In revealing the prior austenitic Grain boundaries using oxidation method how to do oxidation of steels surface procedure pls
Prior austenite grain boundaries can be revealed by thermal etching. For this you need to polish the specimen before heat treating it to austenitizing temperature. Here is a nice review about this method:
Article Revealing austenite grain boundaries by thermal etching: Adv...
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Controlled oxidation for revealing prior austenite grain boundaries is also known as "Kohn Method". You may find detailed explanation of this technique in DIN EN ISO 643 Standard.
Preparation Procedure;
One surface of the specimen shall be polished. The rest of its surface shall not show any traces of oxide. The specimen shall be placed in a laboratory furnace in which either a vacuum of 1 Pa is attained or an inert gas is circulated (e.g. purified argon). Heat treat the specimen in accordance with the austenitizing procedure specified by the customer, or as defined by the International Standard governing the product. At the end of this specified heating period, air shall be introduced into the furnace for a period of 10 s to 15 s. The specimen shall then be water-quenched. The specimen can usually be directly examined using a microscope. NOTE The oxidation method can be done without the inert atmosphere.
The oxide adhering to the previously polished surface should be removed by light polishing with a fine abrasive, taking care that the oxide network which has formed on the grain boundaries is retained; then the polishing should be completed by the usual methods. The specimen should then be etched using Vilella's reagent:
— picric acid 1 g — hydrochloric acid 5 ml — ethanol 100 ml
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