Beware of Hot Tearing in Lifting Lugs – a Case Study in Mechanical Engineering

Abstract

Abstract One out of eight lifting lugs of a large gas turbine engine exhibited indications upon magnetic particle surface crack inspection (MT), that were later verified by ultrasonic testing (UT). The subject part was made of a low-alloy steel casting. It had already been used in service and was supposed to be re-used, together with the other lifting lugs of the set. In order to determine the cause of the indications, non-destructive metallographic examinations by means of the replica technique were performed. In that investigation, the indications were determined to have been caused by crack-like decohesions of material. In one of the examined areas, these imperfections were associated with small casting defects, namely micro shrinkage. This might have contributed to crack initiation. The described small surface cracks at the cylindrical part of the lifting lug itself were probably caused by bending stresses in service. Indications were found only on one out of the entire set of eight parts. There, only the upper side of the short spigot, were axial tension on the surface is to be expected in service when the bollards are in lifting action, was affected. In a second investigation, that same part was sectioned in the area with the largest UT indications. The microstructure of this steel casting is unremarkable. However, several rather large macrocracks were found in the sectioning plane. Scanning electron micrographs confirmed this. There is a moderate carbon segregation in the subject area that is unremarkable. The longest crack in the metallographic section was forced open for microfractography in the SEM. A small pre-existing crack that could not have been caused by service loads was found. Within that area, microfractographic features of transgranular hot tearing were detected, as evidenced by a dendritic appearance of the fracture surface. This is a hot cracking mechanism.