What is action hard

Case hardening is the process of machining a workpiece made of steel by carburizing, hardening and tempering. The so-called case hardening gives you workpieces that have a hard surface and a soft, tough core at the same time. Case-hardening steels are primarily suitable for treatment with this heat treatment process.

Case hardening process

During case hardening, the surface layer of the workpiece is enriched with carbon in a medium suitable for carburizing. As a result of the process, the carbon diffuses from the enriched surface layer into the core of the workpiece. This creates a carbon profile from the edge of the workpiece to the workpiece core, which shows a gradient in the carbon content from the edge to the core. After carburizing, the next process steps for setting the surface hardness and case hardening depth are hardening and tempering

Carburizing

Carburization takes place at temperatures between 880 and 950 ° Celsius in the austenitic state of the steel. If the carburizing process takes place at temperatures of more than 950 ° C, it is called high-temperature carburizing.

Case hardening process sequence

The surface layer is first enriched with carbon through the carburization process. The carbon then diffuses from the enriched surface towards the core of the workpiece. The carbon required for this is obtained from a carbon-releasing medium. During the carburization, the core usually retains its basic carbon content according to the carbon content of the alloy used. The carburization depths are often between 0.1 and 4.0 mm.

Typical methods of carburizing are

  • Carburizing in molten salts
  • Carburizing in carbon powder or carbon granulate
  • Carburization in gas atmospheres
  • Carburizing in negative pressure with or without plasma assistance
Structure and functionality of case hardening in a gas atmosphere

Hardening

In case hardening, the carburization is followed by the hardening of the workpiece. Hardening and tempering give the workpiece high strength and surface hardness, while the workpiece core remains in a tough, tempered condition.

The surface hardness of a case-hardened steel is largely determined by the surface carbon content. According to the course of the carbon present in the surface layer, quenching results in a course of the hardness depth due to the two properties Edge hardness and Case hardening depth can be defined.

The case hardening depth is influenced by three factors: the carburizing depth set during carburizing, the hardenability of the steel and the quenching intensity of the quenching medium used.

Typical hardening processes and media

Typical methods and media for hardening consist of quenching in liquid quenching media or quenching in gaseous quenching media. Water, polymer, hardening oil, molten salt or molten metal are typically used for quenching in liquid quenching media. The gaseous media used for quenching are nitrogen, helium, a gas nozzle field or high-pressure gas quenching.

Quenching media:

  • water
  • polymer
  • Hardening oil
  • Molten salt
  • Molten metal
  • nitrogen
  • helium
  • Gas nozzle field quenching
  • High pressure gas quenching

The hardening of a workpiece is followed by tempering as directly as possible in order to increase the ductility of the initially relatively hard martensite of the carburized surface layer.

Suitable materials for case hardening

Materials suitable for hardening are case-hardening steels or structural steels, which are characterized by a relatively low carbon content and can be unalloyed or low-alloyed. Suitable steels have a base carbon content of <0.25 mass% carbon. Materials that are particularly frequently used for hardening are therefore:

  • 1.6587 - 17CrNiMo6
  • 1.0301 - C10
  • 1.7131 - 16MnCr5
  • 1.7147 - 20MnCr5

Objective of the procedure

The aim of the case hardening process is to improve the mechanical properties of a steel workpiece. This includes, among other things, an increased surface hardness to increase wear resistance and resilience, a tough core to improve flexural fatigue strength and overload tolerance, as well as the improvement of fatigue strength. The latter is significantly influenced by the martensite formation during hardening. This leads to an increase in volume, which is higher in the carbon-rich surface layers than in the low-carbon workpiece core. Due to this effect, internal compressive stresses build up on the surface, which counteract the tensile stresses caused by bending and torsional loads. Cracks then only appear at higher stresses. Case hardening is therefore preferred in the manufacture of highly stressed parts such as those used in drives and gears.

Alternative methods of hardening steel

Carbonitriding

Carbonitriding is a process related to carburization. In addition to carbon, nitrogen is also introduced into the surface layer.

Surface hardening

Laser hardening, electron hardening or inductive hardening are alternative methods for case hardening.