Metallurgical Factors in Forged and Machined Gear Surface Durability

Abstract

Forged and machined bevel gears have been tested to compare surface durability for the two manufacturing processes. The tests and results which showed the machined gears to have 30.6 percent more pitted teeth per gear are discussed in a companion paper. This paper assesses the differences in resistance to pitting fatigue based on metallurgical examination. The gears were manufactured to be as nearly identical as possible and they were subjected to simultaneous heat treatment. Differences in depth of hardness, retained austenite, and grain size are considered to be within normal statistical variations of these parameters, although the sample size was small. The strongest evidence indicates the improved pitting resistance of forged gears is due to the random locations of pit initiation sites. Machined surfaces pit in regular patterns along the tool marks. These pits tend to combine by microcracking more rapidly because the pits are more favorably oriented.