Duplex stainless steel is called "duplex" because its metallographic microstructure consists of two stainless steel grains, ferrite and austenite. The blue-colored austenite phase appears as an "island" surrounded by a sea of light-colored ferrite phases. When the duplex stainless steel is melted, it first solidifies from the liquid state to a fully ferritic structure, and as the material cools to room temperature, about half of the ferrite grains transform into austenite grains ("islands") . As a result, about 50% of the microstructure is austenite phase and 50% is ferrite phase.
The duplex structure allows this type of stainless steel to combine many excellent properties. The advantage of this combination is high strength: the strength of duplex stainless steel is approximately twice that of conventional austenitic or ferritic stainless steels. So designers can reduce the wall thickness in some applications.
Despite their high strength, duplex stainless steels exhibit good ductility and toughness. The toughness and ductility of duplex stainless steels are significantly better than those of ferritic stainless steels and carbon steels, and they maintain good toughness even at very low temperatures such as -40°C/F. But it has not reached the excellent level of austenitic stainless steel.
The corrosion resistance of stainless steel mainly depends on its chemical composition. In most applications, duplex stainless steels exhibit high corrosion resistance due to their high chromium content, which is favorable in oxidizing acids, and sufficient amounts of molybdenum and nickel to resist moderately reducing acids Corrosion of the medium.
The resistance of duplex stainless steel to chloride ion pitting and crevice corrosion depends on its chromium, molybdenum, tungsten and nitrogen content. The relatively high chromium, molybdenum and nitrogen contents of duplex stainless steels make them very resistant to chloride pitting and crevice corrosion. They are available in a range of different corrosion resistance properties, ranging from grades equivalent to the corrosion resistance of 316 stainless steel, such as the economical duplex stainless steel 21011, to grades equivalent to the corrosion resistance of 6% molybdenum stainless steel, such as SAF 25077.
Duplex stainless steels have very good resistance to stress corrosion cracking (SCC), a property "inherited" from the ferrite side. The resistance to chloride stress corrosion cracking of all duplex stainless steels is significantly better than that of 300 series austenitic stainless steels. Standard austenitic stainless steel grades such as 304 and 316 may experience stress corrosion cracking in the presence of chloride ions, humid air and elevated temperatures. Therefore, in many applications in the chemical industry where there is a greater risk of stress corrosion, duplex stainless steels are often used instead of austenitic stainless steels.
In terms of physical and physical properties, it is between austenitic stainless steel and ferritic stainless steel, but closer to ferritic stainless steel and carbon steel. Compared with austenitic stainless steel grades with the same corrosion resistance, the content of nickel and molybdenum in duplex stainless steel is lower. Due to the low alloying element content, duplex stainless steels may have a price advantage, especially when alloying surcharges are high. In addition, due to the higher yield strength of duplex stainless steels, their cross-sectional dimensions can often be reduced. The use of duplex stainless steels offers significant cost and weight savings compared to solutions using austenitic stainless steels.
Development history of duplex stainless steel
Duplex stainless steels have been around for over 80 years. Early grades were alloys of chromium, nickel and molybdenum. The first wrought duplex stainless steels were produced in Sweden in 1930 and used in the sulfite paper industry. These grades were developed to reduce the intergranular corrosion problems of early high carbon austenitic stainless steels. Duplex stainless steel castings were produced in Finland in 1930. The predecessor of the Uranus 50 was patented in France in 1936. After World War II, AISI type 329 stainless steel became a mature grade and was widely used in heat exchanger piping for nitric acid plants. 3RE60 is one of the first generation of duplex stainless steel grades specially developed to improve resistance to chloride stress corrosion cracking (SCC). Later, both forged and cast duplex grades were used in a variety of process industry applications, including vessels, heat exchangers, and pumps.
The first-generation duplex stainless steels have good performance, but have limitations in the as-welded state. The heat-affected zone (HAZ) of the weld has low toughness due to excess ferrite and significantly lower corrosion resistance than the base metal. These limitations limit the application of first-generation duplex stainless steels to specific applications in the non-welded state. The invention of stainless steel refining and the Argon Oxygen Decarburization (AOD) process in 1968 enabled the creation of a range of new stainless steel grades. One of the many advances that AOD has brought about 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 as-welded state approach the properties of the base metal. Nitrogen also reduces the rate of formation of detrimental intermetallic phases along with increased austenite stability.
Nitrogen-containing duplex stainless steels are called second-generation duplex stainless steels. This new commercialization progress, which began in the late 1970s, coincided with the development of offshore oil and gas fields in the North Sea and the market demand for stainless steel with excellent resistance to chloride ion corrosion, good processability and high strength. 2205 has become the main grade of 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 steel, the wall thickness can be reduced, which can reduce the weight of the platform, so the application of this stainless steel is very attractive.
Duplex stainless steels comprise a range of grades with varying corrosion resistance properties depending on their alloy composition.
This article divides modern duplex stainless steels into five categories based on corrosion resistance:
1. Economical duplex stainless steel without intentionally adding molybdenum, such as 2304.
2. Economical duplex stainless steel containing molybdenum, such as S32003.
3. Standard duplex stainless steel with Cr content of about 22% and Mo content of 3%, such as 2205, is the main grade, accounting for about 60% of the amount of duplex stainless steel.
4. Super duplex stainless steel with Cr content of about 25% and Mo content of 3%, PREN value of 40 to 45, such as 2507.
5. For super duplex 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, such as S32707.
The local corrosion resistance of stainless steel has a strong correlation with its alloying 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 that of Mo (in weight percent). The relationship between the relative pitting resistance of stainless steel in chloride ion solution and the stainless steel composition can be described by an empirical relationship called the pitting resistance equivalent number (PREN).