Case hardening

Case hardening is a thermochemical process for surface hardening. The process improves the mechanical properties of components made of steel. Case hardening consists of the process steps carburizing, hardening and tempering.

The process makes it possible to impart a soft, tough core to a workpiece while at the same time achieving a higher surface hardness. Workpieces processed by case hardening are characterized by increased wear protection and improved reverse bending resistance.

The steps in case hardening

The basis of the process is the enrichment of the surface layer of a material with carbon. This results in a workpiece with a carbon content that decreases progressively toward the core carbon content. The process steps in case hardening are carburizing, hardening and tempering.

Carburizing

In carburizing, the steel is in the austenitic phase. The process temperature is therefore between 880 and 950 °C. With the help of a suitable medium for carbon release, the heat treatment enriches the surface layer of the workpiece with carbon. Diffusion then takes place from the surface layer towards the core, with the core normally retaining its carbon content. During carburizing, the carburization depth is set, which is usually between 0.1 and 4 mm. 

Suitable methods for carburize are:

  • Carburizing in the molten salt.
  • Carburization in carbon powder or granules.
  • Carburizing in gas atmosphere
  • Carburizing in the low pressure process

Hardening

The second process step is the actual hardening, which is carried out by cooling the workpiece. Here, the surface hardness is determined by the carbon content in the uppermost layers of the material. Decisive factors are also the set carburization depth, the quenching intensity (cooling rate) and the hardenability of the machined steel.

The following liquid and gaseous substances serve as quenching media:

  • water
  • hardening oil
  • molten salt
  • polymer
  • molten metal

Tempering

After hardening, the final process step is tempering. The aim of heat treatment is to increase toughness. In case hardening, the process temperature for this step is between 160 and 220° C. As a result, the surface layer of the workpiece increases in ductility.

Advantages of the process

Case hardening is a heat treatment used to improve the mechanical properties of a steel workpiece. It has the following advantages:

  • The increased surface hardness leads to improved wear resistance.
  • It also increases the load capacity of the workpiece.
  • The continued toughness of the core leads, and the compressive internal stresses leads to increased reversed bending resistance and overload tolerance
  • Partial hardening is possible with the process.

Variations of case hardening

Single hardening

Single hardening is a variant of case hardening. It is often used for steels that tend to form coarse grains. After carburizing, the workpiece is first cooled slowly after carburizing. In the following step, further heat treatment is carried out. As a result, a recrystallization effect occurs, which leads to grain refinement. 

Single hardening is aimed at the desired properties of the surface layer of the workpiece. Due to the low process temperatures of around 750° C, the low-carbon core is not completely austenitized. Accordingly, a completely martensitic core structure does not form after quenching. This process is also called surface hardening.

Hardening of the core is also possible. Here, the process temperature is adapted to the desired properties of the core and is approximately 900° C. The core is then hardened at a higher temperature. The higher temperatures cause the edge layer to become coarsely needled. The process is also called core hardening.

Double hardening

Double hardening is a combination of surface hardening and core hardening. Here, two hardening processes are carried out, the first at the hardening temperature of the core, the second at a hardening temperature of the surface layer. However, the many temperature changes can cause distortion of the workpiece.

Compared to direct hardening, single and double hardening are more cost-intensive because they are time and energy consuming. For this reason, the processes are mostly used for steels that tend to form coarse grains.

Which materials is case hardening suitable for?

The aim of case hardening is to increase the carbon content. For this reason, only steels with a basic carbon content of less than 0.25% by mass are normally considered. Commonly, the treated materials are unalloyed or low-alloyed. The following steel grades are frequently used for the process:

  • 1.6587/18CrNiMo7-6
  • 1.0301/C10
  • 1.7131/16MnCr5
  • 1.7147/20MnCr5

Typical applications of case hardening include the manufacture of gear parts and gears, as these require high wear protection and good flexural fatigue strength.

A process similar to case hardening is carbonitriding. In this process, nitrogen is diffused into the surface layer in addition to carbon.