What is S31803?

S31803 is the Unified Numbering System (UNS) designation for the original duplex stainless steel. The UNS system was created by a number of trade groups working together in the 1970’s, to reduce confusion from the same alloy being called different things or vice-versa. Each metal is indicated by a letter followed by five numbers, where the letter indicates the family of metals i.e. S for stainless steels.

Duplex stainless steels were developed in the 1930’s, for demanding applications in paper mills where lesser-alloyed grades were not sufficiently corrosion resistance. Based upon a 22% chromium content, they possess the combined austenite: ferrite microstructure that provides 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The UNS S31803 (ASTM F51) specification has largely been replaced by UNS S32205 (1.4462, ASTM F60).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 their desire to maximise the corrosion performance of the alloy, aided by the development of the AOD steelmaking process that allowed tighter control of composition. In addition, it also allowed the level of nitrogen additions to be influenced, rather than just present as a background element. Therefore, the highest performing duplex grade sought to maximise the levels of chromium (Cr), molybdenum (Mo) and nitrogen (N).

S31803 has a superior resistance to corrosion than austenitic grades such as Alloy 316L in most environments, including seawater. This grade offers excellent resistance to intergranular corrosion, pitting and crevice corrosion with its resistance to chloride stress corrosion cracking at temperatures up to 150ºC.

Please note, it is not generally suitable for use at temperatures above 250°C due to the formation of embrittling precipitates, nor below -50°C; this is because of a ductile-to-brittle-transition leading to lower impact toughness.

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