SAE 4340 Alloy Steel
SAE 4340 is a high-strength nickel-chromium-molybdenum alloy steel renowned for its exceptional toughness, high tensile strength, and excellent hardenability. It is widely regarded as a workhorse in industries requiring components to withstand heavy loads, impact, and cyclic stress, such as aerospace, automotive, and heavy machinery.
Basic Information
Definition: SAE 4340 is a medium-carbon alloy steel containing nickel, chromium, and molybdenum, formulated to achieve superior strength and toughness through heat treatment. Its balanced chemistry allows for deep hardening, making it suitable for large or thick-section parts.
Corresponding Standards: Conforms to SAE J404 (USA) and AISI 4340. Internationally, it is equivalent to DIN 1.6563 (Germany), GB 40CrNiMoA (China), and BS EN 10083-3 34CrNiMo6 (Europe).
Key Characteristics: High tensile strength (up to 1900 MPa after heat treatment), excellent toughness (even at low temperatures), good fatigue resistance, and deep hardenability.
Chemical Composition
The precise composition of SAE 4340 is tightly controlled to ensure consistent performance, with typical ranges as follows:
Element Content Range Role in the Steel
Carbon (C) 0.38–0.43% Provides the base for strength; enables martensite formation during quenching.
Manganese (Mn) 0.60–0.80% Enhances hardenability and strengthens the matrix; improves hot workability.
Phosphorus (P) ≤0.035% Harmful impurity; controlled to prevent brittleness, especially in cold conditions.
Sulfur (S) ≤0.040% Controlled to avoid reduced toughness and hot cracking during forging.
Silicon (Si) 0.15–0.35% Acts as a deoxidizer; enhances strength and oxidation resistance.
Nickel (Ni) 1.65–2.00% A key element for toughness; improves ductility and low-temperature impact resistance.
Chromium (Cr) 0.70–0.90% Boosts hardenability, wear resistance, and corrosion resistance; aids in carbide formation.
Molybdenum (Mo) 0.20–0.30% Enhances high-temperature strength, creep resistance, and hardenability; reduces temper brittleness.
Physical Properties
Density: Approximately 7.85 g/cm³ (consistent with most carbon and alloy steels).
Melting Point: Around 1426–1453°C.
Thermal Conductivity: ~42 W/(m·K) at room temperature.
Coefficient of Thermal Expansion: ~12.3×10⁻⁶/°C (20–100°C).
Elastic Modulus: ~207 GPa (similar to other high-strength steels).
Mechanical Properties
SAE 4340’s mechanical properties are highly dependent on heat treatment, with dramatic improvements after quenching and tempering. Key properties include:
Annealed State (for Machining)
Hardness: ≤217 HB (easily machinable with conventional tools).
Tensile Strength: ~655 MPa.
Yield Strength: ~345 MPa.
Elongation: ~25% (good ductility for forming).
After Quenching + Tempering (Typical Heat Treatments)
Tempering Temperature Tensile Strength Yield Strength Elongation Hardness Toughness (Impact Energy)
200°C (392°F) ~1900 MPa ~1600 MPa ~10% 57–60 HRC ~20 J (low-temp temper, high strength)
425°C (800°F) ~1450 MPa ~1300 MPa ~15% 45–48 HRC ~40 J (balanced strength/toughness)
650°C (1200°F) ~930 MPa ~760 MPa ~22% 2828–32 HRC ~80 J (high toughness, moderate strength)
Fatigue Strength: ~550–700 MPa (depending on surface finish and heat treatment), making it resistant to cyclic loading.
Notch Toughness: Excellent, even at sub-zero temperatures (e.g., -40°C), due to nickel content.
Heat Treatment Process
SAE 4340’s performance is maximized through quenching and tempering, leveraging its deep hardenability:
Austenitizing (Heating for Quenching)
Temperature: 820–860°C (1500–1580°F).
Purpose: Transform the microstructure to austenite, ensuring uniform carbon distribution for subsequent hardening.
Quenching
Cooling Medium: Oil-quenched (for thick sections) or water-quenched (for maximum hardness in thin parts).
Result: Forms martensite, a hard but brittle microstructure, requiring tempering to relieve stress.
Tempering
Temperature Range: 200–650°C (392–1200°F), depending on desired properties.
Low Temperatures (200–300°C): Retain high strength and hardness but with lower toughness.
Medium Temperatures (400–500°C): Balance strength and toughness (most common for structural applications).
High Temperatures (600–650°C): Prioritize toughness and ductility over maximum strength.
Annealing
For improved machinability: Heat to 815–845°C (1500–1550°F), slow cool to 650°C (1200°F), then air-cool. Reduces hardness to ≤217 HB.
Processing Performance
Machinability: Good in the annealed state (≤217 HB) using high-speed steel or carbide tools. After heat treatment (high hardness), machining becomes more challenging and may require grinding or EDM.
Weldability: Moderate; preheating to 200–300°C (392–572°F) and post-weld tempering are recommended to prevent cracking, as the steel is prone to hydrogen-induced cracking.
Formability: Cold formability is limited in the annealed state due to moderate strength, but it can be hot-formed at 1100–1200°C (2012–2192°F) with good ductility.
Forging: Excellent hot forgeability; forged at 1100–1200°C, followed by slow cooling to avoid cracking.
Application Fields
SAE 4340’s unique combination of high strength, toughness, and hardenability makes it indispensable in demanding applications:
Aerospace & Defense:
Aircraft landing gear components, engine shafts, and structural parts (requires high strength and impact resistance).
Missile and rocket components, armor plate, and artillery parts.
Automotive & Racing:
High-performance crankshafts, connecting rods, and transmission gears (withstands high cyclic loads).
Racing vehicle chassis components and suspension parts.
Heavy Machinery:
Hydraulic cylinder rods, gears, and shafts for industrial equipment (handles heavy loads and wear).
Mining and construction machinery parts subject to impact and abrasion.
Tooling & Die Making:
Large dies, punches, and high-strength tool holders (requires toughness and wear resistance).
In summary, SAE 4340 is a premium high-strength alloy steel that excels in applications demanding extreme strength, toughness, and reliability under harsh conditions. Its versatility across heat treatments and broad industry adoption solidify its status as a critical material in high-performance engineering.