4130 ALLOY StEEL

4. Heat treatment

The heat treatments for 4130 steel include normalizing, annealing, hardening, tempering, and stress relief.

4.1 Normalizing

The purpose of normalizing is to refine the grain size and obtain a complete solution of the original structure with the formation of austenite.

We suggest that the normalizing temperature be 870-925°C (1600-1700°F), with a soaking time of at least 1 hour or 15 to 20 minutes per 25 mm (1 inch) of maximum section thickness, followed by air cooling.

4.2 Annealing

Annealing is used to soften AISI 4130, thereby increasing its workability and machinability.

The annealing temperature range is 830 to 870°C (1525 to 1600°F), with the holding time varying depending on the section thickness or furnace load. It is then cooled at a rate of 15 °C/h (30 °F/h) to 480 °C (900 °F) and finally air-cooled.

4.3 Hardening (Quenching)

Hardening aims to transform the 4130 steel’s microstructure into martensite or bainite, providing high hardness and toughness. 

The quenching medium is water or oil. The hardening temperature is 860 to 885 °C (1575 to 1625 °F) for oil quenching or 845 to 870 °C (1550 to 1600 °F) for water quenching. 

Because 4130 material has low hardenability, its section thickness must be carefully considered when aiming for high hardness or strength. When compared to 4140 steel, which has higher carbon content and thus greater hardenability, 4130 material are less prone to cracking and distortion during quenching.

4.4 Tempering

Tempering is performed after hardening to reduce internal stresses, increase ductility and toughness, and adjust hardness to the desired level.

The tempering temperature range for 4130 steel is broad, from 200 to 700 °C (400 to 1300 °F). The specific temperature chosen will depend on the desired final properties. Tempering should occur as soon as the 4130 parts reach 52 to 65 °C (125 to 150 °F) after quenching.

Tempering significantly influences the room-temperature mechanical properties of  AISI 4130, including tensile strength, yield strength, and Izod impact energy.

4.5 Stress Relieving

Stress relief heat treatment is used to eliminate stresses generated in 4130 material during processing, such as straightening, forming, or machining. 

We recommend a temperature range of 650 to 675°C (1200 to 1250°F). For hardened 4130 material, the stress relief temperature should not exceed the tempering temperatures previously applied.