gear-materials

name	gear-materials
description	Gear materials — carburized/nitrided/through-hardened steels, material grades (AGMA 923), case depth, core hardness, 8620/9310/4340/Nitralloy steels, AMS specifications, shot peening, gear material selection for automotive/aerospace/industrial.
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Gear Materials — Complete Skill

Gear Material Selection Criteria

Primary requirements:

• Bending fatigue strength at tooth root (S_t per AGMA; σ_Flim per ISO 6336)
• Contact fatigue strength at flank (S_c per AGMA; σ_Hlim per ISO 6336)
• Core toughness (K_IC; Charpy) — impact resistance
• Machinability and hardenability through section
• Dimensional stability during heat treatment (distortion)
• Cost and availability

Secondary requirements:

• Corrosion resistance (stainless or coated)
• Weight (aluminum gears for aerospace accessory drives)
• Operating temperature (above 200°C → special materials)

Carburized and Case-Hardened Steels (AGMA Grade 2–3)

Process: low-carbon steel → carburize in carbon-rich atmosphere → surface carbon enriched to 0.7–1.0% C → quench → temper → surface HRC 58–63, tough core HRC 28–38

SAE 8620 (Most Common Automotive and Industrial)

Composition: 0.18–0.23% C; 0.7–0.9% Mn; 0.4–0.7% Ni; 0.4–0.6% Cr; 0.15–0.25% Mo Hardenability: moderate; suitable for small-to-medium sections (< 100 mm) Core (after carburize + quench + 165°C temper): σ_y ≈ 750 MPa; HRC 30–38 Carburized case (HRC 58–62): σ_Flim ≈ 380 MPa; σ_Hlim ≈ 1,380 MPa (AGMA Grade 2) AMS: AMS 6274 (bar); AMS 6275 (tubing) Applications: automotive transmission gears, differential gears, industrial gearboxes

SAE 9310 (Aerospace Standard Carburizing Steel)

Composition: 0.08–0.13% C; 3.0–3.5% Ni; 1.0–1.4% Cr; 0.08–0.15% Mo High nickel: excellent toughness and case hardenability Core: σ_y ≈ 900 MPa; elongation 14%; Charpy 80 J (tough core for aerospace demands) Carburized case (HRC 60–63): σ_Flim ≈ 420 MPa; σ_Hlim ≈ 1,500 MPa (AGMA Grade 3 possible with clean steel practice) AMS: AMS 6265 (bar); AMS 6267 (billet) Applications: helicopter gearboxes (CH-47, AH-64), turbofan accessory drives, aircraft main drives

SAE 4320 (Moderate Hardenability, Lower Cost than 9310)

Composition: 0.17–0.22% C; 1.65–2.00% Ni; 0.40–0.60% Cr; 0.20–0.30% Mo Intermediate between 8620 and 9310: better toughness than 8620; cheaper than 9310 AMS 6299: aerospace bar; military gear applications Core: HRC 28–34; σ_y ≈ 800–900 MPa

AMS 6308 / Pyrowear 53 (Corrosion-Resistant Carburizing)

Composition: 0.10% C; 2.0% Cr; 2.0% Mo; 13% Ni; Co additions Stainless-like corrosion resistance + carburizable: high-speed steel flight surface actuators S_t ≈ 380 MPa; S_c ≈ 1,380 MPa (similar to 9310 with corrosion benefit) AMS: AMS 6308; temperature limit 315°C for application

Nitrided Steels (AGMA Grade 2)

Process: heat in nitrogen/ammonia atmosphere (500–570°C); no quench; surface nitride compound layer forms → white layer 5–25 μm; diffusion zone below Advantages: minimal distortion (no quench); good surface hardness (HRC 55–70 depending on compound layer); through-hardenable core before nitriding Disadvantages: thinner case (0.2–0.5 mm) than carburized; brittle white layer if too thick → must remove before service; slower process

Nitralloy 135M (AMS 6475)

Composition: 1.35% Al; 0.25–0.35% C; 0.20–0.30% Mo; standard steel with Al → forms coherent AlN nitrides Surface hardness: HV 950–1050 (HRC 68–70 equivalent); highest nitriding hardness Compound layer: must be ≤ 10 μm; remove by honing for gears Case depth (effective, 550 HV): 0.30–0.50 mm after 20–50 hours at 520°C σ_Flim = 310 MPa; σ_Hlim = 1,100–1,250 MPa (lower than carburized) Applications: aircraft engine accessory drives; precision instruments; high-speed spindles

SAE 4140 / 4340 Nitrided

Less specialized alloys: Can be nitrided for moderate-duty applications 4140 nitrided: HRC 55–60 surface; case 0.2–0.35 mm; σ_Hlim ≈ 950 MPa 4340 nitrided: core σ_y ≈ 1,300 MPa; strong core + nitrided surface → aircraft structural parts

Through-Hardened Steels (AGMA Grade 1)

Process: austenitize → quench → temper to desired HRC → no case depth; uniform hardness throughout Applications: low-to-moderate duty; when distortion is critical (nitriding preferred); large gear blanks where carburize depth insufficient

SAE 4340 (AMS 6415)

Composition: 0.38–0.43% C; 1.65–2.00% Ni; 0.70–0.90% Cr; 0.20–0.30% Mo At HRC 32 (tempered): σ_y = 1,000 MPa; σ_u = 1,100 MPa; K_IC = 100 MPa√m At HRC 50 (harder temper): σ_y = 1,500 MPa; σ_u = 1,700 MPa; K_IC = 50 MPa√m (more brittle) S_t = 0.533 × HB + 88.3 MPa; S_c = 2.22 × HB + 200 MPa (AGMA formula) AMS 6415: aircraft structural 4340; vacuum arc melted (VAR) for cleanliness

SAE 4150 / 4150 HP

Slightly higher carbon (0.48–0.53% C): higher hardenability; used in large cross-sections Through-hardened to HRC 28–38 for industrial gearboxes; lower K_IC at higher hardness

Non-Ferrous Gear Materials

Aluminum Alloys

7075-T651: σ_y = 503 MPa; σ_u = 572 MPa; low density (2,810 kg/m³); reduced inertia Applications: aerospace accessory gears (weight-critical); limited to < 200°C S_t ≈ 135 MPa; S_c ≈ 600 MPa (significantly lower than steel)

Limitations: lower surface hardness than steel → higher wear → not for continuous service without coating Anodizing: increases wear resistance; Al-hard anodize (HV 400–500); limited improvement

Copper Alloys

Phosphor bronze (C93700): σ_y = 140 MPa; good machinability; used for worm gears (mates with steel worm) Aluminum bronze (C95400): σ_y = 250–350 MPa; stronger; wear resistant; spur/helical in corrosive environments

Plastic Gears

Acetal (POM/Delrin): most common; self-lubricating; σ_y = 70 MPa; service -40 to +90°C; low noise Nylon (PA66): absorbs moisture → dimensional change; stronger than acetal; σ_y = 80 MPa PPS, PEEK: high-temperature plastics; continuous service to 220°C (PEEK); for harsh environments

PV limit for plastic gears: p_contact × V_sliding < PV_limit [PV_limit = 0.1–0.2 MPa·m/s for acetal; 0.2–0.3 for nylon]

Metallurgical Specifications and Quality

AGMA 923-B05 (Metallurgical Specifications for Gears):

• Surface hardness: measure at 5 points on tooth face
• Retained austenite: < 25% for carburized gears (too much → lower yield strength)
• Core hardness range: ±5 HRC from specification
• Effective case depth (ECD): per drawing at 550 HV intersection
• Grain size: ASTM E112 grain size ≥ 6 (fine grain → better fatigue)
• Hydrogen content (for steels after plating): < 5 ppm for HRC > 40 (hydrogen embrittlement prevention)

Vacuum Arc Remelting (VAR) / Electroslag Remelting (ESR): Reduces inclusions → higher K_IC and fatigue limit Required for: AGMA Grade 3; AMS specs for critical aerospace gears S_t improvement vs. air melt: ~15–25% at Grade 3

Shot peening for gears: SAE AMS 2430 / AMS 2432: shot peening specifications for gears Induces -300 to -700 MPa compressive residual stress at surface S_t increase: 10–30%; eliminates grinding burns' tensile RS Coverage: 100% min; Almen intensity: 0.008–0.018A for small gears

Standards

Standard	Scope
AGMA 923-B05	Metallurgical specifications for steel gears
AMS 6265	9310 alloy steel bar for aircraft gears
AMS 6415	4340 alloy steel for aircraft structural/gear
AMS 2750	Pyrometry (furnace qualification for heat treat)
AMS 2430	Shot peening (coverage, intensity)
AMS 2432	Shot peening computer monitored
ASTM E112	Grain size determination

Output

Provide: gear application and duty (automotive/aerospace/industrial), material selected with grade (AGMA 923 Grade 1/2/3), heat treatment process (carburize/nitride/through-harden), AMS designation, surface hardness (HRC or HV), core hardness (HRC), effective case depth ECD [mm], retained austenite [%] (carburized), S_t [MPa] and S_c [MPa] (AGMA values), vacuum melt required (VAR/ESR), shot peening spec (AMS 2430 Almen intensity), grain size (ASTM E112 number), fatigue life improvement vs. base condition [%], and applicable standard (AGMA 923, AMS 6265/6415, AMS 2430).