Carburization

What is Carburization?

Carburization of steel is a process that steel parts with low carbon content are subjected to heating or thermal insulation in carburizing medium, and make carbon atoms into the surface, then obtain a certain carbon content on surface, which get a high hardness after quenching. However the core of the parts still has low hardness, but good toughness. It’s used to improve surface hardness, wear resistance, contact fatigue strength and bending fatigue strength, and is mainly applied to the parts that have to bear wear, alternating contact stress, bending stress and impact load, such as axles, gears, cam shafts, etc. These parts require surface with high hardness and the cores with enough strength and toughness.

Different Types of Carburization

Carburization consists of solid carburization, liquid carburization, gas carburization and vacuum carburization.

Solid carburization is a process that put the parts into the solid carburizing medium (comprised of charcoal powder, BaCO₃, Na₂CO₃ and other promoters), then it’s put in carburization container and heated to 900 ℃-930 ℃ for a while. Liquid carburization is a process that immerse the parts into the fused-salt bath (mainly consists of NaCN, NaCl, NaCO₃, Na₂CO₃ and other additives) and get the C and N on surface generated from NaCN. Gas carburization is to put the parts in container where there is CH₄ and CO and is subjected to heating, and then make carbon atoms into then surface.

Carburized layer depth can reach several millimeters, and the depth raises with the increase of carburization time, and the increase of temperature. However its speed goes down with the extension of time. Normally, we can adjust to copper plating or reserved machining allowance to protect the area from carburization.

The parts need to be treated by quenching and low temperature tempering after carburization to get the target hardness (HRC55-65). Notice that start tempering around 150℃ when high hardness is important; And start tempering around 180℃-200℃, when you aim at keeping the accuracy and preventing time-dependent deformation.