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Superduplex Stainless Steel
Description of material
V257MWU is a super- duplex stainless steel with a very high pitting resistance equivalent PRE and is highly alloyed with Chromium, Nitrogen, Tungsten and Molybdenum. This grade is resistant to localized corrosion in aggressive environments, such as Chloride containing solutions, and is extensively used in seawater applications.
Off shore systems, petro-chemical and oil & gas industries, heat exchangers, water treatment systems, pressure vessels, desalination and seawater devices, propeller shafts, off shore plant and all applications where both high corrosion resistance and high mechanical properties are indispensable conditions.
Argon Oxygen Decarburization
V257MWU offers a high pitting resistance, as well as crevice and stress corrosion resistance in Chloride environments. This grade also offers hydrogen sulphide stress corrosion resistance. It should also be noted that for this grade, as for every kind of stainless steel, surfaces should be free of contaminants, heat tint and scale, and passivated for optimum resistance corrosion.
This super-duplex has a yield strength and cold working hardening factor higher than other duplex grades, and does not have the ductility and cold heading capacity of austenitic type 300 series stainless steels. Solution annealing and fast cooling must be carried out after cold processing.
V257MWU, as with 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. In large shapes or forgings, V257MWU may show a structure with more intermetallic phases, due to a not fast enough cooling rate, and this can result in a certain reduction of machining. For better performance in machinability, this grade could be substituted by MV274MDE or V2101MN, but only if these grades are able to offer similar or acceptable results in terms of corrosion, toughness and weldability according to a specific Norm or Design.
V257MWU 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 with more Nickel and/or shielding Argon-Nitrogen gas should be used, in order to obtain comparable properties of the base metal or, at least, a reduction of the ferrite 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 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 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 but it recommended to avoid high temperatures that cause an increase of Ferrite content. A slow or improper cooling rate generates a precipitation of deleterious intermetallic phases, causing a considerable reduction of both toughness and corrosion resistance.