Duplex stainless steels are iron, nickel, and chromium (Fe-Ni-Cr) alloys. They are a popular grade in the stainless-steel family, and consist of two-phases in their metallurgical structure, that is, both the austenite and ferrite in equal proportions. Hence, they get their name from the austenite-ferrite micro-structure. They are useful in applications that require good corrosion resistance and strength.
Alloy 2205 duplex pipe finds varied use in different industries. It is typically used in applications for chemical processing, transfer, and storage, and in those systems that are subject to high stresses in chloride-containing environment. These pipes help to transport oil and gas, and also forms a part of heat exchangers and hydraulic tubing systems. The conception of duplex stainless steel was wrought on in the early 1930s.Duplex stainless steels has a chromium content of 22%–25%, with the additional presence of molybdenum, tungsten, and nitrogen. The alloy of austenite-ferrite in its microstructure imparts it a favorable combination of the parent grades.
This article lists the important aspects of duplex stainless-steel pipes.
Stress Corrosion Cracking (Scc) Resistance
SCC is the growth of cracks due to the effect of static and tensile stress in reactive environments. As duplex stainless steel pipe have higher strength and better toughness than ferritic grades, it does not succumb to SCC. It possesses good mechanical properties and shows excellent resistance to corrosion. The higher resistance to SCC is because of its microstructure, which makes it more difficult for a crack in duplex systems.
Fabrication And Welding
Duplex alloys have low risks of hydrogen cracking, and are more resistant to solidification cracking as compared to austenitic grades. It shows easy welding and machining, enabling welding with relatively high heat input and low interpass temperatures. Welding duplex stainless steel is cheaper and easier, and one can use various welding methods like resistance welding, laser welding, and electron beam welding for forming the desired shape.
Different Forms of Duplex Stainless Steel
Based on the presence of different production methods, there can be four basic duplex forms. Lean Duplex has lower Nickel content, and does not contain molybdenum. It is mainly used in storage tank construction. 2205 duplex stainless steel is regular duplex possessing standard amounts of both nickel and molybdenum and finds wide use in pharmaceutical sector. Super Duplex has higher molybdenum and chromium (24-26%) content, and is used for constructing pipes for oil, gas, and chemical industries. Hyper Duplex contains even higher volumes of molybdenum and chromium (27-32%), and finds use in energy and oil sectors.
Cleaning Duplex Stainless Steel
Different fabrication and welding processes can damage duplex stainless steel pipes which can damage its structure. For protecting the steel surface against formation of rust and other impurities, one needs to clean the steel surface before and after welding. Electrochemical cleaning, manual grinding, chemical pickling are common methods for cleaning the surface and restoring the passivation layer that imparts anti-corrosive properties to the structure. It helps to take care of the common side-effects such as oxidation and heat tinting that occurs during the welding process.
These are some of the important properties that characterize the duplex stainless steel grade. It is a highly valuable with enhanced properties such as improved weldability, good toughness, and corrosion resistance, which makes it a useful material in different industries.