Introduction
40Cr is a quality alloy structural steel belonging to the medium-carbon quenched-and-tempered (Q&T) steel family. It is one of the most widely used alloy steels in mechanical manufacturing, combining good hardenability, high strength, and adequate toughness.
The key differentiator of 40Cr versus plain carbon steels (such as 45 steel) is the addition of chromium (Cr, 0.80–1.10%), which significantly improves hardenability and strength. After quenching and tempering, 40Cr achieves tensile strength of 800–1000 MPa — substantially higher than 45 steel (~600 MPa) — while maintaining good toughness and ductility for structural applications.
40Cr is governed by the Chinese national standard GB/T 3077 (Alloy Structural Steels). It is approximately equivalent to:
- AISI/SAE 5140 (USA)
- 41Cr4 / 1.7035 (DIN EN, Germany)
- SCr440 (JIS G4053, Japan)
- 40X (GOST, Russia)
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Chemical Composition
Reference: GB/T 3077 (Alloy Structural Steels)
| Grade | C (%) | Si (%) | Mn (%) | Cr (%) | S (%) | P (%) | Ni (%) | Cu (%) |
|---|---|---|---|---|---|---|---|---|
| 40Cr | 0.37–0.44 | 0.17–0.37 | 0.50–0.80 | 0.80–1.10 | ≤ 0.035 | ≤ 0.035 | ≤ 0.30 | ≤ 0.30 |
Key alloying elements:
- Carbon (C, 0.37–0.44%): Medium carbon content provides the base strength and hardenability. Sufficient for significant hardening through quenching.
- Chromium (Cr, 0.80–1.10%): The defining alloying element. Cr improves hardenability (deeper hardening depth), increases tempering resistance, and enhances wear resistance and corrosion resistance compared to plain carbon steel.
- Manganese (Mn, 0.50–0.80%): Provides solid solution strengthening and improves hardenability.
- Silicon (Si, 0.17–0.37%): Deoxidizer; slightly improves strength and elastic limit.
Mechanical Properties
General Properties (GB/T 3077)
| Grade | Tensile Strength σb (MPa) | Yield Strength σ0.2 (MPa) | Elongation δ5 (%) | Reduction of Area ψ (%) | Hardness | Density (g/cm³) | Melting Point (°C) |
|---|---|---|---|---|---|---|---|
| 40Cr | ≥ 800 | ≥ 650 | ≥ 13 | ≥ 45 | ≤ 207 HBW (annealed) / 25–38 HRC (Q&T) | 7.85 | 1420–1460 |
Mechanical Properties by Sample Size (Q&T Condition)
| Sample Diameter (mm) | Heat Treatment | Tensile Strength σb (MPa) | Yield Strength σs (MPa) | Elongation δ (%) | Reduction of Area ψ (%) | Hardness (HBW) | Impact Energy KV2 (J) |
|---|---|---|---|---|---|---|---|
| d = 25 | Quench + Temper | ≥ 980 | ≥ 785 | ≥ 9 | ≥ 45 | ≤ 207 | ≥ 47 |
Heat Treatment
40Cr responds excellently to heat treatment. Standard procedures:
| Process | Temperature | Cooling Method | Result |
|---|---|---|---|
| Normalizing | 850–900 °C | Air cool | Improved machinability, uniform structure |
| Annealing | 800–850 °C | Furnace cool | ≤ 207 HBW, maximum softness for machining |
| Quenching | 850 °C | Oil quench | High hardness (up to ~55 HRC before tempering) |
| Tempering (low) | 200–300 °C | Air cool | High hardness (45–55 HRC), wear-resistant surface |
| Tempering (medium) | 400–500 °C | Air cool | High strength + toughness balance (35–45 HRC) |
| Tempering (high / Q&T) | 500–650 °C | Air cool | σb ≥ 800–1000 MPa, good toughness (25–38 HRC) |
Quenching medium: Oil quench is standard for 40Cr. Water quenching risks cracking due to the higher hardenability from Cr addition.
40Cr vs 45 Steel: Key Differences
| Criteria | 45 Steel (plain carbon) | 40Cr (alloy steel) |
|---|---|---|
| Cr content | None | 0.80–1.10% |
| Hardenability | Limited (shallow hardening) | Good (deeper hardening) |
| Tensile strength (Q&T) | ~600 MPa | ≥ 800–1000 MPa |
| Toughness | Good | Good |
| Quenching medium | Water | Oil (safer, less distortion) |
| Cost | Lower | Slightly higher |
| Best for | General structural parts, moderate loads | High-load shafts, gears, bolts, critical structural parts |
Applications of 40Cr Alloy Steel
- Mechanical Manufacturing — machine tool spindles, transmission shafts, gears, bolts, and nuts subject to high loads and impact forces
- Automotive Industry — camshafts, connecting rods, transmission gears, and other high-strength drivetrain components
- Construction Machinery — hydraulic system components, transmission parts in excavators, cranes, and heavy equipment operating in harsh environments
- High-strength fasteners — Grade 10.9 and 12.9 bolts and studs (after quenching and tempering)
- Tooling & dies — mold bases, jig components, and tooling fixtures requiring high strength
Standards reference: GB/T 3077 | AISI/SAE 5140 | DIN EN 41Cr4 (1.7035) | JIS G4053 SCr440 | GOST 40X
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