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Duplex Stainless Steel
Description of material
V234N is a ferritic-austenitic duplex stainless steel with a lower Molybdenum content, whose duplex structure, containing approximately equal amounts of Ferrite and Austenite, is able to offer a sufficient resistance to pitting and uniform corrosion, a very good stress corrosion cracking resistance, together with high mechanical properties and toughness.
Pump and valve parts, pressure vessels, desalination plants, tanks, bolts, nuts, fittings, water flanges, rings, stirrers and shafts where corrosion fatigue resistance is required, and reinforcing bars for structural applications such as roads, bridges, harbors, landing zones, and buildings.
Argon Oxygen Decarburization.
V234N has very good resistance in environments containing aggressive solutions. Particularly, it shows an acceptable resistance to general and pitting corrosion, comparable to type 304 and similar austenitic grades, as well as a very high resistance to intergranular corrosion. It also has a good resistance to crevice corrosion thanks to high Chromium and Nitrogen contents. However, in this case, an accurate evaluation of the design of pieces should be carried out, in order to avoid very narrow crevices situations. Stress corrosion resistance is guaranteed thanks to the amount of Ferritic phase in its duplex structure. It’s important to point out that the surface of every kind of stainless steel should be free of contaminants, tint and scale and passivated for optimum resistance corrosion.
V234N 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 corrosion resistance are restored by a new annealing and fast cooling.
V234N, as with other 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, providing that MV274MDE is able to offer similar or acceptable results in terms of corrosion, toughness and weldability require by specific Norms, or the design.
V234N can be welded with the same techniques used for austenitic stainless steels, but special care and suitable choices must be used. No preheating is required and the structure of the HAZ should show an acceptable austenite content if the right welding parameters are applied. Autogenous welding could jeopardize the pitting resistance corrosion of the weld metal (fused zone). Therefore, over-alloyed fillers with more Ni or Ni-Mo, should be used in order to obtain comparable properties of base metal or, at least, a higher austenite content. In very aggressive environments, fillers of Nickel alloys or high alloy Austenitic grades should be used. Matching filler duplex alloys could be used if 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 the ferrite content. Both small pieces, rolled rings or bars could be either air or rapid quenched after forging. However, an annealing with fast cooling after every kind of hot working is mandatory for best mechanical properties and corrosion resistance. Even though V234N has a structure less prone to generate intermetallic phases, when compared to high Mo Duplex grades, a slow or improper cooling rate should be avoided.