SAE 4340 – UNS G43400
High-hardenability nickel-chromium-molybdenum low-alloy steel
Medium-carbon Ni-Cr-Mo steel with hardenability superior to 4140, which allows high strength and toughness in the core of large sections. Supplied annealed or quenched and tempered; VAR version (AMS 6414) for aerospace use.
Designations and equivalents
| System | Designation |
|---|---|
| UNS | G43400 |
| W.Nr./EN | ≈ 34CrNiMo6 – 1.6582 (EN ISO 683-2); ≈ 36CrNiMo4 – 1.6511 |
| DIN | 34CrNiMo6 |
| JIS | SNCM439 |
| Trade names | 4340, BS 817M40 (EN24) |
| Others | AMS 6414 (VAR), AMS 6415; E4340 (electric furnace) |
Standards by product form
| Form | Standard |
|---|---|
| Bar (composition) | SAE J404 / ASTM A29 |
| Quenched and tempered bar | ASTM A434 |
| Aerospace bar/forging | AMS 6414 (VAR) / AMS 6415 |
| Forging | ASTM A668 / to customer specification |
| Mechanical tubing | ASTM A519 |
| Plate | ASTM A829 |
| Bolting | ASTM A320 L43 (A194 grade 4 or 7 nuts) |
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 |
| Plate (> 10 mm, width ≥ 600 mm) | 7225.40.90 | 7225.40 | 7225.40.3050 | 7225 40 40 |
| 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 (%)
SAE J404 / ASTM A29
| Element | Range (%) |
|---|---|
| C | 0.38 – 0.43 |
| Mn | 0.60 – 0.80 |
| P | ≤ 0.035 |
| S | ≤ 0.040 |
| Si | 0.15 – 0.35 |
| Ni | 1.65 – 2.00 |
| Cr | 0.70 – 0.90 |
| Mo | 0.20 – 0.30 |
| Fe | balance |
Mechanical properties
Quenched and tempered, section Ø 40–100 mm (reference 34CrNiMo6 +QT, EN 10083-3 / ISO 683-2)
| Property | Value |
|---|---|
| Yield strength | ≥ 800 MPa |
| Tensile strength | 1,000 – 1,200 MPa |
| Elongation | ≥ 11% |
| Charpy V (20 °C) | ≥ 45 J |
| Hardness, commercial Q&T | ≈ 28 – 36 HRC (typical; UTS 930–1,080 MPa) |
| Hardness, annealed | ≤ 248 HB |
Typical physical properties
| Property | Value |
|---|---|
| Density | 7.85 g/cm³ |
| Melting range | ≈ 1,425 – 1,500 °C |
| Modulus of elasticity | ≈ 205 GPa |
| Thermal conductivity (20 °C) | ≈ 44 W/m·K |
| Thermal expansion (20–100 °C) | ≈ 12.3 µm/m·K |
Heat treatment, welding and machining
| Item | Usual practice |
|---|---|
| Service temperature | Up to ≈ 400 °C in structural service, always below the tempering temperature. Not indicated for prolonged high temperature (use Cr-Mo-V). |
| Heat treatment | Annealing 830–850 °C, slow cooling. Normalizing 850–880 °C. Hardening: austenitize 830–860 °C, oil. Tempering 200–650 °C according to strength (avoid 250–400 °C, temper embrittlement). Accepts nitriding and induction hardening. |
| Welding | Difficult welding (CE ≈ 0.85, cold cracking risk): preheat 250–350 °C, low-hydrogen consumables (AWS E11018-M / E12018-M or E9018-B3 according to strength; ER100S-1/ER120S-1), immediate post-weld tempering. Prefer welding annealed and heat treating afterwards. |
| Machining | Fair in the annealed condition (≈ 50–55% of 1212); quenched and tempered it requires carbide and rigidity. |
Filler metal for welding
The wire composition is usually not identical to the material: each page explains why.
Typical applications
- Large-section shafts and spindles
- Heavy transmission gears and crown wheels
- Landing gear and aircraft structural components (VAR)
- Highly stressed crankshafts and connecting rods
- Forgings for oil and gas and mining
- High-strength bolts
- Drilling tools and mandrels
- Pressure vessels and high-pressure hydraulic components
- Rocket motor cases and defense components
Alternatives: 4140 for smaller sections and lower cost; 300M (4340 + Si, V) for strength > 1,900 MPa; 18Ni maraging for ultra-high strength; nitrided 8550 for surface wear.
SAE 4340: complete guide
Why choose 4340
Nickel (1.65 – 2.00%), added to chromium and molybdenum, gives 4340 greater hardenability than 4140. The part thus reaches high strength and toughness also in the core of large sections, where other alloy steels fall short. That is why it is chosen for highly stressed parts: transmission shafts and gears, landing gear, pressure vessels and defense components.
Supply condition
It is supplied annealed or quenched and tempered, typically with a tensile strength of 930 to 1,080 MPa. Annealed, it has good ductility: it can be bent, formed and machined by the usual methods. Oil quenching and tempering, or normalizing and tempering, raise strength and fatigue resistance.
Surface hardening
Parts already quenched and tempered accept induction or flame hardening and nitriding. The result is a hard surface, resistant to wear and abrasion, over a tough core that withstands shock and impact.
4340 for large-diameter bolting (A320 L43, "4340 PET")
In low-temperature studs above Ø 2½", 4140 (A320 L7) no longer hardens well at the core. ASTM A320 grade L43 uses 4340: C 0.38 – 0.43%, Mn 0.75 – 1.00%, Ni 1.65 – 2.00%, Cr 0.70 – 0.90% and Mo 0.20 – 0.30%, quenched and tempered. Up to Ø 4" it requires tensile ≥ 860 MPa (125 ksi), yield ≥ 724 MPa (105 ksi), elongation ≥ 16%, reduction of area ≥ 50% and Charpy ≥ 27 J (20 ft·lbf) at −101 °C. A194 grade 4 or 7 nuts.
In orders for Petrobras this material sometimes appears as "4340 PET"*. As with 4140, the purchase name should be the ASTM bolting specification. L43 does not appear in the materials table of one of the bolting supply procedures consulted (which lists B7, B7M, L7 and L7M); confirm in the requisition that it is accepted*. Rolled thread, stress relief and coating requirements are the same as for B7 studs.
Frequently asked questions
What is the chemical composition of SAE 4340?
Per SAE J404 / ASTM A29: C 0.38 – 0.43; Mn 0.60 – 0.80; P ≤ 0.035; S ≤ 0.040; Si 0.15 – 0.35; Ni 1.65 – 2.00.
Which standards cover SAE 4340?
Bar (composition): SAE J404 / ASTM A29. Quenched and tempered bar: ASTM A434. Aerospace bar/forging: AMS 6414 (VAR) / AMS 6415. Forging: ASTM A668 / to customer specification. Mechanical tubing: ASTM A519. Plate: ASTM A829. Bolting: ASTM A320 L43 (A194 grade 4 or 7 nuts).
Up to what temperature can SAE 4340 be used?
Up to ≈ 400 °C in structural service, always below the tempering temperature. Not indicated for prolonged high temperature (use Cr-Mo-V).
Which materials can replace SAE 4340?
4140 for smaller sections and lower cost; 300M (4340 + Si, V) for strength > 1,900 MPa; 18Ni maraging for ultra-high strength; nitrided 8550 for surface wear.
When should 4340 be used instead of 4140?
When the section is large or the part requires more core toughness. For the same reference (Ø 40 – 100 mm, quenched and tempered), 34CrNiMo6 has a yield strength ≥ 800 MPa, versus ≥ 650 MPa for 42CrMo4.
Can 4340 steel be welded?
Yes, with care: carbon equivalent ≈ 0.85, preheat of 250 – 350 °C, low-hydrogen consumable and tempering right after welding. The safest route is to weld annealed and quench and temper afterwards.
What is 4340 PET steel?
It is the name used in orders for 4340 intended for Petrobras bolts and studs*, usually large-diameter and low-temperature. The corresponding specification is ASTM A320 L43 (quenched and tempered 4340, Charpy ≥ 27 J at −101 °C, A194 grade 4 or 7 nuts). Order by the ASTM grade, not the nickname.
