Glühen | in Wasserstoff-Schutzatmosphäre

what is hydrogen annealing?

At Aalberts surface technologies we have the very unique capability to anneal parts in a hydrogen atmosphere. Hydrogen is a very reducing gas which creates very clean surfaces especially at high temperatures. Reducing annealing means removing oxides and other reaction products from the metal surface.

hydrogen annealing process

The process at Aalberts surface technologies can be processed as classic heat treatment in a retort or chamber furnace, but we also can perform hydrogen annealing in a vacuum furnace. This is very unique for a commercial heat treater and the process combines the advantages of classic heat hydrogen annealing and processing in a vacuum furnace.

step 1: recovery

Recovery annealing is literally the restoration of microstructure to its original state and consequently its mechanical properties.

Cold deformation, for example bending, deep drawing, cold rolling changes the microstructure of the metal which manifests itself, for example, in higher hardness, higher yield strength and tensile strength, lower toughness

Stress relieving annealing provides a limited degree of recovery, with the highest stresses being reduced, but the microstructural changes remain.

In soft annealing, the temperature is higher, where the microstructure takes on its original state with the corresponding mechanical properties.

Some metal alloys are not suitable for Stress relieving annealing or soft annealing. However, these alloys can usually be solution annealed where they regain their original microstructure.

Solution annealing causes precipitates such as intermetallic particles to disintegrate back into the matrix, returning the microstructure to its original initial state.

step 2: recrystallisation

The microstructure of metal alloys consists of small crystals. These are visible under a light microscope after sanding, polishing and chemical etching.

Recrystallization is the restoration by annealing of the crystal structure (microstructure) of a metal alloy that has been distorted by cold deformation (see recovery annealing).

Recrystallization depends on the degree of plastic deformation (cold deformation) and annealing temperature, and in addition, of course, on the specific metal alloy.

step 3: grain growth

Prolonged high-temperature annealing allows the crystals to grow into larger crystals. In the process, crystals will disappear and merge into other, larger crystals. It should be noted that metal alloys are susceptible to grain growth to varying degrees.

combination with nitrogen or argon gases

As written above, basically annealing under hydrogen gives a clean and bright surface by reacting with oxides and other substances on the metal surface.

Below 650°C, those reactions with oxygen are unstable in an air-circulation furnace and there is a danger of explosion. For this reason, hydrogen is often mixed with nitrogen. Above 650°C the reaction with oxygen proceeds stably.

Argon is a noble gas that does not react with other elements and therefore acts as a protective gas against oxidation during heat treatments at temperatures below approximately 850°C. Above 850°C, gases become very high in energy, affecting the interior insulation of a vacuum furnace. For this reason, heat treatment is then carried out under partial pressure or high vacuum. Above 850°C, the high vacuum also works surface cleaning, although less intensively than under hydrogen.

suitable materials for hydrogen annealing

It is good to determine with an expert whether heat treating under hydrogen is beneficial and necessary in the specific case. Titanium alloys are not suitable for treatment under hydrogen.

With certain steel alloys, hydrogen can cause problems.

fields of application

The process is used widely within the semiconductor or electrical industry to improve magnetic properties the so-called permeability of metal parts. Additionally, the process is capable of removing oxides or even other impurities from the surface of metal parts.