Decarburization

Decarburization is the loss of carbon from steel or iron, typically occurring at or near the surface during high-temperature processing. It involves the diffusion of carbon from the metal toward the exterior, where it is removed as gases such as carbon monoxide (CO) or carbon dioxide (CO2) or reacts with the surrounding atmosphere. The phenomenon is most common during heat treatments conducted in furnace environments that contain oxygen or that otherwise promote carbon removal.

The process generally takes place at elevated temperatures, often above 800–900°C, and is influenced by exposure time, temperature, carbon content, alloy composition, and the chemistry of the surrounding atmosphere. Materials with higher carbon content and those subjected to prolonged heating are more prone to decarburization. Conditions that promote decarburization include air, oxygen-containing atmospheres, or oxidizing, mildly reducing, or carburizing environments where diffusion gradients enable carbon to migrate outward.

The main effect of decarburization is the formation of a softer, lower-carbon surface layer. This decarburized layer typically exhibits reduced hardness, strength, and wear resistance compared to the core, and it can alter fatigue life and dimensional stability. Depth of decarburization is a critical concern in components that require surface hardness, such as gears, shafts, and bearings.

Prevention strategies focus on maintaining protective processing environments and controlling exposure time. Methods include using inert, reducing, or vacuum atmospheres, applying surface coatings or oxide scales to hinder diffusion, shortening heating cycles, or selecting alloys less susceptible to carbon loss. Decarburization is generally undesirable for most structural steel applications, though it can be managed or mitigated through careful process control.