The objective of this project was to identify a solution that would increase the reliability and strength of these fasteners, while reducing life-cycle costs.
Surface ship and submarine components face unique challenges caused by their exposure to harsh marine environments. Of particular concern are the large diameter fasteners used to attach various components on the Seawolf (SSN-21) and Virginia (SSN-774) Class Submarines. Corrosion concerns require that the existing fasteners be replaced at periodic intervals to preclude catastrophic failure.
A wide variety of materials was evaluated for strength and corrosion resistance before the cobalt-nickel alloy MP98T was selected as the best candidate material. The material manufacturing process was then adjusted to obtain the required strength and toughness levels and to ensure that the fasteners could be produced from MP98T without extraordinary measures or a deterioration of the material properties. Smaller diameter MP98T fasteners were fabricated to test the material properties, including corrosion resistance. Once satisfactory test results were achieved, a large diameter (3.5 inch) hex-head fastener was fabricated. (The hexagonal shape was selected for the proof-of-concept fasteners because it was deemed the most difficult head geometry to fabricate.) Material testing conducted on the finished, large diameter fasteners confirmed that the desired material properties were not adversely affected by the fabrication processes. The data from this project will be used to create a new fastener material specification for MP98T.
The MP98T fasteners are expected to last for the life of the submarine without mechanical or corrosion failure. Smaller diameter MP98T fasteners have been authorized for use for the pad eyes of the Advanced SEAL Delivery Systems used on the SSN-21 Seawolf, SSN-22 Connecticut, SSN-23 Jimmy Carter, USS Virginia and the USS Texas submarines.