Case-Hardening Basics: Nitrocarburizing vs. Carbonitriding
The terms sound alike and often cause confusion, but nitrocarburizing and carbonitriding are distinct heat-treating processes. Each has their advantages, depending on the material used and the intended finished quality of a part.
Confusion surrounding the case-hardening techniques of nitrocarburizing and carbonitriding prove the point that it is easy to get lost in the nomenclature behind heat-treating processes. That comes with the territory. Metallurgy is complicated. But there’s value to explaining the differences between these techniques and the benefits that result from their use, including cutting down on the confusion to help manufacturers better understand what goes on in the heat-treater’s furnaces.
Case hardening refers to the “case” that develops around a part that is subjected to a hardening treatment. Nitrocarburizing and carbonitriding both make a workpiece surface harder by imparting carbon, or carbon and nitrogen, to its surface. Material, part specs and intended uses dictate whether nitrocarburizing or carbonitriding is the best case-hardening method.
During carbonitriding, parts are heated in a sealed chamber into the austenitic range – around 1600°F (871°C) – before nitrogen and carbon are added. Because the part is heated into the austenitic range, a phase change in the steel’s crystal structure occurs that allows carbon and nitrogen atoms to diffuse into the part.
Nitrogen is added to low-carbon, low-alloy steels because they don’t harden well without the boost the nitrogen provides. The nitrogen comes in the form of ammonia gas molecules that crack apart 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-temperature operational conditions.
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 the time and resources one is willing to spend to achieve certain case depths.
The carbonitriding process takes from a few hours up to a day or more to 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 less-expensive and easily machined steels, like stamped automotive parts or wood screws. The process makes parts more resistant to wear and increases fatigue strength.
Nitrocarburizing also entails the dissolution of carbon and nitrogen into a workpiece, but more nitrogen is used in nitrocarburizing compared to carbonitriding. 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. Ferritic nitrocarburizing is unique in that it offers case hardening without the need to heat metal parts into a phase change (it is done at 975-1125°F). Within that temperature range, nitrogen atoms are soluble in iron, 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.
Case depths as a result of nitrocarburizing are typically shallower compared to carbonitriding.
Workpieces improved by nitrocarburizing include drivetrain 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.
Knowledge is Power
The nitrocarburizing and carbonitriding processes can be complicated, but they are also critical to ensuring parts can stand up to the environments in which they’ll be used. By learning more about these and other heat-treating processes, you are taking a big step toward more productive discussions and a stronger relationship with your heat-treatment partner.
About the Author
Rob Simons is a metallurgical engineer specializing in ferrous heat treatment with 35 years of experience in the industry. He earned a degree in metallurgical engineering from the University of Missouri-Rolla in 1982 and will be a featured presenter at the ASM Heat Treat 2017 conference. He has been at Paulo for 30 years. Founded in 1943, Paulo is one of the largest providers of heat treating, brazing and metal finishing solutions in North America.