Heat-treatment

Heat-treatment of materials principally improves their properties regarding to their intended use. Often the following material properties are required and achieved by heat-treatment:

• high strength in the case of mechanically high stressed structural parts

• high hardness for tribologically high strained parts

• high ductility to avoid formation of cracks in case of alternating stress

• increase of corrosion resistance

• stress relieving for improvement of mechanical treatment after forming processes

• higher purity for ultra-high vacuum applications

Hardening
Hardening of steels serves the adjustment of defined properties concerning strength and ductility. Hardening is a two-step process consisting of quenching and tempering. In the first step the material is annealed to form a complete austenitic microstructure. Depending on the material, temperatures in the range 900°C-1000°C are regulated in the vacuum furnace. At this temperature, the structure of the steel completely turns into the austenitic structure. After holding the temperature for a sufficient long time the steel is quenched. In PVA vacuum furnaces, this is carried out by a special rapid cooling device, blowing gases like N2 or Ar on the charge, directly. For increase of the cooling effect the rapid cooling process can be carried out in the overpressure-range up to 1,8 bar. The cooling-rates achieved thereby are sufficient for stabilisation of the martensitic structure in air-hardening steels.

In the martensitic structure the carbon content of the steel is in constraint solution. This leads to a strong distortion of the lattice and a high hardness of the material. For technical applications, a material treated this way is unsuitable because of its brittleness. Therefore the quenched material will be annealed in a second process step to achieve higher toughness. By variation of annealing temperature and time, materials properties, such as toughness and hardness, can be adjusted precisely.

Recrystallisation annealing
Recrystallisation annealing is carried out to eliminate the deformed and distorted microstructure of a cold rolled material. During annealing the material recrystallises and a new grain-microstructure will be formed, thereby leading to a softening of the material. Typical annealing temperatures are 450°C to 600°C. The recrystallisation annealing is usually applied after cold rolling processes to prevent the material from cracking due to the embrittlement of the part.

Diffusion annealing
During diffusion annealing microstructure inhomogeneities and concentration differences in the part will be eliminated. As solid state diffusion processes are temperature and time controlled processes, diffusion annealing takes place at very high temperatures with long annealing times. The diffusion annealing of high temperature brazed steel parts is an application example. During annealing at approx. 1000°C certain elements start to diffuse from the brazing seam into the bulk material, thus leading to a dissolution of brittle phases in the joint. This however results in increased joint strength and enhanced corrosion resistance properties of the part.

During all these heat-treatments the vacuum atmosphere prevents the materials and parts from undesired interaction with the atmosphere.

Cleaning annealing
During annealing for cleaning purposes the vacuum not only serves as a protective atmosphere but also as a functional atmosphere. The annealing of oxidised materials is a typical example. During vacuum annealing most of the metal-oxides can be reduced physically. Oxidised copper, for example, will be annealed at 900°C under high vacuum atmosphere to achieve a metallic and shiny surface.

Degassing
During degassing, dissolved gas in the material will be set free and pumped away by the pumping unit. The degassing treatment leads to an elimination of gas contaminations in the materials, which is evident for special high-temperature applications under ultra-high vacuum conditions. The hydrogen-degassing of stainless steels is a typical example.

Annealing with special gas atmosphere
Cleaning annealing under hydrogen partial pressure atmosphere is used for elimination of surface contamination, which cannot be removed in a liquid cleaning bath. These surface contaminations are very often small carbon-particles, which will react with the hydrogen atmosphere to form vapour carbon-hydrogens. In case of organic contaminations a high vacuum atmosphere is used, which leads to an evaporation of the undesired residues.