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Heat treatment

The range of applications for industrial furnaces is extremely diverse. A distinction is made here between numerous different applications in which the products to be heat-treated are subjected to specific temperatures depending on the application. A wide variety of materials (such as steel, aluminium, plastic, rubber, silicone, carbon, composites, composite materials, glass, ceramics etc.) in their various, sometimes complex, component geometries with parameters specific to the product are not simply heated in the industrial furnace, but sometimes follow different time/temperature levels for the respective application. In addition, depending on the customer requirements, application, product and filling level of the industrial furnace, there are further parameters to be considered or met, such as heating and cooling rates, temperature homogeneity, the safe handling of any flammable materials escaping from the product, and the measurement, recording and documentation of the process.

At Airtec, the focus at all times is on implementing the targeted and necessary process and customer requirements. The many years of material and application-specific know-how, in connection with the application of continuously developed and flexible Airtec technology, guarantee the optimal implementation of all heat treatment processes for the benefit and success of our customers.

The industrial furnace and heat recovery systems from Airtec are used for the manufacture of a wide and sometimes highly complex range of products in different industries. All procedures required for the heat process technology are purposefully applied here within the scope of the product to be treated.

A distinction is primarily made between the following heat treatment applications:

Annealing, ageing, heating/warming, hardening, solution heat treatment, tempering, drying

The following descriptions show, without claiming to be exhaustive, the diversity of the applications as well as what needs to be considered when mapping the process, among other things.

Annealing process

During the annealing process, the product to be treated is heated to a temperature below its melting point for an extended period of time.

The product is heated to a temperature above the lower flash limit and maintained at this level until the product is heated uniformly.

The product is then cooled down in a targeted manner, whereby a distinction is made between two cooling modes (slow, fast).

While slow cooling frees the product to be treated from internal stresses, for example, to avoid unwanted deformations, and increases the toughness of metal products depending on the duration of the tempering process, fast cooling uses the “freezing” of the structural matrix to increase the resistance, namely the thermal and mechanical strength of the material.

After steel hardening or welding, for example, heat treatments such as annealing are necessary to achieve the required material properties (toughness, yield strength).

At temperatures typically between 200 °C and 550 °C, heat treatment can last from minutes to several hours.

During the annealing of springs, for example, the springs are also partly pre-tensioned, namely loaded on the block, in order to prevent subsequent setting.


With the ageing process at temperatures of usually 100 °C to 240 °C, strength values can be increased or adjusted through temperature and time by means of stresses in the crystal lattice of the steel.

Related processes include, for example, soft annealing for vehicle part applications that are relevant in crash tests or further processing after homogenisation.

Heating / heat maintenance process

Many chemical components need to be tempered or heated prior to processing, as they are exposed to the weather conditions, for example, from transport or storage outdoors. When processing liquid products, drums are often stored in industrial furnaces in order to be able to supply them at the right temperature for the production process at short notice.

For the installation of mechanical assemblies, parts are also preheated before shrinking, or tools such as casting tools, rolling tools, forging tools and moulds for processing PU or PA must be preheated before being installed in the manufacturing plant and kept warm until the time of use.

For the immersion of preheated workpieces in liquid coating materials such as PVC, with a subsequent gelling process to smooth the surface, the workpieces must be heated continuously or in batch operation to the melting temperature of the coating material.

Hardening process

In the hardening process, the material matrix is subjected to a targeted structural transformation in the product to be heat-treated by the influence of temperature in the hardening furnace. The surface structure of the material matrix is further developed until the desired or final material structure is achieved.

The aim of this process is to increase the dimensional stability as well as the mechanical resistance of the product. While this property is achieved in plastic products by the reaction of the binding systems responsible for binding the material matrix, metal products require rapid cooling after the tempering process, during which the surface structure is “frozen” to the necessary extent. If further changes in product properties are desired, these can be achieved specifically by a downstream annealing or tempering process in the tempering furnace.

The hardening of phenolic resin compounds, for example in the manufacture of synthetic resin-bonded abrasives or discs, is therefore also referred to as hardening.

This also applies to the processing of components made of carbon or glass fibre reinforced plastics. Numerous applications can also be found in the hardening of epoxy resins, for example, in electrical engineering, such as in the insulation of the copper windings of electric motors.

Silicone tempering is also regarded as a hardening process. In order to enable the reaction partners in the plastic injection moulding part to bond, especially in the case of high-quality products such as silicone, more than one component is provided for reaction or polymerisation.

Especially with silicone products such as babies’ dummies or medical products, it is therefore essential to expel the components remaining from the reaction. In the so-called silicone tempering process, these components are bonded to the supplied oxygen in the furnace at high temperature and removed.

The silicone manufacturers specify a reference value of less than 10 m³/h of fresh air for each kilogram of silicone in the furnace. The high proportion of fresh air to be fed into the industrial furnace therefore also means the introduction of a relatively high heating capacity.

In order to keep this energy requirement as low as possible, heat exchanger systems can be provided here to preheat the volume of fresh air with the exhaust air, which often pay off after a few months. At the same time, the contaminated exhaust air must be safely removed from the process area of the industrial furnace. In order to prevent condensation from forming in the industrial furnace in the examples mentioned above, which can lead to high levels of contamination and even furnace fires, it is also necessary to ensure consistent sealing at all points that can be opened, to avoid thermal bridges on the housing and to avoid return flow from exhaust air pipes.

In order to reduce the cost-intensive pressing time during the shaping or vulcanising of rubber parts and rubber-steel compounds, parts are removed from the press after they retain their shape and then transferred to a post-vulcanising furnace for hardening, namely final structural strengthening.

Solution annealing

In solution annealing, including the quenching of aluminium alloys, the atoms of the workpieces are completely dissolved at temperatures of up to 540 °C.

For this purpose, the temperature uniformity in the solution annealing furnace must be ensured in such a way that no undissolved coarse particles remain in the workpiece due too low a temperature or too short a heat treatment time or which can cause melting processes to occur due to too high a temperature.

The subsequent quenching process serves to “freeze” the dissolved structure.

To achieve this, it is necessary to ensure rapid transport to the quenching process in a few seconds. The quenching medium is usually cold or slightly heated water, polymer solutions or more often air for large parts.

When quenching with air, close cooperation with the customer or even tests are necessary, as the uniformity of the quenching process and the quenching rate within a very short time are critical.


Tempering refers to the heat treatment of different materials to compensate for mechanical properties, for example, the relaxation of the material matrix after the manufacture of glass, thermoplastics such as silicone plastics, but also thermosets.

Some tempering processes also have the capacity to ensure the dimensional accuracy of the workpiece. After a galvanic coating for corrosion protection, a tempering process must be carried out downstream, for example, to avoid hydrogen embrittlement.


Drying is the most widespread topic for industrial furnace applications.

This refers to the drying of components that are made of water-soluble components for production but which must be dry at a later stage.

Drying is also used for solvent-based coatings (lacquers) on the surface in accordance with DIN EN 1539, or after cleaning industrial cleaning systems.

In the drying process, liquid is extracted from the product to be heat-treated by vaporisation or evaporation in order to either simply dry a product or to induce the binding of resin systems.

The vapour pressure of the liquid to be expelled at a certain temperature and pressure is crucial for the drying process in case of convection. If the vapour pressure is identical or lower than the liquid content in the atmosphere, drying is not possible.

However, if the vapour pressure is higher than the liquid content in the atmosphere, the speed of the drying process depends on the gas exchange (convection surface, air flow).

As the vapour pressure usually increases at higher temperatures, thereby facilitating drying, the drying process of the product to be treated is designed to the parameters required for this purpose, depending on the type of liquid to be expelled.

When drying paint for surfaces, uniform air flow is also required.

Fresh air filters, but also recirculating air filters, are used to prevent contamination from being baked into the paint layer.

A special application in paint drying is the drying of bulk materials, such as screw clips or clamps for the automotive industry, which have been coated in painting centrifuges with for example zinc lamella coatings in masses of approx. 6 t/h and have to be evaporated and baked on belts or trays.

All paint baking processes must be carried out in accordance with DIN EN 1539 (dryers and furnaces in which flammable substances are released), which is used, among other things, to determine the amount of exhaust air in order to prevent explosive atmospheres being created by the solvents processed in the furnace.

In systems for the impregnation paint drying of motor windings, since the introduction of water-soluble, but also thinner resins, care must be taken to ensure that the resin is prevented from draining, for example, by turning the workpiece in the dryer or is quickly heated and polymerised by effective ventilation to achieve a high heat transfer, before the resin can flow.

After washing processes in cleaning systems or after galvanic coating, workpieces must also be dried. Effective special ventilation or the use of dried air is recommended for spot-free drying or when drying adhesive water in gaps or blind holes.

Continuous or discontinuous dryers are also used for dissolved or mixed masses in the chemical or process engineering industry.

Material properties according to customer requirements

The customer’s product, namely the targeted achievement of the material properties desired by the customer, is at the heart of every heat treatment process. This is a central element of the actions of the entire Airtec team. Our engineers and technicians develop tailor-made process solutions based on the specific product requirements and, together with our specialists, translate these into modern and energy-efficient industrial furnace and cooling technology, or a combination of both. In this way, system solutions are created from the wealth of experience across industries, from which our customers benefit to the full. You can find examples of the designs on our product subpages.

If you have any further questions about the processes of our heat treatment or cooling systems, please do not hesitate to contact us. We are happy to advise you.

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