Steel 4340M
AISI/SAE 4340M · UNS K44220 · AMS 6419 · also 300M
4340M (300M, UNS K44220) is a silicon- and vanadium-modified 4340 low-alloy steel, usually vacuum-arc remelted, that reaches higher strength, toughness and fatigue resistance than standard 4340.
Values for the HR condition
4340M tempers and conditions
4340M is supplied in 9 conditions, soft to hard. HR is the default FabDigit quotes; the values on this page are for it.
Compare all 9 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
Annealed (AMS 6417 / spheroidize annealed bar) | Softest supply condition — pick for rough machining, deep drilling and forming before final quench and temper. | 690 | 930 | 15 | 26 HRC | 36 W/m·K |
Cold rolled | Thin strip/sheet condition for parts that will be formed then hardened; limited availability and limited cold-work capability. | 830 | 1,000 | 9 | 30 HRC | 7 % IACS |
Cold drawn | Small-diameter annealed bar/wire cold drawn to size for tight tolerance and better surface before heat treatment; rarely stocked in this grade. | 860 | 1,030 | 10 | 31 HRC | 7 % IACS |
Normalized & Tempered | Normalize ~925 °C then temper for uniform grain and moderate hardness — a machining/stock condition and a grain-refining step before final hardening. | 965 | 1,240 | 12 | 40 HRC | — |
HRDefault Hot rolled | Mill product as rolled or forged — air-hardened martensite, too hard and brittle to machine; always annealed or normalized before use. | 1,150 | 1,550 | 6 | 47 HRC | 7 % IACS |
Q&T 1790 MPa | Slightly higher temper for parts where fracture toughness and ductility matter more than peak strength. | 1,520 | 1,790 | 11 | 51 HRC | 7 % IACS |
Quenched and tempered | The standard 300M service condition (AMS 6419): highest strength/toughness balance for landing gear, actuator rods and shafts. | 1,620 | 1,930 | 10 | 54 HRC | — |
VAR / CEVM premium-melt bar | Consumable-electrode vacuum remelted stock — choose for fatigue- and fracture-critical aerospace parts needing best transverse ductility and low inclusion count. | 1,640 | 1,930 | 12 | 54 HRC | 7 % IACS |
Q&T 2070 MPa | Maximum-strength condition for weight-critical rocket motor cases and springs; accept lower toughness and high notch sensitivity. | 1,720 | 2,070 | 8 | 56 HRC | — |
What is 4340M?
4340M (300M, UNS K44220) is a silicon- and vanadium-modified 4340 low-alloy steel, usually vacuum-arc remelted, that reaches higher strength, toughness and fatigue resistance than standard 4340. It is used for ultra-high-strength aerospace structure and landing gear, and is supplied as HR, CR, CD, annealed, normalized, VAR and Q&T bar including the 1790 MPa and 2070 MPa strength levels. Default Q&T values are 1150 MPa yield, 1550 MPa tensile, 6 % elongation and 444 HB (47 HRC). Machinability rates 15, it is not weldable when hardened, has no corrosion resistance and is sensitive to hydrogen embrittlement; 4340 is the lower-cost alternative. 4340M has a density of 7.83 g/cm³ (0.283 lb/in³), lighter than 80% of steel grades. It melts at 1,420°C (2,588 °F). 4340M has a yield strength of 1,150 MPa (167 ksi) and a tensile strength of 1,550 MPa (225 ksi) — stronger than 94% of steel grades. Elongation at break is 6% and the elastic modulus is 205 GPa (29.7 Msi).
What is 4340M used for?
- Aircraft landing gear cylinders and axles
- Arresting hooks
- High-strength airframe fittings
- Rocket motor cases
- Torsion bars
- High-strength drive shafts
- Aerospace fasteners
Advantages
- High strength — 1,150 MPa (167 ksi), better than 94% of steel grades
- Polishes to a fine finish — 75/100, better than 94% of steel grades
Limitations
- Limited formability — 5/100, worse than 99% of steel grades
- Difficult to machine — 15/100, worse than 98% of steel grades
- Limited ductility — 6%, worse than 97% of steel grades
- Difficult to weld — 12/100, worse than 96% of steel grades
- Low fracture toughness — 40 MPa·√m (36.4 ksi·√in), worse than 92% of steel grades
4340M properties
Typical room-temperature values for 4340M-HR; bars are the position among steel grades.
Physical3
4340M has a density of 7.83 g/cm³ (0.283 lb/in³), lighter than 80% of steel grades. It melts at 1,420°C (2,588 °F).
Mechanical16
4340M has a yield strength of 1,150 MPa (167 ksi) and a tensile strength of 1,550 MPa (225 ksi) — stronger than 94% of steel grades. Elongation at break is 6% and the elastic modulus is 205 GPa (29.7 Msi).
Thermal6
4340M is rated for continuous service to 290°C (554 °F). It conducts heat at 34 W/m·K (19.6 BTU/hr·ft·°F), worse than 77% of steel grades. Thermal expansion is 11.7 µm/m·K (6.5 µin/in·°F).
Electrical3
4340M conducts electricity at 7 % IACS, worse than 74% of steel grades.
Chemical & Environmental3
Corrosion resistance is not recommended (14/100), better than 70% of steel grades.
Sustainability4
Producing a kilogram of 4340M takes about 36 MJ of energy and emits 2.6 kg of CO₂ — more than 89% of steel grades. Typical recycled content is 25%.
Manufacturability8
4340M's machinability is poor (15/100, worse than 98% of steel grades); weldability not recommended (12/100); formability not recommended (5/100).
Common Calculations5
Values are nominal handbook figures for design screening. Certified mill or lot data ships with every FabDigit order on request.
Strength versus weight in the steel family
Yield strength against density for every steel grade FabDigit runs. 4340M is highlighted; hover a dot for its name, or open the interactive family chart.
Working with 4340M
Is 4340M easy to machine?
Rough machine in the annealed condition (≈40% of B1112) with carbide and rigid setups; at 52–56 HRC only hard turning with CBN, creep-feed or plunge grinding is practical — grind gently to avoid re-tempering burn and grinding cracks.
Can 4340M be welded?
Not weldable in the hardened state; if welding is unavoidable use electron-beam or GTAW with matching filler, 260–320 °C preheat/interpass, and immediately re-austenitize, quench and temper the whole part.
Can 4340M be formed, bent or molded?
Hot forge/upset at 1050–1200 °C with finish above 900 °C, then normalize and anneal; no meaningful cold forming above the annealed condition — bend and straighten only warm and re-stress-relieve.
Manufacturability
The process limits FabDigit applies to 4340M. Every 4340M part is priced by an engineer; design inside these and the quote comes back without a redesign loop.
CNC machining
| Machinability | Fair15 / 100 |
|---|---|
| ISO material group | P.2 |
| Cutting speed (rough / finish) | 180 / 210 m/min |
| Minimum wall | 0.8 mm |
| Minimum internal radius | 1 mm |
| Drill depth limit | 5× diameter |
| Best surface (machined / polished) | Ra 1.6 / — |
| Hardenable after machining | No |
Sheet metal
| Formability | Fair5 / 100 |
|---|---|
| Weldability | Fair12 / 100 |
4340M chemical composition (wt %)
Limits by weight percent from the governing specification, written the way the spec states them — a single maximum for impurities, a range for alloying elements, and the base element as balance. Nominal is the typical mid-range value.
| Element | Spec limit (wt %) | Nominal |
|---|---|---|
| C | 0.4 – 0.46 | 0.43 |
| Sikey modification vs 4340 — raises temper resistance | 1.45 – 1.8 | 1.6 |
| Mn | 0.6 – 0.9 | 0.75 |
| Cr | 0.7 – 0.95 | 0.82 |
| Ni | 1.65 – 2 | 1.83 |
| Mo | 0.3 – 0.45 | 0.38 |
| Vgrain refinement | 0.05 – 0.1 | 0.07 |
| P | ≤ 0.01 | 0.01 |
| S | ≤ 0.01 | 0 |
| Cu | ≤ 0.35 | 0.1 |
| Fe | balance |
What surface finishes work on 4340M?
Shot peen highly stressed fillets, then low-hydrogen-embrittlement cadmium or IVD aluminium plating (or HVOF WC-Co on journals) with a 190 °C/23 h bake within 4 h of plating; chrome plating requires strict bake and peening control.
Surface treatments FabDigit runs on steel parts — pick them in the quote configurator or call them out on the drawing. Each links to its datasheet in the surface-finish library (thickness, specs, call-out, material verdicts).
Plating
CVD Coating
Black / Phosphate
Thermal Spray
Equivalent designations
Standards and trade names that resolve to Steel 4340M in FabDigit's catalog. Equivalence is nominal — check the exact specification when certification matters.
- UNS
- K44220
- AMS
- 6419
- Trade names
- 300M
- Other
- 6417/6419
How much does 4340M cost?
4340M is a standard-cost steel ($$), cheaper than about 62% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
4340M — frequently asked
What is 4340M used for?
4340M is typically used for aircraft landing gear cylinders and axles, arresting hooks, high-strength airframe fittings, rocket motor cases, torsion bars and high-strength drive shafts. In short: high-strength aerospace bar.
What is the yield strength of 4340M?
4340M has a typical yield strength of 1,150 MPa (167 ksi) and a tensile strength of 1,550 MPa (225 ksi) — stronger than 94% of steel grades. Strength varies by condition: see the 9 listed tempers.
Is 4340M easy to machine?
Not especially — machinability is rated poor (15/100, worse than 98% of steel grades). Rough machine in the annealed condition (≈40% of B1112) with carbide and rigid setups; at 52–56 HRC only hard turning with CBN, creep-feed or plunge grinding is practical — grind gently to avoid re-tempering burn and grinding cracks.
Can 4340M be welded?
Not readily — weldability is rated not recommended (12/100). Not weldable in the hardened state; if welding is unavoidable use electron-beam or GTAW with matching filler, 260–320 °C preheat/interpass, and immediately re-austenitize, quench and temper the whole part.
What surface finishes work on 4340M?
Shot peen highly stressed fillets, then low-hydrogen-embrittlement cadmium or IVD aluminium plating (or HVOF WC-Co on journals) with a 190 °C/23 h bake within 4 h of plating; chrome plating requires strict bake and peening control.
Can 4340M be formed, bent or molded?
Hot forge/upset at 1050–1200 °C with finish above 900 °C, then normalize and anneal; no meaningful cold forming above the annealed condition — bend and straighten only warm and re-stress-relieve.
What is the maximum service temperature of 4340M?
4340M is rated for continuous use to about 290°C (554 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between 4340M-HR and 4340M-VAR?
HR is the default condition — mill product as rolled or forged — air-hardened martensite, too hard and brittle to machine; always annealed or normalized before use. VAR: consumable-electrode vacuum remelted stock — choose for fatigue- and fracture-critical aerospace parts needing best transverse ductility and low inclusion count. Yield strength is 1,150 MPa in HR versus 1,640 MPa in VAR.
How much does 4340M cost?
4340M is a standard-cost steel ($$), cheaper than about 62% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
Can FabDigit make parts in 4340M?
Yes — 4340M is available for CNC machining and sheet metal; every part is priced by an engineer rather than instantly. Upload a STEP file to get a price and a DFM check.
Sources
- AMS 6419 — Steel Bars, Forgings, Tubing 1.6Si-0.82Cr-1.8Ni-0.40Mo-0.08V, Consumable Electrode Melted, 1930 MPa — SAE International (2020)
- AMS 6417 — 300M Steel Bars and Forgings, Normalized and Tempered / Annealed — SAE International (2019)
- MMPDS-2023, Chapter 2 (Low-Alloy Ultrahigh-Strength Steels) — Battelle / FAA (2023)
- Aerospace Structural Metals Handbook — 300M — CINDAS (2015)
- ASM Handbook Vol.1: Properties and Selection — Irons, Steels and High-Performance Alloys — ASM International (1990)
- 300M (UNS K44220) alloy steel bar data — Carpenter Technology / Aubert & Duval (2022)
- 40CrNi2Si2MoVA (2015)
- 300M/40CrNi2Si2MoVA — 1688 / Mysteel (2026)
Property data is compiled from published supplier and standards handbooks and normalised for comparison; hardness values on non-Brinell scales are converted approximately for charting only. Manufacturability limits are FabDigit quote-engine defaults and may be relaxed by engineering review. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.