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Martensitic Stainless Steel
Description of material
VAL2MCV is a Chromium –Molybdenum –Niobium-Vanadium martensitic stainless steel designed to supply high mechanical and creep resisting properties, together with high toughness due to its Nb addition is able to promote fine grain and a good corrosion resistance in mild environments. VAL2MCV/1 has slight differences of chemical composition but offers substantially the same performance.
Thanks to a special chemical composition and exact steel making process, this grade is widely used in the power generation industries and steam turbine blades production, as well as in other applications where high temperature resistance is required, such as turbine discs, bolting, fasteners for high temperature environments, screws, pins and clamp rings, parts of petrochemical plants, gas turbine compressor components, pump parts and valve components.
VAL2MCV 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, VAL2MCV offers an acceptable resistance corrosion 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. In addition, a higher cold upsetting could be obtained after a long lasting annealing and very slow cooling in the furnace. It should be pointed out that VAL2MCV could be prone to cause a rapid surface decarburization. If this were a problem, a protective atmosphere should be considered in the heat treatment 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.
Because turbine blades are the main application of this grade, VAL2MCV is supplied with a low Sulphur content. However, in the annealed condition and in the high temperature tempered, after hardening, condition, it offers a suitable and acceptable machinability. In any case, it is important to know that the productivity gain depends on 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, once the welded part reaches room temperature. VAL2MCV normally limits the grain growth in heat affected zones. In solid state joining such as Friction Welding, it 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 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, VAL2MCV 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.