Influence of Material Property on Micropitting and Pitting Behavior

Abstract

Recent studies have shown that micropitting initiated pitting appears to be the dominant metal fatigue mode in modern bearings and gears. If the formation of micropits can be controlled, the fatigue life of the bearings and gears can be readily lengthened, so the useful life of the engine or transmission can be radically extended. The lack of in-depth understanding of micropitting initiation mechanism hinders progress to control the micropitting-initiated pitting failure mode. In this study, we explore the initial stages of micropitting in relation to material differences that impact the crack propagation process. To investigate these mechanisms, we study micropitting and subsequent pitting using pinion and wheel pairs made from two different carburized steels, SAE 8620 and SAE 4027, in a 91.5mm helical back-to-back gear test rig using a typical transmission fluid. For comparison, a similar study is being carried out with the pinion and wheel pairs made from the same SAE 8620 steel. The preliminary results show that steel material differences may change the ultimate pitting fatigue life but may not significantly influence the micropit formation process and subsequent pitting failure mode.

A Review on Micropitting Studies of Steel Gears

“…As revealed by various experimental observations [28][29][30], the difference of gear steel materials and the heat treatment process could generate distinguished micropitting resistances. As reported in the Reference [31], high strength austempered ductile iron gears were gradually accepted due to the low production cost, the eventual noise and vibration reduction, and the self-lubricant properties through graphite nodules, resulting in remarkable gear tooth flank capacity.…”

Atomistic simulation of martensite decay

“…Martensite decay in bearings has been amply investigated and excellent reviews can be found in [5,6]. There are fewer published reports on martensite decay in gears [3,[7][8][9][10][11][12][13][14][15][16][17][18] but they all describe dark and/or white etching features as in bearings. A major difference between martensite decay in bearings and martensite decay in gears is the location where the transformation occurs.…”