Feasibility of underwater friction stir welding of hardenable alloy steel

Abstract

The objective of this thesis is to determine whether friction stir welding (FSW) is a feasible welding process for steels in an underwater environment. Specific benefits would be underwater weld repairs on steel alloy piping systems and/or structures, and crack repairs on control surfaces of submarines without the need for strict environment controls or in the submarine's case, for drydocking. A single tool made of polycrystaline cubic boron nitride (PCBN) with a Tungsten-Rhenium binder was used to conduct a series of bead-on-plate FSW traverses, approximately 64 inches (1.6 m) in total length, on 0.25 inch (6.4 mm) thick plates of a hardenable alloy steel. The first series of traverses involved various revolutions per minute (RPM) and inches per minute (IPM) combinations on a dry plate. A second series was conducted while a plate was immersed in water in order to assess the potential for inducing hydrogen assisted cracking (HAC) during FSW of susceptible alloys. All traverses were visually defect-free. The FSW nuggets (stir zone) exhibited refined microstructures and increased hardness relative to the base plate. Based on preliminary findings, FSW of hardenable alloy steel is a feasible process and should be further researched and refined.