Technical Methods
Customer and Process Know-How
In a heat treatment process, a material or component is heated in a controlled manner and, if necessary, cooled again in order to specifically change or adjust its physical properties (atomic grid structure, material grade/quality, etc.).
In heat treatment, a basic distinction is made between processes with heat input (drying, tempering, etc.) and processes with heat extraction (cooling / quenching). The processes can be combined in a targeted manner, for example to achieve a specific material strength by heating followed by quenching, or to produce a stress-relieved material structure or material toughness by heating followed by slow cooling. The control and influencing variables (process parameters) are the treatment temperature(s), the extent and speed of the temperature changes (temperature curve) and the respective dwell time in the furnace chamber.
Since 1969 – starting with fluidized bed heat treatment plants – the SCHWING Group specializes in the design of customized industrial furnaces. The following methods are most commonly used today in the field of heat treatment with application temperatures up to 600 °C:
Drying
(also dehydration, desiccation)
During drying, liquid or volatile components are removed from the material or component by controlled evaporation or vaporization. In most cases, this is water or non-hazardous but odorous components that are expelled in this way on the material structure.
However, solvents or flammable substances can also be removed, for example to strengthen a plastic component (e.g. by polymerization), to clean a metal part contaminated with punching oil by evaporation, or to evaporate and bake a previously applied paint layer.
In the planning and design of such drying plants, the requirements of DIN EN 1539 for a safe process flow and for the safe discharge of explosive or flammable substances apply.
Tempering
of duroplastics, thermoplastics, elastomers, glass and galvanized components
In tempering, a solid – usually made of glass, acrylic (PMMA) or other plastics – is heated to a temperature below its melting point over a material-specific period of time (several minutes to days) in order to fundamentally improve its material properties.
The aim is to compensate for structural defects, reduce internal stresses, improve the crystal and/or surface structure, and increase the strength or toughness and chemical resistance of the component.
Examples of applications here include the manufacture of glass products and close-tolerance plastic components (increasing dimensional accuracy) and the prevention of hydrogen embrittlement in electrogalvanized surfaces of fasteners.
Treatment of Plastics and Composites
Material structures made of plastics, composites (composites of GRP, GMT, “CfK”, etc.), silicone and rubber are industrially manufactured into dimensionally stable products for a wide variety of uses. For this purpose, these materials are either pre-crosslinked in a shaping process until a dimensionally stable microstructure matrix has formed, or applied in an uncrosslinked state by means of a coating or lamination process, or joined together to form a component.
In the subsequent heat treatment process, hardening, the primary crosslinking that has taken place in the molding process or the applied plastics that are still in the non-crosslinked state are formed into an irreversible, functionally stable microstructure by the action of temperature. The products thus obtain the required final or form strength through the thermal reaction of their binders (phenolic, epoxy, silicone resins, etc.).
Depending on the product-generating materials and/or binders, flammable liquid or gaseous reaction/separation products occur during the reaction, the concentration of which must be safely handled during the heat treatment process or which must be safely removed.
In the planning and design of curing plants, the requirements of DIN EN 1539 for a safe process flow and for the safe discharge of explosive or flammable substances apply.
Cooling & Quenching
The removal of heat or thermal energy from a material/component is called cooling. Depending on the material property to be achieved, cooling can be slow or fast:
- Slow cooling: reduction of internal mechanical stresses, increase of strength and dimensional stability, avoidance of stress cracks in the material, temperature reduction for manual processing
- Rapid cooling (quenching in circulating air cooling chamber): “freezing” of the thermally generated microstructure matrix, increase in the resistance of the material/component to thermal and mechanical stresses
Solution Hardening
(also: homogenizing, diffusion annealing, equalizing annealing)
Solution hardening is a heat treatment of mostly metallic alloys such as aluminum alloys. Solution hardening reduces differences in concentration of the alloying elements by diffusion or dissolves microstructural inhomogeneities (crystal segregation), homogenizes the microstructure and thus relieves stresses in the workpiece.
Stress Relief Annealing
By hardening materials to increase material strength, the lattice stress induced in the material by means of heat can also have negative effects, such as possible material distortion or hardening cracks in the component.
These undesirable accompanying properties can be avoided in metal materials / steel by means of the stress-relief annealing process, which is usually carried out upstream. In this process, the internal stresses in the material structure are reduced as far as possible, enabling low-distortion industrial processing of the material (e.g. in cutting or non-cutting shaping) and thus extending the service life of metal tools.
Tempering metal, NF metal
Tempering refers to the targeted heating or through-heating of a material or component below its transformation point. In this process, the atomic lattice structure of the material is in a dissolved state, which allows the material structure to relax and its technical properties to be positively influenced or specifically adjusted (cf. pretensioning of steel springs).
The main technical properties achieved by the tempering process are as follows:
- Increasing the toughness of a previously hardened metal
- Achieving desired optical effects (e.g. blue coloring)
Aging Non-Ferrous Metal
(also artificial aging, precipitation, age hardening)
By aging alloys or materials with alloying constituents, the yield strength of these materials can be selectively increased or the strength/hardness of the material adjusted at temperatures between 100 and 200 °C (aluminum alloys) in conjunction with the specific treatment time.
In the material structure of an alloyed material, metastable phases are formed in the microstructure as a result of solution annealing with immediately following quenching, which give rise to a microstructure with only limited mechanical stability. The subsequently applied aging process creates barriers or boundaries in the material structure by depositing these metastable phases in a finely distributed form (precipitation), which prevent the material from flowing or counteract the dislocation movements.
Preheating (Heating Up) / Keeping Warm
During preheating or hold-on-heat, materials and components are heated to their processing temperature or kept at this temperature and prepared for a downstream processing operation.
In mechanical and plant engineering, for example, components are joined together in assembly processes by shrink fitting (heating up/cooling) or preheated for welding.