Bending Fatigue Behavior of Carburized Gear Steels: Four-Point Bend Test Development and Evaluation

The ability to evaluate the bending fatigue behavior of carburized low alloy steels in a laboratory and relate these measurements to performance of high contact ratio helical gears is important to the design and development of transmissions. Typical methods of evaluating bending fatigue performance of carburized gear steels do not directly represent helical planetary gears because they lack the geometric and loading conditions of planetary pinions. The purpose of this study is twofold; 1) development of a lab fatigue test to represent the fatigue performance of planetary pinion gears tested in a dynamometer and 2) evaluation of the influence of alloy content on bending fatigue performance of two steel alloys. The steels under evaluation were modified 8620M and 4615M alloys machined into bend bars with a notch representing a gear root and carburized to a case depth of approximately 0.35 mm (using the same carburizing cycle as the planetary pinion gears). The samples were fatigued, under a fully reversed load condition (R=-1), in 4-point bending. Optical and electron microscopic evaluation of the case revealed the presence of internal oxides and NMTP (non-martensitic transformation products) at the surface of the modified 8620 alloy, the result of oxidation of Mn, Cr and Si. The modified 4615 alloy only showed Si and Mn oxidation without NMTP. 4615 modified alloy performance was more than 20% better in fatigue than 8620 modified. These differences in fatigue results are caused by the oxidation and resultant NMTP. These same materials were also evaluated as part of a dynamometer test program which resulted in similar relative fatigue behavior as found in lab experiments.