Liquid Nitriding vs. Gas Nitriding in Case Hardening: What is the Difference?

26 June 2019

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.

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