The next two heat treatment processes are similar. They both use a nitrogen-rich substance to harden soft metal workpieces. But, as the post title implies, the nitrogen arrives in two different states. In one, the nitriding process is applied as a liquid. In the next, the nitrogen coats the workpiece as an all-enveloping gas. Which is better? Which nitride hardening state gets the job done?
Liquid Nitriding Advantages
In most cases, you’ll see a workpiece entering a furnace. If an outside element is required to further treat the component, it’s funnelled into that controlled environment. You’ll hear more about that application system later. As for liquid nitriding, things are done differently here, with a workpiece finding itself lowered into a cyanide-salted bath. There’s less thermal energy required when this route is optioned. Furthermore, the transformatively hardened layer goes deeper in this submerged environment. Last of all, and perhaps more importantly, at least from a financial standpoint, the process is cheaper than a gas-fuelled case hardening approach. Of some interest here, there are several forms of liquid nitriding, including the liquid pressure method and the aerated variant.
Gas-Enriched Nitriding Benefits
Not so fast, there are several major drawbacks to mention. For starters, that cyanide bath is toxic. It’ll need to be disposed of at some point in the future, and that’s not a straightforward proposal. Ammonia gas, applied in a heated furnace as a case hardening atmosphere, is easier to dispose of after a nitriding run has concluded. Better yet, gas levels can be controlled. It’s not as easy to regulate the density of a cyanide bath. By regulating the ammonia atmosphere, case hardening depths can be applied with utmost precision. Clearly, although capable of delivering a deeper casing, liquid nitriding is a more cumbersome process. Last of all, liquid baths fill with contaminants over a fairly short period of time. With gas nitriding, by virtue of its being held in a gaseous state, there are very few process contaminants to worry about. Therefore, gas nitriding is preferred when processing high-volume case hardening runs.
Gas nitriding is arguably the more versatile case hardening option. Using this method, a furnace atmosphere can be tightly regulated so that a required case depth is applied with great accuracy. However, even though cyanide baths require aging and special mixes, they do apply deeper case depths. Additionally, lower temperature baths can be used when the pool is pressurized. Even the addition of an aeration system helps, with the air mix acting as a fluid agitation mechanism. All the same, there’s still that poisonous, environmentally harmful bath to dispose of after a liquid nitriding process has run its course.
Atmospheric control is a powerful tool in a heat treatment engineer’s metal hardening armoury. For example, by adding gaseous nitrogen to a furnace, the gas nitride hardening effect is initiated. This process sees the gas come into contact with a heated low carbon workpiece. As the surface metal and nitrogen brush against one another, the gas diffuses. The result of this gaseous interplay is a stronger, nitride hardened low carbon component.
The Benefits of the Gas Nitriding Process
Well, there’s one immediate upshot, which we’ll reveal right now. Without further delay, because the gaseous ammonia is diffused, there’s no quenching operation required after the process completes. That means there can’t be any quench-induced dimensional changes or material distortions. A scuff and corrosion resistant casing are applied, but there’s no workpiece warping occurring during the hardening procedure. Better yet, the material properties underneath the hardened casing remain fundamentally unaltered.
Viewing the Process Advantages from the Other End
In-process, the benefits are a little harder to define. The nitrogen gas, which is usually applied as an ammonia base (NH3), produces a thin, abrasion-proof coating. Underneath that coating, the alloy workpiece remains unaltered. Like any other mild steel component, it remains malleable and workable. Coming out of the atmospherically sealed furnace, though, more benefits are witnessed. The tough outer coating is there, yet the workpiece is inexpensive and easy to source. The process repeats in no time at all, probably because there’s no additional quenching work to perform. Better yet, the gas output can be controlled with the twist of a valve. That’s a massive plus in the pros column. It’s not always so easy to control furnace heat. Thermal loads require pre-heat phases and cool down periods. That part of the process is obviously harder to master. With convection currents and a sealed furnace, gas diffusion work is that much simpler to manage.
Finally, there are no unidentifiable surface residues to clean off when gas nitriding is the chosen surface hardening process. The nitrogen diffuses cleanly, the nitride hardening effect is triggered, and the job comes to a close. Indeed, this heat treatment approach has all the hallmarks of a turnkey alloy tempering procedure. It starts inside a sealed furnace, absorbs its nitrogen load, yet stays malleable and machinable. Then, right at the end, as the workpiece exits the furnace, the job is done. There’s no quenching phase, which would alter the geometry of the part and cause workpiece distortion. With the cooled part shuttled away to a storage room, clean and without any in-process residue, the next job commences.