Comprehensive Comparison: 4340 vs 34CrNiMo6

34CrNiMo6 (European EN standard) and 4340 steel (American SAE/AISI standard) are both medium-carbon alloy structural steels, which are widely used in key components with high stress and high fatigue strength. The following is a comparison from multiple dimensions:

1. Chemical Composition Comparison

Elements	34CrNiMo6 (EN 10083-3)	4340 (ASTM A29)
C (Carbon)	0.30-0.38%	0.38-0.43%
Cr (Chromium)	1.30-1.70%	0.70-0.90%
Ni (Nickel)	1.30-1.70%	1.65-2.00%
Mo (Molybdenum)	0.15-0.30%	0.20-0.30%
Mn (Manganese)	0.50-0.80%	0.60-0.80%
Si (Silicon)	≤0.40%	0.15-0.35%
Other	Trace S and P impurities	Trace S and P impurities

Key differences:

Carbon content: 4340 has higher carbon content, better hardenability, but slightly lower toughness.
Nickel content: 4340 has higher nickel content, which improves low-temperature toughness; 34CrNiMo6 has higher chromium content and slightly better corrosion resistance.

2. Comparison of Mechanical Properties (quenched + tempered)

Parameters	34CrNiMo6	4340
Tensile strength (MPa)	900-1100	930-1080
Yield strength (MPa)	≥700	≥785
Elongation (%)	≥12	≥12
Impact toughness (J, 20℃)	≥40 (V-notch)	≥35 (Charpy impact)
Hardness (HRC)	28-32 (quenched and tempered)	28-34 (quenched and tempered)

Key differences:

Low-temperature toughness: 34CrNiMo6 has better corrosion resistance and high-temperature strength due to its higher chromium content; 4340 has better impact toughness at low temperatures (below -40°C) due to its high nickel content.
Fatigue strength: 4340 is slightly higher and suitable for high-cycle fatigue applications (such as aviation components).

3. Heat Treatment Process

34CrNiMo6:
– Quenching: 850-880℃ oil quenching.
– Tempering: 540-680℃ (adjusted according to strength requirements), air cooling.
– Surface carburizing or nitriding can be performed to enhance wear resistance.
4340:
– Quenching: 815-845℃ oil quenching or water quenching (depending on the cross-sectional size).
– Tempering: 425-650℃, air cooling.
– Be aware of temper brittleness (avoid long stays in the 300-400℃ range).

Key differences:
4340 is easier to achieve deep hardening (suitable for large cross-section parts), but the tempering process needs to be strictly controlled.

4. Application Fields

34CrNiMo6:
– Heavy gears, crankshafts, high-load bolts, ship drive shafts.
– European automotive/heavy industry (such as German DIN standard applications).
4340:
– Aircraft landing gear, connecting rods, oil drill pipes, military equipment.
– American aviation and military fields (such as Boeing, Lockheed Martin commonly used).

5. Corrosion Resistance

Neither of them is stainless steel, has limited chromium content, and has average corrosion resistance.
34CrNiMo6 has slightly better oxidation resistance in humid environments than 4340 due to its higher chromium content.
Corrosion resistance needs to be improved by plating (chrome plating, nickel plating) or painting.

6. Machinability & Welding Performance

Machinability: Both can be machined in the annealed state, and 4340 has slightly faster tool wear due to its high carbon content.
Weldability:
– Both require preheating (200-300℃) and post-heat treatment to avoid cracks.
– 34CrNiMo6 has slightly poorer weldability (higher alloy content leads to greater hardening tendency).

7. Cost & Supply

Raw material cost: 4340 is usually 10-15% higher than 34CrNiMo6 due to its high nickel content.
Regional supply: 34CrNiMo6 is more popular in Europe, and 4340 is more common in North America and Asia Pacific.

Summary & Material Selection Recommendations

Choose 34CrNiMo6:
– Requires high corrosion resistance or high temperature strength (≤400℃).
– European standard projects or cost-sensitive industrial parts.
Choose 4340:
– Low temperature environment (such as polar equipment) or large cross-section high hardenability requirements.
– Aviation/military fields or projects that follow US standards.

Note: The final material selection needs to be combined with specific working conditions (load, temperature, corrosion environment) and local supply chain capabilities.