Case Hardening Basics: Nitrocarburizing vs. Carbonitriding The terms sound alike and often cause confusion, but nitrocarburizing and carbonitriding are distinct heat-treating processes that have their advantages depending on the material used and the intended finished quality of a part. By Rob Simons

Confusion surrounding the case-hardening benefits that result from their uses, includ- triding both make a workpiece surface harder techniques of nitrocarburizing and carbo- ing cutting down on the confusion to help by imparting carbon, or carbon and nitrogen, nitriding prove the point that it’s easy to manufacturers better understand what goes to its surface. get lost in the nomenclature behind heat- on in the heat treater’s furnaces. Metallurgist Adolph Machlet developed treating processes. nitriding by accident in 1906. That year, he That comes with the territory. Metallurgy CASE HARDENING applied for a patent that called for replacing is complicated. Case hardening2 refers to the “case” that atmosphere air in a furnace with ammonia But there’s value to explaining the dif- develops around a part subjected to a harden- to avoid oxidation of steel parts. Shortly C 1 ferences between these techniques and the ing treatment. Nitrocarburizing and carboni- after he sent the patent application off, he 30 | Thermal Processing CARBONITRIDING (It’s done between 975 and 1,125 degrees During carbonitriding, parts are heated in a Fahrenheit.) Within that temperature range, sealed chamber well into the austenitic range nitrogen atoms are soluble in iron, but the — about 1,600 degrees Fahrenheit — before risk of distortion is decreased. Due to their nitrogen and carbon are added. Because the shape and size, carbon atoms cannot diffuse part is heated into the austenitic range, a into the part in this low-temperature process. phase change in the steel’s crystal structure Workpieces improved by nitrocarburizing occurs that allows carbon and nitrogen atoms include drive-train components in automo- to diffuse into the part. biles and heavy equipment, firearm compo- Nitrogen is added to low-carbon, low-alloy nents such as barrels and slides, and dies for steels because they don’t harden well without manufacturing processes. the boost the nitrogen provides. The nitrogen Nitrocarburizing decreases the potential for comes in the form of ammonia gas molecules corrosion in parts and enhances their appear- that crack apart on the surface of the part to ance. The process generally takes a few hours. provide nitrogen that diffuses into the steel. Adding nitrogen also helps a part maintain INCREASING POPULARITY hardness during use in high-temperature Because case hardening offers superior sur- operational conditions. face qualities with less risk of distortion, it’s Carbonitriding typically achieves greater become a mainstay treatment of parts across case depths compared to nitrocarburizing. a variety of industries: There’s no theoretical limit to how deep a Manufacturers of automotive parts choose case can be achieved in either process, but a to nitride gears, crankshafts, and valve parts practical limit is the time and resources one is because the process imparts hard diffusion willing to spend to achieve certain case depths. layers to the part surface. The increased The carbonitriding process takes from a fatigue strength resists the formation of sur- few hours up to a day or more to achieve the face and subsurface cracks. desired results: a part with high surface hard- Nitriding has become an attractive heat- ness but with a relatively ductile core. The treatment option for makers of tool steels and process concludes with a quench. forging dies because it imparts critical surface Carbonitriding is used to harden surfaces hardness without the risk of distortion that of parts made of relatively less expensive and accompanies higher-temperature treatments. easily-machined steels, like stamped automo- Makers of firearms nitride components tive parts or wood screws. The process makes such as gun barrels and slides because the pro- parts more resistant to wear and increases cess decreases friction coefficients, increases fatigue strength. wear resistance and fatigue strength, and imparts moderate corrosion control. NITROCARBURIZING Nitrocarburizing also entails the dissolution KNOWLEDGE IS POWER of carbon and nitrogen into a workpiece, but, The nitrocarburizing and carbonitriding pro- compared to carbonitriding, more nitrogen is cesses can be complicated, but they’re also noticed that treating parts in an ammonia used in nitrocarburizing. There are two forms critical to ensuring parts can stand up to the atmosphere at elevated temperatures caused of nitrocarburizing: austenitic and ferritic. environments in which they’ll be used. By a “skin, casing, shell, or coating” to develop Austenitic nitrocarburizing refers to the tem- learning more about these and other heat- around a piece that was extremely difficult perature of the nitrogen-enriched zone at the treating processes3, a big step can be taken to corrode or tarnish. surface of a part. A phase change occurs in that toward more productive future discussions Also in 1906, German metallurgist Adolph zone, allowing the nitrogen to diffuse. Ferritic and a stronger relationship with heat-treat- Fry led a research program during which he nitrocarburizing is conducted at a lower tem- ment partners. made the same discoveries Machlet made. perature where no phase change occurs. He also noticed that adding alloying ele- Case depths as a result of nitrocarburizing REFERENCES ments to iron heavily influenced the results are typically shallower compared to carbo- 1. www.totalmateria.com/page.aspx?ID=Ch of nitriding. nitriding. eckArticle&LN=EN&site=kts&NM=117 Material, part specs, and intended uses Ferritic nitrocarburizing is unique in that 2. www.paulo.com/services/heat-treating/ dictate whether nitrocarburizing or carbo- it offers case hardening without the need case-hardening/ nitriding is the best case-hardening method. to heat metal parts into a phase change. 3. www.paulo.com/services/heat-treating/

ABOUT THE AUTHOR: Rob Simons is a metallurgical engineer specializing in ferrous heat treatments 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. Headquartered in St. Louis, Missouri, Paulo operates five divisions servicing the Mid-West, Great Lakes, and Southeast regions of the United States. thermalprocessing.com | 31