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Martensitic Stainless Steel
Description of material
VAL4 martensitic stainless steels exhibit high tensile properties combined with a fair ductility and better corrosion resistance than martensitic type 400 series steels thanks to their higher Chromium and Nickel contents. In general, their corrosion resistance is lower to that of type 300 series steels, however in certain environments they could offer the same performance, but providing higher values of hardness.
VAL4 are used where hardness and corrosion resistance are indispensable, such as dive knives and underwater multi-tools. Their main applications are blade-dive knives in either serrated, not- serrated or mixed blades. These grades, after hardening and tempering, provides a hardness a little bit lower than Chromium martensitic grades but offer a better general and pitting corrosion resistance.
EAF + AOD
The corrosion resistance of these grades is at its best when in the hardened and low temperature tempered condition at their maximum hardness. Their use in the annealed condition, or any other situation able to reduce the surface hardness (i.e. back and surface of knives or serrated cutting edge of blades overheated by grinding) should be avoided. It’s important to point out that the corrosion resistance of these grades is strongly dependent on the care in manufacturing. In addition, it should be considered that the dive blades of VAL4 are made to give a compromise between corrosion resistance and cutting edge properties but cannot compete with Titanium dive knives 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, free of stains and pitting. This means taking care to avoid prolonged immersion in salt water, or contact with sea water for a long time. It’s important to remember that marine water is corrosive, and strongly increases its corrosive effect if it dries on the blade, forming a salt film, where pits and stains quickly form. If this situation happens, little rust spots and tanning 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. An immediate washing by tap / drinkable water and a drying operation, must be done after diving in order to perfectly remove marine water and residual salt that are strongly corrosive and prime a rapid pitting if left on the surface of pieces. It should be always remembered that blades and knives will rust if they come into regular contact with marine water and if they are not cleaned, dried and, if able, oiled or surface protected after use.
Due to high resistance and hardness even in the tempered or annealed conditions, these grades are not suitable for heavy cold forming. A certain cold formability and cold shearing could be obtained after a very long lasting annealing and very slow cooling in the furnace. Grinding and polishing of blades must not generate any surface overheating, causing a strong reduction of corrosion resistance of cutting edges, or grinding cracks in the serrated edge of blades.
VAL4 can be readily hot forged but it’s important to remember that, such as with all the martensitic grades, they easily harden after air cooling. Overheating must be always avoided, because a large amount of scale and cracks may form. An accurate choice of heating and forging parameters produce a good morphology of structures after the hardening and tempering of blades.
Double tempering may be useful for some kinds of professional dive blades, however is not normally used in recreational knives or in certain table blades production where a single temper should be sufficient. Temperatures and soaking should be accurate in order to avoid coarse grains. 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. A diving knife has to strike a balance between being hard to be able to hold cutting and serrated edges, strong to resist breaking, and stable to avoid rapid corrosion in salt water. Useful to remember that a high hardening temperature warrants a best impact strength ( Kv ) when the tempering temperature is less of 350°C maintaining approximately the same hardness values. ( For instance: Hardening t° = 980° + tempering t°= 300° Kv=25 Joule / Hardening t° = 1030° + tempering t°= 300° Kv = 48 joule).