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Martensitic Stainless Steel
Description of material
VAL2BCT is the most popular Chromium martensitic stainless with a medium Carbon content able to give a high hardness after heat treatment together with a moderate corrosion resistance. This grade is designed for cutlery production and is processed by a special steel making operative practice, including a particular chemical balance and a specific heat treatment in order to obtain a suitable structure with a good polishability or mirror-like finishing.
VAL2BCT is widely used where high hardness and corrosion resistance are indispensable, such as table cutlery, kitchen knives and certain kinds of dental and surgical instruments. This grade, after hardening and tempering, provides a hardness higher than VAL2ACT.
EAF + AOD
The corrosion resistance of VAL2BCT is at its maximum when hardened and low temperature tempered condition to its maximum hardness. Its use in the annealed condition (i.e. cutlery with an annealed handle) or any other situation able to reduce the surface hardness (i.e. the back and surface of knives or the serrated cutting edge of blades overheated by grinding) should be resolutely avoided. It’s important to point out that the corrosion resistance of this grade is strongly dependent on the care in manufacturing. In addition, it should be considered that the blades or knives of the martensitic steel VAL2BCT are made to give a compromise between corrosion resistance and cutting edge properties, and cannot compete with the austenitic steel of spoons and forks in terms of pitting and general corrosion resistance. In any case, a few simple precautions can be carried out to maintain blades and knives in good condition, and keep them free of stains and pitting. This means care to avoid prolonged immersion in salt water, vinegar or vinegar with salt, or to maintain cutlery in contact with water containing certain levels of chlorides and chlorine for a long time. It’s important to remember that salt and disinfectants are corrosive and strongly increase the corrosive effect of water, particularly hot water, where pits and stains, can quickly form. Rainbow colored stains are usually caused by detergents, grease and juices and could be removed by rubbing with a non-abrasive polishing paste. The same operation can be done in the case of isolated micro pits that, if not removed, could generate wider rust stain zones. In the case of blades with silver handles, the tarnish remover must never be in contact with the blade and immediately washed in the case of contamination. As a rule, when a dishwasher is used to for washing, knives should not be left in a damp environment overnight, and cycles such as “rinse and hold” should be avoided. In the manufacturing of blades and knives, an immediate washing must be done after the grinding process in order to completely remove /wash away cutting fluids that are strongly corrosive and prime a rapid pitting if left on the surface of pieces.
In the annealed condition, VAL2BCT is suitable for cold forming. Knives or blades can be shaped by cold by rolling or cold drop forging of pickled wire rods or bars. In the case of warm working, the temperature should be well evaluated. Attention should be paid to avoid that a significant increase in temperature caused by plastic deformation of drop pieces exceeds critical points Ac1 or Ac3 entering in austenitic field. On cooling, the austenite will transform to martensite, causing cracking to happen in the flash line zones during deburring operations. When grinding and polishing knives, excessive surface heating can create a significant reduction of corrosion resistance, or grinding cracks, in the serrated edge of blades.
VAL2BCT can be readily forged but it’s important to remember that, as with all the martensitic grades, it easily hardens if air cooled, and is prone to cracking in the case of a delayed annealing operation or where there is an improper cooling rate. In addition, overheating must be always avoided because a large amount of scale, with a deep layer of decarburization and cracks may form. An accurate choice of heating and forging parameters will produce a good morphology of structures after the hardening and tempering of blades.
Double tempering may be useful for some kinds of professional blades but is not normally used in table cutlery production where a single one should be sufficient. It’s recommended to choose temperatures of heat treatment able to offer a compromise between maximum hardness and impact strength together with a good corrosion resistance. Temperatures and soaking times should be accurate, in order to avoid coarse grains and surface decarburization.