What Is the Difference Between AISI 4340 and 300M?

In aerospace specifications, AISI 4340 and 300M often appear side by side. There is a reason for that: 300M is not a separate alloy designed from scratch but a strengthened derivative of 4340. The nickel, chromium and molybdenum backbone is almost identical; the difference comes down to two elements.​‌​​‌​

Silicon and vanadium

In AISI 4340 silicon is in the 0.15–0.30% range. In 300M it rises to 1.45–1.80%, roughly six times as much. To that is added 0.05–0.10% vanadium, and molybdenum is raised from 0.20–0.30% to 0.30–0.50%.​‌​​‌​

The raised silicon content makes the steel resist softening during tempering. That allows the part to be tempered at a higher temperature; stress is relieved while strength is retained. Vanadium, in turn, refines the grain structure. The result shows directly in the figures on our datasheets: where 4340 gives a tensile strength of 1793 MPa when hardened and tempered, 300M reaches 1861–1931 MPa with the same treatment.

Comparison in numbers​‌​​‌​

Values are taken from our product datasheets; section thickness and the applied heat treatment regime change the result.​‌​​‌​

Which one, and when?

AISI 4340 is preferred — shafts, gear bodies, critical fasteners and general aerospace parts where strength in the 1700–1800 MPa class is enough. It is easier to source, and its machining and heat treatment are more widely understood.​‌​​‌​

300M is required — landing gear components, aircraft arresting hooks, highly stressed forgings. Strength is gained at the cost of a somewhat lower elongation than 4340; 300M is used where that trade-off is acceptable. If the specification calls out AMS 6417 or AMS 6419, 4340 is not an acceptable substitute.

A step up​‌​​‌​

Where even 300M is not enough and the same strength is wanted with far higher toughness, AerMet 100 comes into the picture; our comparison of the two is here. For the step below 4340, see the AISI 4140 – AISI 4340 comparison.

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