51312-01421-Qualification of Alloy 686 (UNS N06686) as a high strength corrosion resistant fastener for marine a

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Product Number: 51312-01421-SG

ISBN: 01421 2012 CP

Author: Debajyoti Maitra

Publication Date: 2012

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Corrosion resistant Ni based Alloy UNS N06686 has been suggested as a possible candidate fastener material for marine applications. Yield strength in excess of 150 ksi can be achieved by cold working while retaining significant ductility impact strength and cracking resistance. A qualification program is being conducted by the US Navy in collaboration with Special Metals Corporation to qualify UNS N06686 bolting for use in the new ship designs. Navy has reported intergranular failure in a four point bend test in air and also in a step load intergranular test program on UNS N06686 bar of size 2 inch and 2.5 inch diameter. However slow strain rate testing confirmed the toughness of the material. Further investigation revealed presence of mu phase in the UNS N06686 rods of size more than 1 inch diameter. Phase diagram of UNS N06686 as obtained using a software capable of predicting material Properties showed the presence of mu phase below 1100°C. It was perceived that intergranular mu phase can be reduced to large extent by using a faster quench technique. Various sizes of bar products were made by incorporating improved cooling after annealing. The microstructure of the heats showed significant reduction in mu phase.