In the early days, duplex steels were used for castings, bars and plates for pulp, paper and high temperature applications due to their good corrosion resistance and castability. By 1932, duplex steels accounted for 6.5% of the 5500 ton global stainless steel market.
Over the next few decades, metallurgists developed techniques to reduce carbon content and introduce nitrogen to control plasticity and weldability. Nickel shortages during World War II prompted greater interest in duplex stainless steels. Improvements in the 1950s included the development of duplex grades with nitrogen additions to reduce the risk of cracking during casting and welding.
The development of duplex steels reached a climax in the 1970s, with the second generation of duplex stainless steels, which had better resistance to intergranular corrosion, gaining more applications and greater commercial success.
From a metallurgical perspective, these steels utilize added nitrogen to enhance the formation of austenite. This prevents the duplex structure from reducing to full ferrite in the heat affected zone after welding or hot working. Nitrogen also helps the steel resist pitting corrosion and improves strength at the same time.
Developments in the 1970s and 1980s were dominated by the third generation of duplex steels - the more highly alloyed superduplex grades Forta SDX 2507, Forta SDX 100. These grades contain more alloying elements and therefore have better properties, but at a higher cost. Superduplex stainless steels meet the needs of the offshore oil and gas, food and beverage, and chemical industries.
The most common definition of superduplex stainless steel is that its pitting resistance (PRE or PREN) should be at least 40. To achieve this level of corrosion resistance, metallurgists use alloying elements such as chromium, molybdenum, tungsten, nickel and copper. Of course, more alloying elements in superduplex steels also mean higher costs, but they are generally more competitive in price than austenitic steels and can be considered as a substitute for austenitic steels.
polished stainless steel pipe
