Duplex Stainless Steels - A Simplified Guide

Duplex Stainless Steels – A Simple Guide

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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• Higher strength leading to weight saving
• Greater corrosion resistance particularly to stress corrosion cracking
• Better price stability
• Lower price

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.

Principle of Duplex Stainless Steels

The idea of duplex stainless steels dates back to the s with the first cast being made at Avesta in Sweden in . 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.), and ferritic steels like 430, (1.), 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:
  o Pressure Vessels and Storage Tanks
  o 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:
  o Hot water tanks
  o Brewing tanks
  o Process plant
  o Swimming pool structures

How the Austenite/Ferrite Balance is Achieved

To understand how duplex steels work, first compare the composition of two familiar steels austenitic 304, (1.), and ferritic 430, (1.).

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:

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.

Corrosion Resistance of Duplex Steels

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.

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.

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.), and 316, (1.), 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:

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• Hot water boilers
• Brewing tanks
• Desalination

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 , (1.), and the superduplex grades can be considered.

Barriers to Using Duplex Steels

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..

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:

Both of these phases lead to embrittlement, i.e. loss of impact toughness.

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

• Twice design strength of austenitic and ferritic stainless steels
• Wide range of corrosion resistance to match application
• Good toughness down to minus 80 deg. C, but not genuine cryogenic applications
• Particular resistance to stress corrosion cracking
• Weldable with care in thick sections
• More difficult to form and machine than austenitics
• Restricted to 300 deg. C maximum

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Super Duplex Steel Pipes have excellent strength, better corrosion protection, and resistance to wear. These pipes are designed and manufactured from a combination of austenitic and ferritic stainless steel, enabling them to cope with pressures and harsh environments. The presence of chromium and molybdenum in stainless steel in a high quantity makes Super Duplex Steel corrosion resistant to pitting, crevice corrosion, and stress corrosion cracking.

Characteristics of Super Duplex Steel Pipes

Here are some key features of Super Duplex Steel Pipes:

• High Strength: Super Duplex Steel Pipes show better tensile and yield strengths than conventional duplex and austenitic stainless steel pipes and tubes.
• Excellent Corrosion Resistance: This type resists pitting, crevice corrosion, and stress corrosion cracking in the presence of chlorides at excellent levels.
• Enhanced Durability: These pipes are therefore tailored to meet these generally unfavorable conditions, such as high or low temperatures and a wide range of chemically corrosive environments, among others, to meet their intended operational life and efficiency.
• Good Toughness: Even though Super Duplex Steel Pipes are very strong, they also offer good toughness, thus making them suitable for use in materials prone to impact loads.
• Cost Efficiency: Due to the high strength and corrosion resistance, the wall thickness and the material are smaller than other high-strength alloys.
• High Chromium and Molybdenum Content: This is because chromium and molybdenum concentrations are relatively high, thus giving it the advantage of resisting corrosion over other materials in oxidizing circumstances.
• Good Weldability: Conventional welding techniques can be used, but certain practices may be necessary to maintain the composites' high-performance characteristics.
• Resistance to Sulfide Stress Corrosion: These pipes, therefore, afford protection against SSC since this is a major threat in oil and gas-related applications.
• Low Thermal Expansion: Super Duplex Steel Pipes offer a lower thermal expansion/contraction coefficient and, thus, good dimensional stability under heat.

Therefore, Super Duplex Steel Pipes are used in various industry sectors.

Uses of Super Duplex Steel Pipes

Super Duplex Steel Pipes are used in various industries due to their mechanical strength and corrosion-resistant corrosive resistance.

• Oil and Gas Industry: Seawater and high pressure are two elements that affect pipelines, offshore platforms, and downhole tubing in particular.
• Chemical ProcessingChemical processing is applied in chemical reactor vessels, pipelines, and storage tanks where chemicals or corrosive environments affect the materials.
• Marine Industry: Construction ships and marine equipment because of resistance to seawater attack and relatively high strength-to-weight ratio.
• Power Generation: Used to produce boilers for electricity generation establishments, especially in regions exposed to air and fluids with high temperature and corrosive characteristics, such as the boiler tubes and heat exchangers.
• Desalination Plants: These are found in desalination systems used in seawater purification, where materials must withstand saline environments.
• Pulp and Paper Industry: Whenever contact with abrasive or corrosive substances or where equipment is particularly subjected to using pulping chemicals in certain processes.
• Aerospace Industry: Aerospace parts are employed in aerospace parts, for instance, when strength or resistance to other environmental conditions is necessary.
• Water Treatment: Used in processes such as high-pressure water treatment systems and pumps due to their corrosion resistance and hardness.
• Construction: Used in areas such as the construction of bridges, towers, and rise buildings where strength and durability against harsh weather are required.

Therefore, we are using Super Duplex Steel Pipes in different applications.

Super Duplex Steel Pipes have excellent strength, better corrosion protection, and resistance to wear. These pipes are designed and manufactured from a combination of austenitic and ferritic stainless steel, enabling them to cope with pressures and harsh environments. The presence of chromium and molybdenum in stainless steel in a high quantity makes Super Duplex Steel corrosion resistant to pitting, crevice corrosion, and stress corrosion cracking.

Characteristics of Super Duplex Steel Pipes

Here are some key features of Super Duplex Steel Pipes:

• High Strength: Super Duplex Steel Pipes show better tensile and yield strengths than conventional duplex and austenitic stainless steel pipes and tubes.
• Excellent Corrosion Resistance: This type resists pitting, crevice corrosion, and stress corrosion cracking in the presence of chlorides at excellent levels.
• Enhanced Durability: These pipes are therefore tailored to meet these generally unfavorable conditions, such as high or low temperatures and a wide range of chemically corrosive environments, among others, to meet their intended operational life and efficiency.
• Good Toughness: Even though Super Duplex Steel Pipes are very strong, they also offer good toughness, thus making them suitable for use in materials prone to impact loads.
• Cost Efficiency: Due to the high strength and corrosion resistance, the wall thickness and the material are smaller than other high-strength alloys.
• High Chromium and Molybdenum Content: This is because chromium and molybdenum concentrations are relatively high, thus giving it the advantage of resisting corrosion over other materials in oxidizing circumstances.
• Good Weldability: Conventional welding techniques can be used, but certain practices may be necessary to maintain the composites' high-performance characteristics.
• Resistance to Sulfide Stress Corrosion: These pipes, therefore, afford protection against SSC since this is a major threat in oil and gas-related applications.
• Low Thermal Expansion: Super Duplex Steel Pipes offer a lower thermal expansion/contraction coefficient and, thus, good dimensional stability under heat.

Therefore, Super Duplex Steel Pipes are used in various industry sectors.

Uses of Super Duplex Steel Pipes

Super Duplex Steel Pipes are used in various industries due to their mechanical strength and corrosion-resistant corrosive resistance.

• Oil and Gas Industry: Seawater and high pressure are two elements that affect pipelines, offshore platforms, and downhole tubing in particular.
• Chemical ProcessingChemical processing is applied in chemical reactor vessels, pipelines, and storage tanks where chemicals or corrosive environments affect the materials.
• Marine Industry: Construction ships and marine equipment because of resistance to seawater attack and relatively high strength-to-weight ratio.
• Power Generation: Used to produce boilers for electricity generation establishments, especially in regions exposed to air and fluids with high temperature and corrosive characteristics, such as the boiler tubes and heat exchangers.
• Desalination Plants: These are found in desalination systems used in seawater purification, where materials must withstand saline environments.
• Pulp and Paper Industry: Whenever contact with abrasive or corrosive substances or where equipment is particularly subjected to using pulping chemicals in certain processes.
• Aerospace Industry: Aerospace parts are employed in aerospace parts, for instance, when strength or resistance to other environmental conditions is necessary.
• Water Treatment: Used in processes such as high-pressure water treatment systems and pumps due to their corrosion resistance and hardness.
• Construction: Used in areas such as the construction of bridges, towers, and rise buildings where strength and durability against harsh weather are required.

Therefore, we are using Super Duplex Steel Pipes in different applications.