Steel 4340
AISI/SAE 4340 · UNS G43400 · EN 1.6582, 34CrNiMo6 · JIS SNCM439 · GB 40CrNiMoA
4340 is a medium-carbon Ni-Cr-Mo low-alloy through-hardening steel with deep hardenability and an excellent strength-to-toughness balance after quenching and tempering, usable from about 850 MPa up to nearly 2000 MPa tensile.
Values for the HR condition
4340 tempers and conditions
4340 is supplied in 13 conditions, soft to hard. HR is the default FabDigit quotes; the values on this page are for it.
- Annealed
- HR
- CR
- CD
- Normalized
- Q&T 28-32 HRC
- Q&T
- Q&T 32-36 HRC
- Q&T+Nitrided
- Q&T 36-40 HRC
- Q&T 40-44 HRC
- Q&T 46-50 HRC
- Q&T 50-54 HRC
Compare all 13 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
full/spheroidize annealed (≈815 °C, slow cool) | Softest and most machinable condition; used as the supply state for parts that will be finish heat treated. | 470 | 745 | 22 | 217 HB | 45 W/m·K |
HRDefault Hot rolled | Cheapest stock form for parts that will be rough-machined and then heat treated by the customer. | 620 | 950 | 18 | 29 HRC | — |
Cold rolled | Thin sheet/strip form for blanked parts that will be heat treated after forming. | 620 | 800 | 15 | 22 HRC | 7 % IACS |
Cold drawn | Choose for close-tolerance, bright-finish shafting used without further heat treatment. | 830 | 965 | 12 | 29 HRC | 44 W/m·K |
870 °C, air cooled | Uniform fine-grained structure with high as-cooled strength; usually a preparatory step before machining and Q&T. | 860 | 1,280 | 12 | 39 HRC | — |
Tempered ~600 °C | Toughest useful Q&T level; for shafts and machine parts needing impact resistance and easy finishing. | 860 | 1,020 | 20 | 30 HRC | — |
Quenched and tempered | Standard commercial heat-treated stock ("4340HT"): high strength with good toughness and still machinable. | 880 | 1,030 | 18 | 31 HRC | 43 W/m·K |
Tempered ~565 °C | Balanced strength/toughness for gears, couplings and heavy-duty shafting. | 1,010 | 1,130 | 18 | 34 HRC | — |
Q&T + gas/ion nitrided case | Pick when a Q&T core (typically 32–36 HRC) needs a hard, wear- and fatigue-resistant 0.2–0.5 mm case without distortion. | 1,010 | 1,130 | 15 | 58 HRC | 7 % IACS |
Tempered ~525 °C | Higher strength for torsion bars, tool holders and mid-strength aerospace fittings still needing decent toughness. | 1,160 | 1,240 | 17 | 38 HRC | — |
Tempered ~450 °C | High-strength condition for aircraft structural fittings and highly stressed pins; machining requires carbide tooling. | 1,360 | 1,500 | 14 | 42 HRC | — |
Tempered ~315 °C | Very high strength for landing-gear and heavy-section tooling; toughness and hydrogen tolerance drop sharply. | 1,620 | 1,760 | 12 | 48 HRC | — |
Tempered ~205 °C | Maximum-strength condition; only for wear/strength-critical parts where low toughness and embrittlement risk are acceptable. | 1,860 | 1,980 | 11 | 52 HRC | — |
What is 4340?
4340 is a medium-carbon Ni-Cr-Mo low-alloy through-hardening steel with deep hardenability and an excellent strength-to-toughness balance after quenching and tempering, usable from about 850 MPa up to nearly 2000 MPa tensile. It is the default choice for highly loaded shafts, landing-gear parts, gears, crankshafts, spindles and tooling holders where section sizes are too large for shallow-hardening steels. Limits are poor corrosion resistance (needs plating/paint/oil), difficult welding (preheat and post-weld temper mandatory), hydrogen-embrittlement and temper-embrittlement sensitivity, and falling fracture toughness above ~1600 MPa. 4340 has a density of 7.85 g/cm³ (0.284 lb/in³), heavier than 56% of steel grades. It melts at 1,416°C (2,581 °F). 4340 has a yield strength of 620 MPa (89.9 ksi) and a tensile strength of 950 MPa (138 ksi) — stronger than 78% of steel grades. Elongation at break is 18% and the elastic modulus is 205 GPa (29.7 Msi).
What is 4340 used for?
- Landing gear components
- Drive shafts
- Heavy-duty gears
- Crankshafts
- Machine tool spindles
- Tool holders and arbors
- High-strength fasteners and studs
Advantages
- Polishes to a fine finish — 70/100, better than 87% of steel grades
- High strength — 620 MPa (89.9 ksi), better than 78% of steel grades
4340 properties
Typical room-temperature values for 4340-HR; bars are the position among steel grades.
Physical3
4340 has a density of 7.85 g/cm³ (0.284 lb/in³), heavier than 56% of steel grades. It melts at 1,416°C (2,581 °F).
Mechanical17
4340 has a yield strength of 620 MPa (89.9 ksi) and a tensile strength of 950 MPa (138 ksi) — stronger than 78% of steel grades. Elongation at break is 18% and the elastic modulus is 205 GPa (29.7 Msi).
Thermal6
4340 is rated for continuous service to 400°C (752 °F). It conducts heat at 44.5 W/m·K (25.7 BTU/hr·ft·°F), better than 63% of steel grades. Thermal expansion is 12.3 µm/m·K (6.83 µin/in·°F).
Electrical3
4340 conducts electricity at 7 % IACS, worse than 74% of steel grades.
Chemical & Environmental3
Corrosion resistance is not recommended (12/100), worse than 53% of steel grades.
Sustainability4
Producing a kilogram of 4340 takes about 32 MJ of energy and emits 2.3 kg of CO₂ — more than 70% of steel grades. Typical recycled content is 30%.
Manufacturability7
4340's machinability is fair (48/100, worse than 57% of steel grades); weldability poor (33/100); formability fair (35/100).
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. 4340 is highlighted; hover a dot for its name, or open the interactive family chart.
Working with 4340
Is 4340 easy to machine?
Machine in the annealed or ≤34 HRC condition with rigid setups; above 40 HRC use coated carbide/CBN, low speeds and heavy positive feed, and finish-grind critical surfaces.
Can 4340 be welded?
Weldable only with 250–350 °C preheat, low-hydrogen or matching Ni-Cr-Mo filler and immediate post-weld temper below the part's tempering temperature; avoid welding fully hardened parts.
Can 4340 be formed, bent or molded?
Hot forge/roll at 1050–1200 °C and always normalize afterwards; cold bending is limited to the annealed state with generous radii.
Manufacturability
The process limits FabDigit's quote engine applies to 4340. Design inside these and the part quotes instantly; outside them it routes to an engineer.
CNC machining
| Machinability | Good48 / 100 |
|---|---|
| ISO material group | P.2 |
| Cutting speed (rough / finish) | 150 / 180 m/min |
| Minimum wall | 0.8 mm |
| Minimum internal radius | 1 mm |
| Drill depth limit | 4.5× diameter |
| Best surface (machined / polished) | Ra 1.6 / 0.8 µm |
| Hardenable after machining | Yes |
Sheet metal
| Formability | Fair35 / 100 |
|---|---|
| Weldability | Good33 / 100 |
4340 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 |
|---|---|---|
| CASTM A29 / SAE J404 | 0.38 – 0.43 | 0.4 |
| Mn | 0.6 – 0.8 | 0.7 |
| Si | 0.15 – 0.35 | 0.25 |
| Ni | 1.65 – 2 | 1.8 |
| Cr | 0.7 – 0.9 | 0.8 |
| Mo | 0.2 – 0.3 | 0.25 |
| Pmax; aircraft quality (AMS 6414) ≤0.010 | ≤ 0.04 | |
| Smax; aircraft quality ≤0.010 | ≤ 0.04 | |
| Fe | balance |
What surface finishes work on 4340?
Requires protection — zinc/cadmium or Cr plating, phosphate + oil, or paint; bake at 190 °C after plating to relieve hydrogen, and shot-peen highly loaded fillets to restore fatigue life.
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 4340 in FabDigit's catalog. Equivalence is nominal — check the exact specification when certification matters.
- UNS
- G43400
- EN / DIN / Werkstoff-Nr.
- 1.658234CrNiMo61.651136CrNiMo430NiCrMo16
- GB (China)
- 40CrNiMoA
- JIS
- SNCM439
How much does 4340 cost?
4340 is a premium-cost steel ($$$), cheaper than about 60% 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.
4340 — frequently asked
What is 4340 used for?
4340 is typically used for landing gear components, drive shafts, heavy-duty gears, crankshafts, machine tool spindles and tool holders and arbors. In short: ultra-high fatigue strength.
What is the yield strength of 4340?
4340 has a typical yield strength of 620 MPa (89.9 ksi) and a tensile strength of 950 MPa (138 ksi) — stronger than 78% of steel grades. Strength varies by condition: see the 13 listed tempers.
Is 4340 easy to machine?
Reasonably — machinability is rated fair (48/100, worse than 57% of steel grades). Machine in the annealed or ≤34 HRC condition with rigid setups; above 40 HRC use coated carbide/CBN, low speeds and heavy positive feed, and finish-grind critical surfaces.
Can 4340 be welded?
Not readily — weldability is rated poor (33/100). Weldable only with 250–350 °C preheat, low-hydrogen or matching Ni-Cr-Mo filler and immediate post-weld temper below the part's tempering temperature; avoid welding fully hardened parts.
What surface finishes work on 4340?
Requires protection — zinc/cadmium or Cr plating, phosphate + oil, or paint; bake at 190 °C after plating to relieve hydrogen, and shot-peen highly loaded fillets to restore fatigue life.
Can 4340 be formed, bent or molded?
Hot forge/roll at 1050–1200 °C and always normalize afterwards; cold bending is limited to the annealed state with generous radii.
What is the maximum service temperature of 4340?
4340 is rated for continuous use to about 400°C (752 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between 4340-HR and 4340-CD?
HR is the default condition — cheapest stock form for parts that will be rough-machined and then heat treated by the customer. CD: choose for close-tolerance, bright-finish shafting used without further heat treatment. Yield strength is 620 MPa in HR versus 830 MPa in CD.
How much does 4340 cost?
4340 is a premium-cost steel ($$$), cheaper than about 60% 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 4340?
Yes — 4340 is available for CNC machining and sheet metal with instant online pricing. Upload a STEP file to get a price and a DFM check.
Sources
- ASTM A29/A29M — Steel Bars, Carbon and Alloy, Hot-Wrought — ASTM (2020)
- SAE J404 — Chemical Compositions of SAE Alloy Steels — SAE International (2021)
- AMS 6414 — Steel Bars, Forgings, Tubing 4340 (Vacuum Consumable Electrode Remelted) — SAE Aerospace (2019)
- GB/T 3077 — (40CrNiMoA) — SAC (2015)
- EN 10083-3 — 34CrNiMo6 (1.6582) — CEN (2006)
- ASM Handbook Vol.1 — Properties and Selection: Irons, Steels — ASM International (1990)
- ASM Handbook Vol.19 — Fatigue and Fracture — ASM International (1996)
- MMPDS-2023 typical low-alloy steel data — Battelle/FAA (2023)
- Machinery's Handbook, 31st ed. (machinability ratings) — Industrial Press (2020)
- Mysteel / SMM (2025)
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.