A metallurgical failure analysis of mTBM disc cutters under hard rock excavation

Abstract

Predicting wear in Tunnel Boring Machine (TBM) disc cutters remains a complex challenge due to extreme operational conditions and numerous variables involved in hard rock excavation. This study comprehensively investigates the failure modes and wear mechanisms of mTBM disc cutters, aiming to refine existing wear classifications and improve understanding of the degradation processes. Through a multi-scale approach combining macroscopic visual inspection, optical microscopy (OM), scanning electron microscopy (SEM), microhardness profiling, and X-Ray diffraction (XRD), twelve worn disc cutters from an urban sanitation tunnel were analyzed. Our findings reveal normal/sharpening wear as the most prevalent (58 %), followed by blockage (25 %) and mushrooming (17 %), with distinct distributions based on cutter position. Crucially, microscopic analysis identified the systematic formation of White Etching Layers (WLs) and Dark Layers (DLs) on worn surfaces, where the WL exhibits increased hardness, grain refinement, and a higher content of retained austenite compared to the substrate. Crack nucleation was predominantly observed at the WL/DL interface, a zone of pronounced mechanical property mismatch, leading to progressive material chipping. This observation, supported by detailed microstructural evidence, challenges previous interpretations that dismiss sliding contact in field conditions, strongly suggesting a hybrid rolling-sliding contact mode at the rock-cutter interface. The study underscores the critical role of these subsurface microstructural changes in cutter degradation, advocating for a refined wear classification that incorporates these mechanisms. These insights are vital for optimizing TBM cutter design and enhancing durability under severe excavation conditions.