Case Hardening Processes: Carburising, Nitriding, and Carbonitriding Compared

Introduction to Case Hardening

Case hardening produces a hard, wear-resistant outer layer (case) while maintaining a tough, ductile core. This combination is essential for components subjected to simultaneous surface wear and bulk impact loading — gears, camshafts, bearing races, gudgeon pins, and splines. Unlike through-hardening (which hardens the entire section), case hardening restricts the metallurgical change to a controlled depth, preserving core toughness.

Three major thermochemical diffusion processes dominate industrial case hardening: gas carburising, nitriding (gas or plasma), and carbonitriding. Each diffuses different atomic species into the steel surface at specific temperatures, producing distinct case depths, hardness levels, and property profiles.

Gas Carburising

Process Description

Gas carburising introduces carbon into the surface of low-carbon steel (0.10–0.25% C) by exposing it to a carbon-rich atmosphere at 900–980°C — within the austenite phase field. The atmosphere typically consists of endothermic carrier gas (N₂ + CO + H₂) enriched with propane or natural gas. Carbon potential (Cp) — the equilibrium carbon content the atmosphere imposes on the steel surface — is controlled by monitoring CO₂ or dew point and adjusting enrichment gas flow.

Carbon Diffusion and Case Depth

Carbon diffuses inward from the surface according to Fick’s second law:

∂C/∂t = D × (∂²C/∂x²)

Where D is the diffusion coefficient of carbon in austenite (~5×10⁻¹¹ m²/s at 925°C). Case depth is proportional to √(D×t), where t is time. Typical carburising cycles:

Target Case Depth (mm)	Carburising Time at 925°C	Process Stages
0.5	3–4 hours	Boost (Cp 1.1–1.2%) → Diffuse (Cp 0.85%)
1.0	7–9 hours	Boost → Diffuse → Quench
1.5	14–18 hours	Boost → Diffuse × 2 → Quench
2.5	30–40 hours	Multiple boost-diffuse cycles

Quenching and Hardness After Carburising

After carburising, parts are either direct quenched (from carburising temperature) or slow cooled and re-hardened. Surface carbon target is 0.80–0.95% C — sufficient for 58–62 HRC surface hardness after quenching without excessive retained austenite. Core hardness depends on core carbon content and quench severity: typically 30–40 HRC for most case-hardening grades.

Common steel grades for gas carburising: 8620, 9310, 4320, 3310, EN36 (UK), 17CrNi6-6 (Europe).

Gas Nitriding

Process Description

Nitriding introduces nitrogen into the surface of alloy steel at 490–570°C — well below Ac1, so no phase transformation occurs and distortion is minimal. The conventional gas nitriding atmosphere is dissociated ammonia (NH₃), which decomposes at the steel surface to provide nascent nitrogen.

Nitrogen diffuses into the steel and reacts with nitride-forming alloying elements (Al, Cr, Mo, V) to precipitate very fine alloy nitride particles (AlN, CrN, Mo₂N), producing hardness of 650–1,200 HV (65–72 HRC equivalent) — higher than achievable by carburising.

The Nitrided Case Structure

The nitrided case consists of two layers:

• Compound layer (white layer, 5–25 µm thick): A mixture of iron nitrides (ε-Fe₂₋₃N and γ’-Fe₄N). Very hard (700–1,100 HV) but brittle. May be mechanically removed for fatigue-critical applications.
• Diffusion zone (0.1–0.6 mm deep): Alloy nitride precipitates in the matrix. Provides the majority of hardness and fatigue improvement. Hardness gradient is more gradual than carburising.

Controlled Nitriding: Nitriding Potential

Modern gas nitriding uses controlled atmospheres with defined nitriding potential (Kn = pNH₃ / pH₂^1.5) to produce the required case without excessive compound layer thickness. Two-stage Floe process: Stage 1 at high Kn establishes the compound layer; Stage 2 at lower Kn develops the diffusion zone with a thinner, more controlled compound layer.

Plasma (Ion) Nitriding

Plasma nitriding uses a DC glow discharge between the furnace vessel (anode) and the parts (cathode) in a low-pressure N₂/H₂ atmosphere. Nitrogen ions bombard the steel surface energetically, accelerating nitrogen diffusion. Advantages over gas nitriding:

• Faster process (2–4× shorter cycles)
• Ability to control compound layer composition and thickness precisely
• Selective hardening possible by masking with bore plugs or mechanical screens
• Lower process temperature achievable (400–450°C) for distortion-critical parts

Carbonitriding

Process Description

Carbonitriding simultaneously introduces carbon and nitrogen by operating in an endothermic atmosphere with ammonia addition (typically 3–15% NH₃) at 700–900°C. It is a hybrid of carburising and nitriding, producing shallower but harder cases than conventional carburising, at lower process temperatures.

The co-diffusion of nitrogen and carbon has synergistic effects: nitrogen retards the diffusion of carbon (allowing case formation at lower temperature) and significantly improves hardenability of the case (allowing lighter quenching). Parts can often be marquenched or polymer-quenched with minimal distortion.

Typical case depth: 0.075–0.75 mm. Surface hardness: 58–65 HRC. Applications: small gears, fasteners, retaining rings, bushings, and mass-produced components where short cycle times are economically critical.

Ferritic Nitrocarburising (FNC)

FNC (including proprietary processes Tenifer®, Tufftride®, Nitrotec®) is performed in the ferritic phase field at 570–590°C in a salt bath or atmosphere containing both carbon and nitrogen sources. The short cycle time (1–4 hours) produces a thin epsilon-iron carbonitride compound layer (5–25 µm) with excellent wear resistance and fatigue strength improvement.

FNC is widely used for automotive gearbox components, camshafts, and crankshafts where moderate wear improvement at low cost is needed. It does not produce deep case depths and is unsuitable for heavily loaded gears requiring >0.5 mm effective case depth.

Comprehensive Process Comparison

Parameter	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	0.1–0.75 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	Moderate	Minimal	Very minimal	Low	Very minimal
Cycle Time	4–40 hours	20–80 hours	8–40 hours	1–6 hours	1–4 hours
Best Steel Grades	Low-C (8620, 9310)	Alloy (Nitralloy, 4140)	Alloy steels	Low-C alloy	Any ferrous
Best For	Heavy gears, shafts	Precision dies, crankshafts	Aerospace, tooling	Small gears, fasteners	Mass production parts

Steel Selection for Case Hardening

The right steel grade must be matched to the case hardening process:

• Carburising grades must have low core carbon (0.10–0.25%) to retain core toughness. Nickel addition (1.5–3.5% Ni, as in 9310 and EN36) provides excellent core toughness at low temperatures — essential for aerospace and heavy-duty gears.
• Nitriding grades must contain sufficient nitride-forming elements. Nitralloy 135M (1% Al, 1.6% Cr, 0.2% Mo) achieves the highest hardness (~1,100 HV). Standard alloy steels (4140, 4340) are also widely nitrided to 600–700 HV.
• Carbonitriding grades are typically standard low-alloy or case-hardening grades similar to carburising steels.

Industrial Case Study: Wind Turbine Main Shaft Bearing

Wind turbine main shaft bearings (diameter 1–2 m) are among the most demanding case-hardening applications. Fatigue contact stresses at the rolling element contact exceed 3,000 MPa under full load. Requirements: 2.0–2.5 mm effective case depth, 58–62 HRC surface hardness, ≥35 HRC at case-core transition, and core tensile strength >1,000 MPa.

Solution: Large ring blanks of 18CrNiMo7-6 are gas carburised in a pit furnace at 930°C for 36 hours (boost phase, Cp = 1.15%) followed by 12 hours diffusion at 880°C (Cp = 0.80%). Parts are then oil quenched (60°C) and tempered at 180°C. Final case depth 2.3 mm, surface hardness 60 HRC. Cost justification: improved bearing life from 8 to 20+ years reduces turbine maintenance costs dramatically.

Frequently Asked Questions

Q: Can I nitride stainless steel?
A: Standard gas nitriding is ineffective on austenitic stainless steels because the Cr₂O₃ passive film prevents nitrogen diffusion. Special processes — S-phase nitriding (plasma or low-temperature salt bath at 350–400°C) — can nitride stainless steels to high surface hardness without sensitisation or loss of corrosion resistance.

Q: Why is the compound layer sometimes undesirable in nitriding?
A: The compound layer is brittle and can spall under impact loading or cyclic bending stresses, potentially initiating fatigue cracks. For rotating bending fatigue applications (crankshafts, gears), the compound layer is often mechanically removed by gentle honing or lapping after nitriding.

Q: What is the difference between effective and total case depth in carburising?
A: Total case depth is measured to the point where the carbon content equals the core carbon level, visible on a hardness traverse. Effective case depth is the depth to a specified hardness (typically 550 HV = 52 HRC in ISO 2639), which is the functionally relevant depth for load calculations.

Conclusion

Case hardening is not a single process but a family of thermochemical treatments, each optimised for specific performance requirements. Carburising produces the deepest, most mechanically robust cases for heavy-duty gears and bearing races. Nitriding delivers the highest surface hardness with minimal distortion for precision components. Carbonitriding is the economic choice for high-volume small components. Selecting the wrong process — or the wrong steel grade — leads to premature surface fatigue or core fracture. See also: Quenching Steel Guide and Heat Affected Zone in Steel Welds.

References

• ASM Handbook Vol. 4A: Steel Heat Treating Fundamentals and Processes. ASM International, 2013.
• Davis, J.R. (ed.), Surface Hardening of Steels. ASM International, 2002.
• Pye, D., Practical Nitriding and Ferritic Nitrocarburising. ASM International, 2003.