Just as a refresher, what is heat treatment technology? Certainly, the exposure of a metal part to heat changes its material and mechanical characteristics. And those changes occur over time. That controlled thermal treatment sequence can even be dramatically curtailed by quenching the metal workpiece. What about material oxidation in heat treatment, though? Where does this occur in the sequence?
The Metal Oxidation Issue
Metals, especially ferrous metals, react with oxygen. Even aluminium alloys change when they’re exposed to oxygen. And, as any scientist will inform you, roughly twenty-one percent of the atmosphere we breathe is made up of this gaseous element. The rest of the atmosphere contains mostly nitrogen and a few trace elements, but they don’t bind themselves to metal, not like oxygen does when the electrons in the iron atoms leap towards the openly receptive atmospheric atoms. The resulting iron oxide coating corrodes the metal workpiece. Alternatively, in the case of aluminium, an invisible aluminium oxide skin protects the lightweight alloy. What if massive quantities of heat are added to this scenario?
Observing Thermally Induced Oxidation
Let’s get down to brass tacks, as the older generation says. Heat, massive amounts of furnace-hot heat, will accelerate the effects described above. Rapid heat treatment oxidation takes place. The metal in question, assigned a quotient of oxygen solubility, gains an oxide coating. Now, in certain situations at least, this coating is desirable. Aluminium oxides, for example, are added to the lightweight alloy intentionally so that the metal gains a limited corrosion-resistance feature. In heat treatment terms, this process fights fire with fire, because the controlled process prevents further oxidation. However, ferrous materials do not react in this manner. The corrosive gas causes inter-granular and a nasty orange bloom, which then corrupts the intended microcrystalline finish. The mechanical structure of the part is undermined and so is the part’s strength. Time-consuming post treatment work will correct the issue, but that’s not really the point, is it?
Treatment fatigue is an issue here, as is material corrosion. Even the uniform distribution of the thermal envelope becomes entirely unpredictable, which is why there must be a solution to the problem. In the end, that solution is obvious. The atmosphere is removed from the furnace. One method of kicking oxygen out of the equation involves the substitution of an inert gas, such as nitrogen. More effectively, though, vacuum furnaces entirely remove oxygen from the heat treatment work. Oxidation problems simply can’t take place when the atmosphere is sealed off from the heat treatment environment.