Technical Resources

The metallurgy of duplex stainless steel is very interesting indeed. It is made from two different grades of metal (ferritic and austenitic), and it therefore benefits from the properties of both microstructures.

The metallurgy of duplex stainless steel can be characterised by the fact that it is a Fe-Ni-Cr alloy, and it has a two-phase ferritic-austenitic stainless-steel microstructure when it is at room temperature.

Duplex stainless steels are characterised by high chromium (19–28%) and molybdenum (up to 5%) and lower nickel contents than austenitic stainless steels. The most used duplex stainless steels are the 2205 (22% Chromium, 5% Nickel) and the 2507 (25% Chromium, 7% Nickel); 2507 is known as “super duplex” due to its higher resistance to corrosion.

Duplex stainless steel is highly sought after in heavy industries, like oil and gas nuclear and chemical processing.

The metallurgy of duplex stainless steel
The origin of duplex steels can be traced back to the 1920s, with the first cast being made in Sweden in 1930. The popularity of duplex started to rise in the 1990s when steelmaking technology became more advanced.

The production of a metal with a ferritic-austenitic microstructure was ground-breaking as it meant that it had higher strength, good weldability, good toughness and resistance to stress corrosion cracking.

Compared with austenitic grades, duplex steels characteristically have high chromium content but a rather low nickel content. Molybdenum and nitrogen are added to the metal in order to improve corrosion resistance and balance the microstructure. Nitrogen also increases the mechanical strength.

Varieties of Duplex Stainless Steel

Duplex stainless steels are typically divided into three groups based on their pitting corrosion resistance. The three groups are:

– Standard duplex (PREN range: 28–38, Grade 2205): Standard Duplex Stainless Steel is typical of the mid-range of properties and is perhaps the most widely used today.
– Super-duplex (PREN range: 38-45): Super Duplex metal was developed later than standard duplex steel to meet the specific demands of the oil and gas industry, as well as those of the chemical industries. It has a superior resistance to corrosion and a higher strength, but it is more difficult to process because of the higher contents of Chromium, Molybdenum, Nitrogen and Tungsten. Applications of super duplex include deep-water offshore oil production.
– Lean duplex grades (PREN range: 22–27): Lean duplex steel is the most recently developed of the three grades and it is particularly used in the building and construction industry. It has higher mechanical properties meaning that it is often used in applications where strength is important such as in bridges, pressure vessels or tie bars.

Properties of Duplex Stainless Steel

Duplex stainless steel has a variety of useful properties including:

– Strength:Duplex stainless steels have approximately double the strength of regular austenitic or ferritic stainless steels.
– Toughness and ductility:Duplex stainless steels exceed the toughness and ductility of ferritic grades although they are not as tough as austenitic grades.
– Corrosion resistance:As with all stainless steels, corrosion resistance depends mostly on the composition of the stainless steel, with chromium, molybdenum and nitrogen content being the most important. Duplex stainless steels are extremely corrosion resistant and even in chloride and sulphide environments, duplex stainless steels exhibit very high resistance to stress corrosion cracking (SCC). SCC is a type of corrosion that takes place when a particular set of factors are present: Tensile stress, corrosive environment and a sufficiently high temperature.
– Heat Resistance: Duplex stainless steel has higher heat conductivity and lower thermal expansion than austenitic steels. Duplex grades can easily be used at low temperatures such as -50°C because at low temperatures they have better ductility that ferritic grades of steel.
– Cost:Duplex stainless steels have lower nickel and molybdenum contents than their austenitic counterparts. This lower alloying content means that duplex stainless steels can be lower in cost. Further to this, it is also possible that the thickness of duplex stainless steel can be reduced as it has an increased yield strength. Thinner products mean that significant weight savings can be made.
– Weldability: Duplex stainless steels tend to have good weldability and all standard welding processes can be used although they are not quite as easily welded as the austenitic grades.

This array of properties means that duplex stainless steel can be found in many different applications such as:

– Chemical processing, transport and storage
– Pipes for production and transportation of oil and gas
– Oil and gas exploration and offshore rigs
– Oil and gas refining
– Marine environments
– Pollution control equipment
– Pulp & paper manufacturing
– Chemical process plants
– Structural and mechanical components
– Heat exchangers
– Cooling pipes

Weldability of Duplex Stainless Steel

Welding duplex stainless steel is not particularly difficult but it has to be approached differently to other steels. It is generally thought that the weldability and welding characteristics of duplex stainless steels are better than those of ferritic steels, although maybe not as good as austenitic steels. Interestingly, modern duplex steels with improved nitrogen content are readily weldable.

It is important to note that the properties of duplex steel can be affected by welding parameters such as heat input. This is because the duplex microstructure is more sensitive compared to standard austenitic grades.

Rapid cooling from the high temperatures used during the welding process result in high ferrite levels in the weld metal and adjacent base metal. To combat this, filler metals are used which have been specially designed with higher nickel contents to produce a phase balance like that of the base material.

Consequently, this means that welders should be sure to carry out correct welding procedures to obtain the acceptable weldment structure and properties.

Despite the development of newer nickel alloys like Waspaloy, Rene 41, and Alloy 625, Alloy 718 remains the benchmark for a simple reason: It offers the best cost-to-performance ratio across a wide range of operating conditions.
Advantages over other aerospace superalloys:
Easier to weld and fabricate
Lower material and processing costs
Good availability in bar, plate, sheet, and forgings
Established pedigree with extensive aerospace approvals (AMS, ASTM, and ISO standards)
Because of this, Alloy 718 is not only specified in legacy platforms but continues to be incorporated into next-generation engines and launch systems.

Corrosion Resistance: The composition of duplex stainless steels sets it apart from regular austenitic and ferritic grades in terms of corrosion resistance to aqueous chloride solutions. The pitting resistance equivalent number (PREn) is a calculated value that utilizes specific elements in a grade’s chemistry to roughly rank grades with respect to their pitting corrosion resistance when exposed to aqueous chloride-containing solutions. Duplex stainless steels are often lean in more expensive alloying elements such as nickel, which are substituted with alloying elements such as nitrogen, molybdenum, chromium, and tungsten, which greatly enhances the PREn. The formula for PREn is %Cr + 3.3 x %Mo + 16 x %N. For more in-depth information on the PREn, see our blog, Things to Know About PREn. Additionally, duplex stainless steels are less prone to chloride stress corrosion cracking (CSCC) than austenitic stainless steels like 304/304L and 316/316L due to their dual-phase microstructure. For information on stress corrosion cracking, see our blog, Stress Corrosion Cracking.

Ease of Fabrication: Due to their higher ductility, duplex stainless steels are often more formable than ferritic grades. The higher ductility of duplex stainless steels is attributed to them containing approximately 50% austenite. Duplex stainless steels are also highly weldable in thick and thin sections. To better understand the best practices when welding duplex stainless steel, see our webinar, Best Practices When Using Duplex Stainless Steel.

Solution treating is done at a high enough temperature to facilitate the diffusion of all the constituents into solution. Quenching is to freeze everything in solution that would normally not stay in solution. Aging can be a one step or two step process (a step is a temperature or a cycle) that allows a second phase to precipitate. These precipitates increase the strength of the material.

Solution treating can produce a very soft or hard material depending on the phase equilibrium. Solution aging 718 produces a soft material. In 17-4, a hardened material is obtained. The aging process hardens 718. It also hardens 17-4 at the lower aging temperatures, but as the aging temperature increases, the effect of tempering overtakes the increases imparted by the precipitation process.

Below is how one could tell if a material is supplied in the solution-treated condition:

1. Look at the specification itself. AMS 5662 and AMS 5604 require materials to be solution treated and precipitation hardenable. Hardenable is the crucial word, implying that a material is not hardened. Furthermore, the specification states that each product form is furnished in the solution-treated condition.
2. Mill test certification will show the applicable specification. It may also show that the material is capable of meeting a different specification requiring the actual aging heat treatment.
3. On the mill certification under the product description, it will show the alloy and that it is either annealed, solution annealed, or solution heat treated.
4. Some mill certifications will list the actual product heat treatment and show a separate capability heat treatment. When you see the word capability, the material is solution treated.
5. Some other mill certifications will separately list the mill heat treatment as solution anneal and show an additional laboratory heat treatment of a sample only.

Most precipitation-hardenable materials are supplied in the solution heat-treated condition and will be aged much later. To meet specifications, samples are precipitation hardened to demonstrate that the lot can be heat treated (at a later time) and meet all specification requirements.

Conversely, the information that informs one that the material has been aged includes:

1. Look at the specification itself. AMS 5663 and AMS 5643 require material to be furnished in the solution heat-treated and precipitation-hardened condition.
2. The mill certification will show the applicable specification requires the material to be solution heat treated and precipitation hardened.
3. Under the product description, the mill certification will show that the material is supplied in the solution heat-treated and precipitation-hardened condition.
4. Mill certifications will either show that the product received both heat treatment steps, or there will be no statement of laboratory heat treatment, only product heat treatment.