As the temperature rises in a heat treatment furnace, strange transformations occur. In steel workpieces, the grain structure of the component changes size. Alternatively, new grains form. They undergo phase transformative processes. There’s carbon in the mix, too. As those phase changes take place, the alloy-strengthening element becomes more soluble. The carbon diffuses, so the workpiece hardens. Here, this list of phase states should clarify the matter somewhat.
Normalizing the Grain Size
Before moving onto the phase states and grains, let’s check out a straightforward example. Referred to as “Normalizing,” the heat treatment work holds a steel workpiece just above its critical transformative phase. At these austenitizing temperatures, the grain uniformly changes size. Held for a predetermined period at this temperature, the steel is then cooled at room temperature. Using between 750°C and 980°C (the temperature varies because of carbon content) of furnace heat, all of the steel ferrite is transformed into a harder, uniformly distributed pearlite structure.
Charting Grain Structure Changes
Body-centred ferritic steels can’t easily diffuse carbon. Infusing more thermal energy into the process, taking the temperature gradient up to that 750 to 980°C sweet spot, the grain structure transforms. Face-centred austenite allows the carbon to diffuse. Hard islands of cementite dissolve, and now the crucial moment has arrived. If the second half of the heat treatment process mirrors the first half, then the carbon will resurface while the steel assumes its ferritic microcrystalline structure once more. Obviously, that’s not the result we’re after. Taking control of that second stage, the cherry-hot steel is quenched in oil or water. Now, because of the sudden cooling, the carbon becomes locked inside the cubic grains.
Phase Transformations: Desirable Mechanical Properties
Produced after the high temperatures and quenching operations are complete, martensitic steels are super-hard but brittle until tempered. Austenitic processing changes the nickel-to-chromium ratio slightly so that the alloy gains a stronger corrosion resistance feature. They’re also non-magnetic. By the way, the electrical conductivity and magnetic parameters change when the grain structure is altered. Typically, however, the process targets mechanical hardenability, fatigue resistance, malleability and workability, and corrosion resistance, too. Magnetism and conductivity are important, too, of course, just not as process-integral as those mechano-chemical attributes.
At the end of the day, there are ingredients and heat treatment temperatures to manage. The ingredients are already inside a steel workpiece, or they’re added to a furnace’s atmosphere. Then, by managing temperature levels, hold times, and the quench/tempering work, we can create the differently shaped grain structures mentioned above. As those grains alter size and shape, the carbon in the steel becomes more soluble.
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.
Heat-Based Hardening
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.