Fracture failure analysis of coal mill shaft

Abstract

During the operation of No.2 boiler in a power plant, the fracture of a coal mill shaft occurred. The fracture causes of the coal mill shaft were found by macroscopic inspection, metallographic inspection, chemical composition analysis, fracture morphology analysis and mechanical property inspection. The experimental results showed that: 1. The external surface of the coal mill shaft had been welded, and the poor welding process resulted in a large number of welding defects such as cracks, slag inclusions and lack of fusion in the overlaying layer; 2. The improper hot working process of the coal mill shaft resulted in the abnormal metallographic structure of the shaft, and the existence of serious reticulated ferrite and brittle widmanstatten structure resulted in the low strength of the shaft. 3.Because of the existence of the cyclic torsion and bending stress of large load, the initial crack source occurred at the welding position of the hardfacing layer, and then extended in the form of fatigue, eventually causing the coal mill shaft to break. Three factors led the high cycle and low stress fatigue fracture of the coal mill shaft.