nitriding

name	nitriding
description	Nitriding surface hardening — gas nitriding (Lehrer diagram, Kn control), plasma/ion nitriding, nitrocarburizing, compound layer (ε/γ' phases), diffusion zone hardness profiles, case depth definitions (effective/total), AMS 2759/10, steel grade selection (nitralloy 135M, 4140, H13), distortion, white layer control, and hardness/fatigue improvement.
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Nitriding Surface Hardening — Complete Skill

Fundamentals

Nitrogen Thermochemistry

Nitrogen dissociation in gas nitriding: 2NH₃ → N₂ + 3H₂ [endothermic; forward at surface; nitrogen diffuses into steel] Dissociation rate α = (H₂% + N₂%_fraction) from gas analysis; typical α = 15–85%

Nitriding potential Kn: Kn = P_NH₃ / P_H₂^(3/2) [atm^(-1/2); controls phase formation at surface] Kn < 0.2: no compound layer; only diffusion zone (nitriding zone) Kn = 0.2–1.0: γ' (Fe₄N) compound layer forms Kn > 1.0: ε (Fe₂₋₃N) compound layer forms; brittle if thick

Lehrer diagram: Phase stability map in Kn vs. T space; defines γ'/ε/α-Fe zones Operating window for compound-layer-free nitriding: T = 490–530°C, Kn < 0.2 Standard compound layer production: T = 510–530°C, Kn = 0.5–2.0

Diffusion Kinetics

Nitrogen diffusion in ferritic matrix: D_N = D₀ × exp(-Q/RT) [D₀ ≈ 6×10⁻⁷ m²/s; Q = 77 kJ/mol for α-Fe] At 520°C: D_N ≈ 2×10⁻¹² m²/s

Case depth (diffusion-controlled): x_case ≈ 2 × √(D_N × t) [approximate; valid for early stages] More accurately: x_case from hardness profile where H = H_core + 50 HV (effective case depth definition)

Effective case depth (ECD) definition: Depth to 50 HV above core hardness (or to specific hardness per spec) AMS 2759/10: ECD measured to 517 HV (≈50 HRC) threshold by default Total case depth (TCD): depth where N content returns to base level (optical metallography)

Gas Nitriding

Process Parameters

Temperature range: 480–530°C (below A₁ = 723°C; no phase transformation; minimal distortion) Time: 12–120 hours (longer for deeper case) Atmosphere: NH₃ + N₂/H₂ mixture; ammonia concentration 20–100% Typical furnace: retort furnace; gas-tight; recirculation fan; automatic Kn control via dissociation analyzer

Two-stage (Floe process): Stage 1: T = 495°C, α = 20–30%, Kn = 1.5 (compound layer forms, dense γ') Stage 2: T = 525°C, α = 60–85%, Kn = 0.3 (suppress surface ε; diffuse deeper) Result: thin controlled γ' + deep diffusion zone; less distortion than single-stage

Case Depth vs. Time Table

Time (hours)	ECD (mm) — 4140 steel at 520°C
12	0.10–0.15
24	0.18–0.25
48	0.30–0.40
72	0.40–0.55
96	0.50–0.65
120	0.60–0.75

Nitralloy 135M achieves deeper case (Al forms AlN precipitates; higher surface hardness 900–1,100 HV)

Compound Layer (White Layer)

Composition: ε phase (Fe₂N–Fe₃N): high N content; harder (~1,000 HV); more brittle; hexagonal γ' phase (Fe₄N): lower N; ~700 HV; better toughness; FCC

White layer thickness: Controlled nitriding: 2–15 μm (single-stage); minimal or zero (Floe two-stage) Typical allowance: ≤ 25 μm for gears; ≤ 10 μm for precision components

White layer removal: Lapping or honing after nitriding (typically 5–10 μm removal) for precision applications Nitride diffusion zone must not be compromised during removal

Plasma (Ion) Nitriding

Physics

Glow discharge: DC voltage 300–1,000 V between cathode (workpiece) and anode (furnace wall) N₂/H₂ gas at 1–10 mbar; plasma ionizes N₂ → N⁺ ions bombard surface Ion bombardment: sputters oxides; activates surface; heats part

Advantage over gas nitriding: Precise Kn control via N₂/H₂ ratio (no ammonia required) No compound layer if pure N₂/H₂ 20:80 mix (compound-layer-free nitriding) Faster activation; can nitride stainless steel (plasma breaks passive oxide) No pollution (no ammonia emission)

Process parameters: Temperature: 350–600°C (lower than gas nitriding possible for some alloys) Time: 4–24 hours (shorter than gas; faster surface kinetics) Gas: N₂/H₂ = 1:4 (no compound layer) to N₂/H₂ = 4:1 (heavy compound layer)

Advantages for Stainless Steels

Passive oxide layer: Ion bombardment removes Cr₂O₃ → allows nitrogen ingress Gas nitriding cannot nitride stainless without special pre-treatment Plasma nitriding at low T (≤ 450°C) produces S-phase (supersaturated N in austenite): very high hardness (1,200 HV) without Cr carbide precipitation → maintains corrosion resistance

Nitrocarburizing

Definition: simultaneous nitrogen + carbon introduction; always forms compound layer Temperature: 550–590°C (slightly higher than nitriding) Atmosphere: NH₃ + CO₂ or NH₃ + endogas (N + C sources)

Compound layer: primarily ε (Fe₂₋₃N) with C dissolved; tough ε layer (Fe₂₋₃N,C) Thickness: 5–25 μm compound layer; diffusion zone below

Processes: Ferritic nitrocarburizing (FNC): most common; T = 565–590°C; 1–4 hours QPQ (Quench-Polish-Quench): nitrocarburize → oxidize → polish → oxidize; cosmetic + corrosion Salt bath (Tennifer/Tufftride): AMSCO 6501; molten cyanate + cyanide; OSHA-regulated Gas FNC: CO₂ or endogas addition to NH₃; cleaner than salt bath

FNC vs. nitriding comparison:

Property	Gas Nitriding	FNC
Temperature	490–530°C	565–590°C
Time	12–120 h	1–4 h
Compound layer	γ' or ε, controlled	ε (always)
Case depth	0.1–0.8 mm	0.01–0.05 mm
Hardness (surface)	550–1100 HV	400–700 HV
Distortion	Very low	Very low
Cost	Moderate	Low
Application	Gears, dies, high fatigue	Wear + corrosion (automotive)

Steel Selection for Nitriding

Steel Grades and Expected Hardness

Steel	Surface Hardness (HV)	ECD (0.5 mm)	Notes
Nitralloy 135M (AISI 7140)	900–1,100	Yes	Al-alloyed; best surface hardness; Al₄N precipitates
4140 (Cr-Mo)	550–700	Yes	Common; good toughness
4340 (Ni-Cr-Mo)	500–650	Yes	High-strength core; aerospace
H13 (5% Cr tool steel)	1,000–1,200	Moderate	Dies; wear surfaces
17-4 PH stainless	900–1,100	Shallow	Precipitation hardened; corrosion + wear
M2 HSS	1,100–1,300	Very shallow	Cutting tools; plasma only
1018 carbon steel	200–350	Poor	Low alloy content; poor response

Key alloying elements: Cr, Mo, Al, V: form stable alloy nitrides → high hardness precipitates Ni: has no affinity for N; does not contribute to hardness; improves core toughness Plain carbon steel (<0.2% Cr): poor response; use only for FNC (wear improvement)

Pre-treatment

Required core hardness before nitriding: Core must be tempered to final hardness before nitriding; nitriding temperature must not exceed prior tempering temperature (else softening) 4140: QT to 30–40 HRC (600°C temper); nitride at 520°C (safe) Nitralloy 135M: QT to 28–34 HRC; nitride at 495°C

Surface condition: Clean, scale-free; Ra ≤ 1.6 μm (ground preferred); no decarburization from prior heat treatment Silver, copper, nickel plating can be used as stop-off masks (prevents N ingress under plating) Oxide scale: must be removed (acid etch or grit blast); oxide prevents N absorption

Hardness Profiles and Properties

Hardness Profile Shape

Nitralloy 135M: Surface: 900–1,100 HV; decreasing to core 280–320 HV over 0.3–0.5 mm Steep gradient (Al-nitride precipitation zone)

4140: Surface: 550–700 HV; more gradual gradient; softer core transition at ~0.4–0.6 mm ECD

Design implication: Steep profile → better contact fatigue; higher surface compressive stress Gradual profile → less risk of spalling under impact; better for shock loads

Residual Stress

Compressive residual stress (near-surface): σ_residual = -200 to -700 MPa (compressive; beneficial for fatigue) Caused by volume expansion of nitrided layer (lattice expansion from N) Depth of compressive zone: ≈ ECD (all nitrided zone in compression)

Fatigue improvement: Rotating bending fatigue limit improvement: +20–40% vs. un-nitrided base Hertzian contact fatigue (gears, cams): improvement factor 2–4× Distortion: minimal (process below A₁); dimensional change typically < 0.05 mm (predictable)

Distortion

Sources of distortion: Stress relief of machining stresses Volume expansion of nitrided layer (differential expansion) Temperature gradients in furnace

Control: Slow heating/cooling: ≤ 2°C/min above 300°C Racking: hang parts; avoid horizontal trays for slender parts Straightening: permitted if within elastic range; no cold re-straightening after nitriding (cracks compound layer) Grinding allowance: 0.02–0.05 mm post-nitride grinding on precision surfaces

Quality Control and Specifications

AMS 2759/10 (Gas Nitriding)

Requirements: ECD ≥ specified (measured by micro-Vickers traverse to threshold hardness) Surface hardness ≥ specified minimum (HV or HRC) Compound layer thickness ≤ specified maximum (metallographic cross-section) No surface oxides; no decarburization

Testing: Microhardness traverse: Vickers HV 0.3–1 kg load; spacing 0.05–0.1 mm Compound layer: nital etch (2–4% HNO₃ in ethanol); white layer appears at surface Case depth: measure from surface to core hardness + 50 HV

Other specifications: AMS 2759/10: aerospace gas nitriding SAE AMS 2753: plasma nitriding DIN 17211: nitriding steels (German standard; Nitralloy-class) ISO 15787: heat treatment requirements

Standards and References

Standard	Scope
AMS 2759/10	Gas nitriding of steel parts (aerospace)
AMS 2753	Plasma nitriding
SAE J423	Case hardening methods including nitriding
DIN 17211	Nitriding steels — composition requirements
ASTM E18	Rockwell hardness testing
ISO 6507	Vickers hardness testing
ASM Handbook Vol. 4A	Heat treating steels (nitriding chapter)

Output

Provide: process type (gas/plasma/FNC) with justification (alloy, compound layer requirement, stainless), steel grade (4140/Nitralloy 135M/H13; verify Cr/Mo/Al content), pre-treatment (core hardness [HRC]; tempering temperature vs. nitriding temperature check), process parameters (T [°C]; time [h]; atmosphere/Kn or N₂/H₂ ratio), expected ECD [mm] and surface hardness [HV], compound layer thickness [μm] target (or "compound-layer-free" if specified), distortion allowance [mm] and grinding stock, residual stress (compressive [MPa]; fatigue improvement [%]), mask areas (if selective nitriding), quality test (AMS 2759/10 micro-Vickers traverse; nital etch; threshold hardness [HV]), and applicable standard (AMS 2759/10, AMS 2753, DIN 17211).