Black oxide is a pleasing surface conversion coating. As stated in past articles, the process dresses steel and other select alloys with a corrosion resistant skin. Although mild in nature, that rust impeding property binds equitably with the latest oil impregnating methods to produce a superior oxidization barrier. Better yet, the formation of that blackened barrier doesn’t impact a part’s dimensions, which is an especially desirable feature for smaller components.
Benefiting Smaller Metal Components
Intricate geometrical profiles dictate the outlines of essential steel parts as they exit a machining station. Over at the next metal cutting and pressing equipment line, zinc alloyed components are gaining similarly accurate dimensional shapes. Essentially, these undersized workpieces are being shaped according to a high-tolerance design methodology. Exiting the profiling equipment, the post-processing operation is approaching. What if that stage changes the dimensions of the components and alters the tightly imbued geometry to the point the small part no longer satisfies those dimensional constraints? Black oxide coatings solve this tricky issue by simply converting the existing surface. No spatial alterations take place, so the component slots into place as it’s added to an assembling frame.
A Compact Metal Parts Finish
Oiled and equipped with a corrosion resistance feature that acts as a non-dimensional finish, black oxide post-processing technology has a penchant for compact workpiece processing. In terms of productivity gains, costly alternative finishes have trouble treating the large batches that must run down production lines at speed. The post-treatment stage becomes a bottleneck, the dozens of compact fasteners, discrete equipment components, or batch-processed parts slow to a crawl on the line. And where’s the company bottom line going? It’s narrowing because of a simple issue with the parts finishing stage. In black oxide coating, the small parts are dipped in oxidizing salts, and the entire process is over in minutes, not hours. That’s a tidy little benefit when thousands of screws or drill bits are the subjects of a fast-operating finishing stage.
Imagine a heavier coating. It’s tested on blade edges, on drill bits, and it’s finally used to protect screws. The results of the experiment are disastrous. The threads of the screws are messed up, the drill bits are dimensionally altered, and those blade edges are dulled. Black oxide coatings prevent such issues from occurring by sidestepping the additive approach that’s normally associated with post-processing finishes. Instead of an added coating, the surface metal is converted into black oxide, a hardened and corrosion resistant finish that doesn’t alter thread profiles or drill bit edges in any way whatsoever.
This post will address the gas nitriding process and its effects on steel. Essentially, we’ll be looking at a heat treatment technique that addresses alloy hardening concerns and the need for a process that attenuates the corrosion rate of this ferrous-heavy metal. But first, considering the importance of this subject, just why is gas nitriding such a vital part of any thermal treatment arsenal?
Armoured Against Industrial Pressures
All kinds of forces try their best to undermine steel components. High pressures push alloy panels until they bulge. Meanwhile, a process-essential steel component is rusting or experiencing wear. Gas nitriding work case hardens steel. To be specific, this is a heat treatment technique that’s designed to enhance the surface properties of those steel parts. It does this by thermochemically depositing a coating of hard nitride precipitates all around the component.
Gas Nitriding: The Effects on Steel
Now that the process recap is out of the way, just how does this heat treatment technique impact steel parts? Well, inside our gas nitriding furnace, the temperature is about to hit the 525°C mark. The ammonia gas is reacting, soaking into the surface of the hot steel. After forty-five to sixty minutes, the nitrogen in that gaseous compound penetrates the alloy surface and case hardens the component. For this example, we’re using a steel shaft, a part that’s expected to endure when great pressures are applied. It’s operating in a hot and humid factory, and previous shafts have experienced corrosive pitting. Now, thanks to this gaseous heat treatment process, the shaft no longer suffers from pitting, even when the humidity level peaks.
Controlling the Operation
Pure ammonia is the best choice here, especially when an absolutely corrosion proof steel workpiece is the goal. The next controllable element is the time the part spends inside the furnace. A temperature above 500°C is preferred, and 525°C seems to be a sweet spot for the process. Returning to the timeline, a thirty-minute minimum treatment time can be extended to cover a period of two hours, which is when maximum case hardening depth is achieved. Of course, as with any other thermally active processing station, the ratios of iron and carbon, plus any other exotic additives, will impact the final results.
The disassociated nitrogen bonds with the surface steel to produce a corrosion resistant alloy. Granted, there are steel alloys that exhibit similar properties, but the gas nitriding process introduces this feature without incurring the additional expenses we associate with those alloys. Lastly, that newly nitride-coated part is fatigue resistant and hard. Think of it, a heat treatment procedure that gifts steel with these essential features, and no parts-deforming super-high temperatures were needed.
Process economy is an ideal we chase in heat treatment technology. Masses of fuel and heat are devoured during a hard day’s work inside this manufacturing environment. How does the work retain a cost-efficient edge? Particularly in flame hardening, how do we surface harden a large workpiece while we’re also worrying about an economical processing approach? Well, flame hardening is selective, which means there’s already a frugal system in action.
Flame Hardened Frugality
Talk of affordability occupies the mind of a processing engineer. Certainly, there’s no shortage of techniques. Carburizing processes cook away inside some manufacturing facilities. Meanwhile, another factory has chosen a carbonitriding method. Both techniques have a common factor, the inclusion of a chemical alteration system. The chemicals are diffused, with the metal surface then altering as the furnace fills with gas. Feed systems and gases, they’re efficient but expensive. On the other hand, the flame hardening process doesn’t use chemicals. Mounted on a scanner mechanism, the focused heat source selectively hardens targeted workpiece sections. Gear teeth, for instance, only require hardening on their outer edges, not their entire form.
Dance of the Flame
Most of these facts have been covered in other articles. Flame hardened parts receive focused treatment, and that treatment further reduces costs by reducing post-treatment grinding and finishing. Furthermore, the treated parts don’t experience process distortion, not to the degree found in other surface hardening techniques. Beyond those cost-cutting benefits, there’s the manner in which the flame head discharges its thermal load. Scanning heads move in closely compressed scan lines across large surfaces. Elsewhere, a wheel or vehicle axle is spinning in a special mechanism. The flame is fixed, but the cylindrical piece is rotating at a carefully calculated velocity. In a truly advanced heat treatment station, both methods operate concurrently. The progressive scan is in motion, the part is rotating, and a quench operation immediately follows each stage.
Providing superior wear surfaces, the secret to economical case hardening lays in that three-dimensional control system. The gas and flame head is discharging a measured thermal envelope, one that’s sharply focused. It’s now up to the progressive scan velocity and rotating equipment to regulate the heat. When that head, or parts-moving mechanism, is properly regulated, travel speed combines with output temperature and flame shape to ensure calculated surface hardened depth. More cost-effective than chemical diffusion technology, flame hardened parts come in many shapes and sizes. As long as that selective treatment mechanism can get into the geometry of a workpiece, this is a supremely economical heat treatment solution.
To some extent, worked alloys have memory. Stress can be absorbed and stored in the metal. Under these circumstances, heat treatment facilities call upon the services of a stress relieving process, a technique that’s designed to reset that solid-form memory. In a nutshell, the grain of the metal is homogenized. Thermal relaxation takes over at this point, then the workpiece is ready for more processing or a post-processing finish operation.
Challenging Stress Relief Actions
Playing devil’s advocate for a moment, what happens when a heat treatment run skips this crucial material-homogenizing stage? Formed or machined, cold-worked or welded, the alloy piece is saturated with residual mechanical stress. It’s about to be passed back to another processing stage for more machining, with those wildly interacting intragranular still in place. The part deforms, it warps, and it loses dimensional form. Even if those internal forces don’t make themselves known during a secondary heat treatment operation, the workpiece is still packing that trapped energy. Pressed into service by some fly by night heat treatment functionary, the metal part will fail. Prematurely and drastically, it will release those stresses and fail.
Initiating a Heat Treatment Reset
It takes that kind of reverse logic to really gain an insight into the process. Stress relieving technology sits firmly as a looping branch of the thermal conditioning tree. If the workpiece absorbs stress during the initial machining stage, these forces are thermally relaxed until the part is mechanically equalized once more. Passed through this processing loop and into a heat station that introduces a thermal gradient, more heat-induced stress is encountered. Back into the process sub-branch, that newly added stress is mitigated. The end result, now that the part has left all of the machining stations and heat treatment work behind, is a dimensionally stable, geometrically accurate component. This being the case, the finished metal workpiece is now less susceptible to stress corrosion cracking, post-processing dimensional variations, and grain irregularities.
A number of thermal relaxation temperatures work well here, then there are material normalizing techniques and annealing procedures to further regulate the work. Of the fundamental stress relieving techniques, two types of stress relief dominate. First of all, a 150°C to 200°C method operates as a peak force releasing process. That relatively low-temp system offers significant relief without affecting other treatment work. Using this method, parts lose most internal stress while maintaining their case hardened exterior shell. Alternatively, using a thermal spread of between 600°C to 680°C, the furnace will fully homogenize the alloy grain.
On one side of this comparative guide, induction hardening technology employs a strong electromagnetic field as the heat treatment engine. On the other side, a similarly potent case hardening station is busily armouring the surface of a newly arrived workpiece. Both methods harden exterior surfaces. Case in point, flame hardening equipment is incredibly selective, but is this focused blaze more capable than an induction hardening station?
Aggressive Energies Catalyze Case Hardened Components
On the fire and chemical side of things, a focused oxy-gas flare selectively surface hardens the teeth on a gear wheel. Further down that line, a gas carburizing furnace cooks a component while it simultaneously feeds a small percentage of carbon into the sealed heating enclosure. In short, these case hardening techniques employ furnaces and gases, plus a preordained quantity of surface armouring carbon. Induction hardening equipment is more, shall we say, production friendly, and here’s why.
Compared to Production-Friendly Induction Technology
From the start, there are obvious benefits. The process doesn’t take place inside a sealed furnace, nor does it require a specially conditioned atmosphere. Look closer, look at the nature of this beast. It’s an electrical coil, a series of twisted conductors that induce a powerful magnetic field. As that field interacts with a nearby workpiece, heat is induced. That means this electrical equipment doesn’t touch the component while it’s being heat treated. Furthermore, the entire operation is finished in seconds. Incorporated as part of a production line, one that’s processing hundreds of vehicle axles, the entire project is repeatedly heat treated in hours, not days.
Comparing Case Hardening Benefits
Geometrical profiles can stymy certain induction hardening stations. For example, that flame hardening equipment makes short work of a targeted project, perhaps one that accommodates the radial outlines of gear wheels. That circular induction unit, perfect for cylindrical extrusions, can’t exactly handle the radial teeth on a heavy gear. However, there are induction hardening rigs that do suit this rim-turning requirement. Mounted on an arm, a stationary induction hardening head quickly handles those rim protrusions. Still, that little case study does demonstrate a need for selecting the correct equipment form factor.
Meanwhile, furnace stations case harden all workpiece types, no matter their size or shape. However, it can take many hours to case harden a number of components in these cooking chambers. Induction hardening equipment, on the other hand, incorporates seamlessly into an existing component processing line. Finally, it takes great diffusion mastery to control the case hardening depth in those gas-delivered equipment lines. As for the induction hardening station, a change to the frequency controller will deepen the case hardening effect.