Looking for a tough, versatile steel for rugged applications? 4140 steel is always worth a look. To make the most of this steel’s advantages, including high strength-to-weight ratio and high-temperature stability, heat treating 4140 is essential. So let’s explore the key steps and success factors for 4140 heat treatment. But first, some background ...

What Is 4140 Steel?

4140 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.

Important Properties

4140 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

Composition

Common Applications

• Automotive and transportation – Crankshafts, axles, shafts, connecting rods, and other high-stress components, performance bike frames
• Tooling – molds, mold bases, ejectors, fixtures, tool bodies, tool holders
• hydraulic equipment – shafts, 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.

Success Factors & Considerations

There are several key considerations to ensure success when heat treating 4140 steel. Here are some important ones:

1. Protect the workpiece. Use an inert gas in the furnace to avoid surface oxidation and decarburization. This argon gas kit from Hot Shot Ovens can help.
2. Mind the part geometry. To get optimal results, complex or thick parts may require longer hold times at temperature or during quenching.
3. Temper your tempering. Tempering is essential, but if you want to maximize hardness, stay on the low end of temperature and soak times. Test and repeat tempering as desired.
4. Stay safe. Wear protective gear when working with high temperatures and hot oil. Use a well-insulated furnace, such as a Hot Shot Oven with Cool-Touch Technology.

With those thoughts in mind, go ahead and put the toughness of 4140 to the test in your next project.