The head of a socket spanner made of heat-treated 0.40C-0.34Cr steel cracked in service. The pronounced fibrous structure of the component became evident as soon as it was etched with 2% nital. Folds in the material originating from the shaping process were visible, and the micrograph showed that cracks ran along these folds oriented according to the fiber. The fissures, with the exception of the hardening crack, were partly filled with oxide and showed signs of decarburization at the edges. From this it could be assumed that parts of the external skin had been forced into the folds during...