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Superduplex Stainless Steel
Description of material
V257M is a super duplex stainless steel with high alloying contents and is one of the best grades in the family of duplex stainless steels. Balance between the Chromium/ Molybdenum and Nickel/Nitrogen favours the forming of more Austenite than Ferrite in the structure. This improves the cryogenic toughness and pitting/crevice resistance corrosion with little or no influence on stress corrosion cracking resistance.
Petro-chemical and oil & gas industries, heat exchangers, water treatment systems, 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
V257M offers sufficient resistance to intergranular corrosion, and the same resistance to pitting and crevice corrosion as super-austenitic V2018MN. It also offers very good stress corrosion and sulphide stress corrosion cracking resistance. It should also be noted that for this grade, as for every kind of stainless steel, surfaces should be free of contaminants 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.
V257M, 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, V257M 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.
V257M 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 Nitrogen 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 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 content. Both small pieces, rolled ring 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. Pay attention on cooling rate because V257M is more prone to precipitation of intermetallic phases than other duplex grades. Therefore, an insufficient cooling rate could generate a considerable reduction of both toughness and corrosion resistance.