Where are the obstacles to the use of duplex stainless steel

SCC is a form of corrosion related to a specific combination of factors, mainly tensile stress, corrosive environment, sufficiently high temperature, etc. Usually around 50 degrees Celsius, but may occur at lower temperatures around 25 degrees Celsius in certain environments, especially swimming pools.

Among them, standard austenitic steels such as 304 and 316 are the most susceptible to SCC. The following materials are not susceptible to SCC: ferritic stainless steels, duplex stainless steels, high nickel austenitic stainless steels.
The resistance to SCC makes dual phase steels a suitable material for many processes operating at higher temperatures. Stainless steel structures in swimming pools are known to be prone to SCC. Standard austenitic stainless steels such as 304 and 316 are prohibited in this application. The best steel for this purpose is a high nickel austenitic steel such as a 6% molybdenum grade. However, in some cases, duplex steels such as 2205 (1.4462) and super duplex steels can be considered.
The attractive combination of high strength, broad corrosion resistance, and moderate weldability seems to offer great potential to increase the market share of duplex stainless steels. The advantage of high strength immediately becomes a disadvantage when formability and machinability are considered. High strength also has lower ductility than austenitic stainless steels. Therefore, any application that requires a high degree of formability, such as sinks, is excluded from duplex grades. Even when ductility is sufficient, higher forces are required to form materials such as elbows. There is one exception to the normal rule of poor machinability for grade 1.4162.
The metallurgy of duplex stainless steels is much more complex than that of austenitic or ferritic steels. That's why 3-day sessions can be dedicated to duplexes! This factor means they are more difficult to produce and manufacture in factories. Sigma phase formation is most likely to occur when the cooling rate during fabrication or welding is not fast enough. The more alloyed the steel, the higher the probability of sigma phase formation. Therefore, super duplex steels are the most prone to this problem.
Embrittlement at 475 degrees is due to the formation of a phase called α’ (alpha prime). Although the worst temperature is 475 degrees Celsius, it can still form at 300 degrees Celsius, which limits the maximum use temperature of dual-phase steel. This limitation reduces the potential range of applications even further. On the other hand, the low-temperature use of duplex stainless steels is limited compared to austenitic stainless steels. Unlike austenitic steels, dual phase steels exhibit a ductile-brittle transition in impact tests. Typical test temperatures for offshore oil and gas applications are minus 46 degrees Celsius. Minus 80 degrees Celsius is the lowest temperature normally encountered with duplex steels.