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Martensitic Stainless Steel
Description of material
VAL2D is a high Chromium –Molybdenum –Vanadium martensitic stainless with a Carbon content capable of giving a high hardness after heat treatment, together with a better pitting corrosion resistance than other C-Cr martensitic grades. 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 capability.
VAL2D is used where high hardness and corrosion resistance are indispensable, such as table cutlery, kitchen knives and dental or surgical instruments and is widely used for the manufacture of professional blades. This grade, after hardening and tempering, provides a hardness higher than VAL2B.
EAF + AOD
The corrosion resistance of VAL2D 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 VAL2D 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, keeping 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 stain 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 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, VAL2D is suitable for cold forming. Knives or blades can be shaped by cold 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.
A micro-resulphured VAL2D in the annealed condition and in the high temperature tempered after hardening condition offers a good machinability. This property is lower in the case of none micro-resulphured material, necessary for some special purposes. 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 for 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, VAL2D 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.
VAL2D 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. VAL2D, due to its higher Carbon and Molybdenum contents, is prone to generate more decarburization than VAL2BCT.