AISI 4340 Alloy Steel Round Bar
AISI 4340 is a premium Ni-Cr-Mo alloy steel round bar with the highest hardenability, extreme tensile strength (up to 1800 MPa), outstanding low-temperature toughness, and superior fatigue resistance. Diameter 20mm-600mm, VAR/ESR grades available for aerospace and defense applications.
| Material | Alloy Steel (Ni-Cr-Mo) |
|---|---|
| Grade / Standard | AISI 4340 / SAE 4340 |
| Diameter | 20mm - 600mm |
| Length | 3m / 6m / 12m (or cut to length) |
| Delivery Condition | hot_rolled / normalized / annealed |
| Surface Treatment | bare / polished / coated |
| MOQ | 1 Ton |
| Delivery Time | 20-45 Days / Limited Stock |
| Loading Port | Tianjin / Shanghai / Qingdao |
Overview of AISI 4340 Alloy Steel Round Bar
AISI 4340 is a premium nickel-chromium-molybdenum (Ni-Cr-Mo) alloy steel round bar recognized as one of the highest-strength, most versatile through-hardening alloy steels available in global engineering. With carbon content of 0.38-0.43%, nickel 1.65-2.00%, chromium 0.70-0.90%, and molybdenum 0.20-0.30%, AISI 4340 delivers an unrivaled combination of extreme tensile strength, outstanding toughness, superior hardenability, and exceptional fatigue resistance. It is the benchmark material for the most demanding aerospace, defense, oil & gas, and high-performance automotive applications where failure is not an option.
Shandong Tanglu Metal Material Co., Ltd. supplies premium AISI 4340 alloy steel round bar conforming to ASTM A829, AISI/SAE 4340, EN 36CrNiMo4 (1.6511), and GB/T 3077 40CrNiMoA standards. Produced through EAF/LF/VD steelmaking with optional vacuum arc remelting (VAR) or electroslag remelting (ESR) for ultra-clean aerospace-grade material, our 4340 bars offer extremely low inclusion content, uniform microstructure, and reliable mechanical properties from surface to core even in heavy sections up to 150mm diameter. Whether for aircraft landing gear, military vehicle drivetrains, offshore drilling equipment, or Formula 1 crankshafts, AISI 4340 delivers the absolute peak of metallurgical performance.
Key Features and Technical Advantages
The defining advantage of AISI 4340 over other alloy steels is its exceptional hardenability — the synergistic effect of nickel, chromium, and molybdenum enables through-hardening in sections up to 100-150mm (4-6 inches) diameter with oil quenching. Nickel dramatically improves low-temperature toughness and impact resistance, while also enhancing core ductility even at very high strength levels. This means AISI 4340 maintains Charpy impact values above 20J even at -40°C in Q&T condition, a critical requirement for aerospace and defense applications.
AISI 4340 round bar achieves tensile strengths of 1000-1180 MPa in standard Q&T condition and up to 1800 MPa (260 ksi) in ultra-high-strength Q&T condition, making it one of the strongest commercially available alloy steel grades. It also responds superbly to nitriding for surface hardness up to HRC 65-70 and to induction hardening for selective wear surfaces. The grade has excellent fatigue endurance limits (≥550 MPa at 10⁷ cycles) and superior stress corrosion cracking resistance compared to other high-strength steels.
Our 4340 bars are available in diameter range from 20mm to 600mm, with standard lengths of 3m, 6m, and 12m. All bars undergo ultrasonic testing (UT) per SEP 1921 or ASTM A388 for internal soundness, and VAR/ESR grades receive additional macro-etch and micro-cleanliness evaluation per ASTM E45 Method D. Custom heat treatment, surface grinding, and special testing services are available to meet the most stringent aerospace and defense specifications.
Main Applications of AISI 4340 Round Bar
AISI 4340 alloy steel round bar is the gold standard for the aerospace industry, used for aircraft landing gear components (main fitting, side brace, torque links, axle beams), wing attachment fittings, engine mount bolts, flap tracks, and structural bulkhead forgings. VAR-grade 4340 is specified by Boeing, Airbus, Lockheed Martin, and other OEMs for primary structural components requiring maximum fatigue life and damage tolerance.
In defense and military applications, 4340 is used for armored vehicle hull and turret forgings, tank track components, artillery barrel forgings, missile casings, helicopter rotor hubs, and ballistic-resistant structural elements. The oil & gas industry relies on 4340 for the most demanding downhole drilling tools, including heavyweight drill pipe, drill collars in extreme H₂S environments (per NACE MR0175), blowout preventer (BOP) components, and Christmas tree valve stems.
High-performance automotive and motorsport applications include Formula 1 crankshafts and connecting rods, NASCAR rear axles and spindles, IndyCar uprights and gearbox components, and high-end aftermarket performance shafts. Other critical applications include offshore platform structural forgings, nuclear reactor bolting, power generation turbine shaft forgings, heavy press die blocks, and high-strength structural fasteners per ASTM A540 (B21 through B24 grades).
Why Choose Us for AISI 4340 Alloy Steel Round Bar
Shandong Tanglu Metal Material Co., Ltd. sources AISI 4340 alloy steel round bar from Tier-1 aerospace-certified Chinese special steel mills equipped with EAF/LF/VD primary steelmaking and optional VAR or ESR remelting capabilities. We maintain ultra-strict chemistry control with emphasis on residual element minimization (Cu ≤ 0.20%, Sn ≤ 0.015%, As ≤ 0.010%), ultra-low gas content (O₂ ≤ 15ppm, H₂ ≤ 1.5ppm), and ASTM E45 Method D inclusion rating compliance for aerospace applications.
We offer comprehensive heat treatment services including normalizing, annealing, quenching & tempering (standard and ultra-high-strength), stress relieving, nitriding, and deep cryogenic treatment. Bars can be supplied as hot rolled, forged & rough turned, peeled, or precision ground with tolerance h6/h7/h9. Our monthly supply capacity of 3,000 tons of alloy steel bars supports both rapid prototype orders and large-volume production programs. Each shipment includes original mill test certificate (MTC) per EN 10204 3.1 standard, with EN 10204 3.2, NADCAP-approved third-party testing, and full aerospace material traceability available upon request.
📐 Dimension & Size Table
| Diameter (mm) | Length (m) | Weight (kg/m) | Tolerance (Hot Rolled) |
|---|---|---|---|
| 20 | 6 / 12 | 2.466 | ± 0.40mm |
| 25 | 6 / 12 | 3.853 | ± 0.50mm |
| 30 | 6 / 12 | 5.549 | ± 0.50mm |
| 35 | 6 / 12 | 7.553 | ± 0.50mm |
| 40 | 6 / 12 | 9.864 | ± 0.60mm |
| 45 | 6 / 12 | 12.485 | ± 0.60mm |
| 50 | 6 / 12 | 15.413 | ± 0.80mm |
| 55 | 6 / 12 | 18.650 | ± 0.80mm |
| 60 | 6 / 12 | 22.195 | ± 0.80mm |
| 70 | 6 / 12 | 30.210 | ± 1.00mm |
| 80 | 6 / 12 | 39.458 | ± 1.10mm |
| 90 | 6 / 12 | 49.940 | ± 1.20mm |
| 100 | 6 / 12 | 61.654 | ± 1.30mm |
| 120 | 6 / 12 | 88.780 | ± 1.50mm |
| 130 | 6 / 12 | 103.963 | ± 1.60mm |
| 150 | 3 / 6 | 138.722 | ± 1.80mm |
| 180 | 3 / 6 | 199.759 | ± 2.20mm |
| 200 | 3 / 6 | 246.617 | ± 2.50mm |
| 250 | 3 / 6 | 385.339 | ± 3.50mm |
| 300 | 3 / 6 | 554.888 | ± 4.50mm |
| 350 | 3 | 755.269 | ± 5.50mm |
| 400 | 3 | 986.467 | ± 6.00mm |
| 500 | 3 | 1541.354 | ± 8.00mm |
| 600 | 3 | 2219.549 | ± 10.0mm |
* Custom sizes available upon request. Tolerances per relevant international standards.
🔬 Chemical Composition
| Element | Min | Max | Display Value | Note |
|---|---|---|---|---|
| C | 0.38 | 0.43 | 0.38-0.43 | Per AISI 4340 specification |
| Ni | 1.65 | 2.00 | 1.65-2.00 | Core alloy element — toughness |
| Cr | 0.70 | 0.90 | 0.70-0.90 | Hardenability element |
| Mo | 0.20 | 0.30 | 0.20-0.30 | Hardenability & temper resistance |
| Mn | 0.60 | 0.80 | 0.60-0.80 | |
| Si | 0.15 | 0.35 | 0.15-0.35 | |
| P | - | 0.025 | ≤0.025 | Tight limit for aerospace |
| S | - | 0.025 | ≤0.025 | Tight limit for toughness |
| Cu | - | 0.20 | ≤0.20 | Residual element control |
| Sn | - | 0.015 | ≤0.015 | Aerospace residual limit |
| As | - | 0.010 | ≤0.010 | Aerospace residual limit |
* Chemical composition may vary by heat, thickness and specification. Please refer to the actual mill test certificate.
⚙️ Mechanical Properties
| Property | Value | Unit | Test Condition |
|---|---|---|---|
| Tensile Strength (Normalized) | ≥1080 | MPa | Normalized condition |
| Yield Strength (Normalized) | ≥835 | MPa | Normalized condition |
| Tensile Strength (Q&T Standard) | 1000-1180 | MPa | Quenched & tempered 540-650°C |
| Yield Strength (Q&T Standard) | ≥860 | MPa | Q&T condition, ≤75mm section |
| Tensile Strength (Q&T Ultra-High) | 1550-1800 | MPa | Q&T 300-400°C (aerospace) |
| Yield Strength (Q&T Ultra-High) | ≥1380 | MPa | Ultra-high strength Q&T |
| Elongation | ≥12 | % | Gauge length 50mm, Q&T |
| Reduction of Area | ≥40 | % | Q&T condition |
| Brinell Hardness (Annealed) | 217-269 | HBW | Maximum annealed hardness |
| Brinell Hardness (Q&T Standard) | 300-360 | HBW | Typical Q&T range |
| Impact Energy (Charpy V @ 20°C) | ≥30 | J | Q&T standard condition |
| Impact Energy (Charpy V @ -40°C) | ≥20 | J | Q&T standard condition |
| Surface Hardness (Nitriding) | 65-70 | HRC | After gas/plasma nitriding |
| Fatigue Endurance Limit | ≥550 | MPa | 10⁷ cycles, Q&T condition |
| Modulus of Elasticity | 205-215 | GPa | Room temperature |
* Values shown are minimum requirements unless otherwise stated.
📦 Commercial Information
| Packaging | Premium seaworthy export packing. Alloy steel round bars bundled in hexagonal bundles with 4-6 high-strength steel strapping bands per bundle, weight typically 2-4 tons per bundle. Plastic or wooden end caps for all diameters. Each bundle clearly tagged with heat number, VAR/ESR lot, grade, diameter, length, and heat treatment condition. VCI (Volatile Corrosion Inhibitor) wrapping mandatory for precision ground and aerospace-grade bars. Wooden crates available for VAR/ESR grades and air shipment. Anti-corrosion oil coating applied to all exposed surfaces. |
|---|---|
| Payment Terms | T/T (Telegraphic Transfer),L/C (Letter of Credit),D/P (Documents against Payment) |
| Price Term | FOB,CFR,CIF,EXW |
| Supply Capacity | 3,000 Tons/Month (Alloy Steel Bars) |
| Loading Port | Tianjin / Shanghai / Qingdao |
Why Choose Our AISI 4340 Alloy Steel Round Bar?
Maximum Hardenability & Through-Thickness Strength
Ni-Cr-Mo alloy system enables through-hardening in sections up to 150mm (6 inches) with oil quench — double the capacity of 4140 — ensuring uniform high strength from surface to core even in the heaviest forgings.
Ultra-High Strength Capability
Achieves tensile strength up to 1800 MPa (260 ksi) in ultra-high-strength Q&T condition — among the highest of any commercially available alloy steel — while maintaining meaningful ductility and toughness.
Exceptional Low-Temperature Toughness
Nickel alloying provides superior Charpy impact values (≥20J at -40°C) at very high strength levels, critical for aerospace structures, arctic drilling equipment, and defense applications in extreme environments.
VAR/ESR Grade for Aerospace Certification
Vacuum arc remelted (VAR) and electroslag remelted (ESR) grades available with ultra-clean microstructure, ASTM E45 Method D inclusion compliance, and full aerospace material traceability per AMS 6414/6415.
Comprehensive Testing & Full Traceability
100% UT per ASTM A388, macro-etch, micro-cleanliness, grain size, mechanical properties, and chemistry verification. EN 10204 3.1/3.2 MTC with NADCAP-approved third-party testing available for defense and aerospace programs.
🏭 Applications of AISI 4340 Alloy Steel Round Bar
AISI 4340 alloy steel round bar is the ultimate material for the most demanding safety-critical and high-performance applications. In aerospace, it is the standard for aircraft landing gear forgings, wing attachment fittings, engine mount bolts, and primary structural elements. VAR-grade 4340 is specified by all major aircraft OEMs including Boeing, Airbus, and Lockheed Martin. Defense applications include armored vehicle hull forgings, tank track components, artillery barrel forgings, missile casings, and helicopter rotor hubs. The oil & gas industry uses 4340 for extreme-service drill collars, BOP components, Christmas tree valves, and H₂S-resistant downhole tools. High-performance motorsport relies on 4340 for Formula 1 crankshafts, NASCAR axles, and IndyCar gearbox components. Other critical applications include nuclear reactor bolting, offshore platform structural forgings, power generation turbine shaft forgings, and ultra-high-strength fasteners per ASTM A540.
📋 Quality & Certification
Our Certifications
- ✅ ISO 9001:2015
- ✅ CE Marking
- ✅ ABS
- ✅ DNV GL
- ✅ Lloyd's Register (LR)
- ✅ Bureau Veritas (BV)
- ✅ SGS Certified
Mill Certificate Type
- 📋 EN 10204 3.1
- 📋 EN 10204 3.2
- 📋 Original Mill Certificate
- 📋 Third Party Inspection Available
- 📋 Certificate of Origin
❓ Frequently Asked Questions
What is AISI 4340 alloy steel round bar?
AISI 4340 is a nickel-chromium-molybdenum (Ni-Cr-Mo) alloy steel round bar specified under AISI/SAE 4340 standard with carbon 0.38-0.43%, nickel 1.65-2.00%, chromium 0.70-0.90%, and molybdenum 0.20-0.30%. It is one of the highest-strength, most versatile alloy steel grades available, offering superior hardenability (through-hardens in sections up to 150mm), extreme tensile strength (up to 1800 MPa), outstanding low-temperature toughness, and excellent fatigue resistance. It is the material of choice for aerospace landing gear, defense armor, oil & gas drilling tools, and high-performance automotive components.
What is the equivalent grade of AISI 4340?
AISI 4340 has equivalent grades in multiple international standards: SAE 4340 (US), ASTM A829 4340 (US), AMS 6414/6415 (aerospace), EN 36CrNiMo4 / 1.6511 (European EN 10083), DIN 36CrNiMo4 (German), JIS SNCM439 (Japanese JIS G4103), GB 40CrNiMoA (Chinese GB/T 3077), and BS 817M40 (British). For aerospace applications, the VAR-grade equivalent is AMS 6414 (aircraft quality) and the ultra-high-strength specification is AMS 6415. Exact substitution should be verified per specific project and application requirements.
What is the difference between AISI 4340 and AISI 4140?
The key difference is the presence of 1.65-2.00% nickel in AISI 4340 (AISI 4140 has no intentional nickel). This gives AISI 4340: 1) Greater hardenability — through-hardens in 100-150mm sections vs 75mm for 4140. 2) Higher maximum tensile strength — up to 1800 MPa vs 1080 MPa for 4140. 3) Superior low-temperature toughness — ≥20J at -40°C vs limited toughness for 4140. 4) Better fatigue endurance limit — ≥550 MPa vs ≥480 MPa. 5) Better stress corrosion cracking resistance at high strength levels. However, 4340 is significantly more expensive (~30-50% premium) and requires more careful heat treatment. AISI 4140 is the better choice for general high-strength applications; AISI 4340 is reserved for extreme performance, safety-critical, and aerospace/defense applications.
What is VAR and ESR remelting for AISI 4340?
VAR (Vacuum Arc Remelting) and ESR (Electroslag Remelting) are secondary refining processes that produce ultra-clean steel with significantly reduced inclusion content, lower gas levels, improved microstructural uniformity, and better transverse mechanical properties. VAR is performed under vacuum and is the standard for aerospace-grade 4340 (AMS 6414). ESR uses a slag bath and produces excellent surface quality. Both processes reduce oxide inclusions (per ASTM E45 Method D), sulfur content, and gas (O₂, H₂, N₂). VAR/ESR grades are specified for landing gear, nuclear, and safety-critical applications where the highest material integrity is required.
What heat treatments are available for AISI 4340?
AISI 4340 responds to: 1) Normalizing (850-900°C, air cool) — uniformizes grain, establishes baseline properties. 2) Annealing (800-840°C, furnace cool) — softens for maximum machinability (HBW ≤241). 3) Q&T Standard (oil quench 820-850°C, temper 540-650°C) — tensile 1000-1180 MPa with good toughness. 4) Q&T Ultra-High Strength (oil quench, temper 300-400°C) — tensile 1550-1800 MPa for aerospace. 5) Stress relieving (600-650°C) — reduces residual stresses. 6) Nitriding (500-530°C) — surface HRC 65-70 with minimal distortion. 7) Induction hardening — selective surface hardening. 8) Isothermal transformation (austempering) — bainitic structure for superior toughness. The choice depends on the balance of strength, toughness, and fatigue life required by the specific application.
Can AISI 4340 be welded?
AISI 4340 can be welded but requires extremely careful procedures due to its high hardenability and alloy content. Preheating to 250-350°C is mandatory, and interpass temperature must be strictly controlled. Matching-strength welding consumables (such as ER100S-G or ER120S-G) with ultra-low hydrogen content are required. Post-weld heat treatment (PWHT) at 580-620°C is essential to temper the martensitic HAZ and restore toughness. Welding of Q&T 4340 at high strength levels (>1400 MPa) is generally not recommended due to high risk of hydrogen-induced cracking and stress corrosion. For critical welded structures, a formal WPS/PQR qualification per ASME or AWS D1.1 is required.
What is the difference between AISI 4340 and AISI 4340 mod (4340V)?
AISI 4340 Mod (sometimes called 4340V or 4340 modified) is a variant with slightly adjusted chemistry — typically higher vanadium (0.05-0.10%) for improved grain refinement during heat treatment, and sometimes adjusted manganese and silicon ranges. The 'mod' version offers improved through-hardening in very heavy sections (>150mm), better microstructural uniformity, and slightly enhanced temper resistance. It is commonly specified for very large aerospace forgings (landing gear beams, bulkheads) and heavy-section oil & gas components where the standard 4340 may not achieve full through-hardness.
What sizes and lengths are available for AISI 4340 round bar?
AISI 4340 alloy steel round bar is available in diameter range from 20mm to 600mm. Standard lengths include 3 meters, 6 meters, and 12 meters, with custom cut-to-length service supported. For diameters up to 250mm, standard hot rolled or forged bars are available. For diameters 250-600mm, bars are typically forged and rough-turned. Precision ground bars are available in diameters 20-300mm with tolerance h6/h7/h9. VAR/ESR grades are typically available in diameters 50-400mm. Tolerance per ISO 286 or DIN 1013 depending on condition. Special lengths and near-net-shape forgings available upon project request.
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