4340 Steel

AISI 4340 is the premier Nickel-Chromium-Molybdenum alloy steel, renowned for its incredible combination of strength, toughness, and fatigue resistance. Its defining characteristic is its high "hardenability," allowing it to achieve uniform high strength in very thick sections. At Jucheng Precision, we specialize in machining 4340 for mission-critical applications in the Aerospace and Defense sectors. We provide a comprehensive service including heavy-duty CNC milling, high-precision grinding, and managed heat treatment to ensure your structural components meet the most demanding flight and safety specifications.

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4340 Alloy Steel

4340 Steel Machining Capabilities

Capability Technical Description Jucheng Advantage
Heavy Section Machining 4340 maintains its high strength even in very thick sections. We machine large diameter drive components from solid forged billets. Our high-torque Mazak CNC centers handle the extreme toughness of 4340, maintaining dimensional stability across large workpieces.
Through-Hardened Finishing Machining 4340 after it has been heat-treated to 36-40 HRC for maximum fatigue life and strength. We use specialized ceramic and CBN tooling to cut through hardened 4340, providing finished parts with superior surface integrity.
5-Axis Structural Milling Milling complex, high-stress structural airframe components with intricate geometries. Jucheng’s 5-axis capability ensures that complex aerospace fittings are machined in minimal setups, guaranteeing geometric alignment.
Super-Precision Grinding Achieving Ra 0.2μm finishes on critical bearing and seal journals for high-speed rotating parts. Essential for landing gear and crankshafts where surface defects could lead to catastrophic fatigue failure.

Fabrication Guidelines for 4340 Alloy Steel

Ultimate Deep Hardenability:
4340 is famous for its “deep hardening” characteristics due to the Nickel content. It can be hardened to high levels in sections where 4140 would fail. Tip: Ideal for large, critical shafts over 3 inches in diameter.

Machinability Rating:
4340 is tougher and more abrasive than 4140. Machinability is approx. 55% of 1212. Tip: We use lower cutting speeds and higher-rigidity workholding to prevent tool chatter on this tough alloy.

Hydrogen Embrittlement Risk:
High-strength 4340 is susceptible to hydrogen embrittlement during plating. Tip: If parts are Zinc or Cadmium plated, we perform mandatory “bake-out” cycles immediately after plating to ensure structural safety.

Heat Treatment Precision:
To reach peak performance, 4340 requires oil quenching. We recommend normalized and stress-relieved stock to minimize movement during the quenching process.

Primary Applications

Aerospace & Aviation
Aircraft landing gear, structural airframe members, and engine attachment fittings where failure is not an option.

Heavy Power Transmission
High-torque drive shafts, heavy-duty gears, and crankshafts for commercial and military marine engines.

Defense & Ordinance
Gun barrels, breech blocks, and high-strength fasteners that must withstand extreme shock and impact loads.

Motorsport
Ultra-high performance connecting rods and axles for racing engines that operate under extreme cyclic stress.

Properties

Density 7.85 g/cm³
Tensile Strength 1100 - 1250+ MPa
Yield Strength 860 - 1000+ MPa
Hardness 36 - 40 HRC
Elongation at Break 12 - 15%
Thermal Conductivity 44.5 W/m-K
Fatigue Strength Superior
Machinability Rating 55%
Weldability Poor
Hardenability Very High
Max Part Size 2000 x 800 x 600 mm