Carburizing Mild Steel – How it works

Mild steel is a type of carbon steel that has low contents of carbon. This is also why mild steel is known as “low carbon steel” or “plain carbon steel”. Steel types with higher carbon contents and very little other alloys are known as higher carbon steels (0.3 – 2%) or cast iron (>2%). Due to its relatively small amounts of carbon, mild steel is more malleable and therefore easier to work with than higher carbon steels. However, such low carbon content also makes mild steel relatively soft, which is why case hardening mild steel is a popular and (relatively) easy procedure to improve mechanical properties.

About Mild Steel

To learn how case hardening mild steel works, it is recommended to understand the material first. The level of carbon in mild steels range from 0.05 to 0.25%, which is why not all mild steels are the same. The higher the carbon content, the harder the material.

Mild Steel is also more ductile and weldable than most other steel types. This is one of the main reasons why mild steel is currently the most widely used and produced steel type in the world. Processing mild steel is much cheaper than higher carbon steels and alloy steels. While it exhibits comparatively little hardness and strength (yield and tensile), its mechanical properties suffice for many areas of application.

Mild steel possesses a ferritic structure due to its extremely high amount of iron. This structure is responsible for the magnetic properties of mild steel.

Why Low Carbon Steels Should Not be Heat Treated

On the downside, mild steels are generally harder to work with when it comes to only heat treatment and quenching. It is possible to do it, but there would be little to no change. Due to its low carbon and alloy elements content, mild steel does not form a martensite structure when quenched after being heated.

Carburizing mild steel – A Guide

While it is true that only heating and quenching isn’t a good choice to harden low carbon steels, the same isn’t necessarily true for another method of case hardening – namely carburizing.

Carburizing is the process of diffusing carbon into the surface of low-carbon steels to increase hardness. The material is then quenched, so the carbon is locked in place. The specific steps of case hardening mild steels are as follows:

  • First, the object is heated to austenitization temperature, and then exposed to a cas based carburizing atmosphere.
  • Depending on the required depth and carbon level, the object is kept at this constant temperatures for a few minutes to up to several hours.
  • After the diffusion process is finished, the object is quenched in oil or water.
  • Carburizing depth is determined by application and material and can range from microns to several millimeters.

Advantages of case hardening mild steels

Mild steels that have been carburized have a hard surface and a soft core. This means that case hardened low carbon steels are harder but not brittle. The core retains its ductility and toughness to a large degree while being protected by the hard surface. This improved surface also shows better resistance against wear and fatigue. With the help of appropriate protective pastes, partial carburizing is also possible.


Carburizing is a good way to increase the hardness of your materials. However, the BORINOX® PROCESS FOR HARDENING STAINLESS STEEL and nickel based alloys not only hardens the surface of the workpiece, but also creates a wear-resistant surface layer. The process can be carried out at low temperatures, is extremely dimensionally stable and efficiently prevents cold welding. If you need professional advice, contact our material specialist today.