The case-hardening steel 18CrNiMo7-6 is used for large gear wheels in wind turbines. To achieve the requirements for a wear resistant and tough material, the steel is carburized and case hardened at 980 °C for 80 hours. During this heat treatment grain growth occurs, which deteriorates the mechanical properties of the material and the dimensional stability of the gears. To avoid this, the steel is micro-alloyed with aluminium and niobium. These elements pin the grain boundaries in form of carbides and nitrides to reduce the grain growth during the heat treatment. However, a disadvantage of this alloying system is the high affinity of aluminium to oxygen resulting in an increase of non-metallic inclusions, which decrease the fatigue strength of the gear parts. Therefore, an aluminium-reduced modification of this steel is required, which still avoids grain growth. In this work the grain size stability of several micro-alloyed variants of the case-hardening steel 18CrNiMo7-6 under different heat treatment conditions was examined. For this purpose the grain size of prior austenite grains was observed by light microscopy. Furthermore, the thermo-calc calculations were carried out and compared to atom probe measurements. Additionally, the precipitates were investigated in detail to determine the influences of the type of heat treatment. Scanning- and the transmission electron microscopy was applied for this means. The thermo-calc results showed different precipitation potentials of the investigated alloys. This trend was confirmed by the atom probe measurements. The grain size evaluation with the aid of density distribution curves revealed a bimodal grain structure, whereby the individual alloys showed different distributions. A closer analysis of the precipitates by scanning- and transmission electron microscopy revealed a frequency distribution curve with a bimodal distribution. The effective precipitation size range for grain boundary pinning was calculated and the coarsening behaviour was compared to established coarsening theories.
|Translated title of the contribution||Influence of micro-alloying with Nb, Al and N on the grain size stability of the case hardening steel 18CrNiMo7-6|
|Award date||4 Apr 2014|
|Publication status||Published - 2014|
Bibliographical noteembargoed until 10-02-2019
- case hardening steel
- grain size stability