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Martensitic Stainless Steel
Description of material
VAL1BC is a Chromium –Niobium martensitic stainless steel designed to supply a good corrosion resistance in mild environments, high mechanical and creep resisting properties together with high toughness.VAL1BC has a chemical balance substantially similar to VAL1C but it differs from this one in having a slight lower Carbon and Niobium content.
Thanks to a special chemical composition and exact steel making process, this grade is widely used in the power generation industries and in steam turbine blade production. And in other applications where high temperature resistance is required, such as turbine discs, bolting, fasteners, screws, pins and rings, parts of petrochemical plants, gas turbine compressor components, pump parts and valve components.
VAL1BC 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, VAL1BC 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 annealed condition, this grade is suitable for cold forming 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 VAL1BC 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 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.
VAL1BC 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 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. VAL1BC normally limits the grain growth in heat affected zones and in solid state joining such as Friction Welding, 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 is required. Avoid overheating able to cause internal bursts or promote formations of ferrite stringers. Improper cooling could result in stress cooling cracks. Large forging 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, VAL1BC 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 strong reduction of toughness and resistance. It’s important to point out that high tempering temperatures or annealing impairs the corrosion resistance of all martensitic grades.