4140

AISI 4140 is a chromium-molybdenum alloy steel widely used for high-strength machine parts. It is part of the 41xx family and is valued for a combination of strength, toughness, and wear resistance. The alloy is commonly supplied in annealed form for machining or in quenched-and-tempered forms for high-strength applications.

The chemical composition of 4140 is approximately: carbon 0.38–0.43%, manganese 0.75–1.00%, silicon 0.15–0.30%, chromium 0.80–1.10%, molybdenum 0.15–0.25%, with phosphorus and sulfur each limited to about 0.04% or less. This combination provides good hardenability and the ability to achieve high strength with heat treatment.

In terms of properties, 4140 offers a favorable balance of strength, toughness, and wear resistance, especially after heat treatment. In the annealed state it is relatively machinable; when quenched and tempered, it develops high strength and hardness while retaining toughness, and it exhibits good fatigue performance and wear resistance. Hardness and mechanical properties vary with the tempering temperature.

Heat treatment typically involves annealing for machinability, or a quench-and-temper cycle to reach desired strength. A common process is heating to around 850–900°C, quenching in oil, and tempering at temperatures in the mid-range (roughly 400–650°C) to achieve a range of properties suited to the application.

Welding is possible but must be approached with care; the heat-affected zone can weaken joint strength, so preheating, controlled heat input, and post-weld heat treatment are often recommended. The alloy is widely available in bar, plate, tubing, and forged forms.

Applications include gears, shafts, axles, connecting rods, fasteners, valve stems, and other high-strength components in automotive, aerospace, oil and gas, and general engineering.