300M and AerMet 100 sit at the very top of the ultra-high-strength steels. Their tensile strengths are close to one another — which is why the two are often weighed up on the same line when a specification is being written. But they get there by completely different metallurgical routes, and the toughness that results is not the same.
Two different routes
300M is a classic quenched and tempered steel: carbon 0.38–0.45%, raised silicon, a balanced nickel–chromium–molybdenum composition. It takes its strength from the effect of carbon in the martensitic structure.
AerMet 100 belongs to an entirely different class. Its carbon is low (0.21–0.25%) but its composition contains 11–12% nickel, 13–14% cobalt and 2.9–3.3% chromium. It takes its strength not from carbon but from the finely dispersed precipitates formed during ageing. That difference shows clearly in the elongation and reduction-of-area values on our datasheets.
Comparison in numbers
| PROPERTY | 300M | AerMet 100 |
|---|---|---|
| Carbon (C) | 0.38 – 0.45% | 0.21 – 0.25% |
| Nickel (Ni) | 1.65 – 2.00% | 11.0 – 12.0% |
| Cobalt (Co) | — | 13.0 – 14.0% |
| Chromium (Cr) | 0.70 – 0.95% | 2.9 – 3.3% |
| Molybdenum (Mo) | 0.30 – 0.50% | 1.1 – 1.3% |
| Tensile strength Rm | 1861 – 1931 MPa | 1931 – 1965 MPa |
| Yield strength Rp0.2 | 1517 – 1586 MPa | 1620 – 1724 MPa |
| Elongation | 7 – 8% | 10 – 14% (longitudinal) |
| Reduction of area | 25 – 30% | 55 – 65% (longitudinal) |
| Hardness | max 311 HB (normalised + tempered) | 53 HRC |
| UNS | — | K92580 |
| AMS | 6257 · 6417 · 6419 | 6478 · 6532 |
Values are taken from our product datasheets; section thickness and the applied heat treatment regime change the result.
The real difference: toughness
The row to watch in the table is reduction of area. Where 300M gives 25–30%, AerMet 100 is at 55–65% — more than twice as much. That determines how much load a part with an initiated crack can continue to carry. The yield strength is higher in AerMet 100 as well (1620–1724 MPa against 1517–1586 MPa). In other words AerMet 100 is both stronger and tougher.
Which one, and when?
300M is sufficient — landing gear components, forged structural parts, highly stressed shafts. It has been in use for decades, its heat treatment behaviour is well understood, and it is far easier to source than AerMet.
AerMet 100 is required — parts where damage tolerance is critical and fracture toughness is stated as a number in the specification: main aircraft landing gear members, ballistic applications, critical actuator shafts. If the specification calls out AMS 6478 or AMS 6532, no other steel is an acceptable substitute.
Neighbouring alloys
For the step below, see our AISI 4340 – 300M comparison. If you are looking for a different metallurgy in a similar strength class, Maraging 300 and Maraging 350 are alternatives.
Technical datasheets
300M — chemical composition, mechanical values, standard equivalents
AerMet 100 — chemical composition, mechanical values, standard equivalents
Both grades are supplied to order in round bar, flat bar, plate, sheet, pipe and forged forms. Contact us for a quotation.