Duplex stainless steels are becoming more common. They are being offered by all the major stainless steel mills for a number of reasons:
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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, (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:
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, 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. CompositionIn 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, one of the commonly used formula for this parameter is :-
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 24It 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, i.e. where there are load bearing requirements and/or safety considerations. 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.
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 embrittlementBoth of these phases lead to embrittlement, i.e. loss of impact toughness.
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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 is the steel, the higher is 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
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The term super duplex is typically applied to duplex stainless steels with a chromium content of 25% and a PREN value >40. Pitting Resistance Equivalent Number (PREN) is a quick reference to the corrosion resistance of stainless steels and other corrosion resistant alloys (CRA), which are most likely to be impacted by pitting and crevice corrosion. For Alloy 316L, the PREN is only 25, and for duplex grades it is 34, so the uplift for super duplex stainless steels (SDSS) is significant.
As a very crude ‘rule of thumb’, Alloy 316L can be used successfully in rural, urban and estuary environments, but is limited in more exposed coastal settings; 2205 duplex can be comfortably used in estuary, marine and offshore environments; but for applications with long-term immersion then a super duplex may be necessary.
However, when discussing PREN and corrosion resistance, we are overlooking the other significant benefits of super duplex stainless steels. Firstly, the strength levels are twice those of regular duplex grades, and three to four times those of Alloy 316L. This means that not only can more applications fall within the scope of these alloys, but also end users can exploit this virtue to use less metal, saving cost, reducing component size and decreasing the suspended weight.
All duplex grades resist the effect of stress corrosion cracking, again making them suitable for application in a number of aggressive environments. Their higher strength will contribute to this resistance, but the duplex microstructure of austenite and ferrite grains helps to prevent the propagation of cracks through the alloy.
Finally, for an alloy family with this very favourable combination of physical and mechanical properties, alternative metals are generally more highly-alloyed and significantly more expensive. Super duplex stainless steels benefit from a lower nickel content, reducing their cost and limiting their price volatility.
Since their invention in 1967 by Langley Alloys, super duplex stainless steels are now widely accepted in a variety of applications:
a) Oil & Gas – downhole tooling, wellhead and subsea equipment, pumps and valves all make use of super duplex alloys. As a family, these alloys are included in NACE MR1075 / EN15156-3 as being suitable for use in H2S-containing environments i.e. sour service wells.
b) bolts and fasteners – are a very common application of SDSS, due to their very high starting strength and the possibility to work harden them to even higher strength levels.
c) pollution control scrubbers – this has been a successful application for SDSS in the fabrication of precipitators, fans and pumps. These grades strongly resist corrosion in such environments; seawater is frequently used as a coolant, and acids such as sulphuric acid are formed from the emissions of the burnt fuel.
The recent application of marine scrubbers are unable to exploit the excellent properties of SDSS, and are having to use more expensive super austenitic stainless steels instead. Legislation is forcing such vessels to selectively employ their scrubber systems in certain locations. When not in use, the scrubber will see temperatures beyond the sensible working range for SDSS.
d) marine applications – propellers, shafts, rudders and seals are frequently supplied in SDSS when grades such as S32205 and XM-19 are not deemed acceptable on the grounds of either corrosion resistance or strength.
e) chemical process industry – a large number of processes will utilise sulphuric acid, nitric acid and phosphoric acid, such as in the production of PP, PVC, TiOx, dyes and agrochemicals. Fortunately, SDSS are generally resistant to reducing acids, as well as offering good abrasion and wear resistance.
Ferralium 255 is widely specified in the production of fertilisers, as mixers, tanks and vessels, pumps and valves, as it strongly resists corrosion and the mechanical impact of ‘phos rock’.
f) vegetable processing – a less obvious application has been in the construction of equipment for processing grains and vegetables, The severe wear and corrosion (‘erosion corrosion’) conditions involved in sugar cane processing, mixers and centrifuges, have been well-served in Ferralium 255.
g) water treatment – associated applications such as sewage treatment, desalination and swimming pools all use SDSS to resist the threat of corrosion from seawater, contaminated or brackish solutions.
h) paper and pulp – most components throughout the production processes of pulp and paper can make use of super duplex alloys. Duplex and lean duplex grades are typically specified where possible on grounds of cost, but SDSS will be used in areas of greatest risk of failure.
i) pump shafts – this is a ‘sweet spot’ for SDSS, exploiting the combination of high strength, wear and corrosion resistance.
Langley Alloys stocks a comprehensive range of super duplex stainless steels as solid bars and also plate.
Ferralium 255 (UNS S32550, 1.4507) – from ½” to 14” diameter, plus plate up to 3“ thickness.
Alloy 32750 (SAF2507, 1.4410, UNS S32760) – from ½” to 16” diameter, plus plate up to 3“ thickness.
Alloy 32760 (1.4501, UNS S32760, Zeron 100) – from ½” to 16” diameter, plus plate up to 2 1/2“ thickness.
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