Abstract:
Duplex stainless steel as the name suggests is a two phased alloy having equal composition of ferritic as well as austenitic grades of steel. This gives duplex alloy austenite's excellent resistance to corrosion and toughness along with ferrites ability to increase its strength. Higher strength of duplex stainless steels reduces the cost of the product as it requires lower weight product. Duplex stainless steels have a working range of -500C to 250 0C. Right now most of the applications of duplex stainless steels are in corrosive environment in process industries, oil and gas industries and marine environment. Duplex alloys provide wide range of corrosion resistance value and can be used accordingly depending on the requirement. There is no single measure of corrosion resistance however pitting resistance equivalent number (PREN) is used as means of ranking the grades. Duplex stainless steel has a good weldability but poor forming and machining properties due to high strength. But the major drawback of duplex stainless steel is the susceptibility of ferritic phase to 4500C embrittlement. The aim of duplex alloy is to slowly replace stainless steel as the leading steel alloy. At present the global market share of duplex stainless steels is in between 1 to 3 %.
Introduction:
Stainless steel is a steel alloy, but it does not corrode, rust or stain as easily as steel and are also heat resistant. It contains minimum of 10.5% chromium [1]. The chromium content is responsible for the alloys enhanced corrosion resistance. Stainless steel can be classified into ferritic stainless steel, martensitic stainless steel, austenitic stainless steel, precipitation hardening alloy, duplex stainless steel. Austenitic steel have low strength and are susceptible to corrosion particularly in chloride environment. Ferritic stainless steel has excellent resistance to stress corrosion cracking but is less ductile and are prone to hydrogen embrittlement. This led to the development of duplex stainless steel. Duplex steel are called duplex because they have two phase micro structure of austenite and ferrite having roughly 50% of each. Ferritic phase is responsible for the increase in strength and austenite provides better toughness and corrosion resistance [4].
History of duplex stainless steel:
Duplex stainless steel is a newly found alloy of steel. Although the idea of duplex steel can be traced back to the 1920's, it emerged to prominence only in the early 1980's, developed from cast iron. The early grades of duplex steel were alloys of chromium, nickel and molybdenum. These grades were developed to reduce the inter-granular corrosion problems in the early high carbon austenitic stainless steel. AISI type 329 was extensively used after World War II and was used in heat exchanger tubing for nitric acid service. 3RE60 was one of the first duplex grades to be developed specifically for improved resistance to chloride stress corrosion cracking. Initially developed duplex stainless steel provided good performance characteristics but had limitations in the as-welded condition and the heat affected zone of welds had low toughness and lower corrosion resistance than that of the base metal. Due to these limitations duplex steel was initially used in unwelded condition. The advances made in the next generation of duplex steel were the deliberate addition of nitrogen as an alloying element. This happened in the late 1970's. Nitrogen alloying of duplex stainless steels provided the required heat affected zone toughness and corrosion resistance. Duplex stainless steel of grade 2205 was widely used during this period particularly in gas gathering line pipe and process application on off shore platforms [2].
The first dedicated international conference for duplex alloy was held in St Louis, Missouri, USA (1982). At that time the wrought duplex grade was being established. By the time the third conference was held in Beaune, France (1991), the modern duplex family including the super duplex grade was established [3]. This rapid advancement is mainly due to the advances made in steel making techniques particularly with respect to control of nitrogen content.
Morden duplex stainless steel can be divided into five groups. i) Lean which contains no deliberate Mo addition. ii) Standard duplex. iii) 25 Cr duplex which have pitting resistance equivalent number (PREN) less than 40. iv) Super duplex PERN 40-45 with 25-26 Cr and increased Mo and N grades when compared to 25 Cr. v) Hyper duplex with PREN in access of 45 [2].
Structure of duplex stainless steel:
The main alloying elements for duplex stainless steel are chromium, molybdenum, nitrogen and nickel. To achieve a stable structure for processing and fabrication correct level of each of these elements must be obtained. Duplex stainless steel contains increased amount of chromium (18%-28%) and when compared to austenitic steel decreased amount of nickel (4.5%-8%). Molybdenum is used in some forms of duplex stainless steel. Low quantity of nickel ensures that a full austenitic structure is not formed [1].
Chromium elements leads to the formation of ferrite as it forms body centred cubic structures of iron. It also increases the corrosion resistance of stainless steel. But at higher chromium content more of nickel is required to form a duplex structure. Higher chromium also promotes the formation of intermetallic phases.
Molybdenum enhances chromium's ability to provided pitting resistance in stainless steel. Addition of molybdenum becomes three times more effective than the addition of chromium when the chromium content in stainless steel is about 18%. Like chromium molybdenum is also a ferrite former and also increases the chances of stainless steel forming detrimental intermetallic phases. Its usage is restricted to 4% in duplex stainless steels.
Nitrogen is a strong austenitic former and it also increases the pitting and crevice corrosion resistance of duplex stainless steels. It is a low cost alloying element and most effective solid solution strengthening element. Nitrogen delays the formation of intermetallic phases enough to permit processing and fabrication of the duplex grades. It is used in duplex stainless steel having high content of chromium and molybdenum, to offset their tendency to form sigma phase.
Nickel leads to the change of crystal structure of stainless steel from body centred to face centred cubic structure. Nickel is far less effective when compared to nitrogen in delaying the formation of intermetallic phases in duplex stainless steel face centred cubic structure is responsible for the increase in toughness of duplex stainless steel [2].
The ferrite formers like chromium (Cr), molybdenum (Mo), silicon (Si) and tungsten (W) are balanced by austenite formers like nickel (Ni), carbon (C), manganese (Mn), copper (Cu) and nitrogen (N) in the formation of duplex stainless steel structure [5]. The table1 gives some of the composition of duplex stainless steel.
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
-
-
-
RDN 903
1.4482/
S32001
Lean
20
1.8
0.2
0.11
4.2
-
-
2304
1.4482/
S32001
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
Table 1: composition of duplex stainless steel [5]
In some of the recently formed grades nitrogen and manganese have been used to reduce the nickel content which reduces the cost.
Phase formation in duplex stainless steel:
In addition to ferrite (α) and austenite (γ) a variety of other undesirable phases may form in duplex stainless steel in the temperature range of 300- 10000C during aging or incorrect heat treatment. Additional phases include σ phase, Cr2N, CrN, secondary austenite, χ phase, R phase, π phase, M7C3, M23C6, Cu and τ phase. Moreover in range 300-5000C, spinodal decomposition of ferrite can occur. σ phase is most important of these as it has significant influence on toughness and corrosion behaviour. Table 3 shows the phases in duplex stainless steels.
Table 3: phases in duplex stainless steel [6].
Sigma (σ) phase is a chromium and molybdenum rich phase. It is found in large number of duplex stainless steels because most them contain substantial amount of molybdenum and chromium, which leads to the formation of sigma phase. This has to be taken into account during production as sigma adversely affects both hot ductility and room temperature ductility. Precipitation of sigma phase often occurs at triple junctions or at ferrite- austenite phase boundaries. Precipitation of sigma can be influenced by heat treatment temperature. A high solution heat treatment temperature tends to increase the volume fraction of ferrite which will be diluted with respect to ferrite-forming elements. It decreases the rate of sigma formation [6, 7].
The formation of chromium nitrides has become more important with the increased use of nitrogen in duplex stainless steel particularly in super duplex in the temperature range of 700-9000C. Chromium nitrides formation occurs when rapid cooling takes place form high solution temperature because of super saturation of nitrogen in ferrite. Sigma phase is avoided by cooling from a high solution temperature, but this makes conditions for chromium nitrides formation becomes more favourable. This complicates heat treatment for nitrogen alloyed duplex stainless steel.
Secondary austenite phase formation takes place due to the fact that at high temperature the fraction of ferrite in duplex steel is higher than in finished product and upon cooling and aging at lower temperatures in the range of 600-9000C that ferrite decomposes to austenite and the desired phase proportion is obtained. The newly formed secondary austenite has same composition has bulk austenite. This enriches surrounding ferrite with Cr which in turn leads to nucleation of sigma phase.
Chi (χ) phase is commonly found in duplex stainless steel but in low quantities when compared to sigma phase. The chi phase has adverse effect on toughness and corrosion properties but its effect is often difficult to separate from sigma phase as both the phases coexist. Chi phase is of lower importance when compared to sigma phase as it is of lower volume fraction, but it cannot be completely ignored.
R phase has been found to precipitate in duplex stainless steel in the temperature range of 550-7000C. It is Molybdenum rich compound. It has a trigonal crystal structure. R phase formation reduces toughness and critical pitting temperature. Both intergranular and intragranular precipitates have been observed. Intergranular precipitates are considered to be more deleterious with regards to pitting as they contain as much as 40% Mo.
Nitride π phase was recently observed in duplex weld metal 22Cr- 8Ni- 3Mo [6]. It was found with in the grains and led to embrittlement and pitting corrosion in material aged isothermally at 6000C. π phase has a cubical structure.
Carbides which are carbon rich as in M7C3 precipitate in the temperature range of 950- 10500C this can be avoided by cooling past this temperature range in less than 10 minutes, while carbide like M23C6 precipitates at temperatures below 9500C. Carbides precipitate mostly at the α/γ grain boundaries, but precipitates at α/α and γ/γ boundaries have also been observed. But carbides precipitates are of less importance as new generation duplex steels mostly super duplex stainless steels have low carbon content [6].
Copper particles have been found in copper rich duplex stainless steels. Copper particles promote the nucleation of austenite. Copper precipitates have also been observed in copper baring super duplex stainless steel. Another phase that was recently found in 22Cr- 5Ni- 3Mo is the τ phase however its effects on materials is not known [6].
4750C embrittlement has been observed in duplex stainless steels. It was evaluated by means of mechanical test like hardness test and impact test, magnetic measurements and scanning electron microscopy. The best performance of wrought duplex stainless steel was obtained in solution treated condition. However two hardening and embrittlement processes may occur when these metals are heated: (a) sigma phase precipitation in the range of 7000C to 9000C and (b) precipitation of Cr rich phase in the range of 3000C to 6000C. Cr rich precipitate leads to progressive hardening and reduces toughness of the material, and it occurs due to spinodal decomposition. It is called 4750C embrittlement because it occurs rapidly at this temperature, but this may also occur at temperature as low as 3000C after thousands of hours and temperature as high as 6000C in a few minutes depending on the chemical composition of duplex steels [1].
Properties of duplex stainless steels:
Duplex stainless steel has almost equal proportions of ferrite and austenite. Thus duplex stainless steel combines some of the features of these two classes, but it tend to be closer to ferrite and carbon steel [4,2].
Duplex stainless steels are nearly twice as strong as regular austenitic or ferritic stainless steel.
Duplex stainless steels have better toughness and ductility when compared to ferrite but are not as good as austenite alloy
Corrosion resistance mostly depends on the composition of the alloy. For chloride pitting and crevice corrosion resistance chromium, molybdenum and nitrogen content are most important. Duplex stainless steels have a range of corrosion resistance similar to that of austenitic stainless steels. Duplex stainless steel has a wide range and can be matched for corrosion resistance with austenitic and ferritic steel grades. Pitting resistance equivalent number (PERN) is mostly used as means of ranking the grades.
PERN = % Cr + 3.3 - %Mo + 16 - %N
Table 2 compares duplex steel with some austenitic and ferritic steel 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
Table 2: comparison of duplex steel with austenitic and ferritic grade [5].
Duplex steels have very good stress corrosion cracking (SCC) resistance similar to that of ferritic alloy.
At low temperatures the ductility of duplex stainless steel is better than that of ferritic grades. Duplex can be used at temperatures as low as -500C.
Duplex steels cannot be hardened by heat treatment. However they can be work hardened
Machinability
Duplex stainless steels are machinable, but due to their high strength machining is difficult and time consuming. Also factors such as low volume fraction of non-metallic inclusions and the low carbon content in duplex stainless steel also contribute. Machinability can be improved by adding non-metallic inclusions. This has is employed in duplex of type 22Cr- 5Ni- 3Mo by increasing the sulphur contant, however this leads to reduced corrosion resistance and toughness.
Duplex steel have good weldability but not as easy to weld as austenite grade. Most of the welding process can be used to weld duplex steels. Low thermal expansion in duplex grades help in reducing distortion and residual stress post welding [1, 2].
Applications of duplex stainless steels:
Duplex stainless steels have high strength and excellent corrosion resistance. It provides alternate options to nickel rich austenitic steels in aggressive environment. Duplex stainless steels not only provide more corrosion resistance but are also low in expensive nickel. Because of its higher yield strength when compared to austenitic stainless steels it is possible to reduce section thickness of duplex stainless steels. This will also reduce the cost and weight when compared with austenitic grade. It can be used in wide temperature range for -500C to 2500c [5]. 2205 is the most highly used grade of duplex steels. 2205 has a composition of 22% chromium and 5% nickel. It is a second generation duplex stainless steel. 2205 duplex steel has higher pitting resistance and better chloride stress corrosion cracking resistance when compared to 300 series austenitic stainless steel.
Duplex steels are typically used in:
Chemical processing, transport and storage.
Oil and gas exploration and offshore rigs
Marine environment
Pollution control equipment
Pulp and paper manufacturing industries
Chemical process plants
Civil engineering applications like construction of roads, bridges and buildings.
Flue gas desulfurization plants
Desalination plants
Food and drink industry
Bio fuels sector [1, 2]
Drawback of duplex stainless steels:
Duplex stainless steels have high strength, high corrosion resistance and good weldability. However the high strength of duplex steels has its own disadvantages when we look at it in terms of formability and machinability. Applications that require high degree of formability will rule out duplex stainless steels. Even when ductility in adequate higher forces are required to form the material. The metallurgy of duplex steel is complicated when compared to austenitic or ferritic grades. If not give proper processing unwanted phases can be formed in duplex stainless steels, which will lead to embrittlement [5].
Conclusion:
Duplex stainless steel alloy is a relatively new alloy and it is still in the developmental stage. Right now a lot of research and development is going on, on this topic and a lot of effort is being put into it to find the right composition so as to be able to replace austenitic grade which is right now more widely used than duplex steels. Even though a lot of interest is put into the product its overall market share is about 1%. Like any new product this too is facing reluctance from user to try it out especially in civil sector. The higher cost could be the reason why it is not used in construction field where there are wide ranges of low cost alloys. Though it is not a cryogenic it can be used in operating conditions as low as -500C and has a wide range up to 2500C. 4750C embrittlement is one of the major deterrents for the growth of duplex stainless steel. Duplex stainless steels could be a very alternative in the field of construction even though it is of high cost it does provide long life and low maintenance and could also be used in corrosive environment. With improvements still being made with duplex stainless steels, it would soon replace the present 300 series of stainless steel.
