Superduplex Sigma Phase Precipitation
The undesirable precipitation of a brittle, intermetallic iron-chromium-molybdenum phase at inappropriate cooling rates or annealing temperatures. It leads to a drastic loss of toughness and corrosion resistance.
Sigma phase is the hard, brittle phase that precipitates in the microstructure of superduplex when you hold the material too long in the critical temperature range or cool it too slowly. This often happens during welding or improper heat treatment. The result is a tough material that suddenly becomes brittle and loses its corrosion resistance – you don't want that. Therefore, after welding, you must cool the part quickly to pass through the critical range rapidly. And if you do a heat treatment, you must go to the right temperature and then quench quickly, otherwise you'll bring the sigma phase back in.