Damage Features of Bolts Connecting Main-Shaft with Turbo-Disk Used in a Locomotive Turbocharger

The connection between the turbo-disk and the main-shaft was found to be loose when performing a regular inspection on a locomotive turbocharger. When the bolts connecting main-shaft with the turbo-disk were disassembled, it was found that smeared threads and contact pressing deformation, etc., severe plastic deformation took place on the three connecting bolts. Metallurgical examinations indicate that the core microstructure is composed of the banded ferrite and pearlite along the axial other than the sorbite by the standard toughening process (quenching and tempering). Abnormal microstructure leads to low the hardness and strength so that various damages appeared on the flanks when twisting bolts with nuts (loosening or tightening). The plastic deformation took place on the bolts under the axial load due to low hardness, which resulted in loosening of the connection between the turbo-disk and the main-shaft. Not standardized fabrication process of the bolts is responsible for the failure of the bolts.