Engineering terms usually help out by subtly suggesting their meanings. If someone talks about a quenching operation, then the process becomes self-evident. But that’s not always the case, certainly not when we talk about the normalizing process. It’s a generic term, after all, one that has a dozen different meanings. In heat treatment terminology, however, there’s only one possible meaning. Let’s talk about stress relief.

What is The Normalizing Process? 

Steel is generally normalized by inserting a workpiece into an extremely hot furnace. That controlled thermal envelope heats the steel component until it reaches the material’s stated critical transformation range. Hovering just above this transformation point, the grain of the steel alters. It acquires a more uniform structure. After a predetermined hold period, a length of time that maintains the thermal load, the component is allowed to cool in the open air.

What Does Normalizing Mean?

What’s it all about, this normalizing operation? The process has been briefly described, but we’re still left to wonder over the reasoning behind the process. Well, the clues are held in the resulting uniform carbide size and even grain distribution. Cold worked steel has a form of memory. Those ‘memory cells’ form inside the alloy part’s microcrystalline structure. Stress is the hardest memory to eliminate. The normalizing procedure wipes out that memory. It relieves material stress, restores ductility, and generally strengthens that steely backbone.

The Technical Side 

As the soaking temperature is maintained, somewhere between 810°C and 930°C, the grain refinement phase is triggered. The normal air cooling stage further enhances the smaller metal grains and distributes the newly transformed cementite structure. Like a magnet wiping the patterns on an old reel-to-reel tape deck, the normalizing process erases work stress until the structural base of the part is ductile but still hard. Used to improve component workability, this technique is inexpensive, especially when compared with the annealing method that carries out a similar role inside a heat treatment facility.

Loaded with pearlite, with a refined grain type that’s rich in ferrite and cementite, the ferrous part is freed of mechanical irregularities, of work hardened stress, and made ductile. Cooled in the open air or in a nitrogen atmosphere, the consistent metal grain is transformed until it reaches a state of refined uniformity. Listen for other descriptive labels when this work is undertaken, like the heat and hold process. And, just as a by-the-way, do remember that the ferrous composition and thickness of the subject workpiece will affect the results of a normalizing treatment.

Despite appearances, heat treatment work relies on a scientifically accurate process. Sure, the coarse industrial outlines operating here may suggest otherwise, but don’t be fooled, this is engineering fidelity labouring at the highest level. Just how are those optimised results achieved? The answer is simple. Every step in the process has to be governable. There’s no room for an unpredictable control element here, so where does this controllability factor originate?

Optimally Maintaining the Furnace Atmospheres 

In practically every other industrial process, we ignore the atmosphere. Granted, some of these factories filter the air, or they even opt for a clean room environment, but they don’t control every property in that atmosphere. If heat treatment techniques are to be properly optimized, that atmospheric envelope must be entirely controllable. To that end, the air is sometimes delivered by powerful fans, all the better to ensure uniform heat distribution. Conversely, the air is replaced by an inert gas, such as argon or nitrogen. Sometimes, and this is where the processing methods really take flight, the atmosphere is entirely removed. That’s a principle that’s responsible for the development of vacuum heat treating furnaces.

Intelligently Analyzing Current Operations 

If the processing chain is operating, the facility is spitting out heat treated workpieces. Only, much to the distress of all concerned, there’s a higher than average reject rate. The equipment is functioning, but something is going wrong. Fortunately, a process analyzing stratagem can optimize the hardware and the interconnecting linkages that bridge the hardware stations. For example, a process audit not only calibrates the furnace, it also calibrates the instrumentation that records those thermal envelopes. Residual gases are monitored, identified, and addressed. Leakage models are generated and tested. In short, in-house system audits optimize the heat treatment process.

Employing Applied Detective Services 

In-house engineering departments can track down heat treatment problems, obviously. Alternatively, there are special services whose duty it is to scrutinize the results of an operational heat treatment facility. Those auditing agents focus on atmospheric integrity and heat distribution uniformity. If there’s an unsatisfactory mechanical and material finish, the specialized operative addresses the issue. Metallurgically proficient, the engineers who carry out this auditing service are trained to identify and refine every processing defect.

Finally, when that auditing procedure is concluded, there comes a time when the management team must decide whether an elevated maintenance program will improve the heat treatment results. If not, this is an opportunity to upgrade the equipment, to incorporate a system that vacuum-seals or otherwise controls all of the conflicting process elements impacting the heat treatment cycle.

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.

Certain questions always seem to return to torture novice engineers. No sooner has one problem been solved than the next comes careening down the track like a runaway train. It’s the different heat treatment terms that really create the deepest furrowed brows. Once and for all, we’re going to get rid of this tangle by injecting a heavy dose of clarity into metal transformation procedurals.

Abbreviating the Metal Hardening Process 

Furnace-hot temperatures send the metal workpiece towards its transformative thermal threshold. The microstructural components in that material change. They assume a special atomic relationship, which engineering types refer to as austenite. In this metal phase, the component is workable and corrosion resistant, but it’s hardly usable just yet because the metal is still red-hot. That’s why this is an ‘abbreviated’ description.’ Quenching, the next stage in the heat treatment process, rapidly cools the part, completes the treatment, and locks in the properties that were gained at the upper transformative temperature threshold. Metal hardening technology, therefore, injects the part with heat, whereas the metal quenching stage immediately removes that thermal load. In contemporary engineering methodologies, its water or oil that plays the part of the quenching agent.

Acquiring Metal Tempering Improvements 

There are misconceptions regarding the differences between metal tempering and metal quenching. That’s perhaps because of all the blacksmith scenes in old Hollywood movies. Viewers were told that the part was tempered when it was dropped in a wooden bucket of water, so they assumed the blade was now tougher and durable. In fact, the workpiece had been hardened. It was loaded with a martensitic grain, a form of steel that was brittle. The actual tempering operation was still waiting. Think of this as the process balancer, the heat treatment stage that removes brittleness but keeps the component hard enough to serve its purpose. This time the metal is heated to a lower temperature then allowed to cool until the ductility and workability ratio overwhelms the brittleness of the metal.

The tempered component is durable and corrosion resistant. Heat has been applied again, but it’s not the same near-melting hot thermal load that almost transformed the metal into a molten heap of smoking iron. Metal hardening is, therefore, an intensely hot, furnace-driven process. Metal tempering does again apply heat, but the thermal load is designed to transform the martensitic alloy, to cancel out the brittleness added during the hardening work. As for the metal quenching station, this is where the rapid cooling takes place, the immersion process that immediately removes the heat and locks in the desired mechanical properties.