The role of microstructure stability in the failure of exhaust valves in heavy-duty diesel engines

Abstract

Failure of valves in heavy-duty diesel engines is a significant challenge for engine reliability. This study aims to investigate the metallurgical factors that led to the failure of several exhaust valves made from a nitrogen-containing Fe–Mn–Cr–Mo–Nb–V alloy. The failures predominantly occurred in the hot section of the valve stem and were attributed to environmentally assisted fatigue. The primary cause of failure was identified as the low thermal stability of the valve microstructure, characterized by a high-volume fraction of discontinuous cellular precipitation of M23C6/austenite in the hot section of the valve stem. This microstructural instability played a crucial role in initiating and propagating corrosion/oxidation-assisted fatigue cracks due to the precipitation of low toughness Cr-rich carbides and formation of associated Cr-depleted zone the subsequent creation of Cr-depleted areas. To mitigate similar failures in heavy-duty diesel engines operating under increased peak cylinder pressures and temperatures, it is imperative to utilize materials with higher thermal stability and improved corrosion/oxidation resistance.

Keywords

• Valve alloy
• Heavy-duty diesel engine
• Thermal stability
• Discontinuous cellular precipitation
• Environmentally assisted fatigue