| 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).
