Steel 40CDV12 - 40CrMoV12 - FE-PL1507

Nitriding steel 40CDV12

The 40CDV12 steel is a low-alloy chromium-molybdenum-vanadium steel used for its nitriding capability. Undesirable elements in its chemical composition (phosphorus, sulfur) are kept at very low levels (≤0.02% each). This composition provides excellent hardenability and allows the formation of hard nitrides on the surface during thermochemical treatments (thanks to Cr and V). There is also the E40CDV12, a version produced under vacuum using VAR remelting. This process is chosen to achieve better chemical homogeneity, directional solidification, and especially effective reduction of dissolved gases (O, N, H). By avoiding impurities, the alloy becomes more resistant to fatigue : therefore, E40CDV12 is preferred for certain critical applications.

Mechanical properties

Once heat-treated (quenched and tempered), 40CDV12 achieves high performance with tensile strength between 900 and 1100 MPa (up to 1470 MPa in higher-quality grades). Its yield strength ranges around 700 to 1050 MPa, and it typically presents elongation at break of 12 to 15%.

However, the main advantage of 40CDV12 is the very high hardness achieved after nitriding: the nitrided surface layer can reach ~800 HV, in some optimized cases, providing excellent wear resistance.

Heat and surface treatments

40CDV12 is optimized for a combination of heat treatments and nitriding:

• Soft annealing: Heating to 680–720 °C, slow cooling to facilitate machining. This condition facilitates machining of the raw part due to a softened ferritic-pearlitic structure.
• Quenching: Austenitization at 870–970 °C followed by rapid cooling (preferably in oil). Quenching forms hard martensite.
• Tempering: At 580–700 °C depending on desired mechanical properties (lower temperature = higher strength, higher temperature = better toughness).
• Nitriding: Thermochemical treatment at 480–570 °C under a nitrogen-rich atmosphere (gas or plasma) forming a hard surface layer for better wear resistance (typical thickness of 0.3–0.6 mm). Nitriding significantly enhances surface wear and contact fatigue resistance (pitting), while maintaining a relatively ductile core.
• Other possible treatments (light oxidation, PVD coating) depending on specific requirements.

Comparison with other aeronautical steels

Compared to Ni-Cr-Mo steels (30NCD16, 35NCD16), it offers improved wear resistance but has lower weldability and slightly reduced toughness when seeking extreme strengths. 40CDV12 is ideal when surface hardness and wear resistance are prioritized, whereas other grades would be selected based on welding, corrosion resistance, or very high bulk strength criteria.

Examples include:

• 32CDV13: Lower carbon content, better toughness, slightly lower strength, similar excellent nitriding performance.
• 35NCD16: Very high internal strength, better core toughness but less suitable for deep nitriding (lower surface hardness).
• 15CDV6: Excellent weldability and ductility, similar mechanical strength, but limited nitriding capability.
• Maraging and PH stainless steels: Extreme strength or corrosion resistance but less suitable for surface nitriding.

Machinability and processing

The processing of 40CDV12 requires certain precautions but remains comparable to other heat-treatable alloy steels.

Its machinability is satisfactory in the annealed condition, and forming requires the same care as other steels (control of treatments, avoiding overheating and overly aggressive treatments). It becomes more challenging to work after heat treatment.

Careful forging at high temperature (1000–1150 °C) followed by controlled cooling is recommended.

The main caution relates to welding, which should be avoided or performed with strict precautions. Welding requires preheating and post-weld heat treatment.

Finally, nitriding demands particular attention to avoid excessive embrittlement of the surface layer.

Advantages and limitations

Advantages:

• Excellent combination of surface hardness and core strength.
• High wear and fatigue resistance.
• Good dimensional stability up to ~300 °C.
• Well-established steel with a solid history in aeronautics.

Limitations:

• Difficulty in welding.
• Critical requirement for well-controlled nitriding.
• Slightly lower toughness compared to other Ni-Cr-Mo alloy steels.