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Martensitic Stainless Steel
Description of material
BIOVAL® VAL2C is a 12% Chromium martensitic stainless with a Carbon content capable of high hardness after heat treatment together with a moderate corrosion resistance.
All applications where high hardness and corrosion resistance 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 higher than VAL2A or VAL2B. This grade is widely used in the production of sharp blades in professional cutlery manufacturing and in table cutlery, but only if a better cutting edge of blades is required.
BIOVAL® VAL2C has its maximum corrosion resistance when in the hardened + low temperature tempered condition and with its maximum hardness. Its use in the annealed condition or any other situation able to reduce the surface hardness and in environments containing Chloride, should be avoided. BIOVAL® VAL2C 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 the micro-resulphured condition but in the case of mirror finishing, such as table cutlery or similar purposes, this condition should be avoided. In the case of high polish - ability requirements, a special steel making process and/or heat treatment 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 moderate cold forming operations such as cold heading or up-setting. A higher cold formability could be obtained after a long lasting annealing and very slow cooling in the furnace. It should be pointed out that BIOVAL® VAL2C is prone to surface decarburization: a protective atmosphere should be considered in the heat treatment of finished pieces.
Micro-resulphured BIOVAL® VAL2C in the annealed condition and in the high temperature tempered after hardening condition, offers a good machinability. It’s 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. Grinding and polishing of hardened + tempered material at 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 of martensitic stainless grades is always risky and a 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 and 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 an inevitable reduction of these properties. In solid state joining such as Friction Welding, BIOVAL® VAL2C provides a good 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 the forging process creates a structure with no or little retained austenite avoiding delayed cracks.
BIOVAL® VAL2C should be double tempered after hardening in order to reduce or avoid retained austenite obtaining high values of hardness. Double tempering is not normally used in cutlery production where a single one should be sufficient.