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.
Carbon is the element upon which all of life is founded. Also, in heat treatment technology, carbon is the primary alloying element in the steel hardening process. Not surprisingly, there are variations on this theme. For example, high carbon steel is a dense alloy, but it’s also a brittle metal. Let’s follow this hardening process and apply some heat treatment wizardry so we don’t end up creating an inelastic product.
A “By the Numbers” Viewpoint
First of all, we lay out the figures. Hardened steel contains between 0.61 percent and 1.6 percent of carbon. That key element makes the alloy hard and unworkable. The metal typically refuses to be bent or welded when it’s this hard. With that beyond hard mechanical characteristic in mind, the heat treated part tends to enter the furnace in its final form. Drawn into this fiery environment, the metal glows orange hot inside the 820°C furnace. Alternatively, a section of the workpiece is flame hardened selectively by this austenitising heat. In this latter case, we could be talking about gear teeth or the working edge of a chisel.
High Carbon Heat Treatment
Produced in an open-hearth crucible, approximately one-percent of the hardened steel contains carbon. Now we’re back at the heat treatment furnace. The high carbon steel part has been exposed to a massive thermal envelope. Pushed to 820°C, the hot workpiece is fully transformed into an austenitic state. Next, the internal crystal structure is quenched in a brine solution. The goal here is to uniformly cool the hard part without adding internal stresses or changing the dimensions of the part. A balance is struck. The high-percentage carbon blends with the manganese and the changed crystalline structure of the iron to produce a hardened steel.
Illustrating Some Fringe Benefits
This finished component is dense and hard, durable and dimensionally consistent. In addition to those key mechanical properties, the part features superior shape memory. So, on top of providing tool edges and hard-wear surfaces, this rigid steel is commonly used in the manufacturing sector as a spring strengthening tool. Picture it, the strong alloy supporting a massive vehicle, a railway locomotive, or some other gargantuan construct. Every time the spring is stressed and stretched, the heat treated metal pulls the coiled alloy segment back into its original form.
Case hardened steels are heat treated in furnaces while wholly hardened high carbon steels are born in an open-hearth crucible. Cast and poured into shape, these parts are entirely rigid. In fact, they’re so rigid that a fracture will take place if the metal is incautiously bent, which is why the heat treatment cycle is so important here as a hardness-to-ductility balancer.