Super Duplex

Summary Tables

Super Duplex
Super Duplex stainless steels have a ‘duplex’ two-phase microstructure consisting of both austenitic and ferritic grains that gives them a combination of attractive properties. Whilst duplex alloys were developed around a 22% Cr addition level, super duplex alloys utilise a higher 25%Cr base composition to increase the level of corrosion resistance achievable.

In general, they are twice as strong as either austenitic or ferritic stainless steels. They achieve good toughness and ductility, somewhere between the two. Their corrosion resistance is also very good, assuming comparable levels of chromium, molybdenum and nitrogen in selected compositions. By increasing the level of chromium additions, combined with significant levels of molybdenum and nitrogen, super duplex stainless steels achieve a Pitting Resistance Equivalent number (PREN) >40, which will match or beat all but the most highly alloyed austenitic stainless steels and often provide comparable performance to some nickel alloys also. One further advantage over austenitic stainless steels is their resistance to stress corrosion cracking.

The addition of copper to these grades results in additional corrosion resistance in use not directly recognised by the formula used to calculate PREN. It has been shown that the presence of copper can inhibit the formation of pitting corrosion, and it also contributes to exceptional resistance to sulphuric acid. Resistance to sulphuric, nitric and phosphoric acid makes these products attractive for a number of applications in the chemical processing industry. Ferralium® has an appreciable higher copper content than other super duplex stainless steels, imparting superior corrosion resistance.

Super duplex stainless steels offer a cost effective option for many demanding applications – as their design foregoes high nickel additions which are expensive and subject to price volatility. If the application can utilise the high strength levels of these grades then components can be designed with smaller sections, using less material. This can benefit as a direct material cost saving, but can also contribute to overall design savings if the application involves suspended loads i.e. marine sensors, down-hole tooling.

Summary Tables

Summary Tables

AlloyCommon NameRelated SpecificationsTensile StrengthProof TestElongation
BritishEuropeanUnited StatesN/mm2 (ksi)N/mm2 (ksi)(%)
Ferralium 255 – 3SC for Castings UNS S32550 Alloy 255
CD3MCuN Grade 1C
1.4507 UNS S32550 (Super Duplex S32550) 700 (102) 450 (65) 25
Ferralium 255 – FG46 Fastener Grade UNS S32550 Alloy 255 1.4507 ASTM A276/A476
UNS S32550
ASME A182 F61
860 (125) 720 (104) 16
Ferralium 255 (Pipe & Tube) UNS 32550 Alloy 255 (Super Duplex 255) 1.4507 ASTM A790
ASTM A815
760 (110) 550 (80) 15
SAF2507 Alloy 32750 1.4410
X 2 CrNiMoN 25-7-4
UNS S32750
ASTM A479 F53 800 (116) 550 (80) 25
Alloy 32750 UNS S32750 1.4410 ASTM A276/A476
UNS S32750 (Super Duplex 32750)
ASME A182 Duplex F53
800 (116) 550 (80) 25
Alloy 32760 UNS S32760 1.4501 ASTM A276/A476
UNS S32760 (Super Duplex 32760)
ASME A182 F55
750 (109) 550 (80) 25
Ferralium 255 – SD50 UNS S32550 Ferralium 255 – SD50 (Super Duplex 255) 1.4507 ASTM A276/A476
UNS S32550
ASME A182 F61
790 (115) 586 (85) 25
Ferralium 255 – SD50 HS110 UNS S32550 110ksi Alloy 255 1.4507 ASTM A276/A476
UNS S32550
ASME A182 F61
862 (125) 758 - 965 (110 - 140) 16
Ferralium 255 – 3AF Aged Grade UNS S32550 Alloy 255 1.4507 ASTM A276 / A476
UNS S32550
ASME A182 F61
840 (122) 600 (87) 23
Ferralium 255 – 3SF for Forgings UNS S32550 Alloy 255 1.4507 UNS S32550 (Super Duplex 32550) 750 (109) 550 (80) 25
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