Duplex stainless steels have a mixed microstructure of austenite and ferrite, the ideal ratio being a 50:50 mix, though commercial alloys may have ratios of 40:60. They are characterized by higher chromium (19–32%) and molybdenum (up to 5%) and lower nickel contents than austenitic stainless steels. Duplex stainless steels have roughly twice the yield strength of austenitic stainless steel. Their mixed microstructure provides improved resistance to chloride stress corrosion cracking in comparison to austenitic stainless steel Types 304 and 316. The pitting resistance equivalence number, PREN = %Cr + 3.3 %Mo + 16 %N ranges from 28-38 for duplex and ranges from 38-45 for super duplex. Hyper duplex has a PREN equal to 49.