Case hardening basics: Nitrocarburizing vs. carbonitriding

It’s easy to get lost in the chemistry—and the nomenclature—behind heat-treating processes. There’s peace of mind in knowing what processes your parts will undergo, but only if you understand them. Herewe examine two case hardening techniques—nitrocarburizing and carbonitriding—and explain the differences between the techniques and the benefits that result from their use.

Case hardening

Nitrocarburizing and carbonitriding sound somewhat similar and they perform similar functions: to make a workpiece surface harder by imparting carbon, nitrogen or both to its surface.

Case hardening refers to the “case” that develops around a part that is subjected to a hardening treatment. The chemical changes brought on by case hardening are responsible for the added hardness of a part’s surface.

Material, part specs and intended uses dictate whether nitrocarburizing or carbonitriding is the best case hardening method.

Carbonitriding

During carbonitriding, parts are heated in a sealed chamber well into the austenitic range—around 1600 degrees Fahrenheit—before nitrogen and carbon are added. Because the part is heated into the austenitic range, a phase change occurs and carbon and nitrogen atoms can diffuse into the part.

Nitrogen is commonly added to low carbon, low alloy steels that otherwise wouldn’t harden well as intended. The nitrogen comes in the form of ammonia, which cracks on the surface of the part to provide nitrogen that diffuses into the steel. Adding nitrogen also helps a part maintain hardness during use in high-heat settings.

Low-carbon, low-alloy steels with low hardenability are good candidates for carbonitriding because the process incorporates hardness to those materials that otherwise would not result from heat treating followed by quenching.

Carbonitriding typically achieves greater case depths compared to nitrocarburizing. There’s no theoretical limit to how deep a case can be achieved in either process, but a practical limit is how much time and resources one is willing to spend to achieve certain case depths.

The carbonitriding process usually takes a few hours achieve the desired results: a part with high surface hardness but with a relatively ductile core. The process concludes with a quench.

Carbonitriding is used to harden surfaces of parts made of relatively cheaper and easily-machined steels, like stamped automotive parts or wood screws. The process makes parts more resistant to wear and increases fatigue strength.

Nitrocarburizing

Nitrocarburizing also entails the dissolution of carbon and nitrogen into a workpiece, but, compared to carbonitriding, more nitrogen is used in nitrocarburizing. There are two forms of nitrocarburizing: austenitic and ferritic.

Austenitic nitrocarburizing refers to the temperature of the nitrogen-enriched zone at the surface of a part. A phase change occurs in that zone, allowing the nitrogen to diffuse. Ferritic nitrocarburizing is conducted at a lower temperature where no phase change occurs.

Case depths as a result of nitrocarburizing are typically more shallow compared to carbonitriding.

Ferritic nitrocarburizing is unique in that it offers case hardening without the need to heat metal parts into a phase change (it’s done at between 975 and 1,125 degrees Fahrenheit). Within that temperature range, nitrogen atoms can diffuse into the steel but the risk of distortion is decreased. Due to their shape and size, carbon atoms cannot diffuse into the part in this low-temperature process.

Workpieces improved by nitrocarburizing include drive train components in automobiles and heavy equipment, firearm components like barrels and slides and dies for manufacturing processes.

Nitrocarburizing decreases the potential for corrosion in parts and enhances their appearance. The process generally takes only a few hours.