|Super Duplex S2507||1.4410||S32750 / S32760 / S32950|
Duplex stainless steels are becoming more common. They are being offered by all the major stainless steel mills for a number of reasons:
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.
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:
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).
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 |
|2101 LDX||1.4162/ |
|RDN 903||1.4482/ |
|Zeron 100||1.4501/ |
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.
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|
|RDN 903||1.4482/ |
|316|| 1.4401/ |
|316L 2.5 Mo||1.4435||Austenitic||26|
|2101 LDX|| 1.4162/ |
|DX2202||1.4062/ S32202||Duplex||27 |
|2205|| 1.4462/ |
|Zeron 100|| 1.4501/ |
|Ferrinox 255/ |
|6% Mo||1.4547/ |
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.
SCC is a form of corrosion which occurs with a particular combination of factors:
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:
The resistance to SCC makes duplex steels suitable materials for many processes which operate at higher temperatures, notably:
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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