4340 Steel
Also: AISI 4340, SAE 4340, UNS G43400, E4340, 34CrNiMo6 / 1.6582 (similar), EN24 / 817M40 (BS 970, near), SNCM439 (JIS, near), AMS 6414, AMS 6415, 4340 VAR, 4340 aircraft quality, 4340 HT, nickel-chrome-moly steel
4340 is the Ni-Cr-Mo steel for heavy, highly stressed parts: it hardens through thick sections and stays tough at high strength. Values Q&T and annealed.
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At a glance
| Density | 7.85 g/cm³ (0.284 lb/in³) |
|---|---|
| Tensile strength | About 1,170 MPa (170 ksi) Q&T at 540 °C (1,000 °F); 745 MPa (108 ksi) annealed; about 1,900 MPa (275 ksi) tempered at 205 °C (400 °F) |
| Yield strength | About 1,070 MPa (155 ksi) Q&T at 540 °C; 470 MPa (68 ksi) annealed; about 1,650 MPa (240 ksi) tempered at 205 °C |
| Hardness | About 36-39 HRC Q&T at 540 °C; about 217 HB (mid 90s HRB) annealed; about 50-53 HRC tempered at 205 °C |
| Elongation | About 13-15% Q&T at 540 °C; about 22% annealed; about 10% tempered at 205 °C |
| Modulus of elasticity | 205 GPa (29.7 Msi) |
| Thermal | Melts at about 1,420-1,500 °C (2,590-2,730 °F); thermal conductivity ~44 W/m·K |
| Composition | Fe, 0.38-0.43% C, 0.60-0.80% Mn, 1.65-2.00% Ni, 0.70-0.90% Cr, 0.20-0.30% Mo, 0.15-0.35% Si |
Typical values, Oil quenched and tempered at about 540 °C (1,000 °F), the common high-toughness condition; annealed and low-temper (205 °C) figures noted alongside.. Confirm against the mill certificate and the purchase spec.
- MachinabilityFair
- About 50-55% of the 1212 benchmark annealed. Usually machined annealed or pre-hard; at 36-40 HRC it needs carbide or ceramic, rigid setups and lower speeds, and it wears tools faster than 4140.
- WeldabilityPoor
- Its hardenability makes the heat-affected zone hard and crack-prone. If it must be welded: preheat about 250-350 °C (480-660 °F), hold interpass, low-hydrogen or matching filler, and temper or stress relieve before the part cools to room temperature.
- FormabilityPoor
- Cold forming is limited to gentle bends in the annealed state. It hot forges very well, which is how landing gear, crankshafts and heavy shafts start.
- Corrosion resistancePoor
- Rusts like carbon steel despite the nickel and chromium. Aerospace parts get plated and painted; industrial parts get black oxide, nitriding, chrome or paint.
4340 takes the 0.40% carbon chromium-molybdenum base of 4140 and adds 1.65-2.00% nickel. The nickel raises hardenability, so an oil quench hardens sections of 100 mm (4 in) and more right through, and it keeps toughness and fatigue strength high at strength levels where 4140 turns brittle. It is the reference high-strength steel for landing gear, heavy crankshafts, drive shafts, high-load gears and critical bolts, and it has the deepest body of test data of any alloy steel.
Quenched and tempered at about 540 °C (1,000 °F), 4340 runs about 1,170 MPa (170 ksi) tensile at 36-39 HRC with good impact energy; tempered low it reaches about 1,900 MPa (275 ksi). Aerospace parts are bought to the vacuum-remelted AMS 6414 grade, whose cleaner steel carries longer fatigue life, and are heat treated to stated tensile bands. 300M, a silicon-vanadium modified 4340, extends the same idea to the highest-strength landing gear.
The price of that hardenability is poor weldability and harder machining. Welds need high preheat and an immediate post-weld heat treatment, so designs avoid them. At high strength it is sensitive to grinding burns, notches and hydrogen embrittlement from plating, which is why drawings for 4340 parts carry shot peening, magnetic particle inspection and plating bake requirements. For sections under about 50 mm (2 in) at moderate hardness, 4140 does the job for less.
Heat treatment
Normalize at 870-900 °C (1,600-1,650 °F); normalized 4340 comes out near 360 HB, so it is tempered at about 650 °C (1,200 °F) or annealed (about 845 °C / 1,550 °F with a slow furnace cool) before machining. Harden from 815-845 °C (1,500-1,550 °F) with an oil quench; the nickel-chromium-molybdenum hardenability carries full hardness through about 100 mm (4 in). Temper at 200-650 °C (400-1,200 °F) to set the level: about 50-53 HRC at 205 °C, 36-39 HRC at 540 °C, about 30 HRC at 650 °C. Stay out of the 260-370 °C (500-700 °F) band, where toughness drops. Aerospace drawings state a tensile band (for example 1,240-1,380 MPa / 180-200 ksi) and are heat treated with a test coupon. Nitride at 500-525 °C (930-980 °F) over a core tempered above that temperature. Bake promptly after any plating on high-strength parts.
Tempers and conditions
- Annealed
- Normalized and tempered
- Q&T pre-hard (typically about 28-36 HRC)
- Q&T to 36-40 HRC
- Q&T to stated tensile bands (for example 180-200 ksi, 260-280 ksi)
- Aircraft quality, vacuum arc remelted (AMS 6414)
Stock forms
- Round and flat bar (annealed and Q&T)
- Plate
- Forgings and rolled rings
- Seamless mechanical tube
- Aircraft-quality VAR bar and billet
Relative cost
Tier 3 of 5. About 1.3-1.6x the price of 4140 because of the nickel. Aircraft-quality vacuum-remelted bar (AMS 6414) costs several times commercial 4340.
Typical applications
- Aircraft landing gear, actuators and structural fittings
- Heavy-duty drive shafts, axles and crankshafts
- High-load gears and pinions, through hardened or nitrided
- Spline shafts, couplings and power-transmission parts
- High-strength bolts and studs, including ASTM A320 L43 low-temperature bolting
- Tool holders, die holders and heavy press tooling
- Motorsport axles, connecting rods and half shafts
- Mining and oilfield tools that see shock loads
Process compatibility
How 4340 takes each process, and what to specify. Each process links to its own guide and to the shops that do it.
| Process | Rating | Notes |
|---|---|---|
| CNC Machining | GoodGood | Machined annealed or pre-hard; at 36-40 HRC use carbide, rigid setups and lower speeds than for 4140. |
| CNC Turning | GoodGood | Shafts and axles are rough turned soft, heat treated, then finish turned or ground. |
| CNC Grinding | GoodGood | Required on high-strength parts, but gentle: grinding burns are a fatigue risk, so specify temper-etch inspection. |
| Heat Treating | ExcellentExcellent | Oil quenches through about 100 mm (4 in) and tempers to precise strength bands; the reason 4340 exists. |
| Nitriding | ExcellentExcellent | A hard, fatigue-resistant case on gears and crankshafts over a tough, highly tempered core. |
| Induction Hardening | GoodGood | Hardens journals and teeth to about 50-54 HRC over a Q&T core; temper after. |
| Shot Peening | ExcellentExcellent | Standard on landing gear, springs and shafts: compressive surface stress adds fatigue life at fillets. |
| Forging | ExcellentExcellent | The usual start for landing gear, crankshafts and heavy shafts; normalize and temper before machining. |
| Hard Chrome Plating | GoodGood | Landing gear pistons and rods; high-strength parts need a stress relief before and a bake after plating. |
| Electroless Nickel Plating | GoodGood | Uniform wear and corrosion coat on tooling and shafts; bake high-strength parts promptly. |
| Black Oxide | GoodGood | Common cosmetic finish on tool holders and industrial parts; protects only when oiled. |
| Zinc Plating | FairFair | Fine with a bake on pre-hard parts; above about 1,400 MPa (200 ksi) electroplating risks hydrogen cracking and needs a low-embrittlement process. |
| Welding | PoorPoor | High preheat, interpass control and an immediate post-weld heat treatment; design joints out where possible. Can 4340 steel be welded? |
| Carburizing | Not recommendedNot recommended | At 0.40% carbon a carburized case is brittle and adds nothing; use 8620 or 9310-type carburizing grades. |
Compare 4340
Design and sourcing tips
- State the strength as a range and how it is verified: "Q&T to 1,240-1,380 MPa (180-200 ksi) UTS, 40-44 HRC", with the test location for thick sections.
- For fatigue-critical parts call out vacuum-remelted material (AMS 6414), magnetic particle inspection after final heat treat and grind, and shot peening of fillets and radii.
- Leave grind stock and require temper-etch (nital) inspection for grinding burns on anything above about 40 HRC.
- At high strength, specify low-embrittlement plating and a prompt hydrogen-relief bake, or choose a coating that does not charge hydrogen.
- Use generous fillet radii and avoid sharp cross holes; 4340 at 50 HRC is notch sensitive.
- Do not pay for 4340 when 4140 will do: sections under about 50 mm (2 in) at 28-35 HRC harden fine in 4140.
Frequently asked questions
- Can 4340 be welded?
- Only with difficulty, and designs usually avoid it. Its high hardenability means the heat-affected zone forms hard martensite and cracks unless the part is preheated to roughly 250-350 °C (480-660 °F), held there between passes and tempered or stress relieved before it cools to room temperature. Weld in the annealed condition and heat treat the whole part afterward when you can.
- What is the hardness of 4340 steel?
- It depends entirely on the heat treatment. Annealed it is about 217 HB. Pre-hard bar is commonly around 28-36 HRC. Quenched and tempered at 540 °C (1,000 °F) it is about 36-39 HRC, and tempered at about 205 °C (400 °F) it reaches the low 50s HRC. Specify the range you need and confirm it on the cert.
- What is 4340 steel used for?
- Highly stressed parts where both strength and toughness matter and the section is thick: aircraft landing gear and fittings, heavy crankshafts and drive shafts, high-load gears, motorsport axles, critical bolts and heavy tooling. It is chosen over 4140 when a part is too thick for 4140 to harden through or must survive impact at high strength.
- What is AMS 6414?
- AMS 6414 is the aerospace material specification for 4340 bar, forgings and tube made by consumable-electrode vacuum remelting. The remelt removes inclusions, which lengthens fatigue life in parts such as landing gear. AMS 6415 covers the air-melted equivalent. Both are bought annealed and heat treated by the part maker.
Processes that commonly use it
Related materials
Where to verify these values
- ASTM A29/A29M, ASTM A322 and ASTM A434 (bar)
- AMS 6414 and AMS 6415 (aerospace bar, forgings and tube)
- MMPDS (4340 design allowables at several heat-treat levels)
- ASM Handbook Vol. 1 (properties) and Vol. 4 (heat treating)
