Stub Axles That Were Rejected Because of Slag Inclusions

An automobile manufacturer rejected several 1035 steel stub axles because of what appeared to be short longitudinal cracks in the surfaces of the pins. The cracks were found when six axles were examined for defects by magnetic-particle inspection. However, metallographic examination showed that these lines were not cracks but slag inclusions at and immediately below the surface. Analysis (visual inspection, metallographic examination, and 100x/500x magnetic-particle inspection) supported the conclusions that the inclusions consisted of pieces of fireclay from channel brick that were flushed into the ingot mold. Although no true cracks were present, rejection of the stub axles was nevertheless justified. Slag streaks could reduce the strength of the stub axles and lead to the formation of fatigue fractures during operation. No recommendations were made.