Sigma Phase Embrittlement in Duplex Steels
Sigma phase embrittlement in duplex steels occurs due to the precipitation of a hard, brittle phase at elevated temperature or too slow cooling. This leads to loss of toughness and corrosion resistance.
Physico-Chemical Mechanism of Action
In duplex steels, a hard, brittle intermetallic phase of iron, chromium, and molybdenum forms under certain conditions. This phase develops due to unequal diffusion rates of the elements and can become pronounced at elevated temperatures or with too-slow cooling.
Metallurgical Relevance & Welding Risks
Sigma phase embrittlement causes drastic stiffening of the weld, severely impairing toughness and ductility. This increases susceptibility to cracking and hot cracking. Furthermore, it reduces corrosion resistance because the new phase is more susceptible to corrosive media.
Workshop Practice & Action Guidelines
To avoid sigma phase embrittlement, controlling the cooling rate of the weld is crucial. This can be achieved by optimal selection of the welding process and parameters. For duplex steels, a faster cooling or post-weld heat treatment (PWHT) is recommended, where the weld is briefly heated to a higher temperature after welding and then slowly cooled. This prevents the formation of the hard phase and improves the toughness and corrosion resistance of the weld.