Case hardening

Using hardening is defined as the carburizing, hardening and tempering of a workpiece made ​​of steel.

The aim of curing is used, a soft and tough core with a hard surface of the same material. The boundary layer of the workpiece is enriched in a suitable Aufkohlungsmedium with carbon. Due to the diffusion of carbon from the surface layer enriched in the nucleus, a carbon profile is one that typically has an edge with increasing distance to the core falling variation of the carbon content. Following carburization, the hardening and tempering is performed. Thereby, the surface hardness and hardening depth is adjusted.

Carburization

The carburization is carried out in the austenitic state of the steel, that is to say at temperatures above the transformation point Ac3, in general between 880-950 ° C. If temperatures higher than 950 ° C is applied, spoken by Hochtemperaturaufkohlen. The currently technically achieved maximum temperature for a carburizing process with subsequent Direct quenching is at 1050 ° C. In carburizing carbon transmitted emitting medium on the workpiece surface in the component made of a carbon. The diffusion of the carbon takes place from the enriched surface towards the core. The core retains its base carbon content corresponds to the carbon content of the alloy used in the carburization in the rule. When a carburizing surface carbon course is set, which has the characteristics of surface carbon content and depth of carburization. Typical surface carbon levels are from 0.5 to 0.85 wt% carbon content. Depending on the application of the components but also lower or higher surface carbon levels are sought. Common depths up to between 0.1 and 4.0 mm.

Typical methods used for the carburizing, are:

  • Carburization in molten salts
  • Carburization in Kohlungspulver / granular
  • Carburization in gas atmospheres
  • Carburization in vacuum with or without plasma enhancement

Application-specific, it may be necessary to carburize only portions of a workpiece ( Partial carburizing ). When Salzbadaufkohlen this is achieved in that only the regions of the workpiece to be carburized is dipped in the salt bath. It should be noted that this can not give a precise contour fidelity diffusion / curing, because the diffusion in the peripheral portion of the non-immersed workpiece progresses low. When gas carburizing and Unterdruckaufkohlen, currently the most common method, a crisp isolation is possible by applying off coatings: The diffusion of carbon is prevented, so that worked or mechanically after hardening in the isolated areas can be cold formed or welded. Also in threaded areas is often a carburizing undesirable because it would lead to embrittlement of the thread crests.

A related method is the carbonitriding in which besides carbon and nitrogen is introduced into the boundary layer.

Following carburization, the curing of the component takes place. According to the carbon profile in the boundary layer results in quenching a hardness depth profile with the characteristics of surface hardness and case depth. The surface hardness of case-hardened steel is mainly determined by the carbon content. The set in carburizing, carburizing, the hardenability of the steel used and the quenching of the quenching medium used affect the hardening depth. Following curing of the components is as close as possible started to give the first extremely hard martensite edge of the carburized layer again more ductility.

Typical media and processes which are used for curing include:

  • Quenching in liquid quenching media: water
  • Hardening oil
  • Polymer
  • Molten salt
  • Molten metal
  • Nitrogen
  • Helium
  • Gas nozzle field
  • High pressure gas quenching

The hardening and tempering gives the component a high surface hardness and strength. The core, however, remains in a tough tempered condition.

Alternative method for case hardening is the electron, laser hardening or induction hardening.

Execution

The thus achieved hardening is checked by means of a hardness gradient at right angles to the surface at the cross section. In addition usually the method is applied according to Vickers. In the engineering drawing is called the nominal specification case depth in the form Ehtxxx a, a - b, b, b indicated with xxx as Vickers hardness value, a, a and b as the minimum value as the maximum value.

Example: Eht550 1,0 - 1,5 means that the hardness starting from the edge between 1.0 and 1.5 mm depth must fall below the value of 550 HV.

From Vickers hardness testing methods differ (eg Knoop ) are appended to the numerical value of the hardness limit as another name.

Suitable materials

Hardening steels or structural steels with relatively low carbon content, unalloyed or low alloyed. Suitable materials are steel with a base carbon content of less than 0.25 mass % of carbon. Very commonly used materials are, for example, 1.6587/17CrNiMo6; 1.0301/C10; 1.7131/16MnCr5; 1.7147/20MnCr5.

Objective of the procedure

Improvement in the mechanical properties, in particular:

  • Increase the wear resistance due to increased surface hardening
  • Increase the resilience
  • Improve the bending strength and overload tolerance through tough core
  • Increasing the fatigue strength ( The martensite during hardening leads to an increase in volume. This is the carbon-rich surface layers higher than in the low-carbon core, why build up at the surface compressive residual stresses. These counteract the tensile stresses in bending or torsion, which is why a scribe at higher tension occurs. )

Case hardening is the preferred method for driving parts and gears.

See also

  • Cure (steel)
  • Change material properties
  • Steel
  • Metallurgy
  • Metal processing
  • Gear
Tempering (metallurgy) Austenite
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