Steel 4340 properties
AISI 4340 steel is a low-alloy steel with a carbon content between 0.37% and 0.43% that, through heat treatment, achieves high levels of strength and toughness. It is classified as a chromium-nickel-molybdenum steel and is normally supplied in the quenched and tempered or annealed condition. Its ability to harden in thick sections makes it a frequent choice for structural parts subjected to high stresses and fatigue.
Chemical composition
Section titled “Chemical composition”Steel 4340 contains approximately 0.40% carbon and is characterized by the presence of nickel (1.65‑2.00%), chromium (0.70‑0.90%) and molybdenum (0.20‑0.30%). The following table lists the typical composition ranges, with phosphorus and sulfur limits corresponding to the aircraft grade specification.
| Element | Min (%) | Max (%) |
|---|---|---|
| Iron, Fe | 95.10 | 96.27 |
| Nickel, Ni | 1.65 | 2.00 |
| Chromium, Cr | 0.70 | 0.90 |
| Manganese, Mn | 0.65 | 0.90 |
| Carbon, C | 0.38 | 0.43 |
| Molybdenum, Mo | 0.20 | 0.30 |
| Silicon, Si | 0.15 | 0.35 |
| Sulfur, S | — | 0.010 |
| Phosphorus, P | — | 0.010 |
In commercial mechanical construction grades (ASTM A29) the maximum P and S contents can be up to 0.035% and 0.040% respectively.
Physical properties
Section titled “Physical properties”The density of steel 4340 is 7.85 g/cm³ (0.284 lb/in³) and its melting point is 1427 °C (2600 °F). These values, together with other physical constants, are presented below.
| Property | Metric System | Imperial System |
|---|---|---|
| Density | 7.85 g/cm³ | 0.284 lb/in³ |
| Melting point | 1427 °C | 2600 °F |
Mechanical properties
Section titled “Mechanical properties”In the annealed condition steel 4340 has a tensile strength of 745 MPa (108 ksi) and can exceed 1960 MPa (284 ksi) after quenching and low-temperature tempering. The following table summarizes representative mechanical properties for the most common supply and heat treatment conditions.
| Condition | Tensile strength [MPa / ksi] | Yield strength [MPa / ksi] | Elongation in 50 mm [%] | Reduction of area [%] | Brinell hardness [HB] | Rockwell hardness [HRC] |
|---|---|---|---|---|---|---|
| Annealed (844 °C, furnace cool) | 745 / 108 | 470 / 68.2 | 22 | 50 | 217 | 17 |
| Oil quenched + tempered 425 °C | 1965 / 285 | 1725 / 250 | 11 | 45 | — | 53 |
| Oil quenched + tempered 540 °C | 1240 / 180 | 1100 / 160 | 14 | 50 | 380 | 38 |
| Oil quenched + tempered 650 °C | 1000 / 145 | 850 / 123 | 20 | 55 | 300 | 28 |
The elastic constants are practically independent of the heat treatment condition:
- Modulus of elasticity: 190‑210 GPa (27.6‑30.5 Msi)
- Poisson’s ratio: 0.27‑0.30
- Shear modulus: 80 GPa (11.6 Msi)
- Bulk modulus: 140 GPa (20.3 Msi)
Thermal properties
Section titled “Thermal properties”The coefficient of linear thermal expansion at 20 °C is 12.3 µm/m·°C (6.83 µin/in·°F) after oil quenching and tempering at 600 °C. Thermal conductivity and specific heat are shown in the table.
| Property | Metric | Imperial |
|---|---|---|
| Coefficient of thermal expansion (20 °C, quenched + tempered 600 °C) | 12.3 µm/m·°C | 6.83 µin/in·°F |
| Thermal conductivity (typical for steels) | 44.5 W/m·K | 309 BTU·in/hr·ft²·°F |
Heat treatment
Section titled “Heat treatment”Heat treatments for steel 4340 include austenitizing at 816‑857 °C, oil quenching and tempering between 205 °C and 649 °C. Recommended parameters for each stage are:
- Forging: 982‑1121 °C (1800‑2050 °F); cool slowly after forging.
- Annealing: heat to 844 °C (1550 °F), hold and cool in furnace.
- Normalizing: 844‑899 °C (1550‑1650 °F), air cool.
- Austenitizing for hardening: 816‑857 °C (1500‑1575 °F).
- Quenching medium: oil; in thin sections water may be used, but it increases the risk of distortion and cracking.
- Tempering: select between 205 °C (400 °F) for maximum strength and 649 °C (1200 °F) for maximum toughness. Hold at least 1 h per inch of thickness.
Welding
Section titled “Welding”Steel 4340 has acceptable weldability if preheated to 200‑300 °C and a post-weld stress relief heat treatment is performed at temperatures near 600 °C. Low-hydrogen filler material is recommended (e.g., E12018 electrodes or AWS ER80S‑B2) and, whenever possible, a normalizing anneal should be applied after welding to restore the microstructure of the heat-affected zone.
Machining
Section titled “Machining”The machinability of steel 4340 in the annealed and cold-drawn condition is rated at 57%, taking 100% as the reference for free-machining steel AISI 1212. This low machinability requires reduced cutting speeds, abundant coolant, and the use of carbide or high-speed steel tools with advanced coatings.
Applications
Section titled “Applications”Due to its deep hardening capability and high strength, steel 4340 is used in components such as drive shafts up to 500 mm in diameter, structural bolts of class 12.9, and aircraft landing gear. It is commonly found in the following sectors:
- Aerospace: structural supports, connecting rods, bulkheads.
- Automotive and heavy vehicles: crankshafts, axle shafts, gearbox gears.
- Oil and gas: subsea Christmas trees, flanges, pump shafts.
- Heavy machinery: mill shafts, rolling mill mandrels, die holders.
Equivalent designations
Section titled “Equivalent designations”AISI 4340 steel has as its closest European equivalent 34CrNiMo6 (EN 1.6582) and a wide range of international designations.
| Standard / Country | Designation |
|---|---|
| AISI / SAE (USA) | 4340 |
| UNS (USA) | G43400 |
| EN (Europe) | 34CrNiMo6 (1.6582) |
| DIN (Germany) | 34CrNiMo6 |
| JIS (Japan) | SNCM447 |
| AFNOR (France) | 35NCD6 |
| BS (United Kingdom) | 817M40 |
| GOST (Russia) | 40KHN2MA |
| ASTM | A29, A322, A331, A519, A646, A711 (among others) |
| AMS | 6359, 6409, 6414, 6415, 6454 |
Comparison with similar steels
Section titled “Comparison with similar steels”Compared to steel 4140, 4340 includes 1.65‑2.00% nickel, which gives it higher toughness and hardenability, reaching strengths above 1960 MPa compared to approximately 1500 MPa for 4140 under similar conditions. The following table contrasts some key characteristics of three reference steels.
| Property | AISI 4130 | AISI 4140 | AISI 4340 |
|---|---|---|---|
| Typical carbon (%) | 0.30 | 0.40 | 0.40 |
| Main alloying elements | Cr, Mo | Cr, Mo | Cr, Ni, Mo |
| Nickel (%) | — | — | 1.65‑2.00 |
| Maximum achievable strength (MPa) | ~1300 | ~1500 | >1960 |
| Low temperature toughness | Low | Medium | High |
| Critical hardening diameter (inches) | ~1.5 | ~2.5 | ~4.0 |
4340 is usually chosen when the part will be subjected to high levels of fatigue, impact, or large sections requiring homogeneous properties in the core.
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the chemical composition of steel 4340?
Section titled “What is the chemical composition of steel 4340?”It contains 0.38‑0.43% carbon, 0.65‑0.90% manganese, 0.70‑0.90% chromium, 1.65‑2.00% nickel, 0.20‑0.30% molybdenum, 0.15‑0.35% silicon, and the balance iron.
What is the maximum tensile strength that steel 4340 can achieve?
Section titled “What is the maximum tensile strength that steel 4340 can achieve?”It exceeds 1960 MPa (285 ksi) when oil quenched from 845 °C and tempered at about 425 °C.
What is the maximum hardness that can be obtained in steel 4340?
Section titled “What is the maximum hardness that can be obtained in steel 4340?”It reaches up to 53 HRC (approximately 560 HB) in the as-quenched and low-tempered condition, although hardness decreases as tempering temperature increases.
What is the European equivalent of steel 4340?
Section titled “What is the European equivalent of steel 4340?”Grade 34CrNiMo6 (EN 1.6582) is the closest standardized equivalent.
What is the machinability of steel 4340 in the annealed condition?
Section titled “What is the machinability of steel 4340 in the annealed condition?”In the annealed and cold-drawn condition its machinability is 57% compared to steel AISI 1212, which is taken as the 100% reference.
At what temperature does steel 4340 melt?
Section titled “At what temperature does steel 4340 melt?”Its melting point is around 1427 °C (2600 °F).
References
Section titled “References”- azom.com: https://www.azom.com/article.aspx?ArticleID=6772
- engineeringtoolbox.com: https://www.engineeringtoolbox.com/aisi-sae-steel-numbering-system-d_1449.html
- steelnumber.com: https://www.steelnumber.com/en/equivalent_steel_iron_eu.php?zname_id=402