case-hardening

name	case-hardening
description	Case hardening processes — nitriding (gas/plasma/salt bath), induction hardening, flame hardening, laser hardening, case depth, residual stress, Nitrided Compound Layer, applications comparison.
metadata	{"priority":7,"promptSignals":{"phrases":["case hardening","nitriding","induction hardening","flame hardening","surface hardening","laser hardening"],"minScore":3}}

Case Hardening Processes — Complete Skill

Process Comparison Overview

Process	T [°C]	Case depth [mm]	Hardness [HRC]	Distortion	Applicability
Carburizing	850–950	0.5–3.5	58–65	Moderate	Low-C steel
Nitriding (gas)	480–570	0.1–0.8	50–70 (HV)	Very low	Any steel
Plasma nitriding	350–560	0.05–0.5	55–80 (HV)	Very low	Specialty shapes
Induction	At-temp	1.0–10	58–65	Low–medium	Limited geometries
Flame	At-temp	1.5–10	55–62	Low	Large parts
Laser	At-temp	0.2–1.5	55–62	Negligible	Precision zones
Salt bath nitriding	550–580	0.1–0.5	50–65 (HV)	Very low	Complex shapes

Nitriding

Gas Nitriding (Floe Process)

Mechanism: dissociated ammonia at 480–570°C; nitrogen atoms diffuse into steel → iron nitrides (Fe₂N, Fe₃N, Fe₄N, CrN) Compound layer (white layer): 5–25 μm surface ε-nitride zone; very hard (1000+ HV); brittle Diffusion zone: nitrogen in solid solution; 0.1–0.8 mm depth No quench required: hardened by nitriding at low T → minimal distortion

Two-stage Floe process: Stage 1: 480°C, 25% NH₃ dissociation; builds compound layer Stage 2: 570°C, 75–85% dissociation; extends diffusion zone; reduces compound layer brittleness Duration: 20–100 hr total for 0.3–0.7 mm diffusion zone

Hardness (core and case): Core: as-machined (typically 28–32 HRC; must be pre-hardened steel) Case: 600–1200 HV depending on alloy; alloy content (Cr, Al, Mo, V) improves nitride hardness Al-bearing steels (Nitralloy 135): surface HV = 1100–1200 (nitriding response very high)

Steel selection for nitriding: Nitralloy 135M, 38CrMoAl, 31CrMo12 (excellent nitriding response) H13 tool steel, 42CrMo4, 4140: good response 316 SS: low response (passive oxide layer must be removed by acid etch or plasma)

Plasma (Ion) Nitriding

Process: glow discharge plasma; N⁺ ions bombard part surface; nitriding at 350–560°C Advantages:

• No compound layer or controlled thin compound layer (0–5 μm)
• Can nitride complex shapes including blind holes
• Works on stainless steel (plasma removes passive layer)
• Lower temperature (350°C) for distortion-sensitive parts

Parameters: voltage 400–600 V; pressure 1–10 mbar; atmosphere N₂/H₂ mix (20–50% N₂)

Salt Bath Nitriding (Tufftride/Tenifer): Molten salt cyanate at 550–580°C; fast (1–4 hr); compound layer 10–30 μm Combined oxydation post-treatment (QPQ) → corrosion + wear resistance

Induction Hardening

Mechanism: high-frequency AC current induces eddy currents; resistive heating → austenitize; quench Depth of heating (skin effect): δ = 503 √(ρ / (μ_r × f)) [mm; ρ = resistivity [μΩ·m]; μ_r = relative permeability; f = frequency [Hz]] At Curie T (770°C): μ_r → 1; skin depth increases dramatically → depth control

Frequency selection:

• Deep case (3–10 mm): 1–10 kHz
• Medium case (1–3 mm): 10–50 kHz
• Shallow case (0.5–2 mm): 100–400 kHz

Steel: medium-carbon (0.35–0.55% C) required; suitable grades: 1045, 4140, 4340, 52100 Hardness: 58–63 HRC (depends on C content; C% → HRC from martensite hardness chart) Pattern: precise heating zone (gear tooth, journal, inner bore) Distortion: higher than nitriding (quench from austenitizing T); lower than furnace hardening

Residual stress: Compressive residual stress at surface (martensite volume expansion during quench → compressive) Benefit: improved fatigue resistance; +20–50% fatigue life vs. through-hardened

Flame Hardening

Similar to induction but uses oxy-gas torch instead of inductor Control: travel speed; torch geometry; flame intensity; quench spray follows torch Depth: 1.5–10 mm (less control than induction) Applications: large gears, railroad rails, long shafts (hard to induction-heat full length) Disadvantages: low control; fire hazard; non-uniform case depth; not suitable for complex shapes

Laser Hardening

Process: CO₂ or diode laser beam scanned across surface; austenitize → self-quench (very rapid; no external quench needed for most steels) Depth: 0.2–1.5 mm (limited by self-quench rate) Spot size: 1–30 mm; power 0.5–10 kW; scanning speed 2–50 mm/s

Advantages:

• Negligible distortion (minimal heat input; rapid)
• Precise zones (can harden cam lobe without touching adjacent surfaces)
• No distortion; no quench media

Disadvantages:

• Shallow case only; expensive (laser + robot); absorption coating often required for steel

Absorption: steel reflects laser; apply graphite spray or black paint → absorbs laser → heats Or: diode laser wavelength (808 nm) better absorbed by steel than CO₂ (10.6 μm)

Compound Layer Management

ε-nitride compound layer (white layer): Brittle; can spall under bending fatigue Remove by: grinding 0.05–0.1 mm (removes white layer); or plasma nitriding (controlled thin layer or none) For fatigue-critical parts (gears, crankshafts): specify maximum compound layer thickness < 5 μm or none

Residual Stress Comparison

Process	Surface residual stress	Benefit
Gas nitriding	-400 to -600 MPa (compressive)	Excellent fatigue
Induction	-200 to -500 MPa (compressive)	Good fatigue
Carburizing	-300 to -500 MPa (compressive)	Good fatigue
Flame	Variable; -100 to -300 MPa	Moderate

Compressive residual stress closes surface cracks → excellent for rotating bending fatigue

Standards

Standard	Scope
AMS 2759/10	Gas nitriding
AMS 2759/12	Plasma nitriding
AMS 2745	Induction hardening
ISO 11399	Induction hardening
DIN 58197	Nitriding steels

Output

Provide: process selected (gas/plasma nitriding or induction/flame/laser hardening), process temperature T [°C] and duration t [hr], case depth (diffusion zone and compound layer) [mm], surface hardness [HRC or HV] and core hardness [HRC], surface residual stress [MPa] (compressive), distortion risk (low/medium/high), steel grade and pre-treatment condition, post-process grinding requirement (to remove compound layer [mm]), fatigue life improvement estimate [%], and applicable standard (AMS 2759/10, AMS 2745).