As we discuss carbonitriding, expect to be carried through a heat treatment process that relies heavily on the control of local atmospheric variables. In particular, as the label implies, carbon and nitrogen atoms are injected into the controlled environment. Undoubtedly, this is an essential technique, one that surface hardens base steel, but how does it work?
When a soft metal part is heated, its mechanical properties become accessible and readily manipulable. Imagine doubling down on this engineering science by controlling the atmosphere around the part. That’s what this technique does. It uses a branch of engineering science known as gaseous diffusion to incite surface absorption. Essentially, the air around the steel becomes saturated with carbon. The diffusion process takes over, resulting in an interstitial layering effect. Finally, the part is removed from its gas-filled furnace. Case hardened, the newly heat treated surface sports 0.07mm to 0.75mm of wear-resistant martensite.
Nitrogen Infused Hardness
The fundamentals go a long way towards describing the basics, but this heat treatment technology is far from simple when it’s applied in practice. The source of the two gas mediums represents an initial hurdle. Decayed methane or propane provide the carbon component. Meanwhile, nitrogen is also introduced as an ammonia-based gas. The nitrogen increases the hardness quotient and deepens the case beyond what can be achieved by the nitrogen-less carburizing technique. Additionally, only material hardness is affected. No additional layers are deposited, so dimensional stability is not altered in any way.
Two Roads to Success
A furnace with a gas-infused atmosphere has been used as our case study, but one other method exists in a fully-fledged carbonitriding arsenal. The second version dispenses of gasses so that liquids can take on the burden of the case hardening task. Here, metal components are lowered into a special salt bath. The cyanide-heavy salt then acts as the liquid diffusion mechanism. Again, carbon is released with nitrogen. This mode of operation delivers uniform coverage, but it is best suited for small-scale batch processing work.
Relatively lower furnace temperatures are employed when carbonitriding commences. A purely carburized case requires upward of 900°C of heat, for example, whereas the addition of nitrogen lowers this temperature to approximately 800°C. In exchange for lower energy expenditure and the addition of ammonia, the process grants the low carbon steel a much harder and thicker surface layer, one that doesn’t change the dimensions of the worked part. Commonly worked products include gears, pistons, and shafts.