Surface Failure Analysis of a Retained Mandrel for Steel Tube Hot Rolling

By means of digital camera, OM, SEM, and EDS, a scrapped retained mandrel after steel tube rolling was dissected. The surface defects of the scrapped retained mandrel were observed and classified, the cause for each kinds of defect was discussed. The results show that the surface defects of the scrapped retained mandrel consisted of flaking, map cracking, trench defect, and annular crack, etc. High temperature oxidation and high temperature wear were main reasons for the formation of these defects. The chromium coating at the end and the middle part of the retained mandrel were worn at the initial stage of the rolling. Then the oxidation film formed. The oxidation film blistered, crazed and then fell off. The cracks generated from the surface, expanded and met. They fell off because of friction and inter stress, and then heavy wear formed. With the deterioration of the map cracking, oxidation film, cracks and the drop of the mandrel, the wear trench formed. When trench defect appeared at the surface of the retained mandrel, the failure would be sped up.