4140 vs 4340 Alloy Steel: Heat Treatment Response and Hardenability Comparison

Two Alloy Steels, One Application Family

AISI 4140 and 4340 are chromium-molybdenum alloy steels widely specified for shafts, gears, fasteners, and structural components requiring high strength and fatigue resistance. The key compositional difference — 4340 contains 1.65-2.00% nickel vs 4140's trace nickel — transforms hardenability, toughness, and cost.

Hardenability: The Jominy Difference

4340's nickel content shifts the time-temperature-transformation (TTT) curve to the right, delaying pearlite and bainite formation and enabling full martensitic transformation in thicker sections. In practical terms: 4140 achieves full hardness through ~50mm diameter in oil quench; 4340 achieves full hardness through ~150mm. For sections between 50-150mm, the choice between them is effectively made by the hardenability requirement.

Cost Analysis

4340 bar stock costs approximately 40-60% more than 4140 due to nickel content (nickel is ~$18/kg vs chromium at ~$9/kg). For a 100mm diameter shaft 1m long: 4140 material cost ≈ $350; 4340 ≈ $550. The premium is justified when: (1) section thickness >50mm requires through-hardening, (2) operating temperature is below -40°C where 4340's superior ductile-to-brittle transition matters, or (3) fatigue life is the design-limiting criterion and the 4340's higher toughness directly extends service life.

References & Standards

• ASTM International. Steel & Alloy Standards. astm.org
• International Organization for Standardization (ISO). iso.org
• National Institute of Standards and Technology (NIST). Materials Data. nist.gov
• ASM International. Materials Information Society. asminternational.org
• World Steel Association. Steel Statistical Yearbook. worldsteel.org