Failure Analysis and Characterization in Large Excavator Speed Reducer

Abstract Large excavators are critical assets in large-scale mining operations, relying on gearboxes to transmit mechanical energy during lifting and digging activities. Gear tooth failures in these systems lead to high maintenance costs and unplanned downtime, constituting a significant operational challenge. Understanding the root causes of such failures is essential for improving reliability and performance, as gear durability is governed by the interaction between metallurgical characteristics, heat treatment conditions, and operational stresses. In this study, premature fractures of gear teeth in an excavator gearbox were investigated through a multiparametric analysis. Chemical composition was determined by optical emission spectroscopy, confirming compliance with DIN 18CrNiMo 7-6 steel. Optical and scanning electron microscopy revealed the presence of retained austenite, untempered martensite, and cementite along grain boundaries, as well as multiple fatigue crack initiation sites and characteristic propagation striations. Microhardness profiling indicated heterogeneous case-hardened layers with the coexistence of martensitic and retained austenitic phases, while field vibration measurements demonstrated operational stress levels exceeding recommended limits, thereby accelerating fatigue crack propagation. Despite the chemical composition meeting material specifications, the root cause of failure was attributed to excessive retained austenite resulting from insufficient carbon potential during the carburizing process. Under cyclic loading, the retained austenite transformed into martensite, promoting material embrittlement and intensifying high cycle fatigue crack propagation until catastrophic failure occurred. These findings highlight the critical interplay between microstructure, heat treatment, and operational conditions, underscoring the need to optimize carbon potential, carburizing parameters, and vibration control to enhance the reliability of gear systems used in mining applications.