If you’re designing parts that need a tough core and a hard, wear-resistant surface—like automotive gears, industrial shafts, or aerospace fasteners—you need a material that balances ductility and surface strength. AISI 8620 alloy steel is the perfect solution: as a low-carbon nickel-chromium-molybdenum (Ni-Cr-Mo) alloy, it’s ideal for carburizing (a heat treatment that hardens only the surface), delivering a hard outer layer (up to 60 HRC) and a tough inner core. This guide breaks down its properties, real-world applications, manufacturing process, and material comparisons to help you solve “tough core + hard surface” design challenges.
1. Material Properties of AISI 8620 Alloy Steel
AISI 8620’s performance comes from its low-carbon (0.18–0.23%) and Ni-Cr-Mo composition: low carbon keeps the core ductile, nickel boosts toughness, chromium enhances surface hardenability, and molybdenum improves fatigue limit. Let’s explore its key properties in detail.
1.1 Chemical Composition
AISI 8620 adheres to ASTM A29/A29M standards, with elements optimized for carburizing. Below is its typical composition:
| Element | Symbol | Content Range (%) | Key Role |
|---|---|---|---|
| Carbon (C) | C | 0.18 – 0.23 | Low enough to keep the core ductile; reacts with carbon during carburizing to harden the surface |
| Nickel (Ni) | Ni | 0.40 – 0.70 | Core toughness booster; maintains impact toughness at low temperatures (-30 °C) |
| Chromium (Cr) | Cr | 0.40 – 0.60 | Enhances surface hardenability; improves corrosion resistance of the carburized layer |
| Molybdenum (Mo) | Mo | 0.15 – 0.25 | Raises fatigue limit; prevents brittleness in the carburized surface |
| Manganese (Mn) | Mn | 0.70 – 0.90 | Refines grain structure; boosts tensile strength without reducing ductility |
| Silicon (Si) | Si | 0.15 – 0.35 | Aids deoxidation; supports stability during carburizing |
| Phosphorus (P) | P | ≤ 0.035 | Minimized to avoid brittle fracture in the carburized layer |
| Sulfur (S) | S | ≤ 0.040 | Controlled to balance machinability and surface quality (lower S = smoother carburized surfaces) |
| Vanadium (V) | V | ≤ 0.03 | Trace element; refines grains for uniform surface hardening |
| Copper (Cu) | Cu | ≤ 0.30 | Trace element; adds mild atmospheric corrosion resistance for outdoor parts |
1.2 Physical Properties
These traits make AISI 8620 ideal for carburized parts across industries:
- Density: 7.85 g/cm³ (same as standard steels)—simplifies weight calculations for gears, shafts, or fasteners
- Melting Point: 1,420 – 1,450 °C (2,588 – 2,642 °F)—compatible with carburizing and forging processes
- Thermal Conductivity: 42.0 W/(m·K) at 20 °C; 38.0 W/(m·K) at 300 °C—ensures even carbon diffusion during carburizing (no uneven surface hardness)
- Coefficient of Thermal Expansion: 11.5 × 10⁻⁶/°C (20 – 100 °C)—minimizes distortion during carburizing and quenching
- Magnetic Properties: Ferromagnetic—enables non-destructive testing (NDT) like magnetic particle inspection to check surface cracks after carburizing.
1.3 Mechanical Properties
AISI 8620’s mechanical performance depends on carburizing: the surface is hard, while the core remains tough. Below are typical values for carburized and non-carburized conditions:
| Property | Measurement Method | Non-Carburized (Annealed) | Carburized (0.8–1.0% Surface C) |
|---|---|---|---|
| Surface Hardness | HRC | 18 – 22 HRC | 58 – 60 HRC |
| Core Hardness | HRC | 18 – 22 HRC | 30 – 35 HRC |
| Tensile Strength | MPa (ksi) | 600 MPa (87 ksi) | 1,100 MPa (159 ksi) |
| Yield Strength | MPa (ksi) | 350 MPa (51 ksi) | 800 MPa (116 ksi) |
| Elongation | % (in 50 mm) | 28 – 32% | 12 – 15% |
| Impact Toughness | J (at -30 °C) | ≥ 90 J | ≥ 45 J (core toughness) |
| Fatigue Limit | MPa (rotating beam) | 300 MPa | 650 MPa |
1.4 Other Properties
AISI 8620’s traits solve “tough core + hard surface” challenges:
- Weldability: Excellent—low carbon allows welding without preheating (for non-carburized parts); carburized parts need grinding to remove surface carbon before welding.
- Formability: Great—soft in the annealed condition (18–22 HRC), so it can be bent, forged, or stamped into complex shapes (e.g., gear blanks) before carburizing.
- Machinability: Excellent—annealed AISI 8620 cuts easily with HSS or carbide tools; carburized parts require grinding for final shaping.
- Corrosion Resistance: Moderate—chromium in the carburized layer resists mild rust; for harsh environments, add zinc plating or epoxy coating.
- Surface Wear Resistance: Outstanding—carburized surface (58–60 HRC) resists metal-to-metal wear, extending part life by 3–4x vs. non-carburized steels.
2. Applications of AISI 8620 Alloy Steel
AISI 8620’s carburizing advantage makes it ideal for parts that need to resist wear while absorbing impact. Here are its key uses:
- Gears: Automotive transmission gears, industrial gearbox gears, and helicopter rotor gears—hard surface resists tooth wear; tough core absorbs shock from gear meshing.
- Shafts: Drive shafts, camshafts, and axle shafts—hard surface resists abrasion; tough core handles torque and bending.
- Bearings: Bearing races and bushings—smooth, hard surface minimizes friction; tough core prevents cracking from heavy loads.
- Automotive Parts: Pinions, sprockets, and clutch hubs—withstand engine heat and repeated contact; carburized surface resists wear from daily use.
- Industrial Machinery: Chain links, conveyor rollers, and fasteners—hard surface resists dirt/abrasion; tough core handles impact from material handling.
- Aerospace Components: Landing gear pins, engine accessory gears, and fasteners—carburized surface resists wear; tough core tolerates takeoff/landing stress.
3. Manufacturing Techniques for AISI 8620 Alloy Steel
Producing AISI 8620 focuses on carburizing to create a “hard surface + tough core” structure. Here’s the step-by-step process:
- Steelmaking:
- AISI 8620 is made using an Electric Arc Furnace (EAF) (recycles scrap steel) or Basic Oxygen Furnace (BOF). Nickel (0.40–0.70%), chromium (0.40–0.60%), and molybdenum (0.15–0.25%) are added during melting to ensure uniform alloy distribution.
- Rolling & Forging:
- The steel is Hot Rolled (1,100 – 1,200 °C) into bars, plates, or tubes—hot rolling makes it easy to form. For complex parts (e.g., gear blanks), it’s Hot Forged to shape, then annealed to soften (18–22 HRC) for machining.
- Machining:
- Annealed AISI 8620 is machined into near-final shapes (e.g., gear teeth blanks) using turning, milling, or drilling. HSS tools work well for most cuts; carbide tools are used for tight tolerances.
- Carburizing (Critical Step):
- Gas Carburizing: Parts are heated to 880–920 °C in a carbon-rich gas (e.g., methane) for 4–12 hours (longer = thicker hard layer). Carbon diffuses into the surface (0.8–1.0% C), while the core remains low-carbon (0.18–0.23% C).
- Quenching: After carburizing, parts are cooled to 830–850 °C, held briefly, then quenched in oil. This hardens the surface to 58–60 HRC and the core to 30–35 HRC.
- Tempering: Parts are reheated to 180–220 °C for 1–2 hours, then air-cooled. This reduces surface brittleness without lowering hardness.
- Surface Treatment:
- Grinding: Carburized parts are ground to smooth the surface (removes oxidation) and achieve final tolerances (e.g., gear tooth precision).
- Plating: Zinc plating (rust resistance) for outdoor parts; chrome plating (extra wear resistance) for high-friction parts.
- Shot Peening: Optional—blasts the surface with small metal balls to reduce residual stress and boost fatigue limit.
- Quality Control:
- Chemical Analysis: Spectrometry verifies nickel, chromium, and molybdenum levels (per ASTM A29/A29M).
- Hardness Testing: Rockwell testing checks surface (58–60 HRC) and core (30–35 HRC) hardness.
- Microstructural Analysis: Optical microscopy confirms a uniform carburized layer (no gaps or uneven carbon distribution).
- NDT: Ultrasonic testing checks for internal defects; magnetic particle inspection finds surface cracks from carburizing.
4. Case Studies: AISI 8620 in Action
Real projects show how AISI 8620 solves “wear + impact” challenges.
Case Study 1: Automotive Transmission Gears (Germany)
A car manufacturer needed transmission gears that could resist tooth wear and absorb shift shock. They switched from AISI 1045 carbon steel to carburized AISI 8620 gears. The AISI 8620 gears lasted 200,000 km—double the lifespan of 1045 gears—because the carburized surface (59 HRC) prevented tooth pitting, and the tough core (32 HRC) absorbed shift impact. This reduced warranty claims by 40%.
Case Study 2: Industrial Conveyor Shafts (U.S.)
A warehouse had to replace conveyor shafts every 2 years due to surface wear and bending cracks. They used AISI 8620 shafts, carburized to 58 HRC and shot-peened. The new shafts lasted 5 years—no wear or cracks—because the hard surface resisted abrasion from dirt, and the tough core handled conveyor loads. This saved the warehouse $35,000 in replacement costs.
5. AISI 8620 vs. Other Materials
How does AISI 8620 compare to other carburizable and wear-resistant steels?
| Material | Similarities to AISI 8620 | Key Differences | Best For |
|---|---|---|---|
| AISI 4140 | Cr-Mo alloy steel | Higher carbon (0.38–0.43%); not ideal for carburizing (core too hard); 15% cheaper | Non-carburized, medium-wear parts |
| AISI 8630 | Ni-Cr-Mo alloy steel | Higher carbon (0.28–0.33%); harder core after carburizing; 10% pricier | Heavy-load carburized parts (e.g., truck gears) |
| AISI 1018 | Low-carbon steel | No alloying; poor carburized surface strength; 30% cheaper | Low-wear, low-load carburized parts |
| 52100 Bearing Steel | High-carbon steel | Better wear resistance; no nickel (poor toughness); 20% pricier | Precision bearings (no impact) |
| Stainless Steel 410 | Corrosion-resistant | Carburizable; better rust resistance; 3× pricier | Wet-environment carburized parts |
Yigu Technology’s Perspective on AISI 8620 Alloy Steel
At Yigu Technology, AISI 8620 is our top pick for carburized “tough core + hard surface” parts. Its low-carbon Ni-Cr-Mo composition solves the biggest pain point for clients: getting wear resistance without sacrificing impact toughness—critical for gears, shafts, and fasteners. We supply AISI 8620 in bars, forged blanks, or machined components, with custom carburizing (0.5–1.2 mm layer thickness) and shot peening. For clients moving from plain carbon steel, AISI 8620 delivers 3–4x longer part life at a small cost premium—saving money on maintenance and downtime.
FAQ About AISI 8620 Alloy Steel
- How thick is the carburized layer on AISI 8620?
Typical layers are 0.5–1.2 mm thick—adjusted by carburizing time (4 hours = ~0.5 mm; 12 hours = ~1.2 mm). For parts like gears, a 0.8–1.0 mm layer balances wear resistance and flexibility; for shafts, 0.5–0.7 mm avoids surface cracking. - Can AISI 8620 be used without carburizing?
Yes—but it’s not ideal. Non-carburized AISI 8620 has low strength (600 MPa tensile) and wear resistance, so it’s only used for low-load parts (e.g., brackets). Carburizing unlocks its full potential for wear and impact. - Is AISI 8620 suitable for low-temperature applications?
Yes—its nickel content maintains impact toughness at -30 °C (even after carburizing). For temperatures below -30 °C (e.g., arctic machinery), choose AISI 8640 (higher nickel) for extra low-temperature toughness.
