The Duplex Stainless Steels Engineering Essay

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Introduction

Duplex stainless steels are well known for their excellent combination of very good mechanical properties, high corrosion resistance and readily to be fabricated. This family of stainless steel was developed in 1930s (first generation). Basically, its structure is formed by austenite and ferrite phases. Their utilization in the industry was massive after the 1970s, when new manufactures technologies, such as Argon or Vacuum oxygen decarburization (AOD/VOD) were developed in the steel production and N was added to their chemical composition. This generation is called second generation or 22%Cr according their Cr content.

Nowadays, researching and developing continue in this field, designing new materials. So, there are new grades of theses alloys called super duplex and hiper-duplex stainless steel, which aim is to increase their corrosion resistance and save cost.

Hot working capacity of is still a weak feature of this family of stainless steel, due to the dissimilar characteristics at high temperature of austenite and ferrite

Stainless steels

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At the beginning, on this report will be done an introduction on the fundamentals of stainless steels, classification, and a brief metallurgical discussion about these materials. Then, the technical discussion will be focalized on the metallurgy of Duplex Stainless steel.

The term "Stainless steel" means that these materials are resistant to staining, corrosion, rusty in the atmosphere and aggressive environments. In addition to this feature, these steels have others relevant properties depending on their microstructure and chemical composition, such as high temperature strength, workability, weldability and formability.

Properties mentioned above are achieved with different allowing elements. The most relevant is the Cr, which is responsible of its corrosion resistance. Ni, N, Mo and Nb are others relevant allowing elements utilized in some grades of Stainless steel.

Because its inert behaviour in different environments, chemical industries, aggressive atmospheres and even in contact with the human body, they are used in many relevant applications.

Corrosion Resistance

The stainless steel have this feature, because their content of Cr, which can vary between 12 and 30%. This element reacts with the environment to produce a layer of complex Chromium oxide, which is invisible, insoluble, brilliant. This oxide protects the alloys forming a barrier between the material and the media. If this layer is destroyed by any reason, for example a scratch, immediately it is formed again. This phenomenon is Know as passivation or self-passivation. The corrosion resistance is related directly with the content of this element and may be improved with the addition of other allowing elements, such us Mo.

Stainless Steel Classification

They may be classified into 4 groups according their structure at room temperature:

1- Ferritic

2- Martensitic

3- Austenitic

4- Duplex

In order to understand the metallurgy of stainless steels it is necessary to know the influence of Cr and the others main alloying elements (C, Ni, N, Mo).

Effect of Allowing elements

Cr Effect

Chromium not only contributes to corrosion resistance but also it affects the structure of stainless steels at room temperature. Chromium is a ferrite stabiliser element and it is the base metallurgy to obtain Ferritic Stainless stell. According to the binary diagram Fe- Cr (Figure 1), alloys which Cr content is bigger than 12,7% will present ferrite structure (BCC) at room temperature up to melt point. In this context, it will be possible obtain Martensitic structure only when the content of chromium be lower than 12%, because at higher Cr content, the ferrite will not become in austenite during the heating treatment. In contrast, in real alloys there are others alloying elements that modify this equilibrium diagram.

In the following diagram, it is shown an isothermal precipitation for 2205 DSS, annealed at 1050°C, compared with 2304 and 2507 DSS

Fe-Cr Binary Diagram and

Figure 1 [REF ]

C (Carbon) Effect

C is a stabiliser element of austenite. C content increase the loop of Austenite. So, it is possible to get alloys with 17 % C that will transform in Austenite when they are heated (Figure 2). This characteristics allows to produce martensitic stainless steel with chromium content up to 0.6%. In austenitic and Duplex stainless steels the content of C is reduced to a minimum possible to avoid the precipitation of Chromium carbides, which are deleterious for its corrosion resistance.

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Ni (Niquel) Effect

This element is austenite stabilisers. It changes the structure of the steel from BCC (ferrite) to FCC (austenite). It is the main alloying element in the Austenitic Stainless steels, because it allows to get austenite at room temperature. Furthermore, Ni delays the undesirable formation of intermetallic phases. (REF fundamental of stainless steels Book)

The Ni content in Austenitic steel is at least 6 %, whilst in Duplex steel is between 1.5 to 6%.

Mo (Molybdenum) Effect

Molybdenum is a very effective allowing element ( three times more effective than Cr) for increasing corrosion resistance (pitting and crevice) and it is a ferrite promoter. [REF 1] . In contrast, higher proportions of Mo may cause precipitation of detrimental intermetallic phases. Therefore, its content is normally restricted up to 4% in Duplex and 6% in Austenitic stainless steels [REF 2]

N (Nitrogen) Effect

N is a strong austenite stabiliser. Moreover, it improves the austenite strength due it is dissolve in austenite (strengthening by solid solution). It is used to replace Ni in Austenitic and Duplex stainless steel, because its low cost. It is very relevant its utilization in Duplex steels, because it reduces the formation of detrimental intermetallic phases.

Although the replacement of Ni by N is limited, N has been widely used to reduce cost in duplex stainless steels by reducing content Cr and Ni needed to increase the corrosion resistance performance. [REF 3]. The Nitrogen content in Austenitic stainless steels is up to 0.1%, while in Duplex stainless steel is between 0.1 and 0.4% depending on the grade.

In adittion, N decreases stacking fault energies, wich affect the stress corrosion cracking and the work hardening behaviour.

Duplex stainless steels

Classification

Although there are different classifications for this family of steel, it is possible to consider the following classification according their chemical composition:

a- Lean Duplex (2304): 23%Cr and 4% Ni with a restricted Mo content (less than 0.6%)

b- 2205 Standard Duplex (2205): 22% Cr, 5%Ni and Mo (1,4 -3%). This is the most common and the most utilised type of duplex stainless steel.

c- 25 Cr Duplex : 25%Cr, 7%Ni, 3,5%Mo

d- Superduplex: >25%Cr, 7%Ni, higher Mo and N content between 0.20 and 0.40%.

The corrosion resistance increases from lean Duplex to Superduplex stainless steels, due to their alloying elements content. Clark, Gentil and Cuha (1986) determined a equation that correlates the contents of allowing elements with the Pitting Resistance Equivalent Number (PREN). Higher values of PREN indicate higher pitting corrosion resistance.

PREN = %Cr + 3.3%Mo + 15%N

The following tables shows the chemical composition for duplex stainless steels covered in ASTM specifications. (UNS: Unified Numbering system)

[REF 2]

Microestructure

The structure of this alloys is basically compound by ferrite and austenite. Althought the proportion of austenite phase can vary between 30-70%, many of them are used with a microstructure of 50 % of each phases approximately.

The influence of allowing elements on the final structure can be predicted using the Shieffler diagram or other equations reported in literature. Independently of the diagram used, the estimation is made according to the calculation of Cr equivalent and Niquel equivalent, then the diagram is utilised to obtain the final structure.

For example:

a) - (Schaeffler 1949)

Cr equivalente = Cr + Mo + 1.5 Si + 0.5 Nb

Ni equivalente = Ni + 30 C + 0.5 Mn

b) Hammar and Svensson (1979)

Cr equivalente = Cr + 1.37 Mo + 1.5 Si

Ni equivalente = Ni + 0.31 Mn+ 22 C + 14.2 N

Cr equivalent: %Cr + %Mo + 1.5 %Si + 0.5 %Nb %

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Ni equiv. = %Ni + 30 %C + 0.5 %Mn %

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++M

It is important to understand that these diagrams and equations are empirical and they are determined by welding and cooling at room temperature. This concept is mentioned only to understand the effect of different allowing elements simultaneously.

The ratio between each phase (ferrite -austenite) will be relevant for their properties. The transformation from ferrite to austenite occurs during the solidification. The final structure (rate ferrite/austenite) depend on the chemical composition and the thermal conditions during cooling. In this sense, each of these phases has different composition of allowing elements, depending on if the allowing element is alpha o gamma stabiliser. It means, for example that ferrite will be enriched in Cr, Mo, while the austenite will be enriched in Ni and Ni.

Duplex Stainless steels show a complex precipitation behaviour during hot treatment, rolling and cooling (between 1000 and 300°C). According to a section of the Fe-Cr-Ni diagram at 70% (Fig 2), not only it is possible to notice the influence of the Ni and Cr on the amount of both phases (ferrite and austenite), but also the possible intermetallics that can be formed. This intermetallics phases, σ (sigma),  (chi) and ´ (alpha prime) are detrimental for mechanical properties and hot ductility. (Ref Simulation of precipitacion sigma).Besides this intermetallic phases, these steels can precipitate Carbides in the interphase of alpha-gamma which are responsible for intergranular corrosion.

The sigma phase is formed between 870 and 540 °C by nucleation at grain boundaries or over inclusions and then growing into ferrite phase. This phase is brittle and hard and its Cr content is 45%. According to equilibrium diagram Fe-Cr the content of allowing elements promotes the sigma phase. The partition of allowing elements between alpha and gamma phases exacerbates the conditions for the formation of this phases. For example, the Cr and Mo (stabilisers of ferrite) are concentrated in the ferrite phase and they are also stabiliser of sigma phase. The precipitation of sigma may be controlled by cooling as quick as possible after annealing process. REF el informe de resumen"

The ´ phase is stable in ferrite at temperature between 400 and 560°C. This phase is rich in Cr (about 80%). It causes loss of ductility and increases the hardness. The highest velocity of this reaction occurs at 475°C, so this phenomenon is known as "475 °C embrittlement".

Nitrides and carbides are others detrimental intermetallics phases. Because Nitrogen stabilises austenite and the solubility of N in austenite is higher, the carbides formation is not relevant in duplex steel. [REF 4]. Chromium carbides [(Cr,Fe,Mo)23C6] precipitate between 950°C and 500°C. This precipitation is produced at alfa-gamma interphases. This reaction causes depletion of Cr at grain boundaries makes them susceptible at intragranular corrosion. This phenomenon may be reduced by adding allowing elements such as Ti and Nb. These elements react with the C reducing the amount of C available to react with the Cr to produce carbides. In addition, the nitrogen retards the formation of carbides. In Duplex stainless steels the carbides formation is not relevant because the C content is decreased by Vacuum oxygen decarburization.

The formation of this deleterious phases, nitrides and carbides are controlled by the temperature. Thereby, the temperatures in heat treatments during production, fabrication and service, must be taken into account to predict the final structure and ensure the durability.

In the following chart is shown the isothermal precipitation of intermetallic phases and carbides for 2205 DSS [Ref 2] . The precipitation of sigma () for 2304 and 2507 are shown in dotted line.

From the diagram it is possible to conclude that as Cr and Mo contents (2507) increases, there are more likely to form Sigma more quickly. Alpha prime is formed from ferrite, so in duplex steel this precipitation is not so serious as in ferritic steels. The kinetic of this intermetallic (´) is lower than kinetic of sigma phase, then it is rarely probably that this phase forms during fabrication. Nevertheless, this reaction must be take into account to define the service temperature.

Hot ductility

Even though Duplex stainless steels have more advantages than ferritic and austenitic grades, these alloys present a problem related with the presence of a low ductility region during hot working. [REF 5]. Both phases (ferrite and austenite) have different behaviour at high temperature, because the ferrite is softer than austenite at this temperature. So, the strain during the deformation is not homogeneous in both phases. It means the strain is concentrated in the softer phase, ferrite.(REF Hot deformation characteristics of 2205), So, this different mechanical response at high temperature between each phase can produce cracks at the interphase alfa-gamma affecting the hot ductility. Fig 1 shows a example of a micro cracks found at interphase alpha-gamma. [REF 6 - 7]

FIG 1 - [REF 7]

During the hot working process not only the material is deformed but also the structure of the material is modified. To understand the mechanism that affect the structure is useful to analyse a curve of stress against strain at high temperature. Compression uniaxial at high temperature is one method utilised to represent industrial hot working processes.

Material B

 true

 true

Transition stage

Dynamic recrystallization

Only Dynamic recovering

Hardening and

dynamic recovering

Material A

Material C

In this curve (σ true vs  true) it is possible to describe three stages, hardening and dynamic recovery stage, transition stage or dynamic recrystallization and steady state stage. As it is shown in the graph, some materials have different behaviour. For example, the material A only shows dynamic recovery, the material B shows a continue stress fall during the transition stage (single peak dynamic recrystallization) , and the material C presents oscillations during the transition stage (multiple peak dynamic recrystallization). These differences are related with the material characteristics (structure, chemical composition) and the strain conditions (rate). This behaviour may be associated with the stacking fault energy, which is a material characteristic. High stacking fault energy (SFE) allows dynamic recovering, which causes the annihilation of great portion of the dislocations produced during hot forming and. The rest of dislocations becomes in internal grain boundaries (equiaxed subgrains). This behaviour is characteristics of Ferrite. In contrast, materials with low stacking fault energy develop a restricted dynamic recovering, so density of dislocations increases and produces grain refining by dynamic recrystallization. This process is characteristic of Austenite. So, the ferrite shows better hot working properties than Austenite. [REF 8]. At high temperature, during hot forming process, the strain is concentrated in the ferrite because it is softer than austenite. So, the deformation between both phases is different, which can produce cracks in the interface ferrite-austenite. Has been studied and established by several researchers that the proportion of each phase affect hot ductility of stainless steel. [REF 8]Ref 23 Y 25

Torsion Testing, High Temperature Tensile Test and Compression Test are utilised in order to characterise the hot working properties of stainless steels. [REF 9]. With these tests is possible to obtain the effect of temperature and strain rate over temperature to achieve the optimum hot working process conditions of each stainless. Characteristic curves are shown in the Figure 4, where has been graphed Equivalent stress vs Equivalent strain from a torsion test conducted to samples of 2205 and 2304 DSS at different temperature and strain rate. [REF 10]

FIG 4 - REF - Hot workability of 2304 and 2205 E Evangelista]

In this figure it is possible to observe de dependence between the flow stress with the temperature and strain rate. In general, as temperature rises and strain rate decreases, the stress flow decreases and the strain to fracture increases.

Hot compression test and hot tensile test are a very used methods to measure hot ductility. In these test it is possible to graph the reduction of area, Deformability and resistance to deformation vs Temperature. The reduction of area is a measure of ductility. RA is worked out as (A - Ao)/ A, where A is the specimen area at the end of the test (fracture) and Ao is the initial specimen area. Fig 6 and Figure 7 show examples curves for 2205 stainless steel, which allows to observe that the maximum hot ductility is obtained at 1520 K. [REF 9].

Heating at T° test

Cooling at T° test

from 1523K

Fig 6 [REF 9]

Fig 7 [REF 9]

Different process conditions (temperature and strain) produce different microstructures. The microstructures may be observed after each test by quenching the sample after the fracture as soon as possible. The microstructure of as cast2205 grade shows a acicular austenite within a ferrite matrix FIG 10 [REF 9]

Figure 10 [REF 9]

After heat treatment, the volume fraction of austenite is decreased, because the transformation of austenite to ferrite (delta). Moreover, It can be seen that the austenite grains are elongated due to hot forming process, because dynamic recrystallization does not take place. On the other hand, the ferrite shows dynamic recrystallization, which increases as temperature rise. [REF 9]

Fig 11- Microestructure of 2205 DSS after hot defomation = 50% and strain rate = 1 s -1 . a) T=1323K , b) T=1373K, c) T=1423K, d) T=1473K. [REF: 9]

Inclusions and process conditions are another relevant factors that can affect hot ductility in duplex steels. Cracks can start and then can propagate in inclusions. In addition, It is known that there are a lot of causes during continuous casting process which can produce surface cracks, which be also source for loss of ductility. [REF 11]

Conclusion

Because their excellent properties of corrosion resistance and strength is likely to continue growing demand for Duplex stainless steels. In turn, due to their lowest Ni content than Austenitic steels, some grades are becoming more competitive (lower price).

On the other hand, it has been demonstrated that hot ductility is a weak point in these materials, which cause material rejection during hot working process. In spite of a lot of researching conducted in this field, it is necessary to continue investigating the thermo-mechanical process during hot working and their relationship with microstructure. So, this knowledge will provide new tools to new metallurgical developments and will allow to find the optimum process condition to avoid or decreases this negative factor.