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Martensitic Stainless Steel
Description of material
X122MV is a Cr-Mo-Nb-V-N martensitic stainless steel designed to supply high mechanical properties and good creep rupture strength together with a good corrosion resistance and high toughness. Thanks to a special chemical composition and exact steel making process, this grade is widely used in the power generation industries. In the case of aerospace applications, X122MV is processed by special steel making process such as ESR /VAR.
All applications where high temperature resistance is required such as turbine blades and turbine discs, bolting, fasteners, screws, pins and rings, parts for petrochemical plants, gas turbine compressor components, pump parts and valve components.
X122MV has its maximum corrosion resistance when in the hardened + tempered condition. As with most martensitic grades, its use in the annealed condition or any other situation able to strongly reduce the hardness in environments containing Chloride, should be avoided. However, X122MV offers an acceptable corrosion resistance in many industrial applications in not aggressive environments such as fresh water, petroleum products, gasoline fuel oil, alcohol and some chemicals. It should be noted that this grade, as for every kind of stainless steel, surfaces should be free of contaminant and scale, heat tint, and passivated for optimum resistance to corrosion.
In the annealed condition, this grade is suitable for cold forming operations such as cold heading. In addition, a better cold upsetting could be obtained after a long lasting annealing and very slow cooling in the furnace. It should be pointed out that X122MV is not so prone to cause a rapid surface decarburization as high Carbon martensitic grades. If this were a problem, a protective atmosphere should be considered in the heat treatments of finished pieces. Blooms or large cross section billets can be cut by band and circular saw or abrasive wheel. Small billets could be cut by cold shearing paying attention on low temperatures and clearance of tools.
X122MV does not have a micro-resulphured structure. In the annealed condition it hasn’t a very good machinability (mainly due to chips prone to build up edge) while this does improve in the hardened and tempered condition with higher values of resistance than the annealed condition. However, it is important to know that the productivity gain depends on the type of machines used, the kind of tools used and their geometry, cutting fluids and the kind of machine operations on the pieces produced.
Preheating of small sections is necessary and must be particularly done in the case of both large cross section differences and large welds with several interpasses. Post welding heat treatment (PWHT) is mandatory due to the transformation of martensite in heat affected and fused zones and should be immediately done just after the welded part reaches room temperature. In solid state joining such as Friction Welding, X122MV provides a quality bond line. When friction welded with different grades, a tempering or annealing of the welded piece must be done in order to soften the martensitic structure of HAZ and bond line.
Blooms and ingots require a preheating to avoid cracks and a slow cooling in the furnace after forging. Avoid overheating able to cause internal bursts or promote formations of ferrite stringers. Improper cooling could result in stress cooling cracks. Large forgings and large cross –section shapes should be left to cool until their core reaches room temperature in order to allow the complete transformation of Martensite and, then, immediately heat treated.
Depending on thickness, geometry and required mechanical properties of parts, X122MV could be air or oil hardened. This choice of quenching method depends on the thickness, shape and geometry of pieces and their metallurgical-mechanical requirements as well. The tempering temperature has to be chosen in order to offer the best properties, avoiding those ranges of temperatures and cooling rates able to cause a significant reduction in toughness and corrosion resistance.
|RCCM-M MATERIAL REF.||X12CrNiMoV12-3|
|W.N.||1.4938 / 1.4939|