• Improved workability
• No cracking, even in multi-stage transformation processes
• Longer service life