Early development of duplex stainless steel:
Duplex stainless steel, which combines many beneficial properties of ferritic and austenitic stainless steels, was originally developed in the early 1930s. The initial duplex stainless steel provides good performance characteristics but has limitations under welding conditions. The metallurgical process at the time was not suitable for producing grades with the correct austenite-ferrite balance. In addition, the carbon content of these early duplex stainless steels was relatively high because there was no effective decarburization process technology at the time. Therefore, the production of these materials is often the main production limited to some specific applications.
Modern duplex stainless steel development:
In the late 1960s and early 1970s, several factors led to the development of duplex stainless steel. First, the introduction of vacuum and argon deoxygenation (VOD and AOD) processes opened the door to the production of modern biphasic grades. These developments make it possible to achieve a good balance of low carbon content with high chromium content, high nickel content, and ferrite and austenite. This can produce materials with very good properties. Alloy content provides good resistance to local and uniform corrosion. The two-phase microstructure contributes to the high resistance to chloride stress corrosion cracking under many conditions and high strength. Modern duplex stainless steel also has good weldability.
These modern duplex stainless steels appeared in the same period as the offshore industry increased its activities. The industry needs a stainless steel that can handle aggressive environments. Although austenitic steels can also withstand these aggressive environments, the lack of nickel at the time led to higher prices. All these factors combine to encourage the offshore oil industry to adopt duplex stainless steel.
Duplex stainless steel 2205:
Duplex stainless steel 2205 (UNS S31803 / 32205) is the first commercially developed “second generation” duplex steel. It was developed in the mid-1970s and was introduced by the Krupp Steel producer in Germany. Today is still the most common duplex stainless steel that is currently considered the key material for duplex stainless steels. Duplex stainless steel 2205 provides corrosion resistance in many environments superior to Type 304 (UNS S30400), 316 (UNS S31600) and 317 (UNS S31700) austenitic steels. In addition, the yield strength is about twice that of austenitic stainless steel.
Interestingly, the composition range initially set to 2205 (S31803) was later determined to be too wide. According to the original composition specification, biphasic 2205 may form a harmful intermetallic phase at high temperatures. In order to achieve the best corrosion resistance and avoid these intermetallic phases, the content of chromium, molybdenum and nickel needs to be maintained within more than half of S31803. This modified 2205 is called S32205 and is a typical example of the commercial production of Duplex 2205 iv today.
Precision duplex stainless steel:
Although duplex stainless steel 2205 continues to gain momentum in various industries over time, in some cases extraordinary corrosion resistance is already higher than required. This has led to the development of many streamlined duplex stages such as LDX 2101 (S32101), ATI 2003 (UNS 32003) and Duplex 2304 (UNS S32304). These new duplex stainless steels not only contain less than 2205 alloying elements, but also can be used for alternative 304 or even 316 grade applications. For example, duplex alloys are used in many construction applications due to high strength, good corrosion resistance, and lower overall cost, commonly used stainless steel grade 316.
In addition, since the 1980s, the petroleum industry has been one of the main driving forces for the development of even higher-alloy dual-phase materials, known as super duplex and super duplex. These higher alloy biphasic grades are designed to handle extreme environments such as higher corrosive conditions and higher pressures encountered at larger depths in oil and gas fields [V]. Super duplex grades have an equivalent pitting resistance (corrosion resistance, also known as PRE or PREN), higher than 40. The number of PREs at the super duplex level is 48 or higher [v]. Currently, the current product categories include the super-duplex SAF 2507 SD (UNS S32750) and super-duplex SAF 3207 HD (UNS S33207) and SAF 2707 HD (UNS S32707).
Although the current duplex stainless steel market accounts for a very small part of the total stainless steel, duplex stainless steel is a developing industry and its prospects continue to grow. From the International Stainless Steel Forum ISSF, studies have shown that duplex output soared from 6,000 tons a month, increased to 10,000 tons in 2004 to 2005, and up to 22,000 tons in 2008. Duplex steel continues to gain popularity as various industries begin to consider the entire life cycle cost. In addition to the potential immediate material cost savings, the use of duplex stainless steel can also lead to longer life cycles and lower maintenance costs in many cases.
Source: China Duplex Stainless Steel Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)