Materials for Automotive Components — Crankshaft Steels In automobile (motor car) engines, the crankshaft must withstand high torsion, bending, and fatigue. Which steel is most widely used for manufacturing motor car crankshafts?

Introduction / Context:Crankshafts in automotive engines experience severe cyclic bending and torsional loads at high rotational speeds. The material must provide a blend of core toughness, surface hardenability (for journals), fatigue resistance, and dimensional stability during heat treatment. This question targets the standard material choice used in production automotive crankshafts.

Given Data / Assumptions:

• Passenger-car size crankshafts subjected to high-cycle fatigue.
• Need for case/induction hardening at journals and fillets.
• Mass-production requirements (forgeability and heat-treatment response).

Concept / Approach:Nickel improves toughness and hardenability; chromium enhances wear resistance and through-hardening; their combination (Ni–Cr steels like 34CrNiMo6/EN24 variants) offers an excellent balance for crankshafts. These steels take surface hardening well (induction/flame or nitriding on specific grades) and maintain a tough core to resist shock and fatigue.

Step-by-Step Solution:Identify the primary failure mode: high-cycle fatigue under combined bending and torsion.Map desired properties: high hardenability + wear resistance + core toughness.Select alloying system: Ni adds toughness; Cr adds wear and hardenability → Ni–Cr steel fits best.Conclude: nickel–chrome steel is most widely adopted for motor car crankshafts.

Verification / Alternative check:Production practice commonly specifies Ni–Cr or Ni–Cr–Mo forgings for passenger vehicles, with induction-hardened journals and fillets to raise fatigue strength.

Why Other Options Are Wrong:

• Nickel steel: tough but lacks the wear/hardenability balance without Cr.
• Chrome steel: improves wear but may not provide sufficient toughness alone.
• Silicon steel: used for electrical laminations, not crankshafts.
• Manganese steel: very tough austenitic wear steel (Hadfield) used in impact wear parts, not precise rotating shafts.

Common Pitfalls:Assuming surface hardness alone guarantees fatigue life; crankshafts need a tough core plus hardened surfaces and carefully filleted geometries.

Final Answer:nickel-chrome steel