Super Duplex Steel

Super Duplex stainless steels, like Duplex, feature a balanced microstructure composed of approximately 50% austenite and 50% ferrite. This dual-phase structure provides enhanced mechanical strength and exceptional corrosion resistance compared to standard ferritic or austenitic grades.

The key difference lies in the higher levels of Chromium and Molybdenum found in Super Duplex steels. These elevated alloying elements significantly improve resistance to pitting, crevice corrosion, and chloride-induced stress corrosion cracking — making Super Duplex ideal for use in aggressive environments, such as marine, offshore, and chemical processing industries.

In addition to superior corrosion resistance, Super Duplex offers high tensile and yield strength. This allows for reduced material thickness without compromising durability or performance, which results in considerable cost savings. Like Duplex, its relatively lower nickel content also contributes to more economical production when compared to equivalent austenitic and ferritic steels.

Chemical Composition of Super Duplex Steel Alloys

Grade C Mn Si P S Cr Mo Ni N
Alloy 2507 / F53 Min - 0.50 0.10 - - 24.50 3.00 6.00 0.24
Max 0.03 1.20 0.80 0.035 0.020 26.00 5.00 8.00 0.32
Alloy 2507 / F55 Min - - 0.10 - - 24.00 3.00 6.00 0.20
Max - 1.00 0.80 0.025 0.005 26.00 4.00 8.00 0.30
Alloy 255 Min - 0.80 - - - 24.50 3.10 5.50 0.20
Max 0.025 1.20 0.70 0.025 0.005 26.50 3.80 6.50 0.25

Physical Properties of Super Duplex Steel Alloys

Grade Density (kg/m³) Elastic Modulus (GPa) Mean Co-eff of Thermal Expansion (μm/m/°C) Thermal Conductivity (W/m·K) Specific
Heat
(J/kg·K)		 Electrical
Resistivity
(Ω·m)		 Magnetic
Permeability
(20°C)		 Fracture
Toughness
(MPa√m)
0–100°C 0–315°C 0–538°C at 20°C at 500°C
Alloy 2507 / F53 7810 199 11.1 - - 14.2 - 475 0.80×10−6 33 -
Alloy 2507 / F55 7810 199 11.1 - - 14.2 - 475 0.80 33 98
Alloy 255 7810 199 11.1 - - 14.2 - 475 0.80×10−6 33 98

Mechanical Properties of Super Duplex Steel Alloys

Grade Tensile Strength
(MPa / ksi)		 Yield Strength 0.2% Proof
(MPa / ksi)		 Elongation
(% in 50mm)		 Reduction of Area (%) Hardness Impact Strength
(Room Temp)		 Impact Strength
(-46°C)
Rockwell C (HRC) Brinell (HB) J ft.lb J ft.lb
Alloy 2507 / F53 800 / 116 550 / 79.8 25 45 - <270 80 - 45J (min 35J)
Longitudinal / Transverse		 -
Alloy 2507 / F55 750 / 108.8 550 / 79.8 25 45 - <270 80 59 60J avg (min 45J) 44 avg (min 33)
Alloy 255
Bars ≤100mm		 790 / 114.5 586 / 85 25 50 - 220–270 80 59 45J avg (min 35J) 33 avg (min 25.8)
Alloy 255
Bars 100–300mm		 790 / 114.5 586 / 85 25 50 - 220–270 80 59 45J avg (min 35J) 33 avg (min 25.8)
Alloy 255
Bars >300mm		 760 / 110 570 / 83 25 50 - 220–270 80 59 45J avg (min 35J) 33 avg (min 25.8)

Characteristics of Super Duplex Steel

Characteristics of Super Duplex Steel
Physical properties that offer design advantages. Good resistance to stress corrosion cracking (SCC) in chloride-bearing environments. High resistance to general corrosion.
Very high mechanical strength. Good resistance to pitting and crevice corrosion. Good weldability and workability
High resistance to corrosion fatigue Suggested for applications up to 600° F Opportunity for purchases to reduce their material costs without compromising on quality
Greater tensile and yield strength Good ductility and toughness High resistance to erosion corrosion.

Grades of Super Duplex Steel


Alloy - 2507 / F53

Corrosion Resistance Properties of Alloy - 2507 / F53

The duplex microstructure and the low carbon content gives UNS S32750 very good resistance to intercrystalline corrosion. UNS S32750 is also an approved material according to NACE MR0175 ``Standard Material Requirements - Metals for Sulfide Stress Cracking and Stress Corrosion Cracking Resistance in Sour Oilfield Environments”. It also provides resistance against pitting corrosion and crevice corrosion.

Processing Alloy - 2507 / F53
Hot Forming UNS S32750 should be hot worked between 1875ºF and 2250ºF. This should be followed by a solution anneal at 1925ºF minimum and a rapid air or water quench.
Cold Forming Most of the common stainless steel forming methods can be used for cold working UNS S32750.
Machining UNS S32750 is generally more difficult to machine than conventional austenitic steels and have different machining properties than those of high-alloy austenitic steels. The main difference is that duplex steels are relatively easier to machine with high speed tools than with cemented carbide tools compared to austenitic stainless steels with similar alloy content.
Heat Treatment 2507 should be solution annealed and quenched after either hot or cold forming. Solution annealing should be done at a minimum of 1925ºF Annealing should be followed immediately by a rapid air or water quench. To obtain maximum corrosion resistance, heat treated products should be pickled and rinsed.
Welding UNS S32750 possesses good weldability and can be joined to itself or other materials by shielded metal arc welding, gas tungsten arc welding , plasma arc welding, flux cored wire, or submerged arc welding.
Surface finish UNS S32750 is available with pickled or machined and peeled surfaces.

Alloy - 2507 / F55

Corrosion Resistance Properties of Alloy 2507 / F55

UNS S32760 is a super duplex stainless steel grade with a pitting resistance equivalent (PRE) number of more than 40, and provides excellent corrosion resistance and strength properties. High impact strength at sub-zero temperatures, UNS S32760 displays excellent corrosion resistance.Suitable for use in harsh environments UNS S32760 is a cost effective super duplex stainless steel.

Processing Alloy - 2507 / F55
Workability The high-temperature strength of NAS 75N is basically the same as Type 430 in the range of 950~1150°C. It should be noted that the strength increases abruptly below 900°C. Solution annealing should be done after hot working. Regarding cold workability, care is required as proof stress is high and elongation is low in comparison with Type 304.
Weldability Various welding methods are applicable in the same manner as with the standard austenitic stainless steels, including shielded metal arc welding, TIG welding, and plasma welding. Use of welding electrodes for UNS S32760 is recommended. Preheating and postheating are not necessary. In welding, the interpass temperature should be no more than 100°C in order to prevent formation of intermetallic compounds.
Heat Treatment Solution annealing of NAS 75N should be performed at 1100°C and higher followed by being quenched in water or rapidly cooled by other means. (Conditions provided in ASTM A480/A480M)
Pickling A mixture of nitric acid and fluoric acid is used in pickling. However, because descaling is somewhat difficult in comparison with Type 304, alkali immersion before acid pickling, and if possible, shot blasting are extremely effective.

Alloy - 255

Corrosion Resistance Properties of Alloy 255

UNS S32550 Displays superior corrosion resistance in most media. Also, 2550 typically out performs duplex alloy 2205. It performs well in sulphuric, phosphoric, and nitric and shows excellent resistance to organic acids such as acetic and formic acid. Highly reducing media should be avoided.

Processing Alloy - 255
Heat Treatment Duplex stainless steels cannot be hardened by heat treatment. They can however be work hardened. Solution treatment or annealing can be done by rapid cooling after heating to around 1100°C.
Welding Duplex stainless steels have good weldability. All standard welding processes can be used. They are not quite as easily welded as the austenitic grades but low thermal expansion in duplex grades reduces distortion and residual stresses after welding.