Einfluss der Mikrolegierung auf die Randschicht eines carbonitrierten Stahls 18CrNiMo7-6

To further enhance the performance of case hardening steels, the surface layer is thermochemically treated. By carbonitriding, a thermochemical process by which carbon as well as nitrogen are brought into the surface layer, the tooth flank load capacity of case hardening steel gears can be dramatically improved in comparison to those elicited by carburization. In order to characterize these influence factors, hardness measurements, optical light microscopy, scanning electron microscopy (SEM) and x-ray diffraction (XRD) are commonly utilized. Further in-depth analysis of the crystallography and precipitation within the surface layer via higher-resolution methods, such as electron backscatter diffraction (SEM-EBSD), transmission electron microscopy (TEM) and atom probe tomography (APT), have only been sporadically implemented. Within the scope of this work, two micro-alloyed variants of a classic case hardening steel 18CrNiMo7-6 were produced: 18CrNiMoVNb7-6, with 0,1 wt.−% vanadium and 0,03 wt.−% niobium, and 18CrNiMoVNbB7-6, with 0,1 wt.−% vanadium, 0,03 wt.−% niobium and 0,003 wt.−% boron. Samples of all three alloy variations (W2CN4, VNb and VNbB), were machined, carbonitrided, hardened and tested under identical conditions. Microstructural analysis was undergone utilizing REM for an overview of the microstructural constituents, SEM-EBSD for quantitative analyses of the textures and grains, TEM for chemical investigations of the precipitations and APT for local chemical resolutions of the precipitations and the phase boundaries. Furthermore, thermodynamic and kinetic simulations via MatCalc and Thermo-Calc were implemented in order to identify the precipitations and associate their development with the related process steps and thermochemical heat treatments. In order to determine the influence of carbonitriding on gear performance, the tooth flank load capacity sHlim of carbonitrided spur gears produced from all three alloy variants were investigated via load capacity testing. The results obtained via thermodynamic and kinetic simulations, high-resolution and quantitative electron microscopy, local chemical resolution as well mechanical testing confirm the influence of micro-alloying elements and thermochemical heat treatments on the surface layer properties. Furthermore, by coalescing the results provided by TEM and APT investigations, the various precipitations could be identified. Contrary to literature, no carbonitrides or boron nitrides could be measured and instead the chemically distinct AlN, TiN, CrN, VN and/or NbC phases were confirmed. The evaluation of the residual austenite via REM-EBSD proved difficult due to a gradient which is further influenced by the hard finishing and metallographic preparation of the gears. Nonetheless, a substantial influence of the micro-alloying elements on the texture of the material, especially the martensite, could be confirmed throughout the surface layer. Mechanical testing confirmed an improved surface hardness from 700 HV1 to 740 HV1 and 780 HV1 for 18CrNiMo7-6, 18CrNiMoVNb7-6 and 18CrNiMoVNbB7-6, respectively, while the tooth flank load capacity values sHlim obtained via load capacity testing increased from 1700MPa to 1842MPa and 1839MPa, respectively. In addition to the known influence factor on the tooth flank load capacity sHlim, residual austenite contents, a further factor could be identified: by conditioning the austenite prior and during the cabronitriding process by using micro-alloying elements, the texture of the resulting room temperature microstructure was positively influenced. Through the addition of micro-alloying elements vanadium, niobium and boron to 18CrNiMo7-6, the tooth flank load capacity was further elevated