What is the difference between UNS S31803 and SS316?

UNS S31803 is a duplex stainless steel based upon a 22% chromium content, whereas SS316 is an austenitic stainless steel based upon an 18% chromium content. They have significantly different physical and tensile properties due to their different composition and microstructure. S31803 has better corrosion resistance and higher strength, but SS316 may be used for other reasons.

A duplex stainless steel is named because it has a ‘duplex’ microstructure, meaning a balance of austenitic and ferritic grains. This means that duplex stainless steels combine physical and mechanical properties of both, making them an attractive and cost-effective proposition.

Austenitic stainless steels generally provide excellent corrosion resistance, good mechanical properties over a wide range of operating temperatures, including low temperature toughness, and are non-magnetic. Ferritic stainless steels are generally of lower corrosion resistance, and lower cost, but do resist stress corrosion cracking. Duplex stainless steels combine many of these properties to achieve high strength and excellent corrosion resistance, remain highly cost-effective and still resist stress corrosion cracking.

Compared with SS316, UNS S31803 will have around twice the yield strength. The Pitting Resistance Equivalent Number (PREN) of UNS31803 is close to 34, whereas it is 25 for SS316, meaning that its resistance to pitting corrosion is proportionally less. However, SS316 is usually lower cost than UNS S31803 and so may be selected on account of project costs – although the price differential reduces or disappears when nickel costs are higher. Also, UNS31803 will be limited in applications requiring low temperature (less than -50degc) or high temperature (greater than 250degC), so SS316 may be chosen.

Langley Alloys stocks S31803 (S32205, ASTM A182 F51/F60, DIN 1.4462) duplex stainless steels as round bars from 5/8”-20” diameter (15.87-508mm), plate up to 3” (76.2mm) thickness, as well as pipes in scheduled sizes, flanges and fittings.

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