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Duplex Stainless Steel
Description of material
V225MN is a Molybdenum and Nitrogen alloy ferritic-austenitic stainless steel whose duplex structure containing approximately the same amounts of Ferrite and Austenite is able to provide a good pitting and uniform corrosion resistance, as well as very good stress corrosion cracking resistance, together with high mechanical properties and toughness.
Pump and valve parts, pressure vessels, desalination plants, tanks, bolts, nuts, rings, stirrers, hollows bars handling solutions containing Chloride, marine environments, boat shafts where corrosion fatigue resistance is required, and reinforcing bars for structural application such as roads, bridges, harbors landing zones, and buildings.
Argon Oxygen Decarburization
V225MN has very good resistance in environments containing chloride and fluoride and other aggressive solutions or materials. Particularly, it shows better results in general and pitting corrosion than type 316 and similar austenitic stainless steels, and has very good resistance to crevice corrosion thanks to high Chromium, Nitrogen and Molybdenum contents. In this case, an accurate evaluation of piece design is required in order to avoid very narrow crevices situations. Stress corrosion resistance is guaranteed thanks to presence of the Ferritic part of the duplex structure. It’s important to point out that the surface of every kind of stainless steel should be free of contaminants, heat tint, scale and passivated for optimum resistance corrosion.
V225MN is suitable for the same cold heading and cold deforming processes applied to austenitic stainless steels, but it should be considered that its yield strength is higher and, therefore, more difficult to cold strain. Original mechanical properties and resistance corrosion are restored by a new annealing and fast cooling.
V225MN, such as all duplex stainless steels, is more difficult to machine if compared to the typical austenitic grades. A careful choice of machining parameters should partially reduce the gap. For better performance in machinability, this grade could be substituted by MV274MDE, but only if this alternative grade is able to offer similar or acceptable results in terms of corrosion, toughness and weldability according to specific Norms or Design.
V225MN can be welded with the same techniques of austenitic stainless steels but a special care and suitable choices must be used. No preheating is required and the structure of HAZ should show an acceptable austenite content if the right welding parameters are applied. Autogenous welding could jeopardize the pitting corrosion resistance of weld metal (fused zone). Therefore, over -alloyed fillers should be used in order to obtain comparable properties of the base metal or, at least, an austenite content more than 40-60%. In very aggressive environments, fillers of Nickel alloys or high alloy Austenitic grades should be used. Matching filler duplex alloys could be used if the composition of FZ and HAZ is able to supply the expected results. Post welding annealing restores the balance of Ferrite/Austenite and eliminates the welding stresses.
Large shapes and ingots require a suitable preheating. Avoid overheating or reaching the upper limit of forging temperature to avoid an increase of ferrite contents. Both small pieces, rolled rings or bars could be either air or fast quenched after forging. However, an annealing with fast cooling after every kind of hot working is mandatory for best mechanical properties and corrosion resistance. Slow or improper cooling rates generate the precipitation of deleterious intermetallic phases, causing a considerable reduction of both toughness and corrosion resistance.