Improving Fatigue Life of Carburized Pyrowear 53 Gears by Enhanced Quench Hardening

Carburized Pyrowear 53 steel gears are conventionally hardened by oil quenching, followed by deep freezing and double tempering. Residual compressive stress is generated in the carburized case because of the delay in surface martensite formation due to the high surface carbon content. In a number of research and production trials with steel parts, an intensive quenching processes has been shown to produce surface compressive stress on the order of 1.5 to 2 times more compressive than standard oil quenching methods. [1 - 3] Recent work conducted under US Army AATD sponsorship has demonstrated the potential for up to 28% improvement in bending fatigue strength for carburized and intensively quenched 3-point bend coupons. [4] Based on these findings, an evaluation has been made of the relationship between residual compressive stresses and bending fatigue resistance in gears. Using single tooth bend tests conducted on carburized and hardened spur gears processed using both standard oil quenching and intensive quenching, residual stress profiles in the gear roots, distortion/growth of the teeth, and single tooth bending fatigue life were compared. The physical studies were then supplemented by finite element simulation of the heat treatment and subsequent bending fatigue cyclic loading. The FEA results showed that the stress state in the root area is reduced by the presence of the deep surface compression. Furthermore, neglecting the presence of the compressive surface stress, which is the practice of the standard tooth stress calculation methods, results in calculated stresses that are in excess of the actual fatigue strength of the gear steel. Without the presence of surface compression, all the gears would fail quickly by either overload or low cycle fatigue.