What is Decarburization When Heat Treating Alloy Steel Such as 4140?
Decarburization is the loss of carbon from the surface of steel when heat treated above 1292 degrees Fahrenheit, typically in an oxidizing furnace atmosphere, which reduces hardness and tensile strength. For example, to achieve a hardness of R/C 45-53 with a 4140 steel, the furnace carbon atmosphere should ideally be “neutral” not necessarily matching the 40% carbon content of 4140 steel, but in the range of 0.38% – 0.43 % carbon.
Key Aspects of Decarburization:
- Reason for Depletion: Oxygen in the furnace reacts with surface carbon to form gases (CO/CO2), removing it from the surface of the steel.
- Consequences: A soft surface-decarburized layer (ferrite) forms, leading to premature fatigue failure and reduced wear resistance.
- Heat Treatment Goal for 4140 Steel (R/C 45 -53): 4140 steel is typically quenched and tempered for high-strength applications. Decarburization can create a surface layer with a hardness significantly lower than R/C 45-53, rendering the part unfit for use.
- Prevention Methods: To prevent decarburization, use atmosphere-controlled furnaces.
For the best results with material such as 4140, the atmosphere should be controlled to a carbon potential of roughly 0.40% to prevent decarburization, ensuring the surface matches the core hardness.