AISI 4140's mechanical properties are highly dependent on heat treatment, with the quenched and tempered (Q&T) condition being most common for high-performance applications. All properties below are tested according to ASTM standards (tensile tests: ASTM E8; impact tests: ASTM E23; hardness tests: ASTM E10).
1. Annealed State
Annealed AISI 4140 (heated to 815–845°C, slow-cooled) has a microstructure of ferrite and pearlite, offering good machinability for pre-processing.
| Property | Test Method | Typical Value | Significance |
|---|---|---|---|
| Tensile Strength | ASTM E8 | 655–750 MPa | Moderate strength suitable for preliminary machining and forming operations. |
| Yield Strength | ASTM E8 | 415–485 MPa | Low enough to allow cold forming without cracking. |
| Elongation (200mm gauge) | ASTM E8 | 18–22% | Sufficient ductility for bending and machining. |
| Brinell Hardness | ASTM E10 | 197–241 HB | Soft enough for easy cutting with standard tools. |
2. Quenched and Tempered (Q&T) State
The standard heat treatment for AISI 4140 involves quenching (845–870°C, oil-cooled) followed by tempering at various temperatures to achieve desired strength-toughness balances:
| Tempering Temperature | Tensile Strength | Yield Strength | Elongation | Hardness | Impact Toughness (20°C) | Application Focus |
|---|---|---|---|---|---|---|
| 200°C | 1790 MPa | 1650 MPa | 10% | 55 HRC | 20 J | Maximum hardness for wear parts |
| 300°C | 1550 MPa | 1400 MPa | 12% | 49 HRC | 30 J | High-strength components |
| 400°C | 1240 MPa | 1100 MPa | 16% | 42 HRC | 45 J | Balanced strength/toughness |
| 500°C | 1030 MPa | 930 MPa | 18% | 36 HRC | 60 J | General engineering applications |
| 600°C | 860 MPa | 760 MPa | 22% | 28 HRC | 80 J | High-toughness, low-stress parts |
Note: Higher tempering temperatures reduce strength but increase toughness and ductility, allowing customization for specific application requirements.