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Metallographic Sample Preparation

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This mount shows a quench crack which occurred due to re-heat treating a previously case hardened AISI 4140 alloy steel without a sub-critical anneal.

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Optical microstructure analysis is an essential step in determining the cause of failure due to manufacturing process irregularities. Since metals and most non-metallic materials contain a specific crystal structure directly connected to or created by a manufacturing process, optical examination of this crystal structure provides insight to processing irregularities.

For example, iron alloys are allotropic; meaning the crystal structure changes with temperature and alloy content. When a hardening heat treatment is performed, the microstructure changes from a body centered cubic crystal structure to body centered tetragonal. The change in the observable optical microstructure is dramatic. However, if this transformation is incomplete, observation of the microstructure becomes a critical tool in determining the processing deficiency.

Optical microstructure analysis of metals and non-metals begins with careful metallographic sample preparation. Metallic pieces are usually mounted in hard thermal setting plastic bakelite, while softer non-metallic materials may be held by a cured casting resin. In either case, subsequent grinding and polishing procedures quickly become critical to the ability to observe and define the correct optical microstructure. Sample preparation always contends with surface smearing and edge retention; it takes many years to learn correct techniques to avoid problems.

Lastly, etching to delineate or reveal the microstructure for optical examination must be correctly performed, using swab, immersion, thermal or electrolytic technique. Each etching method changes the observable optical microstructure. If performed incorrectly, the selected etchant and/or etching technique may alter the appearance of the microstructure in such a manner as to resemble a microstructure created by another manufacturing process. Many years of practice and experience are necessary to learn and apply the correct methodology to reveal the true microstructure of a material.

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