Metallurgical characterisation of a historical metal tie-rod from Milan Cathedral

One tie-rod from Milan Cathedral (15th century) recently broke in a defective cross-section. In order to infer the causes of failure, a metallurgical characterisation was performed, the results of which are discussed here. First, a visual inspection of the specimen was performed. Next, the fracture surface was analysed using a Scanning Electron Microscope (SEM) combined with Energy Dispersive X-Ray Spectrometry (EDXS) to detect the failure modes involved. A cross-section close to the failure surface was investigated by means of Stereoscopy, Light Optical Microscopy (LOM), SEM and EDXS in order to identify the microstructural compounds and characterise the slag inclusions. Vickers and Rockwell hardness tests were also carried out to correlate microstructural observations with mechanical properties. A strong relationship between failure and defects due to forging was observed, which is a particularly noteworthy result with regard to the representativeness of laboratory tests and the viability of in-situ inspection.