S31803 is the Unified Numbering System (UNS) designation for the original duplex stainless steel. In the UNS system a letter followed by five numbers identifies each metal, with the letter indicating the family - S for stainless steels. The system was created by several trade groups in the 1970s to reduce confusion caused by one alloy being known by different names.
Duplex stainless steels were developed in the 1930s for demanding applications in paper mills, where lesser-alloyed grades were not corrosion resistant enough. Built around a 22% chromium content, they combine austenite and ferrite to provide increased strength, corrosion resistance and cost-effectiveness.
The UNS S31803 (ASTM F51) specification has largely been replaced by UNS S32205 (1.4462, ASTM F60). This reflected a drive to maximise corrosion performance, aided by the AOD steelmaking process, which allowed tighter control of composition and of nitrogen additions. The highest-performing duplex grade therefore maximises chromium (Cr), molybdenum (Mo) and nitrogen (N).
S31803 resists corrosion better than austenitic grades such as Alloy 316L in most environments, including seawater. It offers excellent resistance to intergranular corrosion, pitting and crevice corrosion, and resists chloride stress corrosion cracking at temperatures up to 150ºC.
It is not generally suitable above 250°C, where embrittling precipitates form, nor below -50°C, where a ductile-to-brittle transition reduces impact toughness.