SAE 8620H Alloy Steel
SAE 8620H is a widely used carburizing alloy steel known for its excellent combination of toughness, wear resistance, and hardenability. The “H” in its designation indicates it is produced to hardenability limits, ensuring consistent heat treatment performance across batches. It is widely applied in industries requiring high surface hardness and core toughness, such as automotive, machinery, and aerospace.
Basic Information
Definition: SAE 8620H is a low-carbon nickel-chromium-molybdenum alloy steel designed for carburizing heat treatment. It develops a hard, wear-resistant surface while retaining a tough, ductile core after processing.
Corresponding Standards: Conforms to SAE J404 (USA) and is equivalent to AISI 8620H. Internationally, it corresponds to DIN 1.6523 (Germany) and GB 20CrNiMo (China, with slight compositional variations).
Key Characteristics: Excellent carburizing response, good machinability in the annealed state, and balanced mechanical properties (high surface hardness + core toughness).
Chemical Composition
The composition of SAE 8620H is tightly controlled to ensure consistent performance, with typical ranges as follows:
Element Content Range Role in the Steel
Carbon (C) 0.18–0.23% Low carbon content ensures core toughness; allows deep carburizing for surface hardness.
Manganese (Mn) 0.70–0.90% Improves hardenability and strengthens the matrix.
Phosphorus (P) ≤0.035% Harmful impurity; controlled to avoid brittleness.
Sulfur (S) ≤0.040% Controlled to prevent reduced toughness and hot cracking.
Silicon (Si) 0.15–0.35% Enhances strength and oxidation resistance during heat treatment.
Nickel (Ni) 0.40–0.70% Improves toughness, ductility, and corrosion resistance; enhances carburizing depth.
Chromium (Cr) 0.40–0.60% Boosts hardenability, wear resistance, and carburizing efficiency.
Molybdenum (Mo) 0.15–0.25% Enhances high-temperature strength, creep resistance, and hardenability; reduces temper brittleness.
Physical Properties
Density: Approximately 7.85 g/cm³ (same as most steels).
Melting Point: Around 1425–1475°C.
Thermal Conductivity: ~42 W/(m·K) at room temperature.
Coefficient of Thermal Expansion: ~12.0×10⁻⁶/°C (20–100°C).
Elastic Modulus: ~207 GPa.
Mechanical Properties
Mechanical properties vary significantly with heat treatment, especially carburizing. Key properties after typical processing:
As Annealed (for Machining)
Hardness: ≤207 HB (easy to machine).
Tensile Strength: ~550 MPa.
Yield Strength: ~300 MPa.
Elongation: ~25% (good ductility for forming).
After Carburizing + Quenching + Tempering
Surface Hardness: 58–63 HRC (wear-resistant surface layer, 0.5–1.5 mm deep depending on carburizing time).
Core Hardness: 30–45 HRC (tough core to resist impact and fatigue).
Tensile Strength: ≥1000 MPa (core strength).
Yield Strength: ≥800 MPa (core yield).
Impact Toughness: ≥40 J/cm² (core toughness, critical for shock resistance).
Fatigue Strength: ~500 MPa (excellent resistance to cyclic loading, critical for rotating parts).
Heat Treatment Process
SAE 8620H is primarily used in carburizing applications to optimize surface and core properties. Typical heat treatment steps:
Carburizing
Heating temperature: 900–950°C.
Process: Parts are exposed to a carbon-rich atmosphere to diffuse carbon into the surface (0.8–1.2% C at the surface).
Purpose: Create a high-carbon surface layer for subsequent hardening.
Quenching
After carburizing, parts are rapidly cooled (oil-quenched) from 820–850°C to form martensite in the surface layer, achieving high hardness.
Tempering
Temperature: 150–200°C (low-temperature tempering).
Purpose: Reduce internal stress from quenching while retaining surface hardness; improve toughness.
Annealing (for Machining)
Heating to 830–870°C, slow cooling to soften the steel (hardness ≤207 HB), improving machinability.
Processing Performance
Machinability: Excellent in the annealed state due to low hardness; suitable for turning, milling, drilling, and grinding. Higher hardness after heat treatment increases cutting resistance.
Weldability: Good weldability with proper preheating (200–300°C) and post-weld heat treatment to avoid cold cracking.
Formability: Moderate cold formability in the annealed state; can be forged at 1100–1200°C.
Application Fields
SAE 8620H is valued for its ability to balance surface wear resistance and core toughness, making it ideal for components subject to friction, impact, and cyclic loads:
Automotive Industry:
Transmission gears, axle gears, and differential gears (requires wear resistance and impact toughness).
Crankshafts, camshafts, and piston pins.
Mechanical Engineering:
Roller chains, sprockets, and bearing rings.
Shafts and gears in industrial machinery.
Aerospace:
Small precision gears and fasteners in aircraft systems.
Oil & Gas:
Drill bits and downhole tool components (resists wear and shock).
In summary, SAE 8620H is a versatile carburizing alloy steel that excels in applications demanding a hard, wear-resistant surface paired with a tough core, making it a staple in high-performance mechanical components.