4140 Alloy Steel: Strength, Toughness and Heat Treatment
4140 is a low-alloy chromium-molybdenum (Cr-Mo) steel containing about 0.40% carbon, 0.8–1.1% chromium and 0.15–0.25% molybdenum. The chromium raises hardenability and corrosion resistance slightly, while molybdenum improves high-temperature strength and prevents temper embrittlement. 4140 is renowned for its combination of high tensile strength, good fatigue resistance and reasonable weldability after proper preheat.
In the annealed state 4140 has a tensile strength near 655 MPa. After quenching and tempering it is usually supplied at a hardness of 28–36 HRC, balancing strength and toughness for demanding components such as crankshafts, axles, gears, drill collars, bolts and machinery shafts.
Heat treatment is the key to performance. Austenitise at 840–870 °C, quench in oil (water quenching risks cracking on larger sections), then temper at 200–650 °C depending on the target hardness. Higher tempering temperatures reduce hardness but greatly improve toughness and dimensional stability. Because 4140 hardens deeply, welds should be preheated to 200–300 °C and post-weld stress-relieved to avoid hydrogen cracking.
For buyers, the critical checks are chemistry (especially Mo and Cr), ultrasonic or magnetic-particle inspection for inclusions, and a verified heat-treatment record. Oversized quench cracks and improper tempering are the most common quality failures. Compared with 4340, 4140 is cheaper and easier to machine but offers lower ultimate toughness — choose 4340 only when peak impact resistance is required.
In the annealed state 4140 has a tensile strength near 655 MPa. After quenching and tempering it is usually supplied at a hardness of 28–36 HRC, balancing strength and toughness for demanding components such as crankshafts, axles, gears, drill collars, bolts and machinery shafts.
Heat treatment is the key to performance. Austenitise at 840–870 °C, quench in oil (water quenching risks cracking on larger sections), then temper at 200–650 °C depending on the target hardness. Higher tempering temperatures reduce hardness but greatly improve toughness and dimensional stability. Because 4140 hardens deeply, welds should be preheated to 200–300 °C and post-weld stress-relieved to avoid hydrogen cracking.
For buyers, the critical checks are chemistry (especially Mo and Cr), ultrasonic or magnetic-particle inspection for inclusions, and a verified heat-treatment record. Oversized quench cracks and improper tempering are the most common quality failures. Compared with 4340, 4140 is cheaper and easier to machine but offers lower ultimate toughness — choose 4340 only when peak impact resistance is required.
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