Super duplex 2507 vs 316: when is it worth paying more?
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TL;DR: Super duplex 2507 has roughly 2.7 times the yield strength of annealed 316 and a pitting resistance number (PREN) of about 41 to 42.5. It costs more, welds less forgivingly and has temperature limits, so use it when the environment is hard, not by default. The situation: in my thread on 304 versus 316 near the sea I said that if the part is permanently wet and warm, I would stop arguing about 304 and 316 and look at duplex. Here is the case, with numbers. Strength The 2507 data in our archive (Alleima tube) gives a yield of 550 MPa at 20 C, tensile 800 to 1000 MPa, elongation 25 % and hardness 28 to 32 HRC. Annealed 316 datasheets list 205 MPa yield and 515 MPa tensile. That is about 2.7 times the yield, which can mean thinner walls for the same pressure, or a stronger part at the same thickness. Corrosion PREN is about 41 to 42.5 in the same sources. PREN is a ranking index, not a guarantee, but a number like that puts 2507 in a different league from 304 and 316 for chloride pitting and crevice attack. That is why duplex shows up in subsea, desalination and chemical plant. Other properties Thermal conductivity is low, 14 W/m.K at 20 C, similar to austenitic stainless. Thermal expansion is lower: 13.5 per million per kelvin between 30 and 100 C against 16.5 for 316 on the UpMet sheet, which helps when stainless is bolted to carbon steel. Stiffness is 200 GPa at 20 C and 180 GPa at 300 C. The catches - Temperature. Duplex stainless steels are usually kept below roughly 250 to 300 C in service because long exposure above that can embrittle them. Check the datasheet for your case. - Welding and fabrication. Duplex is less forgiving than 316. Heat input and filler choice matter, and a poor weld can throw away the corrosion advantage. Not every shop is set up for it. - Cost and supply. It costs more per kilogram, though thinner walls sometimes pay that back. How I would decide If the part is only splashed, the water is cool and the budget is tight, 316 with good design may be enough. If it is immersed, warm, under deposits, in a crevice you cannot design out, or costly to repair, start with duplex and turn the question around: what would it take to justify the cheaper grade? For anyone who has specified or welded duplex: what was the most expensive surprise, fabrication, supply, or something about the environment you did not see coming?