Case Hardening Overview: Carburizing, Carbonitriding, and Nitriding
Case hardening is a family of heat treatment processes that produce a hard, wear-resistant surface layer (the "case") on a softer, tougher core. UTEC Industrial provides in-house induction hardening, through-hardening, and quench-and-temper heat treating services for industrial components in the Pacific Northwest, with integrated CNC machining and reverse-engineering capability. These processes differ fundamentally from through-hardening — rather than heating and quenching the whole part to a uniform hardness, case hardening diffuses carbon, nitrogen, or both into the steel surface at elevated temperature, then either quenches (carburizing, carbonitriding) or slow-cools (nitriding) to produce a graded hardness profile from a hard surface (typically 58–62 HRC) down to a tougher core (typically 28–40 HRC). This combination is required for gears, bearing races, cam surfaces, and other components that must resist wear and rolling-contact fatigue while still absorbing shock loads — a profile that neither through-hardening nor induction hardening produces in quite the same way. This article is an overview of the major case-hardening processes so engineers can recognize when a drawing specification calls for one of them, how the processes compare, and how to specify them clearly.
What is case hardening and how does it differ from through-hardening?
Case hardening is defined by what it changes: the chemical composition of the surface layer. Carburizing adds carbon to a low-carbon steel surface so that the surface — but not the core — can respond to quench hardening. Nitriding adds nitrogen, which forms hard alloy nitrides in the surface without requiring a subsequent quench. Carbonitriding adds both. In every case, the core keeps its original composition and therefore its original toughness, while the surface becomes hard enough to resist abrasion and rolling-contact fatigue. Through-hardening, by contrast, does not change composition at all — it heats a uniform-composition part to austenitizing temperature and quenches the whole section, producing hardness that varies with section size and hardenability but not with depth from the surface. Induction hardening is a third approach: it heats the surface selectively to austenitizing temperature without changing composition and quenches, producing a hard surface on steels that already contain enough carbon to harden (ASM Handbook, Vol. 4A, ASM International, 2013).
How does gas carburizing work?
Gas carburizing loads parts into a sealed furnace held at austenitizing temperature — typically 1,650–1,750 °F (900–950 °C) — in a carbon-bearing atmosphere (endothermic gas generated from natural gas and air, enriched with a hydrocarbon to set the carbon potential). Carbon diffuses from the atmosphere into the austenite at the surface and migrates inward along a concentration gradient. The depth of carbon penetration follows the diffusion equation — roughly proportional to the square root of time at temperature — so a 0.030" case takes perhaps 4 hours at 1,700 °F, while doubling the depth to 0.060" takes approximately 16 hours. At the end of the carburize cycle, parts are either direct-quenched from the carburizing temperature or slow-cooled, reheated to a lower austenitizing temperature, and quenched (for grain refinement and better core toughness). After quench, parts are tempered at low temperature (typically 300–375 °F) to relieve quench stresses while retaining surface hardness. Typical outcomes on an 8620-class carburizing grade: 0.020–0.080" case depth at 58–62 HRC surface, with 28–40 HRC core hardness depending on section size and grade hardenability (ASM Handbook, Vol. 4A, ASM International, 2013; Heat Treater's Guide: Irons and Steels, 2nd ed., ASM International, 1995).
What is carbonitriding and when is it specified?
Carbonitriding is a variant of carburizing in which ammonia is added to the carburizing atmosphere so that both carbon and nitrogen diffuse into the steel surface. The process runs at a lower temperature than straight carburizing — typically 1,450–1,575 °F (790–855 °C) — because nitrogen's austenite-stabilizing effect allows the surface to transform adequately at lower temperature. Case depths are correspondingly shallower, usually in the 0.003–0.030" range, which makes carbonitriding the common choice for small parts (fasteners, small gears, pins, bushings) where a thick carburized case would consume too much of the section. The dissolved nitrogen increases the hardenability of the case, which means carbonitriding can produce adequate surface hardness on leaner-alloy or even plain-carbon steels that would not respond well to straight carburizing — a cost advantage on high-volume small-parts work. Surface hardness and core-hardness outcomes are similar to carburizing, with the case typically showing slightly better wear resistance due to the nitrogen content (ASM Handbook, Vol. 4A, ASM International, 2013).
What is gas nitriding and how does it differ from carburizing?
Gas nitriding is fundamentally different from carburizing in several ways. Temperature is much lower — typically 950–1,050 °F (510–565 °C), below the ferrite-to-austenite transformation — so there is no phase change during the process and no quench afterward. Nitrogen from a dissociated ammonia atmosphere diffuses into the steel surface and combines with nitride-forming alloying elements (aluminum, chromium, molybdenum, vanadium) to form very hard alloy nitrides in the surface. Cycle times are long — 20–80 hours is typical — reflecting nitrogen's slow diffusion at these temperatures. Case depths are shallow (0.005–0.025") but the surface hardness achieved is extremely high, often equivalent to 65–70 HRC when measured by conversion from microhardness. Because there is no quench, nitrided parts show almost no distortion, which is the process's dominant advantage — precision gears, cam followers, and spindles can be nitrided after finish grinding and remain within close tolerance. Nitriding requires a steel that contains nitride-forming alloying elements: the classic "nitralloy" grades (Nitralloy 135M, Nitralloy N) are purpose-designed, and 4140 and 4340 are commonly nitrided after prior quench-and-temper to a 30–35 HRC core (ASM Handbook, Vol. 4A, ASM International, 2013; Heat Treater's Guide: Irons and Steels, 2nd ed., ASM International, 1995).
What about salt bath, plasma, and vacuum case-hardening variants?
Beyond the three atmosphere-based processes above, several specialty variants are used where the mainstream gas processes fall short. Salt bath carburizing and nitriding immerse parts in molten cyanide-containing salt at process temperature; the salt provides the carbon or nitrogen source through direct chemical exchange with the steel surface. Salt baths produce rapid, uniform cases but have been largely displaced by gas processes in most markets due to cyanide-handling environmental and waste-disposal regulations (Heat Treater's Guide: Irons and Steels, 2nd ed., ASM International, 1995). Plasma (ion) nitriding uses a low-pressure chamber in which nitrogen gas is ionized by a glow discharge between the chamber wall and the parts; the ionized nitrogen drives into the surface at temperatures as low as 750 °F. Plasma nitriding is cleaner than gas nitriding, allows selective masking of areas that must remain soft, and produces controllable compound-layer structures — it is the modern precision-nitriding process for aerospace, high-performance automotive, and tooling applications. Low-pressure (vacuum) carburizing uses acetylene or propane as the carbon source in a vacuum chamber at temperatures often higher than gas carburizing; oil or gas quenching completes the cycle. Vacuum carburizing is the dominant process in aerospace and Nadcap-regulated gear production because it avoids intergranular oxidation that can occur with atmosphere carburizing (ASM Handbook, Vol. 4A, ASM International, 2013). Each variant requires specialized equipment — none of the three is part of a general commercial heat treater's standard offering.
How is case depth specified and measured?
Case depth is never defined by the visible boundary of the case — the composition gradient is continuous, so any "boundary" is defined by a measurement threshold. Effective case depth is the depth from the surface at which hardness falls to a specified value, most commonly 50 HRC or 513 HV (the microhardness equivalent of approximately 50 HRC). Total case depth is the depth at which the composition returns to essentially the core composition, measured by microhardness, optical microscopy on etched cross-sections, or chemical analysis. Drawings typically specify effective case depth because it correlates directly with the load-carrying depth of the hardened layer — the part will perform as if it had been uniformly hardened to that depth. Measurement is by microhardness traverse on a polished cross-section per ASTM E384: the cross-section is sectioned perpendicular to the surface, mounted, polished, and a series of microhardness indentations is made at increasing depths from the surface, typically at 0.002–0.005" increments. The depth at which the hardness-vs-depth curve crosses the specified threshold is the effective case depth (ASTM E384; ASM Handbook, Vol. 4D, ASM International, 2014).
When is induction hardening an alternative to case hardening?
Induction hardening and case hardening both produce a hard surface on a tougher core, and for many applications either approach will meet the functional requirement. Induction hardening is the better choice when the part is made from a medium-carbon or alloy steel that already has enough carbon (≥0.35% typically) to respond to surface austenitizing and quenching — 1045, 4140, and 4340 are common. The process is fast (seconds per part rather than hours), requires no atmosphere furnace, and allows selective hardening of only the surfaces that need it by shaping the induction coil to the part geometry. It works especially well on rotationally symmetric parts — shafts, rollers, crane wheel treads, gear teeth via tooth-by-tooth induction — where a coil can be designed to produce the required hardened zone. Case hardening is required when the part must be made from a low-carbon grade (for formability, weldability, or cost reasons) that cannot be induction hardened to useful surface hardness, or when the application requires the shallower case and near-zero distortion that nitriding provides, or when the geometry cannot accommodate an induction coil. The two processes are not interchangeable for specification purposes — a drawing that calls for 0.050" carburized case on 8620 cannot be met by induction hardening 8620 because 8620's core carbon is too low to respond (ASM Handbook, Vol. 4C, ASM International, 2014).
How should case hardening be specified on a drawing, and where should buyers source it?
A useful case-hardening callout on a drawing contains, at minimum: the process (carburize and temper, carbonitride and temper, nitride, or plasma nitride — the process determines the grade requirement); the effective case depth with tolerance (e.g., "0.030–0.050 in effective case depth to 50 HRC"); the surface hardness with tolerance (e.g., "58–62 HRC"); the core hardness when it affects function (e.g., "core hardness 30–40 HRC at mid-radius"); the material grade, which must be compatible with the specified process (8620 or 4320 for carburizing; nitralloy, 4140, or 4340 for nitriding; not 4140 for carburizing); and the test location on the part (most critical surface, a specific feature, or a sacrificial test coupon). A callout that reads only "case harden to 60 HRC" is unspecified — it does not tell the heat treater which process, which case depth, or where to verify. Buyers sourcing case hardening typically do not find it at general commercial heat treaters that run carbon-steel annealing and stress relief. Atmosphere carburizing is offered by dedicated carburizing houses (most concentrated in the Midwest and Southeast). Nitriding, particularly plasma nitriding, is offered by specialty shops often regionally. Vacuum carburizing is an aerospace-industry offering tied to Nadcap-accredited heat treaters. A buyer whose drawing calls for any of these processes should source from a heat treater that specifically advertises that process — do not assume that a commercial heat treater with a car-bottom furnace performs case hardening, because atmosphere case hardening requires sealed-atmosphere equipment that most commercial shops do not operate (ASM Handbook, Vol. 4A, ASM International, 2013; Machinery's Handbook, 31st ed., Industrial Press, 2020).
References
- ASM Handbook, Volume 4A: Steel Heat Treating Fundamentals and Processes, ASM International, 2013.
- ASM Handbook, Volume 4C: Induction Heating and Heat Treatment, ASM International, 2014.
- ASM Handbook, Volume 4D: Heat Treating of Irons and Steels, ASM International, 2014.
- Heat Treater's Guide: Practices and Procedures for Irons and Steels, 2nd edition, ASM International, 1995.
- Machinery's Handbook, 31st edition, Industrial Press, 2020.
- ASTM E384, Standard Test Method for Microindentation Hardness of Materials, ASTM International.
- ASTM A29/A29M, Standard Specification for General Requirements for Steel Bars, Carbon and Alloy, Hot-Wrought, ASTM International.
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