Production of Efficient Nodular Cast Iron Gears - Considering the Fatigue Strength and Rolling Contact Fatigue Strength Based on the Local Microstructure

Large gears with diameters of several meters, used in heavy engineering and mining industries, are subjected to high fatigue and rolling contact fatigue stresses with load cycles up to several 109 cycles. These gears are usually made from steel in order to also achieve the necessary strength in severe conditions, such as in mining applications at elevated temperatures. While small gears are mainly manufactured in one piece, large gear diameters are mostly achieved by using different segments that are joined together. Generally, such large gears are produced in small series or even as individual components, which are mounted as up to 20 segments, each with a mass of 2 t. Size, complexity in production and the application in large machines lead to significant design effort in order to ensure a long lifetime and a high reliability. Any unforeseen failure of a gear can lead to high economic losses and thus needs to be avoided. Due to a more straightforward production process, the good lubrication effects of the elemental graphite and the good availability of high strength but low alloyed material, ductile cast iron (DI) is coming more and more into favor for such applications. Nevertheless, there is still a degree of lack of knowledge, concerning the fatigue assessment of DI gears made from high-strength materials with pearlitic or ausferritic matrices. Within the research project “Gusszahnrad,” the applicants and foundries producing cast gears assessed the microstructure-related fatigue strength of different DI grades on the basis of axial fatigue and rolling contact fatigue testing as well as component investigations. The aim was to optimize the design and manufacturing process and to assess safety as well as fatigue capabilities for large gears.