Duplex Stainless Steels - A Simplified 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.

Barriers to Using Duplex Steels

The attractive combination of high strength, wide range of corrosion resistance, moderate weldability would seem to offer great potential for increasing the market share of duplex stainless steels. However, it is important to understand the limitations of duplex stainless steels and why they are always likely to be "niche players". 

The advantage of high strength immediately becomes a disadvantage when considering formability and machinability. The high strength also comes with lower ductility than austenitic grades. Therefore, any application requiring a high degree of formability, for example, a sink, is ruled out for duplex grades. Even when the ductility is adequate, higher forces are required to form the material, for example in tube bending. There is one exception to the normal rule of poorer machinability, grade 1.4162. 

 
The metallurgy of duplex stainless steels is much more complex than for austenitic or ferritic steels. This is why 3 day conferences can be devoted just to duplex! This factor means that they are more difficult to produce at the mill and to fabricate. 

In addition to ferrite and austenite, duplex steels can also form a number of unwanted phases if the steel is not given the correct processing, notably in heat treatment. Two of the most important phases are illustrated in the diagram below:  

Sigma phase
475 degree embrittlement

                                    

Both of these phases lead to embrittlement, i.e. loss of impact toughness. 

The formation of sigma phase is most likely to occur when the cooling rate during manufacture or welding is not fast enough. The more highly alloyed the steel, the higher the probability of sigma phase formation. Therefore, superduplex steels are most prone to this problem. 

475 degree embrittlement is due to the formation of a phase called α′ (alpha prime). Although the worst temperature is 475 deg C, it can still form at temperatures as low as 300 deg C. This leads to a limitation on the maximum service temperature for duplex steels. This restriction reduces the potential range of applications even further.

At the other end of the scale, there is a restriction on the low temperature use of duplex stainless steels compared to austenitic grades. Unlike austenitic steels duplex steels exhibit a ductile-brittle transition in the impact test. A typical test temperature is minus 46 deg C for offshore oil and gas applications. Minus 80 deg C is the lowest temperature that is normally encountered for duplex steels.

Going Further with Duplex Stainless Steels 
More detailed information on duplex can be found in: 

Practical Guidelines for the Fabrication of Duplex Stainless Steels

Summary of Duplex Characteristics

• Twice design strength of austenitic and ferritic stainless steels
• Wide range of corrosion resistance to match application
• Good toughness down to minus 80 deg C but not genuine cryogenic applications
• Particular resistance to stress corrosion cracking
• Weldable with care in thick sections
• More difficult to form and machine than austenitics
• Restricted to 300 deg C maximum 

 

Source: http://www.bssa.org.uk/