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Martensitic Stainless Steel
Description of material
VAL2W is a Cr-Mo-W -V 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, compressors and steam turbines. In the case of aerospace applications, VAL2W is normally 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.
VAL2W 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, VAL2W 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. Moreover, VAL2W offers good resistance to scaling and oxidation in both continuous and intermittent service but it should be considered that the right temperature service depends on the kind of environment atmosphere. 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 moderate cold forming. 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 VAL2W is 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.
VAL2W does not have a micro-resulphured structure and (Mo / W)-carbides have an abrasive action on the tools. In the annealed condition, VAL2W has a fair machinability (mainly due to chips prone to build up edge) but this a little bit improves 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 weld part reaches room temperature. In solid state joining such as Friction Welding, VAL2W provides a quality bond line. When friction welded with different grades, a tempering or annealing of welded pieces 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. This transformation is necessary to order to avoid a mixed tempered structure of fresh and tempered martensite, with the rest being Austenite, able to cause delayed cracks. Because VAL2W is very prone to cause a rapid surface decarburization, long lasting heating cycles should be avoided, unless a suitable stock removal of forging pieces has been estimated.
Depending on thickness, geometry and required mechanical properties of parts, VAL2W 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. VAL2W must be left to cool down to room temperature after hardening in order to allow the complete transformation of martensite and, then, immediately tempered. A double tempering after hardening is particularly recommended. 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.
|AISI||616 / 422 / B4B|