The main super duplex grades share a 25% chromium basis but differ in their copper and tungsten additions and their guaranteed strength.
Ferralium® (F61)
Ferralium® (UNS S32550, 1.4507, F61) is the original super duplex stainless steel, patented by Langley Alloys in 1967 and launched in 1969. It was the very first alloy described as 'super duplex' and is based around a 25% chromium content. It is the only grade to exceed a minimum yield strength of 85ksi, with an increased copper content for superior pitting corrosion resistance.
Alloy 32760 (F55)
Alloy 32760 (UNS S32760, Zeron 100®, 1.4501, F55) was developed in the 1980s by Mather & Platt (UK) around the same 25% chromium basis. It contains less copper but a deliberate addition of tungsten instead, claimed to selectively improve corrosion resistance.
Alloy 32750 (F53)
Alloy 32750 (UNS S32750, 1.4410, F53), the grade most commonly stocked as solid bar, was promoted by Alleima as SAF2507. It contains neither copper nor tungsten as significant additions.