Softening and Loss of Mechanical Strength of Carbon Steels for Extended Periods of Time at Room Temperature. Application to the Specific Case of the Santa Maria de la Asuncion de Laredo Steel Chain

Abstract

Baldomero Brigido, Laura Garcia, Growene W. Queiros, Fernando Penco, Jose M Gomez de Salazar and Antonio J. Criado

Carbon steels are aged for long periods at room temperature. This ageing is caused by the diffusion of carbon through the ferrite towards locations that make the system more stable according to its total free energy. The slow diffusion does not stop and in long periods of time it changes the morphology of the structure of the initial steel phases. These morphological changes imply a change in the mechanical properties of hardness, resistance, etc. In this publication we deal with one more case of those studied by our team: the Laredo steel chain (Laredo, Cantabria (Spain)). This steel has been studied and its 13th century chronology has been certified, thanks to these microstructural changes and its implication in mechanical properties. The ageing of this steel is important, which validates that its resistance is more than 700 years. The research has been carried out by means of optical microscopy and scanning electronics, complemented with a study of the variation of the hardness of the ferrite against a current standardised carbon steel (DIN CK15). This has also been corroborated using X-Ray diffraction, applied to the current steel and to the steel of the Laredo chain.

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