Evaluation of Crystal State and Residual Stress in the Work-Affected Layer of the Casting Old Coin

Abstract

In general, metal products have a characteristic surface layer called a work-affected layer, formed by polishing at the time of manufacture. This layer has many differences in mechanical properties such as crystal state and residual stress depending on the processing conditions. Regarding this, if the crystal and residual stress states of the work-affected layer, formed during manufacturing was able to be clearly evaluated, it may be possible to clarify the manufacturing history from its characteristics. In other words, the evaluation of the work-affected layer can add new information to archaeological research as an appraisal method for metal products. X-ray measurement, which can be evaluated non-destructively, is very effective as a means of examining the state of the work-affected layer of metal. Therefore, in this study, we investigated the possibility of confirming the crystal state and evaluating the residual stress using the X-ray diffraction method for three types of copper-based casting old coins with different elemental compositions. As a result, the Debye rings obtained by X-ray diffraction were diverse, such as uniform continuous rings and discontinuous rings with light spots, depending on the specimens. That is, it was found that the crystal state of the surface layer of old coins has fine crystals, texture, coarse crystals, etc., depending on the elemental composition. In addition, compressive residual stress of about -140 to -370 MPa exists in all specimens. It was confirmed that this value differs depending on the measurement diffraction plane and the measurement direction, and that a clear difference occurs due to the difference in the elemental composition.