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Martensitic Stainless Steel
Description of material
BIOVAL® VAL1HS is a special variation of the most popular free machining martensitic stainless steel with a Carbon content able to give a sufficient values of hardness after heat treatment but with high machinability compared to the standard type 416 grade (i.e. VAL1Z) and similar grades. This characteristic allows a high productivity in terms of machinability and is widely used in automatic machining industries.
All applications where a suitable hardness with a moderate corrosion resistance are indispensable, such as parts of pumps, pump shafts, gears, wear resistant devices, bolts, nuts, valves stems and shafting. BIOVAL® VAL1HS, after hardening and low temperature tempering condition, provides a hardness similar to the type 410 series. 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® VAL1HS has a good resistance to mildly corrosive environments such as fresh water, crude oil, gasoline, alcohol, some beverages and atmospheres. BIOVAL® VAL1HS has its maximum resistance corrosion when 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. Nevertheless, in case BIOVAL® VAL1HS were to be used in the annealed condition, it should be considered that its resistance corrosion will depend on the corrosive capacity of environment. This means that this condition may not be so detrimental in atmospheric corrosion or only slightly reduced in mild aqueous environments. 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.
Even in the annealed condition, this grade is not suitable for cold heading and up-setting due to the embrittlement effect of the high Sulphur content; it has not been specifically designed for cold forming and is usually supplied as cold finished round, hexagonal and square bars for machining processes. Blooms or large cross section billets can be cut by band and circular saw or abrasive wheel. Cold shearing of small billets and bars is not recommended because this process may cause shear or stress cracks due to its structure embrittled by its high Sulphur content.
BIOVAL® VAL1HS offers a machinability better than the typical martensitic free machined grades. 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. As in general rule, machinability depends on either Sulphur content and hardness in addition to a suitable structure of bars or shapes. All these characteristics are typical of BIOVAL® VAL1HS allowing both high cutting speed and cheap-breaking.
BIOVAL® VAL1HS is not recommended for both fusion and friction welding.
BIOVAL® VAL1HS has not been 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 such as this grade with a higher Sulphur, 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 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.
Depending on thickness, geometry and required mechanical properties of parts, BIOVAL® VAL1HS could be air or oil hardened. The 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 temperatures impair the corrosion resistance of all martensitic grades.