Oil and gas drilling can penetrate to four or five kilometres inside the earth. The materials used to drill these wells and to line the well bore, have to sustain massive extremes of temperature, pressure and corrosion. Super duplex stainless steel performs well in these extreme environments.
The oil and gas industry needed to develop metal alloys that could sustain high pressure and temperatures when working as down hole casing and tubing. They had to be resistant to corrosive substances used as muds during the well drilling operations and to the chemical compounds used down hole to fracture rock formations and stimulate the flow of oil and gas. Originally created to withstand aggressive chemical fluids used in the paper and pulp industries in Scandinavia, duplex and super duplex steels are in the forefront of alloy technology for extreme environments.
The key to super duplex steels is their high content of chromium and molybdenum. This provides a resistance to most oxidising and reducing environments. The addition of different percentage contents of nitrogen and tungsten also refine the properties further. The alloy combination results in a specific pitting resistance equivalent number (PREN) that defines the alloy’s resistance to corrosion.
The equation that defines this phenomenon is:
PREN = Cr + 3.3Mo +aN
Where Cr defines the percentage of chromium content in the alloy, Mo is the percentage molybdenum content and N the percentage nitrogen content. The “a” is a constant whose value ranges between 16 and 32. When the PREN number is greater than 40, the steel is classified as a super duplex. Below that it is a duplex steel.
Characteristics Of Elements
In super duplex steels, the chromium content should be at least 25 percent of the total metal whilst the nickel content is 7 percent. The molybdenum content ranges between 3.5 and 4 percent whilst the nitrogen content should be between 0.25 and 0.27 percent. Molybdenum is a very important element that increases the alloy’s a to any chloride compounds that are present in high concentrations in oil industry fluids. On these parameters, the PREN number should range between 42 and 43 for standard super duplex steels.
These types of steels must perform in temperatures that range between 140 and 290 degrees Celsius and pressures of between 60 and 207 million pascals (MPa). So the material has to have not just high strength, but a resistance to corrosion and extremely high temperatures.
Super duplex steels are used to produce everything from pipe flanges and fittings to cargo tanks. They are also used for high resistance electrical wiring. One of their more common uses is as heat exchangers in oil refineries where there is a risk of stress crack corrosion from streams of fluids with a high concentration of chloride compounds.
Super duplex steels have found a range of applications in extreme sea water environments. They are also used widely as heat exchangers in oil refineries. They are highly resistant to chlorine solution and stress corrosion cracking.
Julia Edwards is a metallurgist who writes about steel alloys for a wide range of manufacturing industry websites and blogs. She follows developments of products manufactured by Chemi Petro to create new ideas for alloy applications.