Stainless Steel Pipe Fittings

Duplex Stainless Steel Group By yaang.com

Written by
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. 

  • Low levels of austenite – Poor toughness and general corrosion resistance
  • High levels of austenite: – Some Reduction in strength and reduced resistance to stress corrosion cracking. 

Good impact test results are a good indication that the material has been successfully welded.  The parent metal usually exceeds  200J.  The ductile to brittle transition temperature is about –50°C. The transition is not as steep as that of carbon steel and depends on the welding process used. Flux protected processes, such as MMA; tend to have a steeper transition curve and lower toughness.  Multi run welds tend to promote austenite and thus exhibit higher toughness

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:-

  • A ferrite count using a Ferro scope is probably the most popular.  For best accuracy the ferrite count should be performed manually and include a check for deleterious phases.
  • Good impact test results are also a good indication of a successful welding procedure and are mandatory in BS4515 Part 2.
  • A corrosion test, such as the G48 test, is highly recommended. The test may not model the exact service corrosion environment, but gives a good qualative assessment of the welds general corrosion resistance; this gives a good indication that the welding method is satisfactory. G48 test temperature for standard duplex stainless steel is typically 22°C, for super duplex stainless steel 35°C




Typical Welding Procedure For Zeron 100 (Super duplex stainless steel)

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
Duplex Stainless Steels have a structure that contains both ferrite and austenite. Duplex alloys have higher strength and better stress corrosion cracking resistance than most austenitic alloys and greater toughness than ferritic alloys, especially at low temperatures. The corrosion resistance of duplex alloys depends primarily on their composition, especially the amount of chromium, molybdenum, and nitrogen they contain. Duplex alloys are often divided into three sub-classes: Lean Duplex (ATI 2102 duplex, ATI 2003 duplex, and ATI 2304 duplex alloys), Standard Duplex (ATI 2205� duplex alloy), and Superduplex (ATI 255 duplex and UNS S32760 duplex alloys). 

Duplex Stainless Steel
Alloy
(UNS Designation)
End Use Composition
nominal wt%
Specifications Density
lb/in3 (g/cm3)
Tensile
Strength
ksi. (MPa)
0.2% Yield
Strength
ksi. (MPa)
Elong-
ation %
Hardness
S82011 Piping and tubing in general corrosion and chloride environments, structural components, storage tanks. C 0.03 max, Mn 2.0-3.0, Si 1.0 max, Cr 20.5-23.5, Ni 1.0-2.0, Mo 0.10-1.00, N 0.15-0.27, Fe Balance ASTM A240 0.279
(7.75)
101 min (sheet) / 95 min (plate)
(700 min (sheet) / 655 min (plate))
75 min (sheet) / 65 min (plate)
(515 min (sheet) / 450 min (plate))
30 min 31 Rockwell C max
S32003 Piping, tubing in general corrosion and choride environments, architectural structures, roofing, topside applications on oil platforms C 0.03 max, Mn 2.0 max, Si 1.0 max, Cr 19.5-22.5, Ni 3.0-4.0, Mo 1.5-2.0, N 0.14-0.2, Fe Balance ASTM A240 ASME Code Case 2503 0.279
(7.72)
100 min (sheet) / 95 min (plate)
(690min (sheet) / 655 min (plate)
70 min (sheet) / 65 min (plate)
(485 min (sheet) / 450 min (plate)
25 min 31 Rockwell C max
2304
S32304
Tanks, digesters, pressure vessels, pipes C 0.03 max, Mn 2.50 max, Si 1.0 max, Cr 21.5-24.5, Ni 3.0-5.5, Mo 0.05-0.60, Cu 0.05-0.60, N 0.05-0.20, Fe Balance ASTM A240, ASME SA-240, SAE J405 0.280
(7.8)
87 min
(600 min)
58 min
(400 min)
25 min 32 Rockwell C max
2205
S31803/S32205
Duplex stainless steel Pipes, Tubing in general corrosion and chloride stress corrosion environments C 0.03 max, Mn 2.0 max,Si 1.0 max, Ni 4.5-6.5, S31803: Cr 21.0-23.0, Mo 2.5-3.5, N 0.08-0.20, S32205: Cr 22.0-23.0, Mo 3.0-3.5, N 0.14-0.20, Fe Balance ASTM A240 ASME SA-240 0.283
(7.82)
90 min (31803) / 95 min (32205)
(620 min (31803) / 655 min (32205))
65 min
(450 min)
25 min 31RockwellC max
2507
S32750
Super duplex stainless steel pipes, containers for CPI, Oil & Gas C 0.04 max, Mn 1.5 max, P 0.04 max, S 0.03 max, Si 1.0 max, Cr 24.0-27.0, Ni 4.5-6.5, Mo 2.9-3.9, N 0.1-0.25, Cu 1.5-2.5, Fe Balance ASTM A240 0.279
(7.73)
110 min
(760 min)
80 min
(550 min

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

Article Categories:
Duplex Stainless Steel

Leave a Comment

Your email address will not be published. Required fields are marked *