Heat treatment under vacuum conditions offers numerous application possibilities. Various approaches have become established, depending on the composite material and the required properties of the adherend.
In all of these heat-treatment processes, the vacuum atmosphere serves to avoid undesirable interactions between the workpiece and the environment.
The quenching and tempering of steels is used to set specific strength and toughness properties. The quenching and tempering process consists of two process steps. In the first stage, the material to be treated is austenitized. For this purpose, temperatures of around 900 °C – 1000 °C are set in the vacuum furnace, depending on the material. This converts the entire steel structure into austenite. After a sufficient holding time, which is dependent on the part geometry, the steel is quenched. This takes place in PVA vacuum furnaces using a special rapid-cooling device, which allows cooled process gas such as nitrogen or argon to be blown into the batch directly. To increase the cooling effect, the rapid-cooling process can also be performed in the overpressure range of up to 1.4 bars. The resulting cooling rates that can be achieved are sufficient to bring about the required martensitic structure in air-hardening steels.
In martensite, the carbon of the steel is present in forcibly dissolved form and ensures a strong lattice strain in the structure and therefore a high degree of material hardness. A material treated in this way is not suitable for technical applications due to its high brittleness. For this reason, the quenched workpiece is tempered in a second process stage. The aim of quenching is to improve the toughness properties compared to the hardened state. The tempering temperature and duration can be used to adjust the material properties, especially the strength, hardness, and toughness across wide ranges.
The purpose of recrystallization annealing is to transform a structure that has been straightened due to cold forming and therefore to restore the original material properties. Typical recrystallization temperatures are between 450 and 600 °C for unalloyed steels and between 600 and 800 °C for medium- to high-alloy steels. Recrystallization annealing is carried out chiefly after forming processes in order to relax and re-orient the heavily deformed structure of the workpiece.
Diffusion annealing eliminates structure inhomogeneities or concentration differences in the workpiece. Since diffusion processes in solids are highly temperature-controlled, diffusion annealing is performed at very high temperatures (mostly between 1050 and 1250 °C) and frequently over long annealing durations (up to 50 hours). One example is diffusion annealing of nickel-based brazed joints at temperatures of around 1000 °C. During the annealing processes, the concentration of metalloids dissolved in the solder material shifts in the direction of the base material. This counters the formation of hard phases in the brazing joint and significantly increases the strength as well as the corrosion-resistance of the brazed joint.
During bright annealing, the vacuum serves as a functional rather than a protective atmosphere. Bright annealing is used for lightly oxidized workpieces and is used to eliminate oxides. A typical application example is the bright annealing of copper. As a result, copper oxide can be reduced and removed without difficulty at temperatures as low as 900 °C in the high-vacuum. Likewise, bright annealing processes are used on steels in a high-vacuum.
During degassing annealing, the gases dissolved in the workpiece are released at high temperatures and exhausted by the vacuum pumps. This reduces the gas content in the workpiece, which is important for various high-temperature applications in an ultra-high vacuum atmosphere. A typical application example is the hydrogen degassing of steel.
Conversely, during pure or ultra-pure annealing, adhesive surface contamination such as very thin adhesions of carbon are removed at high temperatures with the help of a hydrogen atmosphere. In this case, use is made of the reducing effect of hydrogen, which reacts with the carbon to form volatile hydrocarbon compounds. Ultra-pure annealing processes are also carried out under a high-vacuum if the surface contamination consists of organic or volatile residues. The high-vacuum atmosphere then causes the contaminating components to vaporize.
Solution annealing is used primarily for austenitic stainless steel, where its main function is to dissolve precipitation phases (e.g. carbides) in mixed crystals. Rapid cooling can be used to prevent repeated separation of carbides. In addition, solution annealing can bring about the degradation of cold hardening, thereby generating a less strained structure. The standard temperature range for this heat treatment is from 900 °C to 1100 °C.