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Martensitic Stainless Steel
Description of material
BIOVAL® VAL2BZ is a free machining martensitic stainless with a Carbon content capable to warrant a high hardness after heat treatment together with a very good machinability.
All applications where high hardness with a moderate corrosion resistance are indispensable, such as parts of pumps, pump shafts, gears, wear resistant devices and guides. BIOVAL® VAL2BZ, after hardening and in the low temperature tempered condition, provides a hardness higher than VAL1 and VAL2A. This grade is not recommended for applications in pressure vessels and shouldn’t be used for high polishability or mirror finishing processes.
It should be well considered that, as with all free machining grades, Sulphur is added to improve machinability and the formation of MnS inclusions could prime points of pitting if exposed to some corrosive environments. BIOVAL® VAL2BZ has a good resistance to mildly corrosive environments such as fresh water, crude oil, gasoline, alcohol, some beverages and atmospheres. BIOVAL® VAL2BZ 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. 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 not suitable for moderate cold forming or cold heading and up-setting due to embrittlement effect of the Sulphur content.
BIOVAL® VAL2BZ has a good machinability typical of martensitic free machined grades. Productivity gain depends on types of machines used, the kind of tools used and their geometry, cutting fluids and the kind of machine operations on the pieces produced. BIOVAL® VAL2BZ is prone to surface decarburization: a protective atmosphere should be considered in the heat treatment of finished pieces. 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.
BIOVAL® VAL2BZ is not recommended for both fusion and friction welding.
BIOVAL® VAL2BZ is not specifically designed for hot working and is usually supplied as cold finished round, hexagonal and square bars for machining processes. It’s important to know that all free machining grades, particularly Martensitic ones, have a poor hot plasticity and this characteristic must be well evaluated in forging processes. 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® VAL2BZ should be double tempered after hardening in order to reduce or avoid retained austenite obtaining high values of hardness. It’s important to point out that high tempering temperatures impair the corrosion resistance of all martensitic grades.