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Martensitic Stainless Steel
Description of material
VAL2A is a Chromium martensitic stainless with a Carbon content capable of suitable hardness after heat treatment, together with a moderate resistance corrosion. This popular grade is provided for different International Norms that allow some variations of Carbon and Chromium contents compared to EN (VAL2A/UK) or a composition substantially the same (VAL2A/UK2).
All applications where high hardness and corrosion resistance, combined, are indispensable, such as dental and surgical instruments, kitchen knives, parts of brakes, valve seats , plastic molds , parts of pumps, pump shafts ,wear resistant devices, gauges, conveyor chains , guides, instruments and table cutlery. This grade after hardening and tempering provides a hardness lower than VAL2B and VAL2DE. VAL2A is widely used in table cutlery, surgical instruments and pump shafts.
EAF + AOD
VAL2A has its maximum corrosion resistance in the hardened + low temperature tempered condition, and with its maximum hardness. Its use in the annealed condition, or any other situations that are able to reduce the surface hardness, and in environments containing Chloride, should be avoided. VAL2A has good corrosion resistance in mild environments such as fresh water, industrial and rural atmospheres, petroleum products, gasoline fuel oil and alcohol. This grade could be supplied in micro-resulphured condition, but in the case of mirror finishing, such as table cutlery or special purposes, this condition should be avoided. In the case of high polishability requirements, a special steel making process and/or heat treatments should be chosen. 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 or up-setting. It should be pointed out that VAL2A is prone to surface decarburization: a protective atmosphere should be considered in heat treatments of finished pieces.
Micro-resulphured VAL2A in the annealed condition, and in the high temperature tempered condition after hardening, has a good machinability. It’s important to know that the productivity gain depends on the type of machines, the kind of tools and their geometry, cutting fluids used, and the kind of machine operations on the pieces produced. Grinding and polishing of hardened + tempered material at the maximum values of hardness must be carried out with great care, in order to avoid the overheating of the surface of the piece, resulting in poor corrosion resistance and/or grinding cracks.
This process for martensitic stainless grades is always risky, and special care must be applied in the choice of welding parameters. In any case, if a welding process were required, a preheating is mandatory and the part must be maintained at temperature, as well as being followed by immediate annealing or tempering. Fillers of the same composition can be used, to obtain mechanical properties close to that of the base metal. Alternatively, austenitic fillers may be used, considering as inevitable a reduction of the properties compared to that of the base metal. In solid state joining such as Friction Welding, VAL2A 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 suitable preheating to avoid cracks and a slow cooling in the furnace after forging. Overheating must always be avoided in order to reduce the risk of internal bursts. An 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 and, then, immediately, heat treated. A right and suitable heat treatment of pieces after forging process warrants a structure with no or poor, retained austenite, avoiding delayed cracks.
VAL2A should be double tempered after hardening, in order to reduce or avoid retained austenite, but obtaining high values of hardness. Double tempering is not normally used in cutlery production, where a single temper should be sufficient.