Dual Phase Stainless Steel And 2205 Stainless Steel

01What is duplex stainless steel

Duplex stainless steel is called "duplex" because its metallographic microstructure consists of two types of stainless steel grains, ferrite and austenite. When duplex stainless steel melts, it first solidifies into a complete ferrite structure when it solidifies from the liquid state. As the material cools to room temperature, about half of the ferrite grains transform into austenite grains. As a result, approximately 50% of the microstructure consists of austenite phase and 50% consists of ferrite phase.

The strength of duplex stainless steel is approximately twice that of conventional austenitic stainless steel or ferritic stainless steel. Therefore, designers can reduce the thickness of thin walls in certain applications. The following figure compares the yield strength of several duplex stainless steels and 316L austenitic stainless steel in the temperature range of room temperature to 300 ℃. Although duplex stainless steels have high strength, they exhibit good plasticity and toughness. The toughness and ductility of duplex stainless steel are significantly better than those of ferritic stainless steel and carbon steel, and it maintains good toughness even at very low temperatures such as -40 ℃/F. But it still cannot reach the excellent level of austenitic stainless steel.

The corrosion resistance of stainless steel mainly depends on its chemical composition. In most application environments, duplex stainless steel exhibits high corrosion resistance due to its high chromium content, which is advantageous in oxidizing acids, and sufficient amounts of molybdenum and nickel to withstand corrosion in medium reducing acid media.

The ability of duplex stainless steel to resist chloride ion pitting and crevice corrosion depends on its chromium, molybdenum, tungsten, and nitrogen content. The relatively high chromium, molybdenum, and nitrogen content of duplex stainless steel gives them excellent resistance to chloride pitting and crevice corrosion. They have a range of different corrosion resistance properties, ranging from grades equivalent to 316 stainless steel corrosion resistance, such as economical duplex stainless steel 2101, to grades equivalent to 6% molybdenum stainless steel corrosion resistance, such as SAF2507.

Dual phase stainless steel has excellent resistance to stress corrosion cracking (SCC), which is inherited from the ferrite side. All duplex stainless steels have significantly better resistance to chloride stress corrosion cracking than 300 series austenitic stainless steels. Standard austenitic stainless steel grades such as 304 and 316 may undergo stress corrosion cracking under conditions of chloride ions, humid air, and elevated temperatures. Therefore, in many applications in the chemical industry where there is a high risk of stress corrosion, duplex stainless steel is often used instead of austenitic stainless steel.

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, duplex stainless steel has lower nickel and molybdenum content. Due to 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 high yield strength of duplex stainless steel, its cross-sectional size can often be reduced. Compared to using austenitic stainless steel, using duplex stainless steel can significantly reduce costs and weight.

Dual phase stainless steel has a history of over 80 years. Early grades were alloys of chromium, nickel, and molybdenum. The first batch of forged and rolled duplex stainless steel was produced in Sweden in 1930 and used in the sulfite paper industry. These grades were developed to reduce intergranular corrosion problems in early high carbon austenitic stainless steel. In 1930, Finland produced duplex stainless steel castings. The predecessor of Uranus 50 was patented in France in 1936. After World War II, AISI 329 stainless steel became a mature grade and was widely used in heat exchanger pipelines for nitric acid plants. 3RE60 is one of the first generation duplex stainless steel grades developed specifically to improve its resistance to chloride stress corrosion cracking (SCC). Later, both forged and cast duplex stainless steel grades were used in various processing industries, including containers, heat exchangers, and pumps.

The first generation duplex stainless steel has good performance, but has limitations in the welded state. The heat affected zone (HAZ) of the weld seam has low toughness due to excessive ferrite, and its corrosion resistance is significantly lower than that of the base metal. These limitations limit the application of the first generation duplex stainless steel to certain specific applications in non welded states. The invention of stainless steel refining and argon oxygen decarburization (AOD) process in 1968 made it possible to produce a series of new stainless steel grades. One of the many advances brought about by AOD is the deliberate addition of nitrogen as an alloying element. Adding nitrogen to duplex stainless steel can make the toughness and corrosion resistance of HAZ in the welded state close to the properties of the base material. With the improvement of austenite stability, nitrogen also reduces the formation rate of harmful intermetallic phases. Nitrogen containing duplex stainless steel is called second-generation duplex stainless steel. This new commercialization progress began in the late 1970s, coinciding 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 processing performance, and high strength. 2205 has become the main grade of second-generation duplex stainless steel and is widely used in offshore oil platform gas gathering pipelines and processing facilities. Due to the high strength of this steel, the wall thickness can be reduced, thereby reducing the weight of the platform. Therefore, the application of this stainless steel has great appeal.

Dual phase stainless steel includes a series of grades with different corrosion resistance characteristics, and its corrosion resistance depends on their alloy composition. According to corrosion resistance, modern duplex stainless steel is divided into five categories:

1. Economical duplex stainless steel without intentionally adding molybdenum, such as 2304;

2. Economic duplex stainless steel containing molybdenum, such as S32003;

Standard duplex stainless steel with a Cr content of about 22% and a Mo content of 3%, such as 2205, is the main grade, accounting for about 60% of the duplex stainless steel usage;

3. Super duplex stainless steel with a Cr content of about 25% and a Mo content of 3%, with a PREN value of 40-45, such as 2507; 5. Super duplex stainless steel with higher Cr and Mo content than super duplex stainless steel, PREN value greater than 45, such as S32707

The local corrosion resistance of stainless steel is strongly correlated with its alloy element content. The elements that can increase the resistance to pitting corrosion are mainly Cr, Mo, and N. W. Although W is not commonly used, its effective contribution is about half of Mo (in weight percentage). 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 equation called the pitting resistance equivalent number (PREN). The PREN calculation formula for austenitic stainless steel and duplex stainless steel is as follows: PREN (pitting corrosion equivalent)=% Cr+3.3 (% Mo+0.5% W)+16% N

It is generally believed that good performance can be achieved when the ratio of ferrite phase to austenite phase in duplex stainless steel is 30% to 70%. However, duplex stainless steel is often considered to have roughly equal proportions of ferrite and austenite. In current commercial production, in order to achieve optimal toughness and processing characteristics, the proportion of austenite tends to be slightly higher. The interactions between the main alloying elements, especially chromium, molybdenum, nitrogen, and nickel, are very complex. In order to obtain a stable biphasic structure that is conducive to processing and manufacturing, it is necessary to pay attention to the appropriate content of each element.

In addition to phase equilibrium, the second major issue regarding duplex stainless steel and its chemical composition is the formation of harmful intermetallic phases at elevated temperatures. The σ phase and χ phase are formed in high chromium and high molybdenum stainless steel, and preferentially precipitate in the ferrite phase. The addition of nitrogen greatly delayed the formation of these phases. Therefore, it is very important to maintain a sufficient amount of nitrogen in the solid solution. With the increasing experience in manufacturing duplex stainless steel, people are increasingly aware of the importance of controlling a narrow range of components.

The initial composition range of 2205 duplex stainless steel (UNS S31803) was too wide. Experience has shown that in order to achieve optimal corrosion resistance and avoid the formation of intermetallic phases, the chromium, molybdenum, and nitrogen content of S31803 should be kept at the upper and middle limits of the content range. This has led to the introduction of the improved 2205 duplex steel UNS S32205 with a narrower composition range. The composition of S32205 is typical of the commercialized 2205 duplex stainless steel today.

02What is 2205 duplex stainless steel

2205 duplex stainless steel is a (austenitic ferritic) duplex stainless steel composed of 22% chromium, 3% molybdenum, and 4.5% nickel nitrogen alloy (referred to as 2205 duplex steel). In its solid solution structure, the ferrite and austenite phases account for about half each, and the minimum phase content generally needs to reach 30%. Therefore, it has the performance characteristics of both ferrite and austenitic stainless steel. 2205 duplex stainless steel is a nitrogen doped duplex stainless steel with high strength, good impact toughness, and excellent overall and local stress corrosion resistance. 2205 duplex steel is particularly suitable for the temperature range of -50 ° F/+600 ° F. Applications beyond this temperature range can also consider this alloy.

The main feature of 2205 duplex stainless steel is that its yield strength can reach 400-550MPa, which is twice that of ordinary austenitic stainless steel. This characteristic allows designers to reduce weight when designing products, thus saving materials. This alloy has a price advantage over 316317L and reduces equipment manufacturing costs. In terms of corrosion resistance, especially under harsh medium environments such as seawater with high chloride ion content, the resistance to pitting corrosion, crevice corrosion, stress corrosion, and corrosion fatigue of duplex stainless steel is significantly better than that of ordinary austenitic stainless steel, and can be comparable to high alloy austenitic stainless steel. Due to the special performance characteristics of 2205 duplex steel, the product types include bar, wire, welded pipe, seamless pipe, steel plate, forged material, strip, etc. It has a wide range of applications and is still the most widely used grade in duplex steel.