Interpretation of a “Rock Candy” Fracture Exhibited by a Steel Railroad Casting

Following a freight train derailment, part of a fractured side frame was retained for study because a portion of its fracture surface exhibited a rock candy appearance and black scale. It was suspected of having failed, thereby precipitating the derailment. Metallography, scanning electron microscopy, EDXA, and x-ray mapping were used to study the steel in the vicinity of this part of the fracture surface. It was found to be contaminated with copper. Debye-Scherrer x-ray diffraction patterns obtained from the scale showed that it consisted of magnetite and hematite. It was concluded that some copper was accidentally left in the mold when the casting was poured. Liquid copper, carrying with it oxygen in solution, penetrated the austenite grain boundaries as the steel cooled. The oxygen reacted with the steel producing a network of scale outlining the austenite grain structure. When the casting fractured as a result of the derailment, the fracture followed the scale in the contaminated region thus creating the “rock candy” fracture.