Carburizing

Longer carburizing times and higher temperatures typically increase the depth of carbon diffusion.

When the iron or steel is cooled rapidly by quenching, the higher carbon content on the outer surface becomes hard due to the transformation from austenite to martensite, while the core remains soft and tough as a ferritic and/or pearlite microstructure.

[3] Early carburization used a direct application of charcoal packed around the sample to be treated (initially referred to as case hardening), but modern techniques use carbon-bearing gases or plasmas (such as carbon dioxide or methane).

The process depends primarily upon ambient gas composition and furnace temperature, which must be carefully controlled, as the heat may also impact the microstructure of the remainder of the material.

For applications where great control over gas composition is desired, carburization may take place under very low pressures in a vacuum chamber.

Plasma carburization is increasingly used to improve the surface characteristics (such as wear, corrosion resistance, hardness, load-bearing capacity, in addition to quality-based variables) of various metals, notably stainless steels.

In pack carburizing, the workpiece and carbon are enclosed in a container to ensure that contact is maintained over as much surface area as possible.

Pack carburizing containers are usually made of carbon steel coated with aluminum or heat-resisting nickel-chromium alloy and sealed at all openings with fire clay.

In liquid carburizing, the carbon is derived from a molten salt composed mainly of sodium cyanide (NaCN) and barium chloride (BaCl2).