Advances in titanium pipe welding and inspection technology for Navy ships

Seawater piping systems on new Navy ships, such as the LPD-17 amphibious transport dock ships, are transitioning from copper-nickel alloys to commercially pure titanium. This change in material will save the Navy millions of dollars per ship in maintenance and support costs because the need to replace piping several times over the ship life will be eliminated since titanium has outstanding erosion and corrosion resistance. Titanium piping fabrication presents new challenges to U.S. shipyards that have limited experience with this material. The Navy Joining Center (NJC), a Navy MANTECH Center of Excellence, has conducted a project to improve welding and inspection technology and promote efficient, high-quality welding practices. This technology was required to accelerate the learning curve and minimize repair, which can be costly. The NJC project was supported by Avondale Shipyard, Naval Sea Systems Command 05M, and the Naval Surface Warfare Center, Carderock Division. A properly made titanium weld will have a glossy silver appearance that is absent of other surface colors. Contamination of titanium weld metal by interstitial elements (oxygen, nitrogen, carbon, and hydrogen) reduces ductility and toughness, while increasing strength and hardness. Weld colors rapidly form during welding operations when hot titanium surfaces are exposed to the slightest amount of air due to oxidation. Assessing the significance of weld color on weld quality has been difficult. There was no technology available to determine whether the molten weld pool was contaminated and embrittled, or the surface was only oxidized and needed burred to restore ductility. Titanium welding technology training workshops and weld color inspection technology, including a sample workmanship kit, were provided to several shipyards to expedite the learning curve. The weld color inspection kit included a training guide, nine different weld color samples, and photo-cards of the samples. The weld color inspection technology has provided the tools to assure accurate assessment of weld color and help determine corrective actions to improve weld process gas shielding minimizing future problems. A portable hardness test method was developed for assessing weld quality. The objective of the portable hardness inspection was to minimize the repair of sound welds, which only have surface oxidation. Through-thickness contaminated titanium welds must not be accepted to assure resistance to hydride embrittlement.