Opportunities and benefits of stabilized retained austenite by carbonitriding for highly loaded parts

Steinbacher Matthias - Leibniz-Institut für Werkstofforientierte Technologien - IWT (Germany)

Today case hardening of highly stressed gear wheels is state of the art, to meet the specific load demands at flank and tooth root. Today, the heat treatment itself is typically including a carburization and quenching step in liquids or gaseous media. The result of the heat treatment of gear wheels is strongly impacting the strength and load capacity at flank and foot of the treated gears. The typical requirement for the microstructure in the surface near case is a well balanced mixture of finely dispersed retained austenite of less than 25 Vol. % and martensite. Hard intermediate phases like cementite or other carbides are objectionable.
Carbonitriding is an up to now unusual thermo chemical treatment for parts made of case hardening steels. In the past it typically was applied to increase hardness or hardenability of low alloy steels of modicum hardenability. To do so, the treatment atmosphere was modified by the addition of fixed ammonia rates at low treatment temperatures (< 870 °C). Newly developed automatically controlled carbonitriding processes make it possible to set up defined carbon- and nitrogen- depth profiles leading into a significant altered phase-composition of the case: the dominant phase is a carbon and nitrogen rich stabilized austenite with martensite and precipitations of the Me-C-N kind. The talk will give an overview on different phase composition being achievable via carbonitriding. The talk will give examples of carbonitrided samples made of case hardening steels, showing the influence of the heat treatment process and its atmosphere parameters.
Additionally a perspective on the latest development of heat treatment technology for a designated set up of independently controlled carbon and nitrogen potentials for defined depth profiles will be given regarding the interdependency of both elements with each other and the alloy elements.

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