Orbital Welding of Titanium Pipe with Arc Machines’ Equipment for US Navy Ships
Orbital Welding of Titanium Pipe with Arc Machines’ Equipment for US Navy Ships
Fig. 1 — LPD 21 New York under construction at NGSB
Avondale will be delivered to the U.S. Navy this year.
The greatest footage of titanium piping ever in a Navy surface ship is now being installed in the San Antonio class of LPD ships.
When Northrop Grumman Shipbuilding (NGSB) was awarded a contract by the U.S. Navy to design and construct the first of an anticipated 12 ships under the Navy’s LPD 17 program in December 1996, it specified titanium piping to be installed using both orbital and manual gas tungsten arc welding (GTAW) technology. The use of titanium and orbital welding is consistent with the mission and the advanced design features of the ship. The LPD 17 (Landing Platform Dock) San Antonio Class is the latest class of amphibious force ships for the U. S. Navy. The LPD 17 ships have a crew of 361 sailors and a mission to transport Marines and their equipment and supplies by air cushion or conventional landing craft and expeditionary fighting vehicles (EFVs), augmented by helicopters and tiltrotor aircraft such as the MV-22 Osprey.
The initial plan was for Northrop Grumman Avondale Facilities in New Orleans, La., to build 12 LPD 17 ships. Currently, the first four ships have been delivered to the Navy, with the fifth ship, New York LPD 21 (Fig. 1) scheduled for delivery in 2009. The sixth through ninth ships, LPD 22–LPD 25, are presently in various stages of construction across NGSB’s Gulf Coast facilities. A Navy advance procurement contract was recently awarded to NGSB for the purchase of long lead time materials for a ninth ship, LPD 26. These ships will replace 40 existing ships. Several of the LPD 17 ships have now been completed with more than 70,000 ft of titanium piping assemblies installed in LPD 17 through LPD 22 or 12,000 feet of titanium per ship (Ref. 1). Although the titanium piping represents only a fraction of the piping on the ship, this is the largest amount of titanium piping ever installed in any U.S. naval surface ship.
Selection of Titanium Pipe
Fig. 2 —The orbital system designed for titanium pipe welding.
Titanium CP Grade 2 was selected by the design team to replace copper-nickel (CuNi) seawater systems, such as ballast and fire main piping, since titanium is virtually corrosion-free in seawater. The service life of CuNi is severely limited by its corrosion in seawater. Thus, while titanium has an initial higher cost, the savings realized by lower maintenance costs are much greater than the higher initial cost of material and fabrication. Also, since the titanium piping will exceed the service life of the ships, the life-cycle cost is considerably less. Titanium is a low-density element and has the added advantage of a significant weight savings when compared to CuNi. This is important since the piping extends over the length of the ship and the use of titanium will result in a lighter, more stable, faster and more maneuverable ship that will make it more effective in its designated role.
Titanium Pipe Fabrication Plan
Although titanium offers many advantages and is readily weldable by the GTA process, it is a reactive metal that will absorb oxygen, nitrogen, and hydrogen when heated above 500°F. Of foremost concern is preventing the loss of ductility that results if atmospheric oxidation occurs during welding.
NGSB Avondale began to work with the Navy (NAVSEA) to develop a pipe fabrication plan to address the unique properties of titanium several years before the start of construction of the first LPD 17 ship in June 2000. Patrick Hoyt, chief welding engineer at Avondale, came to NGSB in 1997 and along with Robert Duhe, pipe shop supervisor, organized the titanium program. Hoyt was familiar with orbital welding when he arrived, and he introduced orbital welding technology to the shipyard. Welding procedures were developed both for manual GTA welding, which was used on pipe sizes from 2–12 in. Schedule 10, and orbital GTA welding, which was used on pipe sizes from 6 to 12 in. Avondale’s titanium pipe procedures including its fabrication plan, test procedures, quality assurance, and welder training were officially approved by NAVSEA to give them the required U.S. Navy certification.
To be certified as titanium welders, the welders had to pass extensive testing, which included classroom work, assignments, and on-the-job training. Orbital welding operators were selected from the pool of qualified manual welders for additional training. A total of 25 manual welders were qualified to weld titanium and at the peak of operations, there were a total of nine orbital systems.
Fig. 3 — A completed orbital weld on a titanium pipe. Note the shiny appearance of the weld bead.
The fabrication document for titanium welding at Avondale is NAVSEA Technical Publication 278, which covers building and installation of Class P2 piping systems with low temperature and pressure requirements. Bend and tensile testing of the joints were done to meet the requirements of NAVSEA Technical Publication 248, which is similar to Section IX of the ASME Boiler and Pressure Vessel Code. The first production weld was done in 1999. After production started, NGSB Avondale, Edison Welding Institute (EWI), Navy Joining Center (NJC), and NAVSEA continued to refine the welding procedures to minimize the cost drivers. Extensive research was done on the requirements for interpass temperature, shield gas dewpoint, and base metal cleaning methods. Improvements included raising the intepass temperature from 250° to 600°F and raising the required dew-point from –60° to –40°F. They also tested and approved a nondestructive hardness tester. These changes were monitored and approved by the Navy as it was clearly demonstrated that the changes could be accomplished without detriment to weld quality. Significant reductions in time per weld were achieved by these developments.
With orbital welding the torch moves automatically in a circumferential path around a stationary weld joint. Orbital GTAW can be done with or without the addition of wire, but most pipe welding is done with wire. The orbital welding systems used (Fig. 2) were Model 227 orbital welding power supplies and Model 15 fullfunction pipe weld heads manufactured by Arc Machines, Inc., Pacoima, Calif.
The power supply is microprocessorbased and stores weld programs for the various pipe sizes. The power supply controls weld parameters, which include primary and background amperage (maximum 225 A pulsed), torch travel speed (in./min), pulse times, and wire feed speed. The weld head mounts on a guide ring or track that is clamped onto the pipe and the head moves around the pipe to make the weld. In addition to the basic wire feed functions, it can also be programmed to oscillate or weave across the weld joint and to electronically control the length of the arc (automatic voltage control [AVC]). A special extension of the weld head was provided to position the torch at a 45-deg angle to the pipe for the fillet welds — Fig. 2. The direction of the AVC is in the same plane as the torch.
The joint configuration is a P80 lap joint with the pipe end expanded into the flange forming a bell shape. Although the joint could readily be done in a single pass, the welding specification Tech Pub 278 requires two passes with the second pass completely covering the first pass. The first pass is a stringer with no oscillation while the second pass is done with oscillation. The lap joint is simpler and easier to achieve than a square butt joint as it does not require complete joint penetration.
A large gas cup is used to protect the tungsten electrode and weld pool from oxidation and an auxiliary shielding device or trailing shield, which covers a significant portion of the joint, is attached to the torch block and moves with the torch. This maintains the inert gas cover until the metal has cooled below the oxidation temperature.
Color Tells the Story
Fig. 4 — A welding operator viewing the progress of an orbital weld on titanium pipe.
Color is the criterion used to determine whether a titanium weld is acceptable or whether ductility has been affected. A color chart consisting of acceptable and unacceptable sample welds is posted on the wall of the titanium shop. The color changes as the thickness of the oxide film increases. Straw color is acceptable, while blue color is not. The worst case is terminal oxidation characterized by a dull grey flaky surface. However, the use of a trailing shield can make an oxidized weld appear acceptable. Brushing a discolored weld may give the weld a silver appearance that may look passable, but which would have lost ductility.
A good titanium weld has very shiny and reflective surface (Fig. 3), while a brushed weld has a duller appearance. he welds are inspected in the as-welded condition. Brushing is not allowed. To be on the safe side, NGSB Avondale uses a portable hardness tester as an additional assurance that the welds are ductile. The trailing shield made it impossible to view the weld and steer the torch from behind the torch, so the operators had to make the adjustment to control the weld from a position in front of the torch which is more difficult. The welder views the weld through the lens on the HUD (Fig. 4) and makes cross joint adjustments to keep the torch centered on the joint. Minor adjustments in welding current, wire feed speed, and AVC are also possible.
Orbital Welding Advantages
Orbital welding permits continuous nonstop welding for 360 deg since it is unaffected by out-of-position welding. Travel speed is 3 to 4 in./min. After each pass the orbital welding operator repositions the weld head and staggers the second pass. This generally allows for sufficient cooldown time to begin welding the second pass. However, if color appeared from heat buildup, welding would be stopped and another joint begun. Time can be saved by mounting multiple guide rings on joints to be welded and jumping from weld to weld rather than trying to complete both passes consecutively on the same joint. Because a complete pass can be made without stopping, orbital welding proved to be advantageous on the larger pipe sizes. Avondale was able to improve productivity on manual welds by raising the interpass temperature specified in TP 278 from 150° to 600°F. This reduced he time for a 2 in. manual weld joint from 90 to 30 min. Interpass temperature is not a factor on the larger diameter pipe sizes on which the total arc time for a 2-pass orbital weld on a 12-in. pipe is only 20 min, significantly faster than manual welding.
After welding 70,000 ft of titanium pipe, NGSB Avondale has achieved a reject rate of less than 0.15%. Hydro testing was done on individual systems and then on entire systems. No leaks were found. Welders undergo random weld hardness checks quarterly to ensure that weld ductility has not been compromised.
Unique Titanium Welding Shop
The special shop for welding titanium has been greatly expanded to triple the available space. It is air conditioned to control humidity with climate-controlled work areas. The shop can now accommodate more welders and additional orbital welding equipment. Electrified tools are sed since oil from compressed air powered tools would contaminate titanium. The company is pioneering ways to make titanium work better in a shipyard by making the process simpler and more economical.
The results with titanium piping reflects teamwork and dedication of all of those involved, and NGSB Avondale has been designated a Center of Excellence for Titanium.
1. Hoyt, P. 2006. Titanium emerges as new seawater piping material for Navy vessels. Welding Journal 85(6): 62–65.
This article is based on a presentation at the West Coast Welding Shipbuilding Symposium in Seattle, Wash., Feb. 27, 28,
2008 by Patrick M. Hoyt, chief welding engineer, Northrop Grumman Shipbuilding, Avondale Facilities, New Orleans, La. The author wishes to acknowledge the contributions of Patrick M. Hoyt and Edward L. Winter, APR, site manager communications, NGSB, Avondale Facilities, in the preparation of this article. All photos are by Ricky Kellum, NGSB, Avondale Facilities.