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Characteristics, Chemical Composition And Development History Of Duplex Stainless Steel

Apr 07, 2021

Duplex stainless steel is called "duplex" because its metallographic microstructure is composed of two kinds of stainless steel grains, ferrite and austenite. In the picture below, the yellow austenite phase looks like an "island" surrounded by a blue ferrite phase ocean. When the duplex stainless steel is melted, it first solidifies into a complete ferrite structure when it solidifies from a liquid state. As the material cools to room temperature, about half of the ferrite grains are transformed into austenite grains ("islands") . As a result, about 50% of the microstructure is the austenite phase and 50% is the ferrite phase. Let's take a look at the characteristics, chemical composition and development history of duplex stainless steel.

Duplex stainless steel has a two-phase microstructure of austenite and ferrite

The dual-phase structure makes this type of stainless steel combine many excellent characteristics.

High Strength:

The strength of duplex stainless steel is about twice the strength of conventional austenitic stainless steel or ferritic stainless steel. Therefore, designers can reduce the wall thickness in some applications. The following figure compares the yield strength of several duplex stainless steels and 316L austenitic stainless steels in the temperature range from room temperature to 300C.

Good toughness and ductility:

Despite the high strength of duplex stainless steels, they exhibit good ductility and toughness. The toughness and ductility of duplex stainless steel are significantly better than ferritic stainless steel and carbon steel, and it maintains good toughness even at very low temperatures such as -40°C/F. However, it has not yet reached the excellent degree of austenitic stainless steel.

The minimum mechanical properties of duplex stainless steel specified by ASTM and EN standards

Excellent corrosion resistance

The corrosion resistance of stainless steel mainly depends on its chemical composition. In most application environments, duplex stainless steels show high corrosion resistance. This is due to their high chromium content, which is advantageous in oxidizing acids, and contains sufficient amounts of molybdenum and nickel, which can withstand moderate reducing acids. Corrosion of the medium.

The resistance of duplex stainless steels to chloride ion pitting and crevice corrosion depends on their chromium, molybdenum, tungsten and nitrogen content. The relatively high content of chromium, molybdenum and nitrogen in duplex stainless steels gives them good resistance to chloride pitting and crevice corrosion. They have a series of different corrosion resistance, including grades equivalent to the corrosion resistance of 316 stainless steel, such as economic duplex stainless steel 2101, and grades equivalent to the corrosion resistance of 6% molybdenum stainless steel, such as SAF 2507.

Duplex stainless steel has very good resistance to stress corrosion cracking (SCC), which is "inherited" from ferrite. The resistance of all duplex stainless steels to chloride stress corrosion cracking is significantly better than that of 300 series austenitic stainless steels. While standard austenitic stainless steel grades such as 304 and 316, under the conditions of chloride ions, humid air and elevated temperature, stress corrosion cracking may occur. Therefore, in many applications in the chemical industry where there is a greater risk of stress corrosion, duplex stainless steel is often used instead of austenitic stainless steel.

Physical properties

Between austenitic stainless steel and ferritic stainless steel, but closer to ferritic stainless steel and carbon steel.

Cost advantage

Compared with austenitic stainless steel grades with the same corrosion resistance, duplex stainless steel has lower nickel and molybdenum content. Because of the low content of alloying elements, duplex stainless steel may have an advantage in price, especially when the alloy surcharge is high. In addition, due to the higher yield strength of duplex stainless steel, its cross-sectional dimensions can often be reduced. Compared with the use of austenitic stainless steel, the use of duplex stainless steel can significantly reduce costs and weight.

The development history of duplex stainless steel

Duplex stainless steel has a history of more than 80 years. The early grades were alloys of chromium, nickel and molybdenum. In 1930, the first batch of wrought-rolled duplex stainless steels were produced in Sweden and used in the sulfite paper industry. These grades were developed to reduce the intergranular corrosion problems of early high-carbon austenitic stainless steels. 

In 1930, Finland produced duplex stainless steel castings. In 1936, the predecessor of Uranus 50 was patented in France. After World War II, AISI 329 stainless steel became a mature brand and was widely used in heat exchanger pipes in nitric acid plants. 3RE60 is one of the first-generation duplex stainless steel grades specially developed to improve the resistance to chloride stress corrosion cracking (SCC). Later, forged-rolled and cast duplex stainless steel grades were used in various processing industry applications, including vessels, heat exchangers, and pumps.

The first-generation duplex stainless steel has good performance, but it has limitations in the welded state. The heat-affected zone (HAZ) of the weld is low in toughness due to excessive ferrite, and its corrosion resistance is significantly lower than that of the base material. These limitations make the application of the first-generation duplex stainless steel limited to some specific applications in the non-welded state. The invention of stainless steel refining and argon oxygen decarburization (AOD) in 1968 made it possible to produce a series of new stainless steel grades. One of the many advancements brought about by AOD is the deliberate addition of nitrogen as an alloying element. The addition of nitrogen to the duplex stainless steel can make the toughness and corrosion resistance of the HAZ in the welded state close to the performance of the base material. As the stability of austenite increases, nitrogen also reduces the rate of formation of harmful intermetallic phases.

Nitrogen-containing duplex stainless steel is called the second-generation duplex stainless steel. This new commercialization progress began in the late 1970s, which coincided with the development of the North Sea offshore oil and gas fields and the market's demand for stainless steel with excellent chloride ion corrosion resistance, good processing performance and high strength. 2205 duplex stainless steel has become the main grade of the second-generation duplex stainless steel and is widely used in gas gathering pipelines and processing facilities on offshore oil platforms. Due to the high strength of this kind of steel, the wall thickness can be reduced, thereby reducing the weight of the platform, so the application of this kind of stainless steel is very attractive.

Duplex stainless steel contains a series of grades with different corrosion resistance properties, and its corrosion resistance depends on their alloy composition. This article divides modern duplex stainless steels into five categories based on corrosion resistance:

1. Economical duplex stainless steel that does not deliberately add molybdenum, such as 2304; 2. Economical duplex stainless steel containing molybdenum, such as S32003; 3. Standard duplex stainless steel with a Cr content of about 22% and a Mo content of 3%, such as 2205 duplex stainless steel, Is the main brand, accounting for about 60% of the amount of duplex stainless steel; 4. Super duplex stainless steel with a Cr content of about 25% and a Mo content of 3%, with a PREN value of 40 to 45, such as 2507 duplex stainless steel; 5. Special super duplex Phase stainless steel, the content of Cr and Mo is higher than that of super duplex stainless steel, and the PREN value is greater than 45. For example, the local corrosion resistance of S32707 stainless steel has a strong correlation with its alloy element content. Elements that can increase pitting resistance are mainly Cr, Mo, and N.W. Although W is not commonly used, its effective contribution is about half of Mo (in weight percent). The relationship between the relative pitting resistance of stainless steel in chloride ion solution and the composition of stainless steel can be described by an empirical relationship, called the pitting resistance equivalent number (PREN). The calculation formula of PREN for austenitic stainless steel and duplex stainless steel is as follows:

PREN (pitting resistance equivalent)=%Cr+3.3(%Mo+0.5%W)+16%N

Table 1 shows the chemical composition and PREN value range of the second-generation wrought-rolled duplex stainless steel and cast duplex stainless steel. For comparison, the first-generation duplex stainless steel and the most commonly used austenitic stainless steel are also included.

Table 1 Chemical composition (wt%) and PREN value range of forged-rolled and cast duplex stainless steel*

(For comparison, austenitic steel grades are also listed)

*Maximum value, unless specified range or stated as minimum value. -Not specified in the standard.