300M steel (modified 4340) – UNS K44220
Ultra-high-strength low-alloy steel: 4340 with silicon and vanadium
Ni-Cr-Mo low-alloy steel derived from SAE 4340, with added silicon (1.45 – 1.80%) and vanadium (0.05 – 0.10%), vacuum melted. Oil quenched and tempered at ≈ 300 °C, it reaches 1,900 – 2,100 MPa tensile strength with better toughness, ductility and fatigue strength than 4340 at the same level. It is the classic landing gear steel. It is not a carbon steel: it contains about 5% total alloying elements.
Designations and equivalents
| System | Designation |
|---|---|
| UNS | K44220 |
| W.Nr./EN | No direct EN equivalent (BS S155 in the UK) |
| Trade names | 300M, modified 4340 (4340 Mod, E4340 Mod), 434M, Lescalloy 300M VAC-ARC |
| Others | AMS 6257, AMS 6417, AMS 6419; MIL-S-8844 Class 3 and MIL-S-83135 (superseded by the AMS); BMS 7-26; ASTM A646 |
Standards by product form
| Form | Standard |
|---|---|
| Bar and forging (normalized and tempered) | AMS 6257 (supersedes MIL-S-8844 Class 3) |
| Vacuum-melted bar, forging and tube (VAR) | AMS 6417 (supersedes MIL-S-83135) |
| Bar, forging and tube | AMS 6419 |
| Aircraft-quality cleanliness | AMS 2300 |
| Bloom and billet for aerospace forgings | ASTM A646 |
Tolerances and HS codes
- Forged bars (round and square): machining allowance from 2 mm (Ø 6–16 mm) to 24 mm (Ø 400–500 mm) on the diameter. E.g. Ø 100–125 mm finished: +7 mm, tolerance +6 / −0 (length up to 3,500 mm).
- Forged discs: allowance of 4 to 9 mm on the diameter and 3 to 11 mm on the height, plus-only tolerance.
- Cutting and machining: band saw −0 / +5 to +15 mm on length; waterjet +10 % of thickness on the diameter (min. 2 mm); machined parts −0 / +2 to +5 mm on the diameter. See the full tables (INF 002 and INF 003, the same for every alloy).
| Form | NCM (Brazil, 8 digits) | HS (international, 6 digits) | HTS (USA, 10 digits)* | CN (European Union, 8 digits)* |
|---|---|---|---|---|
| Forged bar | 7228.40.00 | 7228.40 | 7228.40.0000 | 7228 40 90 |
| Hot-rolled bar | 7228.30.00 | 7228.30 | 7228.30.8041 | 7228 30 61 |
| Seamless pipe | 7304.19.00 | 7304.19 | 7304.19.5050 | 7304 19 10 |
| Welded pipe | 7306.19.00 | 7306.19 | 7306.19.5150 | 7306 19 00 |
| Flange and fitting | 7307.91.00 | 7307.91 | 7307.91.5030 | 7307 91 00 |
Tariff class: alloy steel (by chemical composition, under the Chapter 72 Notes and Section XV Note 5).
Pipe: 7304.19 and 7306.19 are line pipe for oil and gas pipelines; boiler, heat exchanger or process tubes go under 7304.39/7304.59 (seamless) and 7306.30/7306.50 (welded).
*: US and EU codes for reference. NCM per current legislation (General Rules of Interpretation, Notes to Chapters 72, 74 and 75 and Section XV Note 5). Confirm the final code with your customs broker according to the item size and finish. HS = first 6 digits of the NCM.
Chemical composition (%)
AMS 6417 (UNS K44220); AMS 6257 requires C 0.40 – 0.44* and AMS 6419 requires C 0.40 – 0.45
| Element | Range (%) |
|---|---|
| C | 0.38 – 0.43 |
| Mn | 0.65 – 0.90 |
| Si | 1.45 – 1.80 |
| Ni | 1.65 – 2.00 |
| Cr | 0.70 – 0.95 |
| Mo | 0.30 – 0.50 |
| V | 0.05 – 0.10 |
| P | ≤ 0.010 |
| S | ≤ 0.008 |
| Cu | ≤ 0.35 |
| Fe | balance |
Mechanical properties
Oil quenched from 871 °C and double tempered at ≈ 302 °C (575 °F) – AMS 6417 minimums (longitudinal) and AMS 6257 range
| Property | Value |
|---|---|
| Tensile strength | 1,900 – 2,100 MPa (AMS 6257); ≥ 1,862 MPa (270 ksi, AMS 6417) |
| Yield strength (0.2%) | ≥ 1,555 MPa (AMS 6257); ≥ 1,517 MPa (220 ksi, AMS 6417) |
| Elongation | ≥ 8% longitudinal; ≥ 5% transverse |
| Reduction of area | ≥ 30% longitudinal; ≥ 20% transverse |
| Hardness, quenched and tempered | ≥ 52 HRC at the core (sections up to 89 mm) |
| Delivery hardness (normalized and tempered) | ≤ 311 HB |
Typical physical properties
| Property | Value |
|---|---|
| Density | 7.85 g/cm³ |
| Modulus of elasticity | ≈ 205 GPa* |
| Thermal expansion (mean) | ≈ 11.5 µm/m·K* |
Heat treatment, welding and machining
| Item | Usual practice |
|---|---|
| Service temperature | Structural use at room and moderate temperature, always well below the tempering temperature (≈ 300 °C); above it the part loses strength. It is not stainless: it needs corrosion protection (cadmium or zinc-nickel plus paint). |
| Heat treatment | Normalize at 927 °C (1,700 °F), air cool. Harden: austenitize at 871 °C (1,600 °F) and oil quench. Double temper 2 h + 2 h at ≈ 302 °C (575 °F) for the 1,900 – 2,100 MPa range; tempering can range from 204 to 649 °C depending on the strength required. For machining, normalize and temper at ≈ 649 °C (1,200 °F). Forge between 1,066 and 1,232 °C, finishing no lower than 927 °C. Sections up to 89 mm through-harden (≥ 52 HRC). |
| Welding | Weldable by arc or gas processes, but rarely welded in critical parts: high carbon and hardenability bring a risk of cold cracking. When unavoidable, weld with preheat and a low-hydrogen consumable and repeat the full heat treatment afterwards, under a qualified procedure*. |
| Machining | Machine in the normalized and tempered condition (≤ 311 HB), leave stock and only then harden and temper. After hardening (≥ 52 HRC), grinding only, with parameters that avoid burns: grinding burns create untempered martensite and surface cracks*. |
Typical applications
- Aircraft landing gear (cylinders, axles and links)
- Aircraft structural components such as flap tracks
- Ultra-high-strength drive shafts
- High-strength bolts and fasteners
- Highly loaded gears
- Missile and defense components
- Motorsport suspension and driveline parts
Alternatives: SAE 4340 when 1,000 – 1,400 MPa is enough and cost matters; AerMet 100* (Ni-Co, UNS K92580*) when fracture toughness and stress corrosion resistance are critical at the same strength; Maraging 250 to harden without quenching, with almost no distortion and easier welding; 17-4PH or 15-5PH when corrosion matters more than strength.
300M steel (modified 4340): complete guide
What 300M steel is
300M is a modified SAE 4340: it keeps the nickel, chromium and molybdenum of 4340, raises silicon to 1.45 – 1.80% and adds vanadium. It is vacuum arc remelted (VAR), with aircraft-quality cleanliness (AMS 2300), and delivered normalized and tempered for machining. After quenching and tempering it reaches 1,900 – 2,100 MPa tensile strength without higher carbon.
By classification it is a low-alloy steel, not a carbon steel. It sits in the carbon and low-alloy steels group of the Dustre catalog.
Why silicon and vanadium
Silicon allows tempering at ≈ 300 °C while keeping very high strength and shifts tempered martensite embrittlement to higher temperatures*. Vanadium refines the grain. The result is better fatigue strength, ductility, impact strength and fracture toughness than 4340 at the same strength level.
Hydrogen embrittlement, plating and finishing
The higher the strength of the steel, the smaller the amount of hydrogen that causes delayed cracking under load. On 300M landing gear, cadmium plating protects against corrosion, but cleaning and plating must be controlled to reduce the risk of hydrogen embrittlement, followed by a relief bake.
Shot peening introduces compressive residual stresses at the surface and improves fatigue life; grit blasting, on the other hand, roughens the surface and can reduce fatigue strength.
300M, 4340, AerMet 100 and Maraging 250
4340 costs less and covers shafts and gears up to ≈ 1,400 MPa. Maraging 250 reaches ≥ 1,724 MPa yield by aging, without quenching and with almost no distortion, but at a much higher cost. AerMet 100* (Ni-Co) trades cost for more toughness and stress corrosion resistance. When corrosion is the main problem, the choice moves to precipitation-hardening stainless steels such as 17-4PH and 15-5PH.
Supply forms and Dustre services
Round bars and blocks forged in open die, cut to size on a band saw, with rough or finish machining, heat treatment in our own furnaces and hardness, tensile and impact testing. All material is supplied with a 3.1 inspection certificate and heat traceability.
Frequently asked questions
What is the chemical composition of 300M steel (modified 4340)?
Per AMS 6417 (UNS K44220); AMS 6257 requires C 0.40 – 0.44* and AMS 6419 requires C 0.40 – 0.45: C 0.38 – 0.43; Mn 0.65 – 0.90; Si 1.45 – 1.80; Ni 1.65 – 2.00; Cr 0.70 – 0.95; Mo 0.30 – 0.50.
Which standards cover 300M steel (modified 4340)?
Bar and forging (normalized and tempered): AMS 6257 (supersedes MIL-S-8844 Class 3). Vacuum-melted bar, forging and tube (VAR): AMS 6417 (supersedes MIL-S-83135). Bar, forging and tube: AMS 6419. Aircraft-quality cleanliness: AMS 2300. Bloom and billet for aerospace forgings: ASTM A646.
Up to what temperature can 300M steel (modified 4340) be used?
Structural use at room and moderate temperature, always well below the tempering temperature (≈ 300 °C); above it the part loses strength. It is not stainless: it needs corrosion protection (cadmium or zinc-nickel plus paint).
Which materials can replace 300M steel (modified 4340)?
SAE 4340 when 1,000 – 1,400 MPa is enough and cost matters; AerMet 100* (Ni-Co, UNS K92580*) when fracture toughness and stress corrosion resistance are critical at the same strength; Maraging 250 to harden without quenching, with almost no distortion and easier welding; 17-4PH or 15-5PH when corrosion matters more than strength.
Is 300M a carbon steel?
No. It is a low-alloy steel: besides ≈ 0.40% carbon, it contains about 5% total alloying elements (Ni 1.65 – 2.00%, Si 1.45 – 1.80%, Cr, Mo, Mn and V). In the catalog it sits in the carbon and low-alloy steels group, next to 4340.
What is the difference between 300M and 4340?
300M is 4340 with silicon (1.45 – 1.80% versus 0.15 – 0.35%), vanadium (0.05 – 0.10%) and slightly more molybdenum, vacuum melted. Tempered at ≈ 300 °C, it reaches 1,900 – 2,100 MPa tensile strength with better toughness and fatigue strength. Commercial quenched and tempered 4340 works in the 930 – 1,080 MPa range.
Why does 300M need care when plated?
At around 1,900 MPa, tiny amounts of hydrogen (less than 0.1 ppm mobile hydrogen in 4340 at 1,790 – 1,930 MPa) can crack the part under load. After pickling and cadmium or zinc-nickel plating, the part goes to a hydrogen relief bake; the practice cited by the US Navy is 24 h at 190 °C for the highest-strength steels. Always follow the customer's process specification.
What is the tempering temperature of 300M?
Double temper 2 h + 2 h at ≈ 302 °C (575 °F), after oil quenching from 871 °C. This cycle gives 1,900 – 2,100 MPa tensile strength. Higher tempering temperatures (up to 649 °C) lower the strength.
