Optimized gear performance by alloy modification of carburizing steels for application in large gear boxes

Both the tooth root and tooth flank load carrying capacities are parameters that decisively influence gear size, as well as gearbox design. The principal requirement of all modern gearboxes is to comply with the demands for steadily increasing power density and to simultaneously offer high reliability of their components. Consequently, the required performance spectrum of materials used for such components is very high. For larger gear sizes, the loads and stresses increase at both surface and sub-surface locations within the gear tooth. Thus, the material properties and particularly the strength, increase in importance at greater material depths below the surface. The choice of appropriate alloying elements in high performance carburizing steels, in combination with an appropriate heat treatment, allows the adjustment and optimization of strength properties over the material depth. Thereby, the requirements of highly loaded, large gears can be met and a sufficiently high load carrying capacity provided. This paper initially gives an overview of the main failure modes of case carburized gears resulting from material fatigue at the surface and subsurface locations. Furthermore, the underlying load and stress mechanisms, specifically in relation to the gear size, are discussed as these considerations principally define the required material properties. Subsequently, the principles of newly developed, as well as modified, alloy concepts for optimized gear steels with high load carrying capacity are presented. In particular, the impact of modification of the bulk alloying elements molybdenum, manganese and nickel, as well as that of the microalloying element niobium, was investigated. Furthermore, an evaluation of the relevant material properties of these modified steel grades and a comparison to typical reference grades for gears are presented. In experimental work, the load carrying capacities of tooth root and flank were determined using a pulsator, as well as an FZG back-to-back test rig. The results demonstrate the effectiveness of these innovative, new alloy concepts. Furthermore, the test results are compared to the current state-of-the-art and benchmarked against established load carrying characteristics of gears manufactured from conventional reference steel grades. (AU) Copyright © 2018 Companhia Brasileira de Metalurgia e Mineração (CBMM) All rights reserved