duplex steel – properties, grades, hardening methods

History of duplex stainless steels

Although the first cast of duplex stainless steel dates more than 80 years back and was made in the 1930s in Sweden, only in the last 30 years the stainless steel grade got a significant importance in the industry. The first generation had their limitations in the as-welded condition, but already provided good performance properties. The invention of argon oxygen decarburization, however, made it possible to add nitrogen as an alloying element. The provided advantages were a higher austenite stability and a reduction in the rate at which detrimental intermetallic phases form.

The addition of nitrogen to the alloying defined the second generation of duplex stainless steels, which started in the late 1970s. The demand for grades with high yield strength, mechanical strength and good fabricability increased due to the spreading of offshore gas and oil fields in the North Sea. The second generation provided superior strength and reduced the thickness of the walls and the weight of the oil platforms. In particular, 2205 was used in a high amount a became the workhorse of the second generation duplex steels.

Since then, the development has not remained still. Modern duplex steel grades are usually divided into five groups. The categorization of duplex stainless steels is mostly made on the basis of the corrosion resistance, which strongly correlates to the alloy content.

Which Duplex Steel Grades are there?

Duplex steel meets high requirements in corrosion and strength. In principle, three types can be distinguished: Standard Duplex is the most frequently used version. The most common version of this sort is steel with the material number S31803 (1.4462). Standard duplex steel is versatile and is particularly used, if both corrosion resistance and strength are required. Standard duplex steels have a proportion of more than 20% of chromium, about 5% of nickel and about 3% of molybdenum. A simpler version is Mager Duplex (also known as Lean Duplex), which is available among others in the grades S32001 (1.4482), S32101 (1.4162), S32202 (1.4062), S32304 (1.4362) and 1.4655.

Lean duplex, in comparison with standard duplex, has lower proportions of molybdenum and nickel content. This results partly in lower material costs. In addition, lean duplex has a larger tolerance towards tensile stress. Lean duplex is among others used in the construction industry for these uses, in case of which this grade can use its full potential for weight saving. The third variant is the super-duplex grades. They stand out because of a particularly strong resistance to corrosion and a high strength. The most widespread representative of this group is S32750 (1.4410). Others are S32760 (1.4501), S32750 (1.4507) and 1.4477. Super duplex is suitable for demanding areas of application, such as seawater desalination or construction of chemical tankers. Super duplex grades have an increased proportion of chromium (about 25%), nickel (up to 8%) and molybdenum (up to 4%) compared to standard duplex.

The characteristics of the different groups:

• Lean duplex: no addition of Molybdenum (2304)
• Molybdenum-containing lean duplex: (S32003)

• Standard duplex: alloying consists of 22% Cr and 3% Mo (2205)

• Super duplex: higher Cr content around 25% and 3% Mo, pitting resistance equivalent number (PREN) of 40 – 45 (2507)

• Hyper duplex: higher content of Cr and Mo, PREN higher than 45 (S32707)

Properties

The chemical composition of duplex stainless steels

Although duplex stainless steels mostly combine a phase balance of roughly 50% ferrite and 50% austenite, the beneficial characteristics can still be achieved in a range of 30 — 70%. Usually, for higher toughness and better processing properties, austenite is slightly favored by the current commercial production. The main ingredients of the alloying are chromium, molybdenum, nitrogen, and nickel.

Corrosion resistance

Duplex stainless steel grades are characterized by great crevice corrosion resistance and excellent resistance to pitting which, depending on the environment, bests the properties of austenitic grades by far. Due to the high chromium content, duplex stainless steels perform really well in oxidizing acids. The sufficient addition of molybdenum and nickel provides good resistance in mildly reducing acid environments. In addition, duplex stainless steels resist very well to chloride stress corrosion cracking due to the addition of ferrite to the phase balance and outperform austenites like 304 and 316 by far.

However, the addition of ferrite to the microstructure also leads to some disadvantages in comparison to austenitic steel grades. Ferrite is vulnerable to hydrogen embrittlement, therefore duplex steels are less suitable for environments where hydrogen may be charged into the metal.

Applications for duplex stainless steel grades

Duplex stainless steel grades are widely used across different industries. The pulp and paper industry uses them for several applications such as bleaching equipment, chip storage tanks, and suction roll shells. Duplex stainless steels replaced austenitic and carbon steel grades in the paper industry due to their superior properties, which results in lower overall costs.

The oil and gas industry profits extensively from duplex stainless steel grades. Especially the 2205 grade is used for flow lines, pipes and further equipment like pumps. Super duplex stainless steels, which show a higher strength and an outstanding fatigue resistance, are often used for forgings, tubes, and fasteners. Further, the trend to drill for oil in deeper sea levels requires longer umbilicals which meet special requirements. Therefore, hyper duplex stainless steels, which outperform the super duplex steel grades, have been developed.

The construction industry relies heavily on duplex stainless steels. In particular, the 2205 grade play an important role in the building of bridges. For example, the Hong Kong’s Stonecutter Bridge and Singapore’s Helix Bridge are both composed of the 2205 duplex steel grade.

Hardening of stainless steel

Heat treatment processes for duplex steel often result in problems to maintain the properties with respect to corrosion and strength. Using the BORINOX® method, duplex grades can be further improved in their positive properties, without the protection against corrosion being lost. Do you need an increase in the degree of hardness for your duplex steel product? Contact us today!