Stainless Steel Pipe Fittings

Super-Duplex Stainless Steel By yaang.com

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Super-Duplex Stainless Steel By yaang.com

The first-generation duplex stainless steel were developed more than 70 years ago in Sweden for use in the sulfite paper industry. Duplex alloy were originally created to combat corrosion problem caused by chloride-bearing cooling water and other aggressive chemical process fluid. Called duplex because of its mixed microstructure with about equal proportion of ferritic andaustenitic. Duplex stainless steel are a family of grade, which corrosion performance depending on their alloy content. 

The term “Super-Duplex” was first used in the 1980’s to denote highly alloyed, high-performance Duplex stainless steel with a pitting resistance equivalent of >40 (based on Cr% + 3.3Mo% + 16N%). With its high level of chromium, Super duplex stainless steel provides outstanding resistance to acids, acid chlorides, caustic solutions and other environments in the chemical / petrochemical, pulp and paper industries, often replacing 300 series stainless steel, high nickel super-austenitic stainless steel and nickel based alloys.

The chemical composition based on high contents of chromium, nickel and molybdenum improves intergranular and pitting corrosion resistance. Additions of nitrogen promote structural hardening by interstitial solid solution mechanism, which raises the yield strength and ultimate strength values without impairing toughness. Moreover, the two-phase microstructure guarantees higher resistance to pitting and stress corrosion cracking in comparison with conventiona stainless steel. From the introduction of its first-generation, Duplex steel has seen a steady increase in popularity.


Recently, the production of high-strength, corrosion resistant super-duplex coil has been implemented in the marine and chemical industries, architecture and mast riggings, wire lines, lifting and pulley equipment and well service strands. In fact, development of wire processing techniques has enabled the production of steel wires down to 1mm in diameter. 
The various Alloys Super-Duplex falls under the Duplex stainless steel grouping. Duplex stainless steel are graded for their corrosion performance depending on their alloy content. Today, modern Duplex stainless steel can be divided into four groups:

  • Lean Duplex such as 2304, which contains no deliberate Mo addition;
  • 2205, the work-horse grade accounting for more than 80% of duplex usage;
  • 25 Cr duplex such as Alloy 255 and DP-3;
  • Super-Duplex; with 25-26 Cr and increased Mo and N compared with 25 Cr grades, including grades such as 2507, Zeron 100, UR 52N+, and DP-3W

Composition of Duplex Stainless Steel The table lists the duplex stainless steel covered in ASTM specifications for plate, sheet, and bar products.

UNS Number 
Duplex Grades
Typeb C Mn P S Si Cr Ni Mo N Cu Other
S31200 0.030 2.00 0.045 0.030 1.00 24.0-26.0 5.5-6.5 1.20-2.00 0.14-0.20
S31260 0.03 1.00 0.030 0.030 0.75 24.0-26.0 5.5-7.5 2.5-3.5 0.10-0.20 0.20-0.80 W0.10-0.20
S31803 0.030 2.00 0.030 0.020 1.00 21.0-23.0 4.5-6.5 2.5-3.5 0.08-0.20  
S32001 0.030 4.0-6.0 0.040 0.030 1.00 22.0-23.0 1.00-3.00 0.60 0.05-0.17 1.00  
S32205 2205 0.030 2.00 0.030 0.020 1.00 19.5-21.5 4.5-6.5 3.0-3.5 0.14-0.20  
S32304 2304 0.030 2.50 0.040 0.030 1.00 21.5-24.5 3.0-5.5 0.05-0.60 0.05-0.20 0.05-0.60  
S32520 0.030 1.50 0.035 0.020 0.80 24.0-26.0 5.5-8.0 3.0-4.0 0.20-0.35 0.50-2.00  
S32550 255 0.04 1.50 0.040 0.030 1.00 24.0-27.0 4.5-6.5 2.9-3.9 0.10-0.25 1.5-2.5  
S32750 2507 0.030 1.20 0.035 0.020 0.80 24.0-26.0 6.0-8.0 3.0-5.0 0.24-0.32 0.50  
S32760 0.030 1.00 0.030 0.010 1.00 24.0-26.0 6.0-8.0 3.0-4.0 0.20-0.30 0.50-1.00 c
S32900 329d 0.06 1.00 0.040 0.030 0.75 23.0-28.0 2.5-5.0 1.0-2.0  
S32950 0.03 2.00 0.035                

a Weight percent, maximum unless otherwise noted. 
b Unless otherwise indicated, a common name, not a trademark, widely used, not associated with any one producer, as listed in ASTM A 240. 
c W 0.50-1.00; Cr+3.3Mo+16N=40 min. 
d AISI designation BENEFITS

  • High strength,
  • High resistance to pitting, crevice corrosion resistance.
  • High resistance to stress corrosion cracking, corrosion fatigue and erosion,
  • Excellent resistance to chloride stress-corrosion cracking
  • High thermal conductivity
  • Low coefficient of thermal expansion
  • Good sulfide stress corrosion resistance,
  • Low thermal expansion and higher heat conductivity than austenitic steels,
  • Good workability and weldability,
  • High energy absorption.

Applications

  • Duplex Stainless Steel Pipes and duplex Stainless Steel tubes for production and handling of gas and oil,
  • Heat exchanger and duplex Stainless Steel pipes in desalination plants,
  • Mechanical and structural components,
  • Power industry FGD systems,
  • Duplex Stainless Steel pipes in process industries handling solutions containing chlorides,
  • Utility and industrial systems, rotors, fans, shafts and press rolls where the high corrosion fatigue strength can be utilized,
  • Cargo tanks, vessels, piping and welding consumables for chemical tankers.
  • High-strength, highly resistant wiring.
  Seamless Duplex Stainless Steel Pipes
1. ASTM A789/A789M 12.7-1016x 0.5-25.4mm 
2. ASTM A790/A790M 10.3-1016x 0.5-36mm
3. API 6L 
EFW Duplex Stainless Steel Pipes 
1.ASTM A789/A789M:12.7-323.9×0.5-12.7mm 
2:ASTM A790/A790M 10.3-610×0.5-18mm 
3.ASTM A928/A928M 10.3–610x 0.5-18mm 
Material: UNS S31500 S32304 S31803 S2205 S2760 S2750,S 32205, S 32550, S32750, S 32760.
Specifications: Seamless Duplex Stainless Steel Pipes
1. ASTM A789/A789M 12.7-1016x 0.5-25.4mm
2. ASTM A790/A790M 10.3-1016x 0.5-36mm 
EFW Duplex Stainless Steel Pipes
1.ASTM A789/A789M:12.7-323.9×0.5-12.7mm
2:ASTM A790/A790M 10.3-610×0.5-18mm
3.ASTM A928/A928M 10.3-610x 0.5-18mm
Material: UNS S31500 S32304 S31803 S2205 S2760 S2750,S 32205, S 32550, S 32750, S 32760.
Duplex Stainless Steel 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 (AL 2003 alloy), Standard Duplex (AL 2205 alloy), and Superduplex (AL 255 Alloy and UNS S32760).
Standard Met:
Duplex Stainless Steels: Part One 
Abstract: 
Stainless steel is the name given to a family of corrosion and heat resistant steels containing a minimum of 10.5% chromium. Just as there is a range of structural and engineering carbon steels meeting different requirements of strength, weldability and toughness, so there is a wide range of stainless steels with progressively higher levels of corrosion resistance and strength. 
Duplex stainless steels have a mixture of austenitic and ferritic grains in their microstructure; hence they have a duplex structure. This effect is achieved by adding less nickel than would be necessary for making a fully austenitic stainless steel. 

Microstructure
Stainless steel is the name given to a family of corrosion and heat resistant steels containing a minimum of 10.5% chromium. Just as there is a range of structural and engineering carbon steels meeting different requirements of strength, weldability and toughness, so there is a wide range of stainless steels with progressively higher levels of corrosion resistance and strength. This results from the controlled addition of alloying elements, each offering specific attributes in respect of strength and ability to resist different environments. The available grades of stainless steel can be classified into five basic families: ferritic, martensitic, austenitic, duplex and precipitation hardenable. 
The division based on microstructure is useful because the members within one family tend to have similar physical and mechanical properties. However, the properties for one family can be very different from the properties for another family. For example, austenitic stainless steels are non-magnetic, while ferritic and duplex stainless steels are magnetic. 
The difference between the families is fundamental on the atomic level. The arrangement of atoms in the ferrite crystal is different from the one in the austenite crystal: 
Figure 1: The ferritic stainless steel on the left has a body centered cubic (bcc) crystal structure. By adding nickel to this stainless steel the structure changes from bcc to face centered cubic (fcc), which is called austenitic.
In the ferritic stainless steel, the iron and chromium atoms are arranged on the corners of a cube and in the center of that cube. In the austenitic stainless steels the atoms, here iron, chromium and nickel, are arranged on the corners of the cube and in the center of each of the faces of the cube. This seemingly small difference profoundly affects the properties of these steels. 
Table 1: Select properties of austenitic and ferritic stainless steels
Properties Austenitic Ferritic 
Toughness Very high Moderate 
Ductility Very high Moderate 
Weldability Good Limited 
Thermal expansion High Moderate 
Stress corrosion cracking resistance Low Very high 
Magnetic properties Non-magnetic Ferro magnetic 
Because of their good mechanical properties and the ease of fabrication, austenitic stainless steels are much more widely used than ferritic stainless steels. About 75% of all stainless steel used worldwide is austenitic and about 25% is ferritic. The other families, martensitic, duplex and precipitation hardenable stainless steels each represent less than 1% of the total market. 
Besides nickel there are other elements that tend to make the structure austenitic. These elements are called austenite formers. Alloying elements that tend to make the structure ferritic are called ferrite formers. 
Table 2: Alloying elements formers for stainless steel microstructure
Ferrite formers Austenite formers 
Iron Nickel 
Chromium Nitrogen 
Molybdenum Carbon 
Silicon Manganese 
Copper
Duplex stainless steels have a mixture of austenitic and ferritic grains in their microstructure; hence they have a duplex structure. This effect is achieved by adding less nickel than would be necessary for making a fully austenitic stainless steel. 
Figure 2: Adding 8% nickel to a ferritic chromium stainless steel makes an austenitic chromium-nickel stainless steel, for example Type 304 stainless steel. If less nickel is added to a chromium steel, about four or five percent, a duplex structure, a mixture of austenite and ferrite, is created as in 2205 duplex stainless steel. 
Austenitic-ferritic (Duplex) stainless steels contain increased amount of chromium (18% -28%) and decreased (as compared to austenitic steels) amount of nickel (4.5% – 8%) as major alloying elements. As additional alloying element molybdenum is used in some of Duplex steels. Since the quantity of nickel is insufficient for formation of fully austenitic structure, the structure of Duplex steels is mixed: austenitic-ferritic. 
The properties of Duplex steels are somewhere between the properties of austenitic and ferritic steels. Duplex steels have high resistance to the stress corrosion cracking and to chloride ions attack. These steels are weldable and formable and possess high strength 
In the annealed condition, most wrought duplex stainless steels contain about 40-50% austenite in a ferritematrix. When these materials solidify, σ ferrite forms first. Depending upon the composition, a varying amount of austenite is expected to form as the last material solidifies. 
Additional austenite forms by a solid-phase transformation during subsequent annealing. Accordingly, an annealed product is expected to contain more austenite than as-cast or as-welded material. A sufficient amount of austenite must be maintained to provide satisfactory corrosion resistance and mechanical properties. This amount of austenite may vary with the service application and with alloy composition and thermal history. 
Additional phases found in duplex stainless steels can include σ, χ, R, α’, carbides and nitrides. These phases have generally been studied using isothermal heat treatments in the laboratory. 
Sigma Phase
Sigma is a hard, brittle intermetallic phase which is expected to contain iron, chromium and molybdenum in most duplex stainless steels. In these alloys, σ generally can be formed between about 600 and 950°C, with the most rapid formation occurring between 700 and 900°C. 
Sigma typically nucleates in the austenite-ferrite grain boundaries and grows into the adjacent ferrite. Often, additional austenite forms in the areas of chromium depletion adjacent to the σ phase. Elements which stabilize ferrite such as chromium, molybdenum and silicon increase the tendency to form the σ phase. On a weight percent basis, molybdenum can promote σ phase formation much more effectively than chromium, particularly at higher temperatures (e.g. about 900°C). Austenite forming elements such as nickel or nitrogen can also accelerate the nucleation and growth of the σ phase, although these elements may reduce the total amount formed. 
The alloy elements are portioned, and increased levels of each element tend to be present in the phases they stabilize. As nickel or nitrogen stabilize additional austenite, the reduced amount of ferrite becomes enriched in chromium and molybdenum. As a result, σ phase formed may be reduced by nickel or nitrogen, however, because of the smaller volume fraction of ferrite. 
The σ phase can deplete chromium and molybdenum in surrounding areas and reduce resistance to corrosion. As little as about 1% σ phase may reduce impact toughness, while about 10% can cause complete embrittlement of duplex stainless steels. 

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

Article Categories:
Duplex Stainless Steel

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