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Martensitic Stainless Steel
Description of material
VAL1MPV is a Chromium- Molybdenum-Vanadium martensitic stainless steel designed to supply high mechanical properties and a good corrosion resistance and is widely used in making turbine blades.
Bolting, fasteners, screws, pump shafts, parts for petrochemical plants, pump parts and valve components. When VAL1MPV is used for steam turbine blades production, a special chemical composition and exact steel making process are always designed and applied.
VAL1MPV has its maximum resistance corrosion 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, VAL1MPV offers an acceptable resistance corrosion in many industrial and domestic applications, in not aggressive environments such as fresh water, industrial and rural atmospheres, petroleum products, gasoline fuel oil and alcohol. 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 furnace. It should be pointed out that VAL1MPV 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 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 to low temperatures and clearance of tools.
For turbine blade applications, VAL1MPV 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 ). This improves in the hardened and tempered condition, which has 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 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 after welding. In solid state joining such as Friction Welding, VAL1MPV 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. Alternatively, large forgings and large cross –section shapes should be left to cool until their core reach room temperature and, then, immediately heat treated.
Depending on thickness, geometry and required mechanical properties of parts, VAL1MPV 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 resistance. It’s important to point out that high tempering temperatures or annealing temperatures impair the corrosion resistance of all martensitic grades.