Failure Analysis of Cracking in a Muff Used for Railway Vehicle Coupling

Cracking in a muff used as a coupling component in railway vehicles was investigated to identify its root cause and propose preventive measures. The failed component had been in service for approximately 60 months and exhibited multiple cracks during overhaul. Comparative analyses of a cracked service component and an unused counterpart were conducted using micro X-ray computed tomography, chemical composition analysis, mechanical testing, microstructural characterization, and fractography. The failed muff showed numerous cracks and predominantly intergranular brittle fracture. It also exhibited high hardness (approximately HB 450) and low impact toughness, indicating insufficient tempering during heat treatment. Microstructural observations supported this interpretation, revealing acicular carbides in the cracked component and spheroidized carbides in the properly tempered unused one. The results indicate that inadequate tempering caused embrittlement and reduced toughness, thereby promoting crack initiation and propagation under repeated vibration and impact during service.