Ferrous metals are those that contain iron. Laced with carbon and other exotic elements, iron is a durable, structurally capable alloy. It exudes a polished finish when formed into stainless steel, becomes malleable and machine-formable when forged as mild steel, and it even demonstrates a high measure of corrosion resistance when mixed with nickel. Endlessly versatile, a heat treatment process allows ferrous metals to assume even more adaptable forms.
Salt Bath Treatments
Past articles have talked about normalizing, about raising iron to 50°C above its transformative temperature. At this juncture, the metal workpiece is air cooled until it reaches room temperature. The microcrystalline grain of a nominated ferrous metal part becomes uniform when it’s normalized. Above and beyond this thermally active heat treatment method, ferrous workpieces are lowered into special salt baths. For cyaniding, a molten bath of cyanide salts hardens the ferrous surface. Held at over 760°C, the case hardening effect is locked in place after the part is quenched. As another “bathing” procedure, ferrous parts can also be dipped in anhydrous ammonia. This is known as nitriding work.
Special chemical baths transform iron grains, but they only do so at the surface. The chemicals are molten and left in contact with the workpiece until a specified case hardened depth is obtained. For purer material hardening results, heat treatment professionals “soak” parts in massive quantities of thermal energy. The grains transform. The ferrous alloy glows cherry red. Its grain changes and its mechanical properties improve. Unfortunately, as hard as the component undoubtedly becomes, it’s also brittle. Tempered and quenched, the part balances its strength against a newly added measure of malleability.
Emulating the Blast Furnace
So far, we’ve told tales about ferrous metals, about iron and its exotic additives. Nothing has been said about carbon content. This is a factor that’s normally taken care of back at the blast furnace. Happily, there are a few processing decisions that can be taken at a heat treatment facility. They’re designed to add carbon to mild steel. Again, this is a case hardening process, one that diffuses atmospherically sustained quantities of the strengthening element while a carbon bearing material is present. Charcoal is one such furnace additive.
Ferrous metals are remarkably versatile, with their mechanical properties altering as a heat treatment vector is varied. Still, at least for the carbonizing and salting procedures, most of these diffusion-facilitated techniques are used in case hardening work. For true whole-workpiece transformative strength, upper critical transformative temperatures are utilized. The ferrous stress relieved and hardened. Finally, uniformly strengthened but brittle, tempering and oil/water quenching finish the process.