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duplex steel grades, duplex stainless steel material grades astm, duplex steel astm grades equivalent

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ASTM A240 Duplex and Super Duplex Steel Sheets, Plates & Coils Specification

  • Dimensions : ASTM, ASME and API
  • Specifications : ASTM A240 / ASME SA240
  • Size : 5 mm To 100 mm thick in 1000 mm TO 2500 mm width & 2500 mm to 12500 mm
  • Hardness : Soft, Hard, Half Hard, Quarter Hard, Spring Hard etc.
  • Form : Coils, Foils, Rolls, Plain Sheet, Shim Sheet, Perforated Sheet, Chequered Plate, Strip, Flats, Blank (Circle), Ring (Flange)
  • Schedule : SCH 5, SCH10, SCH 40, SCH 80, SCH 80S, SCH 160, SCH XXS, SCH XS
  • Finish : Hot rolled plate (HR), Cold rolled sheet (CR), 2B, 2D, BA NO(8), SATIN (Met with Plastic Coated)
  • Grades : UNS NO. S 31803, S 32205, S 32550, S 32750, S 32760.
  • Our Duplex & Super Duplex Steel Sheets, Plates & Coils conforms to NACE MR0175/ISO 15156

ASME SA240 Duplex and Super Duplex Steel Sheets, Plates & Coils Grades

STANDARD WERKSTOFF NR. UNS
Duplex 1803 1.4462 S318032
Duplex 2205 1.4462 S32205
Super Duplex S2507 1.4410 S32750 / S32760 / S32950

Duplex Stainless Steels - A Simplified Guide

Duplex Stainless Steels - A Simple Guide 

Duplex stainless steels are becoming more common. They are being offered by all the major stainless steel mills for a number of reasons:

  • Higher strength leading to weight saving
  • Greater corrosion resistance particularly stress corrosion cracking
  • Better price stability 
  • Lower price

There is a conference on the subject of duplex every 2-3 years where dozens of highly technical papers are presented. There is a lot of marketing activity surrounding these grades. New grades are being announced frequently. 

Yet, even with all this interest, the best estimates for global market share for duplex are between 1 and 3%. The purpose of this article is to provide a straightforward guide to this steel type. The advantages and disadvantages will be described.

Principle of Duplex Stainless Steels

The idea of duplex stainless steels dates back to the 1920s with the first cast being made at Avesta in Sweden in 1930. However, it is only in the last 30 years that duplex steels have begun to “take off” in a significant way. This is mainly due to advances in steelmaking techniques particularly with respect to control of nitrogen content. 

The standard austenitic steels like 304 (1.4301) and ferritic steels like 430 are relatively easy to make and to fabricate. As their names imply, they consist mainly of one phase, austenite or ferrite. Although these types are fine for a wide range of applications, there are some important technical weaknesses in both types: 

Austenitic – low strength (200 MPa 0.2% PS in solution annealed condition), low resistance to stress corrosion cracking 

Ferritic – low strength (a bit higher than austenitic, 250 MPa 0.2% PS), poor weldability in thick sections, poor low temperature toughness 

In addition, the high nickel content of the austenitic types leads to price volatility which is unwelcome to many end users. 

The basic idea of duplex is to produce a chemical composition that leads to an approximately equal mixture of ferrite and austenite. This balance of phases provides the following:

  • Higher strength – The range of 0.2% PS for the current duplex grades is from 400 – 550 MPa. This can lead to reduced section thicknesses and therefore to reduced weight. This advantage is particularly significant for applications such as: 
    o Pressure Vessels and Storage Tanks 
    o Structural Applications e.g. bridges 
  • Good weldability in thick sections – Not as straightforward as austenitics but much better than ferritics.
  • Good toughness – Much better than ferritics particularly at low temperature, typically down to minus 50 deg C, stretching to minus 80 deg C.
  • Resistance to stress corrosion cracking – Standard austenitic steels are particularly prone to this type of corrosion. The kind of applications where this advantage is important include: 
    o Hot water tanks 
    o Brewing tanks 
    o Process plant 
    o Swimming pool structures

How the Austenite/Ferrite Balance is Achieved 

To understand how duplex steels work, first compare the composition of two familiar steels austenitic 304 (1.4301) and ferritic 430 (1.4016).

Structure Grade  EN Number C Si Mn P S N Cr   Ni Mo
Ferritic 430 1.4016 0.08 1.00 1.00 0.040 0.015 - 16.0/18.0 - -
Austenitic 304 1.4301 0.07 1.00 2.00 0.045 0.015 0.11 17.5/19.5 8.0/10.5 -

The important elements in stainless steels can be classified into ferritisers and austenitisers. Each element favours one structure or the other: 

Ferritisers – Cr (chromium), Si (silicon), Mo (molybdenum), W (tungsten), Ti (titanium), Nb (niobium) 

Austenitisers – C (carbon), Ni (nickel), Mn (manganese), N (nitrogen), Cu (copper) 

Grade 430 has a predominance of ferritisers and so is ferritic in structure. Grade 304 becomes austenitic mainly through the use of about 8% nickel. To arrive at a duplex structure with about 50% of each phase, there has to be a balance between the austenitisers and the ferritisers. This explains why the nickel content of duplex steels is generally lower than for austenitics. 

Here are some typical compositions of duplex stainless steels:

Grade  EN No/UNS Type Approx Composition 
Cr Ni Mo N Mn W Cu
2101 LDX 1.4162/ 
S32101
 Lean 21.5  1.5 0.3 0.22 5 - -
DX2202 1.4062/ S32202 Lean 23 2.5 0.3 0.2 1.5 - -
RDN 903 1.4482/ 
S32001
Lean 20 1.8 0.2  0.11 4.2 - -
2304

1.4362/ 
S32304

 Lean 23 4.8  0.3  0.10 - - -
2205 1.4462/ 
S31803/ 
S32205
Standard  22 5.7 3.1 0.17 - - -
2507 1.4410/ 
S32750
Super 25 7 4 0.27 - - -
Zeron 100 1.4501/ 
S32760
Super 25  7 3.2 0.25 - 0.7 0.7
Ferrinox 
255/ 
Uranus 2507Cu
1.4507/ 
S32520/ 
S32550
Super 25 6.5 3.5 0.25 - - 1.5

   
In some of the recently developed grades, nitrogen and manganese are used together to bring the nickel content to very low levels. This has a beneficial effect on price stability. 

At present, we are still very much in the development phase of duplex steels. Therefore, each mill is promoting its own particular brand. It is generally agreed that there are too many grades. However, this is likely to continue until the “winners” emerge.

Corrosion Resistance of Duplex Steels

The range of duplex steels allows them to be matched for corrosion resistance with the austenitic and ferritic steel grades. There is no single measure of corrosion resistance. However, it is convenient to use the Pitting Resistance Equivalent Number (PREN) as a means of ranking the grades. 

PREN = %Cr + 3.3 x %Mo + 16 x %N 

The following table shows how the duplex steels compare with some austenitic and ferritic grades. 

Grade EN No/UNS Type Typical PREN
430 1.4016/ 
S43000
Ferritic 18
304 1.4301/ 
S30400
Austenitic  19
441 1.4509/ 
S43932
Ferritic 19
RDN 903 1.4482/ 
S32001
Duplex  22
316  1.4401/ 
S31600
Austenitic  24
444 1.4521/ 
S44400
 Ferritic 24 
316L 2.5 Mo 1.4435 Austenitic  26
2101 LDX  1.4162/ 
S32101
Duplex 26
2304 1.4362/ 
S32304
Duplex 26
DX2202 1.4062/ S32202 Duplex 27 
904L 1.4539/ 
N08904
Austenitic 34
2205  1.4462/ 
S31803/ 
S32205
Duplex 35
Zeron 100  1.4501/ 
S32760
 Duplex 41
Ferrinox 255/ 
Uranus 2507Cu
1.4507/ 
S32520/ 
S32550
 Duplex  41
2507 1.4410/ 
S32750
Duplex 43
6% Mo 1.4547/ 
S31254
Austenitic 44

It must be emphasised that this table is only a guide to material selection. It is always important to assess the suitability of a particular with a full knowledge of the corrosive environment.

Stress Corrosion Cracking (SCC)

SCC is a form of corrosion which occurs with a particular combination of factors: 

  • Tensile stress
  • Corrosive environment
  • Sufficiently high temperature. Normally 50 deg C but can occur at lower temperatures around 25 deg C in specific environments, notably swimming pools.

Unfortunately, the standard austenitic steels like 304 (1.4301) and 316 (1.4401) are the most susceptible to SCC. The following materials are much less prone to SCC:

  • Ferritic stainless steels
  • Duplex stainless steels
  • High nickel austenitic stainless steels

The resistance to SCC makes duplex steels suitable materials for many processes which operate at higher temperatures, notably:

  • Hot water boilers
  • Brewing tanks
  • Desalination

Stainless steel structures in swimming pools are known to be prone to SCC. The use of standard austenitic stainless steels like 304 and 316 is forbidden in this application. The best steels to use for this purpose are the high nickel austenitic steels such as the 6% Mo grades. However, in some cases, duplex steels such as 2205 (1.4462) and the superduplex grades can be considered.

 

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