Effects of Different Microstructural and Hardness Gradients Generated by Single and Combined Surface Treatments with a Nitriding Top Layer

Wednesday, April 20, 2016: 2:30 PM
Ballroom B (Hyatt Regency Savannah)
Dr. Anja Buchwalder , TU Bergakademie Freiberg, Institute of Materials Engineering, Freiberg, Germany
Mr. Normann Klose , TU Bergakademie Freiberg, Institute of Materials Engineering, Freiberg, Germany
Prof. Rolf Zenker , TU Bergakademie Freiberg, Institute of Materials Engineering, Freiberg, Germany
Prof. Heinz-Joachim Spies , TU Bergakademie Freiberg, Institute of Materials Engineering, Freiberg, Germany
Mrs. Anne Jung , TU Bergakademie Freiberg, Institute of Materials Engineering, Freiberg, Germany
Due to its lower density in comparison to steel and its excellent, near-net shape castability with very thin wall thicknesses, cast iron exhibits high potential for light-weight constructions, especially for highly loaded automotive components. The presence of material-specific graphite, however, requires additional measures (surface treatment) for improving the corrosion and tribological load behavior. The results of comparative investigations of single and combined surface treatment technologies incorporating electron beam (EB) remelting and nitriding were presented in associated previous works.

This contribution utilizes the depth-dependent microstructure within the treated surfaces and the resulting hardness-depth profiles to discuss the relevant wear and corrosion mechanisms active in surfaces produced by single and combined treatment. The core interests comprise the morphology of the ledeburite (especially the orientation and size of the carbides) in the EB remelted layer, the composition and distribution of phases (γ’-, ε-nitrides), as well as the thickness of the compound and diffusion layers formed after nitriding. In this context, the often controversial role of the typically high silicon content of cast iron during nitriding – including the effect of the formation of silicon nitride on the hardness profile and, therefore, the wear behavior – was analyzed. Furthermore, the significant influence of the nitriding process used (plasma nitriding, gas oxinitriding) on both the layer structure and the load behavior was observed. For the gas oxinitriding process, the special role of the near-surface oxide layer was also examined.

Finally, the potentials and limitations of the single and combined surface treatments with respect to the improvement of the tribological and corrosion load behavior of cast iron were derived based on a cross-evaluation of the results of the treatments investigated.

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