The Heat Treatment Of 4140 Alloy Steel

4140 steel is a low-alloy steel valued for its high strength, toughness, and wear resistance. You might see it referred to as “chromoly,” due to the chromium and molybdenum content. The chrome makes it more suitable for hardening (as well as more corrosion resistant); the molybdenum makes it tougher and more stable at high temperatures.

4140 steel offers several advantages related to its balance of strength and ductility:

• Tensile strength – Great for jobs that require significant strength
• Formability – High ductility allows forming with conventional techniques
• Machinability – Above average for its hardness

Common Applications

• Automotive and transportation – Crankshafts, axles, cylinder shafts, connecting rods, and other high-stress components, performance bike frames
• Tooling – molds, mold bases, ejectors, fixtures, tool bodies, tool holders
• hydraulic equipment –cylinder piston rods, gears, sprockets, gear racks, valves, chain links, spindles,
• Dies – Brake, trim, and forming
• Machinery and wear parts

Heat Treatments for 4140

Heat treat 4140 steel to achieve a wide range of mechanical properties, including high tensile strength, good fatigue strength, and excellent toughness. The following are some guidelines. We always recommend talking to a metals expert about the specifics of your material and your application.

Annealing

Soften the steel to facilitate machining and other work.

1. Heat your workpiece in the furnace up to 1450 – 1600°F.
2. Hold for 1 hour (additional time for thicknesses over 1 inch).
3. Turn off the furnace and allow the metal to cool inside.

Normalizing

Improve mechanical properties.

1. Heat your workpiece in the furnace to 1600 – 1700°F.
2. Hold for at least 30 minutes, preferably more.
3. Remove the metal from the furnace and allow to cool in ambient air.

Hardening & Quenching

Optimize strength and toughness.

1. Heat in your furnace to 1550 – 1600°F.
2. Hold for at least 30 minutes per inch of thickness.
3. Remove from the furnace and quench the workpiece immediately in mineral oil.
4. When the workpiece has cooled to 150°F, proceed to tempering.

Tempering

Minimize brittleness and risk of cracking. Lower temperatures will yield higher tensile strength; higher temperature will reduce it. Expect a solid 225 ksi tensile strength (as well as hardness of ~ Rockwell C 50) at 600°F, dropping to about 130 ksi at 1000°F.

1. Heat in your furnace to 400 – 1200°F.
2. Hold at temperature for 15 minutes per inch of thickness.
3. Remove from the furnace and air cool.