Super Duplex Stainless Steel

Super Duplex Stainless Steel

UNS S32750 or super duplex stainless steel 32750/32760 is a fusion of austenite and ferrite in an equal proportion which improves the strength of ferritic and austenitic steel grades. With a higher content of molybdenum and chromium, super duplex gets better corrosion resistance than standard duplex grades.

One of the very first super duplex stainless steels was Z100, characterized by greater molybdenum and nitrogen content when compared with earlier duplex alloys. Due to its better resistance to seawater super duplex was first developed for such applications but later, it found many other applications in environments with resistance to halides and acids is needed.

Specifications 

GRADE UNS EURONORM AFNOR DIN
NO NAME
Super Duplex 32750/32760 S32205/S31803 1.4501 X2CrNiMoN 25-7-4 Z3 CrNi 25.07 AZ W. Nr 1.4501

Corrosion Resistance of Super Duplex Steel

The mixture of balanced dual-phase microstructure gives UNS S32750 its better strength with cost-effective corrosion resistance especially in high chloride environments. Super duplex stainless steel S32750/32760 has very good localized corrosion resistance along with stress corrosion cracking in combination with higher mechanical strength.

What is super duplex stainless steel?

As per the international standards such as Unified Numbering System (UNS) for super duplex stainless steel is set as UNS 32750 or UNS 32760. Super duplex stainless steel has a higher content of chromium, molybdenum, nickel together with copper and tungsten as compared to duplex stainless steel.

Chemical composition of the super duplex S32750

GRADE   CR C CU MN MO N NI P W Si
S32750/S3260 min 24 0.5 3 0.24 6
max 26 0.03 1 2 5 0.35 8 0.35 0.03 1

The primary difference between duplex and super duplex stainless steel is the content they have. The main differences are the amount of chromium and molybdenum content in a super duplex together with copper and tungsten which make it more corrosion resistant to chloride and acidic environment than duplex steel grades.  Please refer to the above-mentioned table for details.

The combination of copper and nitrogen in super duplex S32760/S32750 gives the alloy resistance to sulfuric and mineral acids like hydrochloric acid. As it is a highly alloyed duplex stainless steel, UNS S32750 has very high strength, better resistance to stress corrosion cracking, and extra-ordinary resistance to pitting.

Physical properties

Grade Density (g/cm3) Melting Range °C Elastic Modulus (GPa) Thermal Expansion coefficient (μm/m/°C) Thermal Conductivity (W/m.K) Specific Heat ( J/Kg.K) Electrical Resistivity (μΩ.cm)
S32750/S32760 7.8 1350°C / 2460°F 200 11.1 14.2 20°C 8.5
475

Due to its high resistance to the crevice, stress corrosion cracking and better strength this alloy is better than the austenitic (300 series) and 22%Cr duplex stainless steels.

Forming and Heat Treatment Process of Super Duplex UNS S32750 steel

The Super Duplex UNS S32750 material can be worked on in a temperature range from 1100°C to 1250°C and if necessary, it can be forged with reheat process and cooled in still air. In the annealing heat treatment process, UNS S32750 can be worked in the range of 1025°C – 1120°C considering sufficient time is given for the center to achieve furnace temperature and hold for a time sufficient with the ruling section, after which, water quenching is necessary.

The super duplex 32750/32760 has to be processed with pickling i.e. a mixture of nitric and fluoric acid to be applied on the surface of the super duplex steel. Because of its better corrosive nature, before the pickling process, super duplex S32750 must be immersed into a fused salt bath.

Weldability of Super Duplex Steel

In the super duplex welding process, the microstructure and the composition of the weld metal are controlled with similar success by modifying the filler material composition. 

The ideal welding process for super duplex UNS S32750 grade is primarily SMAW, GTAW, GMAW, SAW. In addition to these, flux cored arc welding process is considered to be ideal too.

Applications of Super Duplex Stainless Steel

The super duplex S32750 is preferred over duplex 32507/ 31803 under seawater application because it has a Pitting Equivalent Resistance (PRE) number of more than 40 which suggests it has a better resistance against corrosion due to pitting in seawater applications.

The super duplex 32750/32760 with its better pitting and crevice corrosion resistance and effective stress corrosion cracking is popularly used in the chemical industry, pharmaceutical industry, seawater application, pulp and paper industry, and also in the flue gas desulfurization process.

Some of the additional applications of UNS S32750 are:

  • Desalination Equipment
  • Chemical process pressure vessels, piping, and heat exchangers
  • Marine Applications
  • Flue Gas Scrubbing Equipment
  • Pulp & Paper Mill Equipment
  • Offshore Oil production/technology
  • Oil and gas industry equipment