4140 vs 4340: Alloy Steel Bar Compared

The chemistry gap

4140 (EN 1.7225 / 42CrMo4, JIS SCM440) is a chromium-molybdenum steel with about 0.40% C, 1% Cr and 0.2% Mo. 4340 (EN 1.6582 / 34CrNiMo6, JIS SNCM439) keeps the same backbone and adds 1.65–2.0% nickel, with slightly higher molybdenum.

What nickel buys

• Toughness: nickel lowers the ductile-to-brittle transition temperature and raises Charpy impact values, especially in large sections and at sub-zero service temperatures.
• Hardenability: 4340 through-hardens in much larger ruling sections — where a 4140 shaft may only harden to the case depth at big diameters, 4340 can reach target hardness and strength through the core.
• Fatigue: at equal tensile strength, 4340 generally shows better fatigue and fracture toughness, which is why it appears in landing gear, high-strength bolts and critical rotating parts.

Typical quenched-and-tempered strength ranges overlap (roughly 850–1200 MPa depending on temper and section), so the choice is rarely about headline strength — it is about toughness at that strength.

Cost and availability

4340 typically costs noticeably more per kilo — often 30–60% above 4140 depending on market and form — and stockholders carry fewer sizes. Machining and welding are comparable; both need preheat and controlled procedures when welded.

Decision rule

Default to 4140 for shafts, gears, hydraulic rods, bolts and general machinery: it covers most engineering duty at the best price. Step up to 4340 when the design is fracture-critical, the section is large, service is cold, or a specification (aerospace, defence, high-end motorsport) names it. Paying for nickel where loads are benign buys nothing measurable.

Working with the two grades

Both are supplied annealed, normalised, or prehardened (quenched and tempered), and both respond to the same hardening practice — oil or polymer quench from roughly 830–870 °C, tempered to the required strength. 4340 demands a little more care: its higher hardenability makes it more sensitive to quench severity in thin sections and more prone to quench cracking around sharp section changes, and decarburisation control matters because its fatigue advantage is lost with a soft skin. Induction and flame hardening suit 4140 well; 4340 is more often used through-hardened or nitrided. On the certificate, compare impact values at the required test temperature rather than room-temperature tensile results alone — that is where the two grades genuinely separate, and a 4140 heat that merely meets its tensile specification will not match a 4340 heat's toughness in a large, cold, shock-loaded part.

A note on equivalents

The EN pairing mirrors the AISI one: 42CrMo4 (1.7225) is the near-equivalent of 4140, 34CrNiMo6 (1.6582) of 4340, and JIS SCM440 / SNCM439 follow the same pattern. Substitution across systems is routine for this family, subject to the certificate checks described above.