Steel 4140 properties
Chemical composition
Section titled “Chemical composition”AISI 4140 steel contains between 0.38 and 0.43% carbon, along with chromium and molybdenum as main alloying elements.
| Element | Content (%) |
|---|---|
| Carbon, C | 0.380 – 0.430 |
| Chromium, Cr | 0.80 – 1.10 |
| Manganese, Mn | 0.75 – 1.00 |
| Silicon, Si | 0.15 – 0.30 |
| Molybdenum, Mo | 0.15 – 0.25 |
| Phosphorus, P (max.) | 0.035 |
| Sulfur, S (max.) | 0.040 |
| Iron, Fe | Balance |
Physical properties
Section titled “Physical properties”Density of 4140 steel is 7.85 g/cm³ (0.284 lb/in³) and its melting point is around 1416 °C (2580 °F).
| Property | Metric | Imperial |
|---|---|---|
| Density | 7.85 g/cm³ | 0.284 lb/in³ |
| Melting point | 1416 °C | 2580 °F |
Mechanical properties
Section titled “Mechanical properties”Mechanical properties of AISI 4140 vary significantly depending on heat treatment condition. Below are typical values in annealed condition and in quenched and tempered condition.
Annealed condition (as supplied)
Section titled “Annealed condition (as supplied)”| Property | Metric | Imperial |
|---|---|---|
| Tensile strength (Rm) | 655 MPa | 95 000 psi |
| Yield strength (Rp0.2) | 415 MPa | 60 200 psi |
| Elongation (rupture in 50 mm) | 25.7 % | 25.7 % |
| Brinell hardness | 197 HB | 197 |
| Rockwell B hardness (converted) | 92 HRB | 92 |
| Elastic modulus | 190 – 210 GPa | 27.6 – 30.5 Msi |
| Poisson’s ratio | 0.27 – 0.30 | 0.27 – 0.30 |
| Machinability (base AISI 1212 = 100) | 65 % | 65 % |
Quenched and tempered (hardened and tempered)
Section titled “Quenched and tempered (hardened and tempered)”Values depend on the cross-section of the part. Data shown for diameters between 16 and 40 mm (thicknesses from 8 to 20 mm).
| Property | Metric | Imperial |
|---|---|---|
| Tensile strength (Rm) | 1000 – 1200 MPa | 145 – 174 ksi |
| Yield strength (Rp0.2) min. | 750 MPa | 109 ksi |
| Elongation (min.) | 11 % | 11 % |
| Reduction of area (min.) | 45 % | 45 % |
| Impact energy (Charpy V, min.) | 35 J | 26 ft·lbf |
For larger sections (40 – 100 mm) tensile strength is 900 – 1100 MPa, with minimum yield strength of 650 MPa.
Thermal properties
Section titled “Thermal properties”Coefficient of thermal expansion of 4140 is 12.2 µm/m·°C in the range 0 to 100 °C. Thermal conductivity at 100 °C reaches 42.6 W/m·K (296 BTU·in/h·ft²·°F).
| Property | Condition | Metric | Imperial |
|---|---|---|---|
| Thermal expansion coeff. | 0 – 100 °C | 12.2 µm/m·°C | 6.78 µin/in·°F |
| Thermal conductivity | @ 100 °C | 42.6 W/m·K | 296 BTU·in/h·ft²·°F |
Heat treatment
Section titled “Heat treatment”- Austenitizing and quenching: Heat to 845 °C (1550 °F) until homogenization, cool in oil. This obtains maximum hardness martensitic structure.
- Tempering: Perform at a temperature between 200 and 650 °C depending on desired hardness; low tempers maintain high strength, high tempers improve toughness.
- Normalizing: Heat to 913 °C (1675 °F) for a prolonged period and air cool to refine grain and homogenize microstructure.
- Annealing: Heat to 872 °C (1600 °F), then slowly cool in the furnace to soften the material and improve machinability.
Forging and forming
Section titled “Forging and forming”- Forging: Working temperature between 926 and 1205 °C (1700 – 2200 °F). Do not forge below 850 °C.
- Hot working: Perform between 816 and 1038 °C (1500 – 1900 °F).
- Cold working: Only in annealed condition; 4140 is harder than plain carbon steels, thus requires higher forces.
Welding and machining
Section titled “Welding and machining”Welding: Accepts all conventional processes (electric arc, MIG/MAG, TIG). If welded in hardened condition, preheating (200 – 300 °C) and post-weld treatment are mandatory to avoid cold cracking.
Machining: Machinability in annealed condition is good (index 65% relative to AISI 1212 steel). After quenching and tempering to low hardness it can be machined with carbide tools, although more difficult than in annealed condition.
Applications
Section titled “Applications”- Automotive and mechanical industry: crankshafts, shafts, gears, connecting rods, bolts, pins, high-strength fasteners.
- Oil and gas: flanges, studs (ASTM A193 B7), valves, high-pressure piping.
- Aeronautics: structural components such as landing gear tubes and critical supports.
- Armaments: firearm parts (e.g., rifle barrels and breech components).
- Bicycles and motorsports: frames and roll cages thanks to excellent strength/weight ratio.
Comparison with similar steels
Section titled “Comparison with similar steels”| Property | AISI 4140 | AISI 4130 | AISI 4340 |
|---|---|---|---|
| Carbon (%) | 0.38 – 0.43 | 0.28 – 0.33 | 0.38 – 0.43 |
| Main alloy | Cr, Mo | Cr, Mo | Cr, Ni, Mo |
| Mechanical strength | High | Medium‑high | Very high |
| Toughness | Good | Good | Excellent |
| Weldability | Requires pre/post | Good | Requires pre/post |
| Typical applications | Shafts, gears, bolts | Aeronautical tubes, cages | Very demanding components (aviation, heavy shafts) |
4140 steel offers an optimal balance between strength, toughness and cost, being more robust than 4130 but more economical than 4340.
Equivalent designations
Section titled “Equivalent designations”| Standard / Country | Designation |
|---|---|
| AISI / SAE | 4140 |
| UNS | G41400 |
| EN (Europe) | 42CrMo4 (1.7225) / 42CrMoS4 (1.7227) |
| DIN (Germany) | 42CrMo4 |
| JIS (Japan) | SCM440 |
| BS (United Kingdom) | 708M40 |
| UNI (Italy) | 42CrMo4 |
| AFNOR (France) | 42CD4 |
| GOST (Russia) | 38ChM |
| ASTM | A193 B7, A194 7/7M, A29, A322, A519, among others |
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the typical hardness of 4140 steel in annealed condition?
Brinell hardness of annealed 4140 is 197 HB, equivalent to 92 HRB or 207 HV on Vickers scale.
What tensile strength does 4140 reach after quenching and tempering?
In sections 16 to 40 mm, tensile strength between 1000 and 1200 MPa is achieved, with minimum yield strength of 750 MPa.
What is the austenitizing temperature for quenching 4140 steel?
Recommended temperature is 845 °C (1550 °F), followed by rapid cooling in oil.
Up to what temperature does 4140 steel maintain its mechanical properties?
Use is recommended up to approximately 400 °C; above 500 °C the strength drops significantly although it is still used with high tempers.
What machinability index does 4140 have compared to a free-machining steel?
In annealed condition it has an index of 65%, taking AISI 1212 steel with index 100% as reference.
What is the minimum impact energy guaranteed for hardened and tempered 4140?
According to section, a minimum of 35 J (26 ft·lbf) in Charpy V-notch test is required for diameters over 16 mm.
References
Section titled “References”- azom.com: https://www.azom.com/article.aspx?ArticleID=6769
- engineeringtoolbox.com: https://www.engineeringtoolbox.com/compositions-steel-d_1494.html
- steelnumber.com: https://www.steelnumber.com/en/equivalent_steel_iron_eu.php?zname_id=371