1.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, (1.4016), 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:
Pressure Vessels and Storage Tanks
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:
Hot water tanks
Brewing tanks
Process plant
Swimming pool structures
2.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, as follows:
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.
3.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
