Duplex Stainless Steel Group By yaang.com

Duplex Stainless Steel Group is intermediate in terms of structure and alloy content between ferritic and austenitic stainless steel. The main characteristic that differentiates Austenitic-ferritic steel from austenitic and ferritic stainless steel is that they have a higher yield strength and tensile strength. They are therefore often used in dynamically stressed machine parts, e.g. suction rolls for paper machines. New areas of application are within the oil, gas and petrochemical sector, seawater bearing systems and the offshore industry.

Duplex Stainless Steels are normally named Duplex stainless steel pipes due to the two phases present in the microstructure. Typically twice the yield of austenitic stainless steel. Minimum Specified Ultimate Tensile Strength typically 680 to 750N/mm2 (98.6 to 108ksi).  Elongation typically > 25%.

Superior corrosion resistance than a 316. Good Resistance to stress corrosion cracking in a chloride environment. Duplex stainless steel materials have improved over the last decade; further additions of Nitrogen have been made improving weldability. Because of the complex nature of this material it is important that sourced from good quality steel mills and is properly solution annealing. Casting and possibly thick sections may not cool fast when annealing causing sigma and other deleterious phases to form. 

The material work hardens if cold formed; even the strain produced from welding can work harden the material particularly in multi pass welding.  Therefore a full solution anneal is advantageous, particularly if low service temperatures are foreseen.

The high strength of this material can make joint fit up difficult.

Usable temperature range restricted to, -50 to 280°C

Used in Oil & Natural Gas production, chemical plants etc.

The microstructure shows an image of a duplex stainless steel pipes

Metallographic Test - Metallography Testing
Metallographic Test Report 

Standard Duplex Stainless Steel 
S31803 22Cr 5Ni 2.8Mo 0.15N PREN = 32-33

Super Duplex Stainless Steel:   Stronger and more corrosion resistant than standard duplex stainless steel. 
S32750(Zeron 100) 25Cr 7.5Ni 3.5Mo 0.23N  PREN = 40

Duplex stainless steel solidifies initially as ferrite, then transforms on further cooling to a matrix of ferrite and austenite.  In modern raw material the balance should be 50/50 for optimum corrosion resistance, particularly resistance to stress corrosion cracking.  However the materials strength is not significantly effected by the ferrite / austenite phase balance.  
 

The main problem with duplex stainless steel is that it very easily forms brittle intermetalic phases, such as Sigma, Chi and Alpha Prime.  These phases can form rapidly, typically 100 seconds at 900°C.  However shorter exposure has been known to cause a drop in toughness, this has been attribute to the formation of sigma on a microscopic scale.  
Prolonged heating in the range 350 to 550°C can cause 475°C temper embrittlement.  
For this reason the maximum recommended service temperature for suplex stainless steel is about 280°C.

Sigma (55Fe 45Cr) can be a major problem when welding thin walled small bore pipes made of super duplex stainless steel, although it can occur in thicker sections.  It tends to be found in the bulk of the material rather than at the surface, therefore it probably has more effect on toughness than corrosion resistance.  Sigma can also occur in thick sections, such as castings that have not been properly solution annealed (Not cooled fast enough).

However most standards accept that deleterious phases, such as sigma, chi and laves, may be tolerated if the strength and corrosion resistance are satisfactory.

Nitrogen is a strong austenite former and largely responsible for the balance between ferrite and austenite phases and the materials superior corrosion resistance.  Nitrogen can’t be added to filler metal, as it does not transfer across the arc. It can also be lost from molten parent metal during welding.  Its loss can lead to high ferrite and reduced corrosion resistance.  Nitrogen can be added to the shielding gas and backing gas, Up to about 10%; however this makes welding difficult as it can cause porosity and contamination of the Tungsten electrode unless the correct welding technique is used.  Too much Nitrogen will form a layer of Austenite on the weld surface.  In my experience most duplex stainless steel and super duplex stainless steel are TIG welded using pure argon.

Backing / purge gas should contain less than 25ppm Oxygen for optimum corrosion resistance.

Fast cooling from molten will promote the formation of ferrite, slow cooling will promote austenite. During welding fast cooling is most likely, therefore welding consumables usually contain up to 2 - 4% extra Nickel to promote austenite formation in the weld.  Duplex stainless steel should never be welded without filler metal, as this will promote excessive ferrite, unless the welded component is solution annealed. Acceptable phase balance is usually 30 – 70% Ferrite

Duplex welding consumables are suitable for joining duplex to austenitic< stainless steel or carbon steel; they can also be used for corrosion resistant overlays.  Nickel based welding consumables can be used but the weld strength will not be as good as the parent metal, particularly on super duplex stainless steel. 

Tight controls and the use of arc monitors are recommended during welding and automatic or mechanised welding is preferred.  Repair welding can seriously affect corrosion resistance and toughness; therefore any repairs should follow specially developed procedures.  See BS4515 Part 2 for details. 

Production control test plates are recommended for all critical poduction welds.

Welding procedures should be supplemented by additional tests, depending on the application and the requirements of any application code:-

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Super duplex stainless steel pipes 60mm Od x 4mm Thick     Position 6G

Maximum Interpass 100°C     Temperature at the end of welding < 250°C

1.6mm Filler Wire          85 amps  2 weld runs (Root and Cap)

Arc energy  1 to 1,5 KJ/mm         Travel speed  0.75 to 1 mm/sec

Recommended Testing

1. Ferric Chloride Pitting Test To ASTM G48 : Method A
2. Chemical analysis of root
3. Ferrite count

Source: Yaang Pipe Industry Co., Limited (www.yaang.com)

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