Carbonitriding

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The carbonitriding is a special type of case hardening . It is mostly used for low to medium hardening depths (CHD = Case Hardened Depthalt , old designation according to DIN 50190-1: Eht). In addition to carbon , smaller amounts of ammonia are diffused into the surface layer. Ammonia is made up of nitrogen and hydrogen. In this process, the nitrogen usually acts as an alloying element in the edge areas of the workpiece. This improves the hardenability at the edge and it is therefore possible to use cheaper steels (e.g. unalloyed or low-alloy steels, free-cutting steels and deep-drawing steels) for hardening. With free-cutting steels and deep-drawing steels, however, there is a risk of retained austenite forming due to the higher manganese content.

Carbonitriding usually takes place at temperatures between 750 and 930 ° C. The carbonitriding depths produced in this way are mostly between 0.06 and 0.60 mm deep; this depends on the selected temperature: in the upper area the nitrogen absorption is lower than in the lower area. The temperature is therefore chosen according to the desired property. Carbonitriding is done in the upper area, if the hardness of the surface layer is particularly important, and in the lower area, if you want to achieve a high enrichment of the surface layer with nitrogen with regard to the usage properties. It is common practice to temper carbonitrided parts after quenching. In the case of unalloyed steels, this usually happens at 150 to 180 ° C and in the case of alloyed steels at 160 to 200 ° C.

The frictional wear resistance and the emergency running properties of the nitrogen-enriched surface layer are better and the tempering resistance is higher than is achieved with case hardening. The higher the nitrogen content of the operating layer, the higher the tempering temperature that can be selected.

According to DIN 8580, nitriding is one of the manufacturing processes by changing material properties. The carbonitriding process is not a nitriding process, but a hardening process. The reason for this is that only small amounts of N (= nitrogen) diffuse into the structure and therefore it is not a heat treatment that creates a bonding layer .

literature

  • Wolfgang Weißbach: Materials science. 18th edition, Vieweg Teubner Verlag, Wiesbaden 2012, ISBN 978-3-8348-1587-3 .
  • Hans Berns: Steel science for engineers. Springer Verlag Berlin Heidelberg GmbH, Berlin 1991, ISBN 978-3-540-54557-6 .

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