Case Hardening: Carburising, Nitriding and Carbonitriding — Processes, Parameters and Selection Guide
Case hardening encompasses a family of thermochemical diffusion treatments that produce a hard, wear-resistant surface layer while preserving a tough, ductile core. This combination is indispensable for components subjected to simultaneous surface fatigue and bulk impact — gears, camshafts, bearing races, gudgeon pins, and splines. This guide provides a rigorous technical comparison of gas carburising, gas nitriding, plasma nitriding, carbonitriding, and ferritic nitrocarburising (FNC), covering diffusion mechanics, process control, steel selection, and industrial applications.
Key Takeaways
- Carburising (900–980°C) produces the deepest cases (0.5–2.5 mm) and is the preferred route for heavy-duty gears and bearing races.
- Nitriding (490–570°C) achieves the highest surface hardness (650–1,200 HV) with minimal distortion because no quench is required.
- Carbonitriding uses simultaneous C and N diffusion at 700–900°C; dissolved nitrogen increases hardenability, enabling lighter quenches and lower distortion.
- Ferritic nitrocarburising (FNC) produces a thin epsilon-carbonitride compound layer in short cycles (1–4 h) at 570–590°C — suited to high-volume mass-production parts.
- Steel grade selection is process-critical: nitriding requires Al, Cr, or Mo nitride-formers; carburising requires low core carbon (0.10–0.25%) for core toughness.
- Effective case depth (to 550 HV / 52 HRC) governs load-carrying capacity in gear and bearing design; total case depth is a microstructural measurement only.
The Metallurgical Rationale for Case Hardening
Through-hardening increases strength and hardness uniformly through a cross-section, but in doing so it eliminates the toughness and impact resistance the core must provide. Case hardening resolves this trade-off by confining the metallurgical change to a controlled surface depth. The hard case resists contact stress, sliding wear, and surface fatigue (pitting); the soft, tough core resists bending fatigue and impact fracture. The iron-carbon phase diagram governs whether the process involves austenitising and quenching (carburising, carbonitriding) or sub-critical diffusion without phase transformation (nitriding, FNC).
Unlike annealing and normalising, which homogenise the microstructure, or quenching and tempering, which produce uniform hardening, case hardening deliberately creates a hardness gradient. The depth and slope of that gradient — and the residual compressive stress it generates at the surface — are the engineered outputs of the process.
Gas Carburising
Process Principles
Gas carburising introduces carbon into the surface of low-carbon steel (0.10–0.25% C) by holding components in a carbon-rich atmosphere at 900–980°C, entirely within the austenite phase field. At these temperatures, austenite can dissolve up to ~1.2% C, enabling significant carbon uptake from the atmosphere. The furnace atmosphere typically consists of an endothermic carrier gas (nominally 40% N2, 40% CO, 20% H2) enriched with propane or natural gas to maintain the required carbon potential (Cp).
Carbon potential is defined as the carbon content (wt%) that a steel in equilibrium with the atmosphere would adopt at the process temperature. It is monitored continuously by measuring CO2 concentration (infrared analysis) or dew point and is adjusted by varying enrichment gas flow. Maintaining precise Cp control prevents both decarburisation (Cp too low) and carbide network formation from excessive surface carbon (Cp > 1.1%).
Fick’s Second Law and Case Depth Prediction
Carbon diffusion from the enriched surface into the steel interior follows Fick’s second law:
∂C/∂t = D × (∂²C/∂x²)
Where:
C = carbon concentration (wt%)
t = time (s)
x = depth below surface (m)
D = diffusivity of C in austenite (m²/s)
At 925°C: D ≈ 5×10⁻¹¹ m²/s
At 980°C: D ≈ 9×10⁻¹¹ m²/s
Approximate case depth scaling:
x_case ∝ √(D × t)For an infinite plate with surface concentration Cs and initial concentration C0, the solution gives:
(Cx - C0) / (Cs - C0) = 1 - erf( x / 2√(Dt) )
In practice, carburising uses a two-stage boost–diffuse cycle: a high Cp boost stage (Cp = 1.1–1.2%) maximises carbon flux into the surface; a lower Cp diffuse stage (Cp = 0.80–0.85%) allows the carbon gradient to flatten toward the target surface carbon of 0.80–0.95% C, which yields 58–62 HRC after quenching without excessive retained austenite.
Carburising Cycle Parameters
| Target Effective Case Depth | Approx. Cycle Time at 925°C | Boost Cp | Diffuse Cp | Post-Treatment |
|---|---|---|---|---|
| 0.5 mm | 3–4 h | 1.15% | 0.85% | Direct oil quench + 180°C temper |
| 1.0 mm | 7–9 h | 1.15% | 0.85% | Direct oil quench + 180°C temper |
| 1.5 mm | 14–18 h | 1.20% | 0.85% | Slow cool, reheat, oil quench |
| 2.5 mm | 30–40 h | 1.20% | 0.80% | Multiple boost–diffuse, oil quench |
Quenching, Hardness, and Retained Austenite
Following carburising, parts are direct-quenched from the carburising temperature (avoiding an additional reheat) or slow-cooled and reheated to the appropriate austenitising temperature before quenching. Oil quench at 60°C is the most common medium for case-hardening grades. Typical results: surface 58–62 HRC, core 30–40 HRC depending on core composition and section thickness.
When surface carbon exceeds ~0.95%, retained austenite increases significantly, reducing surface hardness and dimensional stability. Cryogenic treatment (−70 to −80°C) after quenching converts most retained austenite and is specified for bearing races and precision gears. See the guide on martensite formation in steel for the crystallographic mechanism.
Common Case-Carburising Steel Grades
- AISI 8620 (0.20% C, 0.55% Ni, 0.50% Cr, 0.20% Mo) — Widely used for automotive gears; good case hardenability and core toughness.
- AISI 9310 (0.10% C, 3.2% Ni, 1.2% Cr, 0.12% Mo) — Aerospace standard; very high core toughness at low temperature.
- AISI 4320 — Higher hardenability than 8620; used for large cross-section shafts.
- EN36 / 655M13 (UK) — 3.5% Ni, 0.8% Cr; equivalent to AISI 3310; heavy-duty gearing.
- 18CrNiMo7-6 (Europe) — The European standard for wind turbine and heavy industrial gearing.
Gas Nitriding
Process Principles
Gas nitriding introduces nitrogen into the surface of alloy steel at 490–570°C — well below the eutectoid temperature (Ac1 ≈ 723°C for plain carbon steels) — so no austenite forms and no quench is required. The atmosphere is dissociated ammonia (NH3), which decomposes at the steel surface:
2 NH3 → 2 [N] + 3 H2 (at steel surface)
Nascent nitrogen [N] dissolves in ferrite and diffuses inward, reacting with nitride-forming alloying elements to precipitate very fine, coherent alloy nitride particles (AlN, CrN, Mo2N, VN). These precipitates produce hardness by precipitation strengthening, achieving 650–1,200 HV — higher than is achievable by any carburising process.
The Two-Layer Nitrided Case
The nitrided case comprises two structurally distinct zones:
- Compound layer (white layer), 5–25 μm: A continuous layer of iron nitrides — ε-Fe2–3N and γ′-Fe4N. Hardness 700–1,100 HV. Exceptionally resistant to sliding wear and surface fatigue in the correct phase balance. However, it is brittle and can initiate fatigue cracks under bending or impact; for rotating-bending fatigue applications it is mechanically removed by honing after nitriding.
- Diffusion zone, 0.1–0.6 mm deep: Alloy nitride precipitates dispersed in the ferritic matrix. Provides the bulk of the fatigue resistance, surface compressive residual stress (typically −400 to −600 MPa), and hardness. Hardness declines gradually from peak near the surface to core hardness, avoiding the abrupt gradient that can cause spalling under hertzian contact loading.
Nitriding Potential and Process Control
Modern controlled gas nitriding specifies the nitriding potential Kn:
Kn = p(NH3) / [p(H2)]1.5
High Kn (>5): Promotes ε (epsilon) compound layer (Fe2-3N)
Low Kn (<1): Promotes γ′ (gamma-prime) compound layer (Fe4N)
Very low Kn: Minimal compound layer; diffusion zone development only
The two-stage Floe process exploits this relationship: Stage 1 operates at high Kn to establish initial nitrogen saturation; Stage 2 reduces Kn to develop the diffusion zone at controlled compound layer thickness. This allows compound layer thickness to be specified within ±2 μm — critical for precision die and tooling applications.
Steel Grades for Nitriding
| Steel Grade | Key Alloying | Peak Case Hardness | Typical Application |
|---|---|---|---|
| Nitralloy 135M | 1% Al, 1.6% Cr, 0.2% Mo | ~1,100 HV | Aircraft components, precision gears |
| AISI 4140 / EN19 | 1% Cr, 0.2% Mo | 650–700 HV | Crankshafts, camshafts, dies |
| AISI 4340 / EN24 | 1.8% Ni, 0.8% Cr, 0.25% Mo | 650–750 HV | High-strength shafts, tooling |
| H13 tool steel | 5% Cr, 1.35% Mo, 1% V | 900–1,050 HV | Hot-work tooling, die casting dies |
| AISI 1045 / plain C | Minimal alloy | 250–350 HV | Not recommended; poor response |
Plasma (Ion) Nitriding
Plasma nitriding uses a DC glow discharge between the furnace vessel (anode) and the components (cathode) in a low-pressure (1–10 mbar) N2/H2 atmosphere. High-energy nitrogen ions bombard the steel surface, sputtering iron atoms that react with nitrogen to form iron nitrides which are redeposited, creating a catalytic surface for nitrogen absorption. The ion bombardment provides energy directly to the surface rather than relying on thermal diffusion alone, accelerating the process by 2–4× compared to gas nitriding at equivalent temperature.
Plasma nitriding offers significant advantages for aerospace and precision tooling applications:
- Process temperature can be reduced to 400–450°C, minimising dimensional change in tight-tolerance components such as injection moulding dies and aerospace actuators.
- Compound layer phase (ε vs γ′) and thickness are controlled by adjusting N2/H2 ratio, temperature, and pressure independently.
- Selective area hardening is achieved by masking with bore plugs or metallic screens — no coating or stop-off paste required.
- Stainless steel and titanium alloys can be processed in specially configured systems (active screen plasma nitriding).
Carbonitriding
Process Principles
Carbonitriding simultaneously diffuses both carbon and nitrogen into steel in an endothermic atmosphere with ammonia addition (3–15 vol% NH3) at 700–900°C. It is conceptually a hybrid of carburising and nitriding, but the synergistic effects of co-diffusion produce properties that differ from either process alone.
The critical metallurgical effect of dissolved nitrogen is hardenability enhancement. Nitrogen dissolved in austenite shifts the TTT and CCT curves significantly to the right, suppressing pearlitic and bainitic transformations. A carbonitrided case may achieve full martensite on polymer or marquench — quench media that would produce mixed microstructures in an equivalent carburised case without nitrogen. This allows:
- Smaller quench distortion, critical for near-net-shape mass-produced components.
- Use of lower-alloy (and lower-cost) steel grades while maintaining surface hardness.
- Shorter cycle times because the lower process temperature (vs carburising) reduces thermal expansion mismatch.
Carbonitriding Parameters and Applications
| Parameter | Typical Range | Comments |
|---|---|---|
| Process temperature | 700–900°C | Lower than carburising; above Ac1 for most steels |
| NH3 addition | 3–15 vol% | Higher NH3 at lower temperatures |
| Case depth | 0.075–0.75 mm | Shallower than carburising |
| Surface hardness | 58–65 HRC | Higher than carburising due to N in martensite |
| Quench medium | Polymer, marquench, or oil | Lighter quench than carburising due to N hardenability |
| Typical cycle time | 1–6 h | Short; economical for high-volume production |
Carbonitriding is the process of choice for high-volume small components: small automotive gears, fasteners (nuts, bolts, studs), retaining rings, bushings, rocker arm pivots. See also the bainite microstructure guide for how TTT diagram shifts under nitrogen influence relate to transformation products in the case.
Ferritic Nitrocarburising (FNC)
FNC — marketed under proprietary trade names Tufftride®, Tenifer®, and Nitrotec® — introduces both carbon and nitrogen in the ferritic phase field at 570–590°C in a salt bath (molten cyanate/carbonate), controlled atmosphere, or fluidised bed. The process temperature is above the eutectoid for most steels, but components are cooled slowly (or quenched into salt) to avoid austenite formation.
FNC produces a thin ε-iron carbonitride compound layer (5–25 μm) with excellent sliding wear resistance and moderate fatigue improvement due to surface compressive stress. The short cycle (1–4 h) and minimal distortion make it highly cost-effective for mass-production ferrous components that do not require deep effective case depth. It is widely applied to automotive crankshafts, camshafts, gearbox sliding forks, and small hydraulic components.
Comprehensive Process Comparison
| Parameter | Gas Carburising | Gas Nitriding | Plasma Nitriding | Carbonitriding | FNC |
|---|---|---|---|---|---|
| Temperature (°C) | 900–980 | 490–570 | 400–570 | 700–900 | 570–590 |
| Case depth | 0.5–2.5 mm | 0.1–0.6 mm | 0.1–0.6 mm | 0.075–0.75 mm | 0.01–0.025 mm |
| Surface hardness | 58–62 HRC | 65–72 HRC | 65–72 HRC | 58–65 HRC | 400–700 HV |
| Quench required? | Yes (oil/polymer) | No | No | Yes (light) | No |
| Distortion risk | Moderate | Minimal | Very minimal | Low | Very minimal |
| Typical cycle time | 4–40 h | 20–80 h | 8–40 h | 1–6 h | 1–4 h |
| Best steel grades | 8620, 9310, 4320 | Nitralloy, 4140, 4340 | Alloy steels, tool steels | Low-C alloy steels | Any ferrous alloy |
| Primary application | Heavy gears, bearing races | Precision dies, crankshafts | Aerospace tooling, dies | Small gears, fasteners | Mass-production components |
Steel Selection for Case Hardening
Matching the steel grade to the case hardening process is as important as process parameter selection. Mismatches produce either a soft, inadequate case or a brittle core incapable of providing the impact resistance for which case hardening was selected.
Carburising Grade Selection Principles
Core carbon content must be low (0.10–0.25%) to ensure the core does not harden to unacceptable brittleness during quenching. Nickel additions (1.5–3.5% Ni) markedly increase core toughness at sub-zero temperatures, which is why 9310 and EN36 are the standard for aerospace gearboxes operating at −40°C to −55°C. Chromium and molybdenum provide hardenability; boron (in some grades) can substitute for more expensive alloying elements in lightly loaded automotive applications. The hardness testing guide covers verification of core and case hardness by Vickers and Rockwell methods.
Nitriding Grade Selection Principles
The response to nitriding depends entirely on the type and concentration of nitride-forming elements present in the steel. Aluminium is the most potent: even 1% Al produces AlN precipitates with hardness contributions up to 400 HV above the matrix. Chromium and molybdenum are also strong nitride formers; vanadium is less common but effective in tool steels. Plain carbon steels (1018, 1045) respond poorly to nitriding — they develop only a thin, predominantly iron nitride compound layer with minimal diffusion zone hardening, and are not cost-effective to nitride.
Effective Case Depth vs Total Case Depth
The distinction between effective and total case depth is critical for component design and acceptance testing. Total case depth, measured on an etched metallographic section as the depth to the point where carbon content visually approaches the core level, is a microstructural description only. Effective case depth is measured to a specified hardness — 550 HV (52 HRC) per ISO 2639 for carburised cases — and directly governs the load-carrying capacity used in gear and bearing life calculations per AGMA and ISO 6336.
Specifying total case depth on a drawing without specifying effective case depth can lead to acceptance of components where the carbon gradient is extended but shallow, providing inadequate fatigue resistance at contact loads. Engineers should always specify effective case depth at a defined hardness with a ± tolerance, not total case depth alone. For nitrided components, the analogous measurement is typically to 400 HV or 50 HV above core hardness, as defined in ISO 15787.
Residual Stress and Fatigue Performance
All case hardening processes that introduce interstitials (C and/or N) into a constrained surface layer produce compressive residual stresses at the surface. These compressive stresses are beneficial: they oppose crack opening under bending or contact fatigue loading, increasing the fatigue endurance limit. The magnitude of residual compression depends on the process:
- Carburising + quench: Surface residual stress typically −200 to −400 MPa (compressive), resulting from case transformation volume change and thermal gradient during quenching. Shot peening after case hardening can increase this to −600 to −900 MPa for fatigue-critical applications such as aircraft gear teeth.
- Nitriding: The volumetric expansion of the diffusion zone (nitride precipitation) produces compressive residual stress of −400 to −600 MPa without any quench. This is a major advantage: the stress state is produced isothermally without the risk of quench cracking.
- FNC: Residual compressive stress −100 to −300 MPa in the compound layer and sub-surface. Lower magnitude but still beneficial for sliding wear and fretting fatigue.
For a deeper treatment of how pearlite colony growth and prior austenite grain size influence case hardenability, refer to the relevant metallurgical guides. Fine prior austenite grain size (ASTM 7–9) is actively beneficial for case-hardened fatigue performance.
Industrial Case Study
Wind Turbine Main Shaft Gearing: 18CrNiMo7-6 Gas Carburised
Wind turbine main shaft bearings and planetary gear sets (component diameters 1–2 m) represent among the most demanding case hardening applications in industry. Hertzian contact stresses at rolling element contacts under full rated load exceed 3,000 MPa, and the components must sustain 20-year design lives with minimal planned maintenance access.
Requirements: 2.0–2.5 mm effective case depth (to 550 HV), 58–62 HRC surface hardness, ≥35 HRC at the case–core interface, core tensile strength >1,000 MPa, and maximum retained austenite <20% (to limit dimensional instability).
Process: Ring blanks of 18CrNiMo7-6 (0.17% C, 1.65% Ni, 1.60% Cr, 0.30% Mo) are gas carburised in a large pit furnace at 930°C for 36 h (boost phase, Cp = 1.15%) followed by 12 h diffusion at 880°C (Cp = 0.80%), oil quenched at 60°C, and tempered at 180°C for 4 h.
Result: Effective case depth 2.3 mm, surface hardness 60 HRC, retained austenite 12% (within specification). Component service life extended from approximately 8 years (through-hardened predecessor design) to >20 years, dramatically reducing turbine maintenance cost over lifetime.
Process Selection Guide
Selecting the appropriate case hardening process requires balancing mechanical performance requirements against cost, cycle time, dimensional tolerance, and steel availability:
- Choose gas carburising when effective case depth >0.5 mm is required, components are large, and distortion can be managed by fixture quenching or post-process grinding.
- Choose gas or plasma nitriding when dimensional accuracy is paramount (precision dies, hydraulic actuators), or when maximum surface hardness and fatigue resistance are needed without post-treatment grinding.
- Choose plasma nitriding over gas nitriding when compound layer control is critical, when stainless steels or titanium alloys must be treated, or when cycle time reduction justifies the higher equipment cost.
- Choose carbonitriding for high-volume small components where case depth <0.75 mm is acceptable and distortion must be minimised for net-shape part geometry.
- Choose FNC for moderate wear improvement of mass-production ferrous components where deep case depth is not required and cost per part must be minimised.
Understanding heat-affected zone microstructure principles is also valuable when evaluating secondary operations (welding repairs, induction hardening touchups) that may inadvertently affect the hardened case.
Frequently Asked Questions
What is the difference between effective case depth and total case depth in carburising?
Total case depth is measured metallographically to the point where carbon content equals the core carbon level and is a microstructural observation only. Effective case depth is measured to a specified hardness limit — typically 550 HV (52 HRC) per ISO 2639 for carburised parts — and is the functionally relevant dimension used in gear and bearing load-capacity calculations. Specifying only total case depth on engineering drawings risks accepting parts with extended but shallow carbon gradients that provide insufficient fatigue resistance.
Can stainless steel be nitrided?
Standard gas nitriding cannot penetrate the Cr2O3 passive film on austenitic stainless steels and is therefore ineffective. Low-temperature plasma nitriding (350–400°C) can produce an expanded austenite or “S-phase” layer 20–40 μm deep with hardness up to 1,200 HV, without sensitisation or significant corrosion resistance loss. Active screen plasma nitriding systems are commercially available specifically for this application in medical device, pump, and food-processing component manufacture.
Why is the compound layer removed in fatigue-critical nitrided components?
The compound (white) layer of iron nitrides (ε-Fe2–3N, γ′-Fe4N) is very hard but brittle. Under cyclic bending stress or rolling contact fatigue, microcracks can initiate at compound layer discontinuities (pores, grain boundaries) and propagate into the underlying diffusion zone, causing early fatigue failure. For rotating-bending or gear tooth fatigue applications, the compound layer is removed by light honing or lapping (typically removing 5–10 μm) to expose the compressively stressed diffusion zone as the working surface.
What steels are best suited for gas nitriding?
Steels containing strong nitride-forming elements (Al, Cr, Mo, V) respond best. Nitralloy 135M (1% Al, 1.6% Cr, 0.2% Mo) achieves approximately 1,100 HV surface hardness. AISI 4140 and 4340 nitride to 650–750 HV and are the workhorses for crankshafts and dies. H13 and other Cr-Mo-V hot work tool steels reach 900–1,050 HV. Plain carbon steels (1045, 1020) develop only a thin iron nitride compound layer and minimal diffusion zone hardness, making them uneconomical to nitride conventionally.
How does nitrogen improve hardenability in carbonitriding?
Nitrogen dissolved in austenite retards nucleation of pearlite and bainite by stabilising the austenite phase, shifting TTT and CCT curves to the right. Effectively, nitrogen raises the critical cooling rate required to avoid transformation products, which is the definition of increased hardenability. A carbonitrided case can achieve full martensite on polymer quench or marquench at cooling rates that would produce mixed pearlite-bainite-martensite microstructures in an equivalent carburised (nitrogen-free) case, thus allowing reduced quench severity and dramatically lower distortion.
What is nitriding potential and how is it controlled in practice?
Nitriding potential Kn = p(NH3) / [p(H2)]1.5 is the thermodynamic parameter governing nitrogen activity at the steel surface. High Kn promotes the ε-phase (Fe2–3N) compound layer; lower Kn favours γ′ (Fe4N); very low Kn produces minimal compound layer with diffusion zone development only. In practice, Kn is measured by analysing the furnace atmosphere composition using mass spectrometry or infrared gas analysis, and is adjusted by varying the ratio of incoming NH3 to cracked NH3 (dissociated ammonia) flow rates.
What is ferritic nitrocarburising and how does it differ from nitriding?
Ferritic nitrocarburising (FNC) simultaneously diffuses carbon and nitrogen in the ferritic phase field at 570–590°C, producing a thin ε-iron carbonitride compound layer (5–25 μm) and a shallow diffusion zone. Unlike nitriding, which develops a deep diffusion zone (0.1–0.6 mm) through long cycles (20–80 h), FNC uses short cycles of 1–4 h and is optimised for wear resistance and modest fatigue improvement in mass-production ferrous parts. It cannot achieve effective case depths required for heavily loaded gears or bearing raceways.
How does plasma nitriding differ from gas nitriding?
Plasma nitriding uses a DC glow discharge to generate nitrogen ions that bombard the steel surface energetically, providing a direct activation mechanism for nitrogen absorption rather than relying solely on gas-surface equilibrium. This allows process temperatures 50–100°C lower than equivalent gas nitriding cycles, cycle times 2–4× shorter, and independent control of compound layer phase and thickness by adjusting N2/H2 ratio, temperature, and pressure. Plasma nitriding also enables selective area hardening through mechanical masking and is the only practical route for surface hardening of austenitic stainless steels and titanium alloys.
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