S31803 is the Unified Numbering System (UNS) designation for the original duplex stainless steel. The UNS system was created by multiple trade groups in the 1970s to reduce confusion when the same alloy was called different things, and vice versa. Each metal is represented by a letter followed by five numbers, where the letter represents the metal series, i.e. S for stainless steel.
For paper mill applications requiring lower alloy grades that do not have sufficient corrosion resistance. Based on a 22% chromium content, they have a combined austenitic:ferritic microstructure that provides greater strength, corrosion resistance and cost-effectiveness.
The UNS S31803 (ASTM F51) specification has been largely superseded by UNS S32205 (1.4462, ASTM F60). This reflects their desire to maximize the alloy’s corrosion properties, thanks to the development of the AOD steelmaking process, which allows for tighter control of composition. In addition, it allows to influence the nitrogen addition level rather than just being present as a background element. Therefore, the highest performing duplex grades seek to maximize the content of chromium (Cr), molybdenum (Mo) and nitrogen (N).