carburizing

name	carburizing
description	Carburizing heat treatment — gas/pack/vacuum carburizing, case depth, carbon profile, diffusion kinetics, core vs. case hardness, quench and temper, AMS 2759/7, AGMA 2101 gear case depth.
metadata	{"priority":7,"promptSignals":{"phrases":["carburizing","case carburizing","gas carburizing","vacuum carburizing","case depth","surface hardening carburize"],"minScore":3}}

Carburizing Heat Treatment — Complete Skill

Process Overview

Goal: enrich steel surface with carbon → harder case + tough core Mechanism: carbon atoms from atmosphere diffuse into austenite at high T → retained as high-carbon martensite after quench Steel requirement: low-carbon core (0.10–0.25% C) → after carburizing: surface 0.7–1.0% C; core unchanged

Methods

Gas Carburizing (Most Common)

Atmosphere: endothermic gas (N₂/H₂/CO mix) + enriching gas (natural gas or propane) Temperature: 850–950°C (typical 920–940°C); austenite stable; high carbon diffusivity Carbon potential (Cp): atmosphere carbon activity; set at 0.8–1.0% (controlled by CO₂/O₂ probe)

Boost-diffuse cycle: Phase 1 (boost): Cp = 1.0–1.2%; rapid carbon pickup; surface reaches ~1.0% C Phase 2 (diffuse): Cp = 0.8%; reduce surface C; allow carbon gradient to flatten; better toughness

Advantage of gas carburizing: batch or continuous; large loads; well-established Disadvantage: requires combustible atmosphere; quench from carburizing temperature or re-heat

Vacuum Carburizing (LPC — Low Pressure Carburizing)

Atmosphere: acetylene (C₂H₂) or propane at 1–20 mbar; pulsed injection Temperature: 880–1050°C (higher T possible without oxidation → faster diffusion) Process: boost (acetylene exposure) + diffuse (evacuate → let carbon diffuse) pulses

Advantages: no surface oxidation; no intergranular oxidation (IGA); faster cycle; better for complex geometry (blind holes carburize uniformly under vacuum) Quench: high-pressure gas quench (HPGQ) under nitrogen or helium (5–20 bar); less distortion than oil quench

Pack Carburizing

Embedding in carbon-bearing solid (wood charcoal + BaCO₃ energizer) T = 850–950°C; batch process; slow; high surface carbon (~1.0%) Disadvantage: slow, non-uniform, poor control; rarely used in modern production

Carbon Diffusion and Case Depth

Fick's second law (semi-infinite solid approximation): C(x,t) = C_s - (C_s - C_core) × erf(x / (2√(D t)))

C_s = surface carbon [%]; C_core = core carbon [%]; D = diffusion coefficient [m²/s]; t = time [s]

Diffusion coefficient for carbon in austenite (γ-iron): D = D₀ × exp(-Q / (R T)) D₀ = 2.0×10⁻⁵ m²/s; Q = 142,000 J/mol; R = 8.314 J/(mol·K) At 920°C (1193 K): D = 2.0×10⁻⁵ × exp(-142000/(8.314 × 1193)) = ≈ 1.2×10⁻¹¹ m²/s

Effective case depth (ECD): ECD = depth at which hardness = 550 HV (corresponding to ~0.35% C after quench) Or: total case depth (TCD) to microstructurally distinguishable limit (~0.20% C change)

Time for target ECD: ECD ≈ 2√(D t) × erfinv((C_s - 0.35)/(C_s - C_core)) For C_s = 0.85%, C_core = 0.18%: ECD ≈ √(3 D t) [rough approximation]

Example: ECD target = 0.8 mm; D at 920°C = 1.2×10⁻¹¹ m²/s t ≈ ECD² / (3D) = (0.0008)² / (3 × 1.2×10⁻¹¹) = 6.4×10⁻⁷ / 3.6×10⁻¹¹ ≈ 18,000 s ≈ 5 hr

Post-Carburize Thermal Processing

Direct quench (from carburize temperature): Most common; quench in oil or HPGQ from 850°C Case: high-carbon martensite (60–65 HRC) Core: low-carbon martensite or bainite (30–45 HRC)

Reheat quench: Slow cool from carburize T → reheat to 800–830°C → quench Grain refinement; better core toughness; more distortion

Tempering (required after quench): 150–200°C temper; 1–2 hr; reduces brittleness without significant hardness loss Tempering at > 200°C → hardness drops rapidly (not for most carburized parts)

Retained austenite: Surface C = 1.0% → M_s temperature < 20°C → significant retained austenite (15–30%) Reduces case hardness; can transform to martensite under load (TRIP effect) Mitigate with: cryogenic treatment (-80°C), sub-zero after quench

Gear Case Depth (AGMA 2101)

Minimum effective case depth for gears: ECD_min = 0.16 × m_n [mm; m_n = normal module in mm] ECD_max = 0.30 × m_n [recommended range] Purpose: case should extend to root; prevent core yielding at root

Root fillet case: After grinding (required for precision gears): remove 0.05–0.15 mm Design ECD before grinding = ECD_after + grinding stock

Hardness Profile

Typical profile (8620 steel, 920°C × 5 hr, ECD 0.8 mm):

Depth [mm]	Approx HRC
0 (surface)	60–62
0.3	58–60
0.6	55–58
ECD (0.8)	50 (=550 HV)
1.2	40–45
Core (> 2 mm)	30–38

Material Selection for Carburizing

Common grades:

Grade	C_core [%]	Core strength	Applications
8620	0.20	High	Gears, cams; standard
4320	0.20	Very high	Heavy-duty gears
9310	0.10	Exceptional	Aerospace gears (AMS 6265)
5120	0.20	Medium	Lighter applications

Standards

Standard	Scope
AMS 2759/7	Gas carburizing of steels
AMS 2759/12	Vacuum carburizing
AGMA 2101	Gear rating; includes case depth
ISO 6336	Gear strength calculation (case depth factor)
ASTM A313	Carbon steel for heat treatment

Output

Provide: carburizing method (gas/vacuum), temperature T [°C] and time t [hr], boost-diffuse cycle (Cp and duration), surface carbon C_s [%] and core carbon C_core [%], diffusion coefficient D [m²/s] at T, effective case depth ECD [mm], total case depth TCD [mm], hardness profile (surface to core [HRC vs. depth]), quench medium and temper cycle, retained austenite estimate [%] and mitigation if needed, gear case depth check vs. AGMA 2101 (if applicable), and applicable standard (AMS 2759/7, AGMA 2101).