Is super duplex better than 316?

Super duplex stainless steels provide far higher strength and corrosion resistance than 316. Therefore, the immediate answer is yes – super duplex is better than 316.

‘Duplex’ stainless steels are a family of stainless steels that contain a mixed microstructure of austenite and ferrite phases. They offer a combination of good corrosion resistance and high strength, making them suitable for a wide range of applications. Super duplex stainless steels have a higher content of alloying elements, such as chromium, nickel, and molybdenum, compared to duplex stainless steels. They are based upon 25% chromium content. As such, super duplex stainless steels have up to four times the tensile strength of 316 stainless steel, and considerably better corrosion resistance. An alloys resistance to pitting corrosion can be usefully summarised by the PREN value, which is the ‘Pitting Resistance Equivalent number’. The PREN for super duplex stainless steels is >40, compared with the PREN for 316 of 25.

However, despite the very attractive benefits of super duplex stainless steels over 316, there are some limitations. Super duplex stainless steels lose much of their impact toughness below -50degC, whereas for 316 the impact toughness is largely retained down to cryogenic temperatures. Super duplex stainless steels are limited to long-term operating temperatures below 250degC, whereas 316 can be used up to 870degC (as long as it is the low carbon 316L or titanium stabilised 316Ti variants to avoid intergranular corrosion).

Super duplex stainless steels are certainly harder to machine, given their much higher strength. They are perceived as more difficult to weld but can be readily fabricated with the appropriate equipment and procedures.

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