Crankshaft Material 40Cr vs. 42CrMo

1. 40Cr (Induction Hardened)

· Physical Properties & Microstructure:

· Base Condition: Quenched and tempered (Tempered Sorbite), hardness 270–300 HB.

· Surface Hardening: Induction hardening + low-temperature tempering, surface hardness 42–48 HRC.

· Case Depth: Approx. 2–3 mm (deep, compressive residual stress).

· Core Strength: Tensile strength ≥ 736 MPa, good toughness.

· Effect on Diesel Engine Performance:

· Impact Resistance: Excellent. The deep hardened layer allows the crankshaft to withstand sudden shock loads (e.g., frequent start-stops or abrupt loading in non-road machinery).

· Wear Resistance: Good, particularly in dusty or poor-lubrication conditions due to the thick case.

· Fracture Sensitivity: Moderate. Stress concentration at fillet radii is critical; improper assembly or re-grinding can initiate cracks from the hardened layer.

· Cost & Application: Low cost, mature process. Suitable for general-purpose, cost-sensitive air-cooled diesel engines prioritizing durability under harsh conditions.

2. 42CrMo (Nitrided)

· Physical Properties & Microstructure:

· Base Condition: Quenched and tempered, hardness approx. 260–280 HB.

· Surface Hardening: Plasma (ion) nitriding, surface hardness 750–900 HV (~62–66 HRC).

· Case Depth: Approx. 0.3–0.5 mm (thin, very high compressive residual stress).

· Core Strength: High tensile strength with excellent support for the thin nitrided layer.

· Effect on Diesel Engine Performance:

· Wear & Fatigue Resistance: Excellent. The extremely hard surface significantly improves high-cycle fatigue strength and resists scuffing and seizure, ideal for high-speed or turbocharged engines.

· Friction Efficiency: Lower friction losses contribute to better mechanical efficiency.

· Thermal & Contamination Sensitivity: High. The thin case softens above ~450°C (e.g., during bearing seizure). Also less tolerant to hard particles (e.g., metal debris), which can penetrate the thin layer.

· Dimensional Stability: Excellent (low process temperature), virtually no distortion. Suitable for high-performance, high-reliability engines (e.g., aviation or racing-grade diesels).

Summary

· 40Cr (Induction Hardened): Tough, impact-resistant, and economical. Best for standard air-cooled diesels where shock loads and low maintenance costs are priorities.

· 42CrMo (Nitrided): Siwei Power's Choice, precision, high load capacity, and low friction. Best for high-power-density, long-service-life engines where mechanical efficiency and fatigue limits are critical.