Automotive design materials: A connecting rod for internal combustion engines is usually manufactured from which class of steel to achieve strength with good forgeability and fatigue resistance?

Introduction / Context:
Connecting rods experience large alternating tensile and compressive loads at high frequency. Material selection balances strength, toughness, fatigue resistance, and manufacturability (forging, machining, and heat treatment).

Given Data / Assumptions:

• Application: automotive/IC engine connecting rod.
• Manufacturing: hot forging followed by heat treatment and machining.
• Target properties: high fatigue strength, good toughness, moderate cost.

Concept / Approach:
Medium carbon steels (about 0.35–0.5% C) such as 40C8/45C8 or alloyed variants (e.g., 42CrMo4) provide an optimal combination of quench-temper response and forgeability. Low-carbon steels lack strength; high-carbon steels can be too brittle and difficult to forge; high-speed steels are tool materials, not structural. Stainless variants are cost-ineffective and unnecessary for this duty.

Step-by-Step Solution:
Identify property needs: fatigue strength and toughness.Match with material class responsive to quenching/tempering: medium carbon steels.Eliminate classes: low carbon (weak), high carbon (brittle), HSS (tool), stainless (unnecessary and heavy).Select medium carbon steel.

Verification / Alternative check:
Industry practice shows forged medium-carbon steels (often micro-alloyed or Cr–Mo alloyed) as standard connecting-rod materials.

Why Other Options Are Wrong:
Low carbon steel: inadequate strength; high carbon: poor toughness; HSS: intended for hot cutting tools; austenitic stainless: excessive cost and density, no fatigue advantage.

Common Pitfalls:
Assuming “strongest” tool steels are ideal for all loads; forgetting the essential role of toughness and manufacturability in high-cycle parts.

Final Answer:
medium carbon steel