Superduplex sigma phase precipitation

If you cool superduplex incorrectly or hold it too long in a critical temperature range, the structure transforms and sigma phase forms. That makes the material hard and brittle, the weld loses its toughness and can simply crack under load. So you have to keep heat input under control and ensure clean, controlled cooling to avoid falling into that. Especially with thicker plates, that means: don't let it cool too slowly after welding, but don't quench with water either, or you'll get other problems.