Microstructural Modification and Mechanical Properties Evaluation of an Austenitic Stainless Steel Subjected to Friction Stir Based Repair and Mitigation Techniques

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The purpose of this dissertation work is to develop the friction stir-based repair and/or modification techniques for preventing cracks/flaws with possible applications to mitigation chloride induced stress corrosion cracking (CISCC) for austenitic stainless steel 304/304L SS dry storage canisters to ensure their integrity during and possibly beyond their design life. The fusion welded parts of the storage canisters exhibit increased CISCC susceptibility in aggressive chemical environments. The dissertation work focused on two approaches: i) apply friction stir processing (FSP) to heal a pre-existing SCC-induced crack as well as to modify microstructure in the heat affected zone of a fusion weld; ii) introduce molybdenum by friction stir based alloying process to create compositions resistant to SCC. FSP was developed based on the basic principles of friction stir welding (FSW), a solid-state joining technique, which was developed by The Welding Institute (TWI), UK. The FSP runs were conducted at the Pacific Northwest National Laboratory. The work involved studying the effects of FSP on the microstructural characteristics and mechanical properties in terms of Vickers microhardness and tensile properties. The microstructural and mechanical behavior of the FSP specimens is evaluated using different techniques. The main highlights of this dissertation are: a) correlation between process parameters, grain boundary character, and hardness properties; b) development of FSP technique as a repair technique for a SCC induced crack; c) development of FSP as a post-processing technique by modifying the microstructure and mechanical properties of fusion welded stainless steel; and d) development of friction stir alloying technique with molybdenum powder incorporated into the 304L SS matrix. The work brought in new knowledge and understanding on the feasibility of applying FSP as a technique for repair and/or mitigation of cracks/flaws in austenitic stainless steels for potential usage in various industrial sectors including nuclear energy.
Friction Stir Processing, Dry storage canisters