How Do the Chemical Elements of 4340 Steel Affect Its Properties?

4340 steel is a high-strength, high-toughness nickel-chromium-molybdenum alloy structural steel, belonging to the category of ultra-high-strength steel. Its excellent comprehensive performance (especially in the quenched and tempered state) makes it widely used in key fields such as aviation, defense, and energy. The following is a systematic analysis of the performance of 4340 steel by the main chemical elements:

4340 Steel Core Chemical Elements & Functions

1. Carbon (C) :0.38-0.43%

Core function: basic strengthening element, ensuring hardness and strength.
Influence:

• Strength and hardness: Provides basic strength by forming martensite and carbide, and the hardness can reach HRC 55+ after quenching.
• Hardenability: Sufficient carbon is a prerequisite for obtaining full martensite.
• Cost: When the carbon content approaches the upper limit (0.43%), the weldability decreases significantly and strict preheating is required; excessive carbon will reduce toughness.

2. Nickel (Ni) :1.65-2.00% (Core difference between 4340 and 4140)
Core function: Influence: Toughening element, improving low temperature toughness, hardenability and fatigue resistance.
Influence:

• Toughness improvement:
  – Significantly reduce the ductile-brittle transition temperature (DBTT) and improve low temperature impact toughness (maintain high toughness at -40℃).
  – Inhibit crack propagation and improve fracture toughness (key requirement for aviation landing gear and rocket shell).
• Hardenability: Synergizes with chromium and molybdenum to significantly increase the critical diameter (oil quenching critical diameter ≈100mm, higher than ~75mm of 4140).
• Solid solution strengthening: Enhance matrix strength while maintaining good plasticity.
• Corrosion resistance: Slightly improve atmospheric corrosion resistance.

3. Chromium (Cr) :0.70-0.90%
Core function:Hardenability enhancer, auxiliary for wear resistance and corrosion resistance.
Influence:

• Hardenability: Form carbides (Cr₇C₃), which improve the stability of austenite after dissolution, and synergize with Ni and Mo to strengthen hardenability.
• Wear resistance/hardness: Hard carbides improve surface wear resistance.
• High temperature strength: Carbides delay tempering softening (but weaker than molybdenum).

4. Molybdenum (Mo) :0.20-0.30%
Core function: Anti-tempering softening, brittleness suppression, hardenability booster.
Influence:

• Anti-tempering softening:
  – Fine Mo₂C carbides are precipitated during tempering at 500-600℃, producing a secondary hardening effect to maintain high temperature strength (such as engine crankshaft requirements).
• Inhibit temper brittleness:
  – Effectively block the segregation of impurities such as phosphorus at the grain boundary, and avoid the sharp drop in toughness in the 350-575℃ temper brittleness zone (4340 high nickel content increases the risk of brittleness, and the addition of molybdenum is crucial).
• Hardenability: Strongly improve the uniformity of hardness in the core of large cross-sections.

5. Manganese (Mn) :0.60-0.80%
Core function: Hardenability auxiliary element, deoxidation solid solution strengthening.
Influence:

• Hardenability: Expand the γ phase area, stabilize austenite, and improve hardenability (but the effect is weaker than Ni and Cr).
• Strength: Solid solution strengthens ferrite.
• Deoxidation: Reduce oxide inclusions.

6. Silicon (Si) :0.15-0.35%
Core function: deoxidizer, solid solution strengthening.
Influence:

• Strength: Significantly improves yield strength (solid solution strengthening effect is stronger than manganese).
• Oxidation resistance: Improves high temperature oxidation resistance (<500℃).
• Negative impact: Too high content (>0.5%) will promote decarburization and reduce toughness.

7.Impurity element restrictions

4340 Element Synergistic Effect & Performance Summary

Key Differences Between 4340 Steel and 4140 Steel

1. Addition of nickel (Ni):
– The ultra-high toughness (especially at low temperatures) and higher hardenability of 4340 are mainly due to nickel (4140 does not contain nickel).
2. Strength-toughness balance:
– The toughness of 4340 is significantly better than that of 4140 at the same strength level (due to the toughening effect of nickel).
3. Application scenario upgrade:
– 4340 is used in extreme environments: aircraft landing gear (AMS 6414), armor plates, rocket engine casings (high stress resistance + low temperature brittle fracture).
– 4140 is mostly used for general engineering parts (gears, shafts).

Typical Heat Treatment Process and Performance of 4340 Steel

Process:
`850°C austenitization → oil quenching → tempering as required (425-650°C)`

Performance range:

• Tensile strength: 900-2000 MPa (adjusted by tempering temperature)
• Impact energy (room temperature): 40-100 J (nickel ensures high toughness under high strength)

4340 Steel Application Industry

• Aerospace: landing gear, turbine shaft, helicopter rotor
• Defense and military industry: tank crankshaft, armor parts, gun receiver
• Energy equipment: ultra-high pressure valve, key bolts of drilling platform
• Racing industry: high-performance connecting rod, transmission shaft

Summary: 4340 steel achieves the golden triangle balance of strength-toughness-hardenability through the combination of “high nickel + molybdenum controlled brittleness”, becoming the preferred material for ultra-high stress and low temperature environment. Its element design logic highlights the essence of “synergistic enhancement” in alloying.

MORE ABOUT 4340 STEEL PRODUCT