Super Duplex Stainless Steel

2023-05-17 00:22:19

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 co

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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 


GRADEUNSEURONORMAFNORDIN
NONAME
Super Duplex 32750/32760S32205/S318031.4501X2CrNiMoN 25-7-4Z3 CrNi 25.07 AZW. 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 CRCCUMNMONNIPWSi
S32750/S3260min240.530.2460.5
max260.031250.3580.3511

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

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