Carbonitriding microalloyed case-hardening steels for the automobile powertrain

  OLM-depiction of the case-hardening steels carbonitrided surface area Copyright: © IEHK Figure 1: OLM-depiction of the case-hardening steels carbonitrided surface area, different martensite structures (blue & red brown) and retained austenite (white brown) can be seen

Gears that are used in the automobile powertrain are thermochemically treated to enhance the mechanical properties of the tooth flank. The most common treatments are case-hardening, in which the surface is charged with carbon, and carbonitriding, in which the surface is charged with carbon and nitrogen.

In comparison to common case-hardening treatment, the carbonitriding treatment not only increases the surface hardness and wear resistance even further, but also the long life fatigue strength and temper resistance. At the same time the carbon stabilizes retained austenite (Figure 1), and thus increasing the damage tolerance. It is assumed, that the content of retained austenite, together with precipitated carbonitrides and residual compressive stresses, inhibit crack formation and propagation and therefore increasing the load capacity of the component. The retained austenite absorbs plastic deformation and transforms into martensite under load. In this way crack growth is hindered and the mechanical strength rises locally.

In a joint research project, a new case-hardening steel with a specifically adapted heat treatment during carbonitriding, was developed. The IEHK main focus lies on the adjustments of microstructure and precipate state, as well as its characterization and mechanical testing of the gear.