Case hardening

from Wikipedia, the free encyclopedia

Case hardening is a process for surface hardening ( surface hardening ), consisting of carburizing , hardening and tempering a workpiece made of steel .

The aim of case hardening is a soft and tough core with a hard surface of the material at the same time. The surface layer of the workpiece is enriched with carbon in a suitable carburizing medium. As a result of the diffusion of the carbon from the enriched surface layer into the core, a carbon profile is established which typically has a progression of the carbon content that decreases with increasing distance from the edge to the core. Hardening and tempering are carried out after the carburization. This sets the edge hardness and case hardening depth.

Carburization

Carburization takes place in the austenitic state of the steel, that is to say at temperatures above the transformation point A c 3 , usually between 880 and 950 ° C. If temperatures of more than 950 ° C are used, we speak of high-temperature carburizing. The currently technically realized maximum temperature for a carburizing process with subsequent direct hardening is 1050 ° C. During carburization, carbon is transferred from a carbon-releasing medium into the component via the workpiece surface. The carbon diffuses from the enriched surface towards the core. During the carburization, the core usually retains its base carbon content, which corresponds to the carbon content of the alloy used. In the case of carburization, a surface carbon profile with a characteristic surface carbon content and a specific carburization depth is set. Typical marginal carbon contents are from 0.5 to 0.85 mass% carbon content . However, depending on the area of ​​application of the components, lower or higher marginal carbon contents are also aimed for. Common carburization depths are between 0.1 and 4.0 mm.

Typical methods used for carburizing are:

  • Carburization in molten salts
  • Carburization in carbon powder / granulate
  • Carburization in gas atmospheres
  • Carburization in negative pressure with or without plasma support

Depending on the application, it may be necessary to carburize only parts of a workpiece (partial carburizing). In the case of salt bath carburizing, this is achieved in that only the workpiece areas to be carburized are immersed in the salt bath. It should be noted that this cannot result in an exact contour-true diffusion / hardening, because the diffusion progresses slightly in the edge area of ​​the non-immersed workpiece. In the case of gas carburizing and vacuum carburizing, the most common methods currently used, the application of hardening protective pastes enables sharp-edged insulation: The diffusion of carbon is prevented, so that after hardening, the insulated areas can still be mechanically processed, cold formed or welded. Carburization is also often undesirable in thread areas because it would lead to embrittlement of the thread tips.

A related process is carbonitriding , in which not only carbon but also nitrogen is introduced into the surface layer, and nitriding , in which only the nitrogen content is increased.

Hardening / quenching

Following the carburization, the component is hardened / quenched , i.e. the heated workpiece is rapidly cooled using quenching agents. When quenching, according to the carbon profile in the surface layer, a hardness depth profile results with the characteristic features: surface hardness and case hardening depth. The surface hardness of a case-hardened steel is largely determined by the surface carbon content. The case hardening depth is influenced by the carburizing depth set during carburizing, the hardenability of the steel used and the quenching intensity of the quenching medium used.

Typical media or processes that are used for hardening / quenching are:

  • Quenching in liquid quenching media:
  • Quenching in gaseous quenching media:
    • nitrogen
    • helium
    • Gas nozzle field

Starting

After hardening, the components are tempered as soon as possible in order to give the initially extremely hard martensite of the carburized surface layer more ductility .

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

An alternative method to case hardening is electron hardening, laser beam hardening or inductive hardening .

execution

The hardening achieved in this way is checked by means of a hardness curve perpendicular to the surface on the cross section. The Vickers method is usually used for this purpose. In the design drawing, the case hardening depth is specified as a target in the form CHD xxx a, a - b, b with xxx as the Vickers hardness value, a, a as the minimum value and b, b as the maximum value.

Example: CHD 550 1.0 - 1.5 means that the hardness from the edge must fall below the value 550 HV between 1.0 and 1.5 mm depth.

Hardness testing methods other than Vickers (e.g. Knoop ) are added to the numerical value of the limit hardness as a further designation.

Suitable materials

Case-hardening steels or engineering steels with a relatively low carbon content, unalloyed or low-alloyed . Suitable materials are steels with a base carbon content of less than 0.25 mass% carbon. Very frequently used materials are z. B. 1.6587 / 18CrNiMo7-6; 1.0301 / C10; 1.7131 / 16MnCr5; 1.7147 / 20MnCr5.

Objective of the procedure

Improvement of the mechanical properties, in particular:

  • Increase in wear resistance through increased surface hardness
  • Increase in resilience
  • Improved flexural fatigue strength and overload tolerance thanks to the tough core
  • Increase in fatigue strength (Martensite formation during hardening leads to an increase in volume. This is higher in the carbon-rich outer layers than in the low-carbon core, which is why internal compressive stresses build up on the surface Tension occurs.)

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

literature

  • Walter Eversheim, Günther Schuh: Production and Management 3 . Design of production systems, Springer Verlag, Berlin 1999, ISBN 3-540-65453-4 .
  • Joh. Schiefer, E. Grün: Course in hardening technology . Springer Verlag Berlin Heidelberg GmbH, Berlin 1927.

Web links

Individual evidence

  1. ↑ Case hardening - how does it work? Techpilot Lexicon. In: Techpilot Lexicon. Accessed December 11, 2019 (German).