Metallurgical Failure Investigation of Replaceable Tooth Flank Used in Gear Rim Applications

Abstract

Tooth flanks used in gear rim applications of iron ore pelletizing plant get failed frequently, and the failures are also likely to take place in other suspected flanks. The objective of the study is to identify crack through nondestructive inspection and ascertain the probable cause and causes of failure to occur in the part based on examination and by studying its chemical, metallurgical, and mechanical properties. Ultrasonic inspection has been carried out on many tooth flanks fitted in the gear rim at plant site and found the presence of a subsurface crack in the core region at 40 mm depth from the side surface of the tooth flank. The inspected and identified tooth flank was used for failure analysis. Liquid penetrant test was carried out and it was found that a longitudinal centerline crack existed which propagated from core to the surface toward the trailing edge. Samples identified near the cracked region and surface were cut by machining, and metallographic characterization was carried out through optical, field emission scanning electron microscope, and electron probe microanalyzer. The microstructural features show the presence of many isolated micro-cracks and their orientation and inclusions. The presence of the martensitic phase at subsurfaces and pro-eutectoid cementite at isolated regions with the presence of high carbon concentration, amount to an increase in hardness in comparison with tempered martensite phase at core region. It was observed that micro-crack nucleated at nonmetallic inclusions. The nonmetallic inclusions, i.e., MnS, the difference in microstructures, a variation of hardness and nucleation of the void, void growth, and void coalescence, were found in the grain interface leading to propagation of crack and caused the rupture of the part during service.