4340 steel (G43400): properties, composition, equivalents
4340 steel (AISI/SAE 4340, UNS G43400): 0.40 % C, 1.8 % Ni, 0.8 % Cr, 0.25 % Mo. Q&T to 1000–1900 MPa with high toughness; through-hardens to 100+ mm. Composition, heat treatment, hardness by temper, 4340 vs 4140, 34CrNiMo6/SNCM439/EN24 equivalents.
What is 4340?
AISI 4340 is the high-strength, high-toughness alloy steel: 0.40 % carbon with 1.8 % nickel, 0.8 % chromium and 0.25 % molybdenum, UNS G43400. Nickel does what chromium and molybdenum alone cannot — it keeps the tempered martensite tough at very high strength and pushes hardenability so far that sections of 100 mm and more through-harden in oil. The result is a steel that can be tempered to any level from 950 to 1900 MPa and still resist impact and fatigue, which is why 4340 is specified for aircraft landing gear (as E4340 or 300M), crankshafts, heavy-duty gears and shafts, power-transmission components, oilfield drilling equipment and armour.
In practice most 4340 is used quenched and tempered to 30–40 HRC (1000–1300 MPa), often bought pre-hardened; the low-temper 1800+ MPa condition is reserved for aerospace parts made from vacuum-melted E4340 with rigorous hydrogen control, since the steel becomes notch- and hydrogen-sensitive above about 1400 MPa. Weldability is poor.
JIS SNCM439, GOST 40KhN2MA, GB 40CrNiMoA and BS 817M40 (EN24) are identical; EN 34CrNiMo6 (1.6582) is the standard European substitute with slightly different Ni/Cr balance.
4340 chemical composition
| Element | Mass % | Note |
|---|---|---|
| C | 0.38 – 0.43 | |
| Mn | 0.6 – 0.8 | |
| P | ≤ 0.035 | E4340 (electric-furnace, AMS 6414): P ≤ 0.015, S ≤ 0.015 |
| S | ≤ 0.04 | |
| Si | 0.15 – 0.35 | |
| Ni | 1.65 – 2 | |
| Cr | 0.7 – 0.9 | |
| Mo | 0.2 – 0.3 |
SAE J404 / ASTM A29 (G43400). E4340 = electric-furnace/vacuum-degassed aerospace quality. 300M (AMS 6257) adds Si 1.45–1.80 and V for 1900–2100 MPa. Compare EN 34CrNiMo6 (1.6582): C 0.30–0.38, Ni 1.30–1.70, Cr 1.30–1.70, Mo 0.15–0.30 — lower C and Ni, higher Cr.
4340 mechanical properties
| Thickness / condition | Yield strength ReH / Rp0.2 MPa (ksi) | Tensile strength Rm MPa (ksi) | Elongation A % | Hardness |
|---|---|---|---|---|
| Annealed (810 °C, furnace cool) — typical | ≈ 470 (≈ 68.2) | ≈ 745 (≈ 108.1) | 22 | ≈ 217 HB |
| Normalized (870 °C, air) — typical | ≈ 860 (≈ 124.7) | ≈ 1280 (≈ 185.6) | 12 | ≈ 363 HB |
| Pre-hardened bar (Q&T 28–34 HRC) — typical | ≈ 800–900 (≈ 116.0–130.5) | ≈ 950–1050 (≈ 137.8–152.3) | 15 | 277–321 HB |
| Q&T, tempered 650 °C (1200 °F), 25 mm — typical | ≈ 855 (≈ 124.0) | ≈ 965 (≈ 140.0) | 19 | ≈ 280 HB |
| Q&T, tempered 540 °C (1000 °F) — typical | ≈ 1075 (≈ 155.9) | ≈ 1170 (≈ 169.7) | 14 | ≈ 350 HB |
| Q&T, tempered 425 °C (800 °F) — typical | ≈ 1360 (≈ 197.3) | ≈ 1470 (≈ 213.2) | 12 | ≈ 430 HB (45 HRC) |
| Q&T, tempered 315 °C (600 °F) — typical | ≈ 1590 (≈ 230.6) | ≈ 1720 (≈ 249.5) | 10 | ≈ 490 HB (50 HRC) |
| Q&T, tempered 205 °C (400 °F) — typical | ≈ 1680 (≈ 243.7) | ≈ 1880 (≈ 272.7) | 9 | ≈ 520 HB (53 HRC) |
| As-quenched (oil, 845 °C) | — | — | — | 55–59 HRC |
Typical values for 25 mm rounds (ASM). Aerospace heat-treat levels (AMS 2759): 125–145 ksi, 150–170 ksi, 180–200 ksi, 260–280 ksi (300M). Fracture toughness KIc ≈ 50–60 MPa√m at 1800 MPa, ≈ 100 MPa√m at 1100 MPa.
Impact toughness
4340 requires a minimum Charpy V-notch energy of 50 J at 20 °C (typical, Q&T 1100 MPa; ≈ 20 J at 1800 MPa). Nickel keeps toughness high at strength levels where 4140 becomes brittle; the reason 4340 is chosen for landing gear and crankshafts. Avoid tempering at 230–370 °C.
4340 equivalent grades
| Grade | System | Match | Why / differences |
|---|---|---|---|
| 817M40 (EN24) | BS (superseded British) | Identical | BS 970 817M40: C 0.36–0.44, Ni 1.30–1.70, Cr 1.00–1.40, Mo 0.20–0.35. |
| 40CrNiMoA | GB/T (China) | Identical | GB/T 3077 40CrNiMoA: C 0.37–0.44, Ni 1.25–1.65, Cr 0.60–0.90, Mo 0.15–0.25 (Ni slightly lower). |
| 40KhN2MA | GOST (Russia/CIS) | Identical | GOST 4543 40KhN2MA: C 0.37–0.44, Ni 1.25–1.65, Cr 0.60–0.90, Mo 0.15–0.25. |
| SNCM439 | JIS (Japan) | Identical | JIS G4053 SNCM439: C 0.36–0.43, Ni 1.60–2.00, Cr 0.60–1.00, Mo 0.15–0.30. |
| 300M | AISI / SAE | Near | Si-V modified 4340 for 1900–2100 MPa (landing gear, high-strength bolts). |
| 4330V / 4330 Mod | AISI / SAE | Near | Lower-carbon (0.30 %) V-modified version with better weldability and toughness for oilfield and aerospace. |
| 34CrNiMo6 | EN (European) | Near | 1.6582: C 0.30–0.38, Ni 1.30–1.70, Cr 1.30–1.70, Mo 0.15–0.30 — lower carbon/nickel, higher chromium; similar hardenability and strength; the standard European substitute. |
| 36CrNiMo4 | EN (European) | Near | 1.6511: C 0.32–0.40, Ni 0.90–1.20, Cr 0.90–1.20, Mo 0.15–0.30 — closer on Cr, lower Ni. |
| 4140 | AISI / SAE | Functional | Ni-free: same carbon, lower hardenability and toughness; adequate below ~75 mm and 1200 MPa. |
Identical = same composition and mechanical limits within rounding. Near = one limit differs (e.g. carbon max or impact temperature). Functional = interchangeable for most uses after an engineering check. Full cross-reference for 4340 →
Old and superseded designations
| Old name | Standard | Note |
|---|---|---|
| G43400 / E4340 | UNS / AMS 6414 | E = electric-furnace (aircraft quality); 4340H = H43400 hardenability band. |
| 34CrNiMo6 / 1.6582 | EN 10083-3 | Nearest European grade; 36CrNiMo4 (1.6511) and 40NiCrMo7 (Italy) also quoted. |
| 817M40 / EN24 | BS 970 | EN24 (BS 970 1955) is still the everyday UK/India name; 817M40 is the 1970+ designation. |
| SNCM439 | JIS G4053 | Formerly SNCM8. |
| 300M | AMS 6257 / 6419 | Si-V modified 4340 for 1900–2100 MPa (landing gear). |
Product forms and tolerance standards
Product forms: bar and forgings (ASTM A29, A322, A108; AMS 6415/6414 (E4340 vacuum-melted)), pre-hardened bar at 28–34 HRC, seamless tube (A519), plate (A829), wire, aerospace: AMS 6414 (E4340), AMS 6257 (300M modified).
Dimensional tolerances: bar: ASTM A29 / A108; forgings: ASTM A788 / AMS 2374; aerospace bar: AMS 6414 / 6415.
Key property values
4340 steel yield strength
Typical: annealed ≈ 470 MPa (68 ksi); normalized ≈ 860 MPa; Q&T tempered at 650 °C ≈ 855 MPa; 540 °C ≈ 1075 MPa (156 ksi); 425 °C ≈ 1360 MPa; 315 °C ≈ 1590 MPa; 205 °C ≈ 1680 MPa (244 ksi). 300M at 260–280 ksi level: ≈ 1650–1720 MPa yield / 1930–2000 MPa tensile.
4340 tensile strength
Typical: annealed ≈ 745 MPa (108 ksi); normalized ≈ 1280 MPa; Q&T 965 MPa (650 °C temper) to 1880 MPa (205 °C temper, 273 ksi). Aerospace levels 125–145 / 150–170 / 180–200 ksi.
4340 hardness
Annealed ≈ 217 HB; normalized ≈ 363 HB; pre-hard 28–34 HRC; Q&T 280 HB (650 °C) → 350 HB (540 °C) → 45 HRC (425 °C) → 50 HRC (315 °C) → 53 HRC (205 °C); as-quenched 55–59 HRC. Nitrided surface 55–60 HRC.
Heat treatment
Anneal 810–845 °C, furnace cool ≤ 15 K/h to 500 °C (or isothermal 650 °C). Normalize 870–900 °C, air. Harden 830–860 °C (1525–1575 °F), oil quench (air-hardens in thin sections); temper 205–650 °C, 2 h, double temper for aerospace; avoid 230–370 °C (tempered-martensite embrittlement) and slow cooling through 450–550 °C. Stress relieve 550–650 °C. Ac1 ≈ 725 °C, Ac3 ≈ 775 °C, Ms ≈ 290 °C. Ideal critical diameter ≈ 150 mm+ (through-hardens to 100 mm in oil).
Hydrogen embrittlement
Above ~1240 MPa (180 ksi, ≈ 40 HRC) 4340 is highly susceptible: bake 190 °C for 4–24 h within 4 h of plating, pickling or cadmium/zinc coating; specify low-hydrogen consumables and limit exposure to acids. Aerospace parts are 100 % baked and often shot-peened.
Physical properties
Density 7.85 g/cm³, E = 205 GPa, expansion 12.3 × 10⁻⁶/K, conductivity 44.5 W/m·K, specific heat 475 J/kg·K.
Weldability
Poor weldability (CE ≈ 0.85–0.95). Weld only annealed material: preheat 300–370 °C, interpass ≥ preheat, low-hydrogen consumables (E11018-M / ER100S-1 or matching Ni-Cr-Mo), slow cool, temper immediately at 550–650 °C. Never weld Q&T parts intended to stay at strength. Repair welding of hardened 4340 requires full re-heat-treatment. Consider 4330V for weldable applications.
Machining, forming and heat treatment
Machinability ≈ 50 % of B1112 annealed (217 HB); pre-hard 28–34 HRC machines well with coated carbide (≈ 45 %). Rough in annealed condition, finish after Q&T where distortion allows; grinding with care (burn). Cold forming only annealed and mild; hot forging 1200–950 °C then anneal or normalize. Surface treatments: nitriding (excellent, 0.3–0.6 mm at 600+ HV), induction hardening (55–58 HRC), shot peening, chrome/cadmium plating with mandatory hydrogen bake.
Typical applications
- Aircraft landing-gear components, actuator pistons, flap tracks (E4340 / 300M)
- Crankshafts, connecting rods and camshafts for high-performance and heavy-duty engines
- Heavy gears, pinions, shafts and spindles in power transmission, mining and steel mills
- Oilfield: drill collars, kelly bars, subs, wellhead and BOP components (within NACE limits)
- High-strength bolts and studs (ASTM A320 L43, SAE Grade 8+, aerospace NAS)
- Armour, gun barrels, ordnance components
- Injection-mould and die-casting holder blocks (pre-hard), machine-tool spindles
- Motorsport: axles, hubs, gears, con-rods
Frequently asked questions
What is the difference between 4340 and 4140?
4340 adds 1.8 % nickel. At equal hardness it is tougher and it through-hardens in sections 2–3× larger (100+ mm vs ~50 mm). 4140 is cheaper and adequate for most parts under 75 mm at ≤ 1200 MPa; 4340 is chosen for larger sections, higher strength (to 1900 MPa) and fatigue/impact-critical parts.
How hard can 4340 get?
55–59 HRC as-quenched; in service typically 30–45 HRC (1000–1400 MPa) and up to 50–53 HRC at low temper for aerospace parts made from E4340/300M. Nitriding gives 55–60 HRC on the surface.
What is E4340?
Electric-furnace (usually vacuum-arc or ESR remelted) aircraft-quality 4340 with P and S ≤ 0.015 %, tighter inclusion control and full traceability (AMS 6414/6415). It is required for landing gear and other flight-critical parts.
What is 300M?
A 4340 modified with 1.6 % silicon and 0.07 % vanadium (AMS 6257) that tempers to 1930–2070 MPa (280–300 ksi) with usable toughness — the standard landing-gear and high-strength bolt steel.
What is 4340 equivalent to in EN?
EN 34CrNiMo6 (1.6582) is the standard substitute (lower C and Ni, higher Cr; similar strength and hardenability); 36CrNiMo4 (1.6511) is a lower-Ni alternative. BS EN24/817M40, JIS SNCM439 and GOST 40KhN2MA are exact matches.
Can 4340 be welded?
Only with difficulty: annealed condition, 300–370 °C preheat, low-hydrogen filler, immediate post-weld tempering, then full re-heat-treatment if strength is required. For welded high-strength structures use 4330V or 4130 instead.
Related grades
Verified against ASTM A29/A29M-20 (SAE J404 chemistry); ASTM A29/A29M-20 Table 2 (G43400); AMS 6414 / 6415 / 6257; AMS 2759/1 (heat treatment); ASM Handbook Vol. 1 and Vol. 4. Last checked: September 2026.