40CrNiMoA Alloy Steel
40CrNiMoA is a high-performance alloy structural steel known for its exceptional strength, toughness, and hardenability. It belongs to the chromium-nickel-molybdenum alloy system, which imparts excellent mechanical properties after heat treatment, making it ideal for high-stress components in automotive, aerospace, and machinery industries.
Basic Information
Definition: 40CrNiMoA is a low-alloy steel strengthened by chromium, nickel, and molybdenum, designed to achieve high tensile strength, fatigue resistance, and toughness through quenching and tempering. The “A” suffix indicates it meets strict quality requirements for purity and uniformity, suitable for critical applications.
Corresponding Standards: Conforms to GB/T 3077-2015 (Alloy structural steels) in China. International equivalents include:
American: AISI 4340
European: 1.6511 (DIN)
Japanese: SNCM439 (JIS)
Key Characteristics: High strength (up to 1400 MPa after heat treatment), excellent impact toughness, good fatigue resistance, and superior hardenability (can be fully hardened in thick sections).
Chemical Composition
The precise composition of 40CrNiMoA balances strength, toughness, and heat treatment response:
Element Content Range Role in the Steel
Carbon (C) 0.37–0.44% Provides a foundation for hardness and strength; balanced to avoid brittleness.
Chromium (Cr) 0.60–0.90% Enhances hardenability, wear resistance, and corrosion resistance; forms stable carbides.
Nickel (Ni) 1.25–1.65% Significantly improves toughness and impact resistance, especially at low temperatures; aids in hardenability.
Molybdenum (Mo) 0.15–0.25% Boosts hardenability, high-temperature strength, and creep resistance; reduces temper brittleness.
Silicon (Si) 0.17–0.37% Acts as a deoxidizer; improves strength and oxidation resistance.
Manganese (Mn) 0.50–0.80% Enhances hardenability and strength; refines grain structure.
Phosphorus (P) ≤0.025% Strictly controlled to prevent brittleness, critical for high-toughness applications.
Sulfur (S) ≤0.025% Minimized to avoid hot cracking during forging or welding and reduce toughness loss.
Physical Properties
Density: ~7.85 g/cm³ (consistent with most alloy structural steels).
Melting Point: ~1430–1480°C.
Thermal Conductivity: ~40 W/(m·K) at room temperature.
Coefficient of Thermal Expansion: ~12×10⁻⁶/°C (20–100°C), ensuring dimensional stability in temperature cycles.
Elastic Modulus: ~206 GPa, providing rigidity in high-load applications.
Mechanical Properties
40CrNiMoA’s mechanical properties are transformed by heat treatment (quenching + tempering), unlocking its full potential for high-stress applications:
Property Annealed State Quenched & Tempered (Typical)
Yield Strength (Rp0.2) ~300 MPa ≥930 MPa
Tensile Strength (Rm) 600–800 MPa 1080–1470 MPa
Elongation (A) ≥19% ≥12%
Reduction of Area (Z) ≥45% ≥55%
Impact Toughness (Akv) ~60 J ≥63 J (at 20°C, V-notch)
Hardness ~200 HB 28–32 HRC (tempered at 600°C); up to 50 HRC at lower tempering temps.
Fatigue Resistance: Excellent—endures cyclic loads in applications like crankshafts and gears (fatigue limit ~500–600 MPa).
Toughness: Maintains high impact resistance even at sub-zero temperatures, making it suitable for cold environments.
Heat Treatment
Heat treatment is critical to maximize 40CrNiMoA’s performance, with quenching and tempering tailored to application requirements:
Annealing
Purpose: Soften the steel for machining and homogenize the microstructure.
Process: Heat to 820–840°C, hold 2–4 hours, cool slowly (≤30°C/hour) to 600°C, then air cool. Results in hardness ≤229 HB.
Quenching
Temperature: 850–880°C (austenitization), held 1–2 hours (based on thickness).
Cooling: Oil quenching to form martensite, achieving as-quenched hardness of 50–55 HRC.
Tempering
Temperature Range: 200–650°C (adjusted for desired strength-toughness balance).
Common Practice: Temper at 550–600°C for 2–3 hours (double tempering for large parts) to achieve 28–32 HRC with optimal toughness.
Effect: Reduces brittleness, relieves internal stress, and stabilizes dimensions.
Processing Performance
Weldability: Moderate—requires preheating (150–250°C) and post-weld tempering to prevent cold cracking, due to high hardenability. Low hydrogen welding methods are recommended to avoid hydrogen-induced cracking.
Machinability: Good in the annealed state (≤229 HB) with high-speed steel or carbide tools. Hardened 40CrNiMoA (HRC ≥30) is more challenging, requiring sharp tools and proper cooling.
Forging Performance: Excellent—forged at 1100–1200°C, followed by slow cooling to avoid cracking. Forging refines the grain structure, enhancing mechanical properties.
Formability: Limited cold formability due to high strength; typically hot-formed or machined to final shape.
Application Fields
40CrNiMoA is the material of choice for high-stress, high-reliability components across industries:
Automotive & Transportation:
Engine crankshafts, connecting rods, and transmission gears (resist cyclic loads and impact).
Heavy-duty truck axles, drive shafts, and suspension components.
Aerospace & Defense:
Aircraft landing gear parts, engine mounts, and structural components (require high strength-to-weight ratios).
Missile and weapon system components needing toughness and fatigue resistance.
Mechanical Engineering:
High-load gears, shafts, and bolts in industrial machinery.
Hydraulic cylinder rods, press components, and power transmission parts.
Energy Industry:
Drill collars, wellhead components, and high-pressure valves in oil and gas extraction.
Turbine shafts and generator components in power plants.
Advantages and Limitations
Advantages
Excellent Strength-Toughness Balance: Combines high tensile strength with exceptional impact toughness, rare in many high-strength steels.
Superior Hardenability: Can be fully hardened in thick sections (up to 100mm), ensuring uniform properties.
Good Fatigue Resistance: Withstands repeated cyclic loads, critical for rotating or vibrating components.
Low-Temperature Performance: Maintains toughness at sub-zero temperatures, suitable for cold environments.
Limitations
Higher Cost: More expensive than carbon steels (e.g., 45#) or low-alloy steels (e.g., 40Cr) due to nickel and molybdenum content.
Welding Complexity: Requires pre- and post-weld heat treatment, increasing production time and costs.
Corrosion Susceptibility: Poor resistance to rust and chemicals; requires protective coatings (painting, plating) for outdoor use.
Comparison with Similar Steels
Steel Grade Key Alloy Elements Tensile Strength (Quenched/Tempered) Toughness (Akv at 20°C) Typical Use Case
40CrNiMoA Cr, Ni, Mo 1080–1470 MPa ≥63 J High-stress components (crankshafts, landing gear).
40Cr Cr 800–1000 MPa ≥45 J General machinery parts (shafts, gears).
4340 (AISI) Cr, Ni, Mo 1030–1600 MPa ≥54 J Aerospace and high-performance automotive parts.
30CrMnSiA Cr, Mn, Si 1080–1380 MPa ≥47 J Structural parts requiring high strength.
In summary, 40CrNiMoA alloy steel is a premium material for critical applications demanding high strength, toughness, and fatigue resistance. Its unique combination of properties makes it indispensable in industries where component failure could have severe consequences, solidifying its role as a high-performance structural alloy.