A chromium-molybdenum engineering steel (AISI 4140 / EN 42CrMo4), one of the most widely used alloy steels for shafts, gears, bolts and tool holders — strong, tough and easy to harden in oil.
What Is 4140 Alloy Steel?
4140 is a low-alloy chromium-molybdenum steel. The chromium and molybdenum give it good hardenability and toughness, so it is the default choice for parts that must be strong and reliable — shafts, gears, bolts, axles and tool holders.
It is cheaper than tool steels, machines well in the annealed state, and oil-hardens to a useful 28 – 34 HRC. It is not stainless and must be protected against corrosion.
Supply Forms and Sizes
Every form is available soft-annealed (for further forming and machining) or heat-treated to your hardness window.
| Form | Condition | Size Range | Typical Use |
|---|---|---|---|
| Round bar | Annealed / pre-hardened | 10 – 300 mm | Shafts, axles, holders |
| Flat bar & plate | Annealed / Q&T | 6 – 200 mm | Gears, blocks, bases |
| Forging stock | Hot rolled | 40 – 300 mm | Forged parts |
| Hex & square bar | Annealed | 12 – 100 mm | Bolts, fasteners |
Chemical Composition (AISI 4140 / EN 10083-3 (42CrMo4))
| C | Mn | Si | Cr | Mo | P max | S max |
|---|---|---|---|---|---|---|
| 0.38 – 0.43 | 0.75 – 1.00 | 0.15 – 0.35 | 0.80 – 1.10 | 0.15 – 0.25 | 0.035 | 0.040 |
Heat analysis is reported on the mill certificate for every lot. If your drawing calls for tighter limits than the standard, flag it at enquiry — ladle-refined heats usually meet it without a surcharge.
Physical Properties
| Property | Value |
|---|---|
| Density | 7.85 g/cm³ |
| Elastic modulus (E) | ~200 GPa |
| Shear modulus (G) | ~79 GPa |
| Thermal conductivity | ~40 W/m·K at room temperature |
| Coefficient of thermal expansion | ~12 ×10⁻₆ /K (20 – 200 °C) |
Mechanical Properties
Typical values for 4140 after oil quenching and tempering near 600 °C. The mill certificate is the binding document.
| Condition | Tensile Strength | Yield Strength | Elongation | Hardness |
|---|---|---|---|---|
| Annealed | ≥ 655 MPa | ≥ 415 MPa | ≥ 18 % | ≤ 217 HB |
| Q&T | 850 – 1000 MPa | ≥ 650 MPa | ≥ 14 % | 28 – 34 HRC |
| Q&T high str. | 1000 – 1200 MPa | ≥ 850 MPa | ≥ 12 % | 34 – 39 HRC |
Equivalent Grades
| Standard | Designation |
|---|---|
| USA AISI / SAE | 4140 |
| Europe EN 10083 | 42CrMo4 (1.7225) |
| Japan JIS G4105 | SCM440 |
| China GB/T 3077 | 42CrMo |
Heat Treatment and Processing
Preheat slowly, then raise to the grade forging range. Work above the lower limit and cool slowly after forging to avoid cracks.
790 – 830 °C, soak, furnace cool. Soft condition for machining.
830 – 860 °C, oil. Full through-harden in moderate sections.
540 – 680 °C by required hardness; lower temp keeps peak strength, higher temp adds toughness.
After machining or forming, 200 – 350 °C for 1 – 2 hours, air cool, to stabilise dimensions and relieve residual stress.
Heat-treated bars and plates are supplied with the hardness and test certificate you specify. For critical parts we can control decarburisation depth and supply a spheroidized or soft-annealed structure on request.
Applications of 4140 Alloy Steel
The classic use; strong and tough in rotation.
Hardened teeth with tough core.
Reliable clamp in machinery.
Stable, machinable bodies.
Buckets, links, spindles in abrasive duty.
Structural parts needing strength.
4140 Alloy Steel vs Alternative Grades
| Grade | vs 4140 Alloy Steel | Choose It When |
|---|---|---|
| 4140 | — | Strong, tough, easy-to-harden engineering steel. |
| 4340 | Adds Ni for more toughness | Aerospace / critical fatigue parts |
| 1045 | Plain carbon, cheaper, weaker | Low-stress general parts |
| 4130 | Lower carbon, more weldable | Tubing, frames where welding dominates |
Quality Control and Mill Certificates
Every shipment ships with an EN 10204 3.1 mill certificate covering heat chemical analysis, hardness and the mechanical test on record. On request we add: ultrasonic test on heavy bar and plate, magnetic particle inspection on machined surfaces, and third-party inspection (SGS / BV) before loading. Dimensional tolerances follow the relevant EN / ASTM / JIS standard or your drawing.
Frequently Asked Questions
What is 4140 equivalent to?
EN 42CrMo4 (1.7225), Japan SCM440 and China 42CrMo are the same chromium-molybdenum steel; confirm against the certificate for critical parts.
What is 4140 steel used for?
Shafts, gears, axles, bolts, tool holders and machine parts that need strength and toughness at moderate cost.
What hardness does 4140 reach?
Typically 28 – 34 HRC in the quenched-and-tempered condition; higher tempers add toughness.
Is 4140 weldable?
It welds with pre-heat and post-heat by a qualified procedure; the hardenable chemistry cracks without it.
What is the minimum order quantity?
Mill lots typically start at 500 kg per item; common bar sizes are often from stock.
Related Products
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