Sigma phase
The sigma phase (also σ phase) is an intermetallic phase that can arise, for example, when welding ferritic and austenitic steels . This is based on the fact that some alloy elements in high-alloy steels and at corresponding temperatures tend to come together in this phase.
It arises during prolonged exposure in a temperature range between about 600 and 900 ° C and can have different compositions - e.g. B. Chromium - molybdenum - nickel - iron or chromium-iron. Alloy components such as molybdenum, titanium and silicon favor the formation of the sigma phase. Since the phase goes back into solution above 900 ° C., it can be removed again by a suitable annealing treatment .
In the case of welding, the Schaeffler diagram can be used to check whether there is a risk of a sigma phase developing .
properties
The sigma phase has the following properties:
- brittle (poor notched impact strength )
- hard
- low ductility
- not magnetizable
- very low corrosion resistance
Emergence
The sigma phase occurs when a space-centered cubic (krz-) and a face-centered (kfz-) metal meet, the atomic radii of which are the same; small deviations in the atomic radii (up to 8%) are tolerated. It is preferably formed from δ-ferrite , since the composition of the δ-ferrite in high-alloy Cr-Ni steel is the same. One possible composition consists of (24% Cr, 18% Mo, 6% Ni, 52% Fe), another of (48% Cr, 52% Fe). At temperatures between 600 and 900 ° C, this undesired phase separates out in high-alloy steels. It is undesirable not only because of its embrittling effect, but also because of its ability to remove chromium from the matrix. The consequence of this is a drastic deterioration in the resistance to corrosion. When a sigma phase separates out of the δ-ferrite, austenite is also formed due to Cr and Mo depletion (ferrite former). The resulting mixture of sigma phase and austenite with any remaining δ-ferrite residues apparently has the character of a eutectic structure.
Countermeasures
The following measures are used to prevent or slow down the formation of the sigma phase:
Prevent emergence
- Limit heat supply / operating temperature (the sigma phase needs a certain amount of time to develop; so if welding can be carried out quickly enough, the formation is stopped in good time)
- Nitrogen and carbon reduce the formation of a sigma phase and can even suppress it entirely.
Post treatment
- Quench annealing (Sigma phase dissolves above 900 ° C)
Individual evidence
- ^ Bargel, Schulze: Material Science 10th Edition, p. 230
- ↑ http://www.metallograf.de/start.htm?/untersuchungen/sigmaphase/sigmaphase.htm
- ↑ http://www.vhi-gmbh.com/de/der-werkstoff/versproedungen
- ↑ http://www.metallograf.de/start.htm?/untersuchungen/sigmaphase/sigmaphase.htm
- ↑ Bargel, Schulze: Material Science 10th Edition, p. 238ff
- ↑ http://www.andernachu.bleck.com/glossary/sigma-phase/