The Navy's first attempts at underwater friction stud welding showed some promise, but were not ground-breaking. The program included torque tests and combination torque/bend tests, followed by detailed metallographic examination and Charpy V-notch impact tests in order to evaluate the quality of friction stud welds made on both HY-80 and DH-36 materials.

Welding was performed under various combinations of ambient temperature, water depths (maximum 30 feet (9m)), and position. None of the environmental, attitude, or material variables had a significant impact on the mechanical properties of the weldments, which met or exceeded the requirements for fusion welded studs as specified by MIL-STD-1689 and MIL-STD-248 for applications not requiring shock qualification.

Although the program was successful, the use of friction stud welding was limited to shallow water applications such as anode attachment, external blanking of sea chests, and temporary attachment points for rigging, all which could be performed by conventional wet welding.

Learning from Disaster
It was not until lessons learned from the unfortunate sinking of the Russian submarine Kursk that friction stud welding had a meaningful application. Rescue of the Kursk at 350 feet (106m) was hampered by the lack of welding capabilities needed at that depth. The Navy's underwater welding capabilities have been limited to 50 feet (15m) and underwater wet welding in general has not produced sound welds beyond 140 feet (42m).

Underwater dry chamber welding, although capable of producing sound welds beyond 750 feet (228m), would not be practical due to the logistics and set-up time. Additionally, Navy diving, for all intent and purpose, is limited to 300 feet (91m). The Navy's submarine rescue ships, which had saturation diving capabilities beyond 300 feet (91m), were decommissioned in the mid 1990s.

In regard to the Kursk, deeper welding and diving capabilities were needed to attach a haul-down cable for a submarine rescue chamber (SRC). In addition, the first and most important step during a rescue is the ability to provide life support gas to the survivors, as demonstrated in the 2002 rescue of coal minors in Pennsylvania.

Commercial Successes
In early 1998, friction stud welding was performed commercially at a depth of 1,300 feet (394m) and involved the friction welding of anode continuity tails to riser base piles using a work-class ROV. The friction welding equipment used was a Circle Technologies HMS 3000, which is hydraulically-driven, electronically-controlled, and rated to a depth of 3,000 feet (910m).

Based on this concept, the Naval Sea Systems Command (NAVSEA) initiated another program to evaluate underwater friction stud welding for use in submarine rescue. The program required interfacing the HMS 3000 friction stud welder with the Navy's atmospheric diving suit (ADS), rated to 2,000 feet (606m). The feasibility of this concept was demonstrated in 2001 by Oceaneering International using their WASP ADS and the HMS 3000 friction stud welding system.

Friction stud welding provides the capability to weld a pattern of studs to the hull of a disable submarine, to which a pad-eye can be attached for the SRC haul-down cable and life support gas can be provided by means of a hot tap process using hollow studs. Combined with an ADS, the system provides rescue capabilities well beyond 300 feet (91m).

Concurrent with the Navy's application for underwater friction stud welding for submarine rescue, Oceaneering pursued the application commercially for offshore platform repairs. However, initial research showed that there was a limited amount of accurate public information on the mechanical properties of underwater friction stud welding. As such, the use of this process for any offshore repair without a full evaluation for mechanical, corrosion, and fatigue would not be acceptable.

The Road to Approval
The initial step for obtaining any approval for use on an offshore platform was to design the friction welded stud. Commercially available underwater friction welding equipment is limited to a one-inch-diameter stud. In order to increase the chances of dependability while on the jobsite with the friction welding equipment, a half-inch friction stud was selected.

Stainless steel friction stud material was recommended by the friction welding equipment vendor, and initial procedure testing was based on 304 stainless steel studs. These test showed Vicker hardness values higher than acceptable, a dissimilar metal between the stud and base metal, and difficulty in accurate torque problems. The stainless steel stud material had a higher yield than the hull material, which added the concern of hull damage should the stud accidentally see a shock load.

As the hull material was EH-36 plate, this was also selected as the material for the friction welded stud. Should any type of overload be applied to the friction-welded stud, it would yield prior to causing damage to the hull material. Using identically matched material for the hull and friction stud also avoided any corrosion concerns.

The next step was procedure testing the process at Ohio's Edison Welding Institute. Several hundred friction welds were made using the half-inch friction studs of EH-36 material to half-inch EH-36 plate. A wide range of stud rotational speeds, duration, and forging pressures were evaluated. Hammer bend, charpy impact, and Vicker hardness tests were performed to establish a range of parameters for the procedure.

Hardness Hassles
The friction welding process provided uniform results throughout a wide range of rotation, duration, and forging pressures. Changes in these parameters to the limits of the equipment, had little to no effect on the quality of the friction welding process. Welded samples provided satisfactory results in the hammer bend, charpy impact testing, but had high hardness values, reading in the high 400 to 500 Vickers.

Changes in the rotational speed, forging duration and forging increments had no noticeable effect on eliminating the high hardness readings on the weld macros. Friction welding in both air and underwater showed little to no change in the hardness readings, which were unacceptably high. Extensive experiments were performed in an attempt to reduce the hardness values, as corrosion and other concerns would not allow hardness values above 325 Vickers. Some of the initial experiments using preheat showed hardness reading could be reduced by applying a preheat of over 600 degrees Fahrenheit prior to the friction welding process. While preheat would reduce the hardness, it was not practical for underwater use.

In a last attempt to reduce the hardness values obtained with the friction welding, the Carbon Equivalent (CE) of the friction studs and sample test plates were reduced using special import EH-36 that had lower CE values than any plate available in the United States.

Testing showed that slight changes in the CE of the materials used in the friction welding process provided a drastic reduction in the hardness values. Going from a CE of .41 to a CE value of .36 in the EH-36 material would reduce the Vickers hardness readings by as much as 200 Vickers. Using an EH-36 stud of CE .36 and a plate material of CE .39 would provide a drastic change in the hardness values from one side of the fusion zone to the other. Both the friction stud and the platform hull material had to have a CE of less than .36 in order for the hardness values to be with in acceptable limits. Just a 0.03 change in the CE would take the hardness readings from almost 400 Vickers on the plate side of the fusion zone to the low 200 Vickers on the stud side.

Magnet Development
Electro-magnets, the final hurdle for the onshore development phase of the project, were needed to counter the forging forces of the friction welding unit. This seemed like a simple enough problem, but the initial magnet trials failed shop load cell testing.

Multiple load cell tests with various magnets gave confirmation that a small, six-inch electro-magnet held the best potential. A six-magnet fixture was built and tested in a tank. The tank test showed the six-magnet fixture worked well on the most challenging locations, which all had a plate thickness of a half-inch.

However, while the magnet fixtures worked well in the shallow test tanks, their initial sea trials at the jobsite were a disaster. Water intrusion into the wiring quickly caused wiring failures and shorted the friction welding equipment, which had no current protection for its submerged computer system. Additional development of the magnet fixture required a change to a fully pressure-compensated system built to protect the wiring and magnet coils, rather than the Scot Cast used on the initial attempts.

A Demanding but Promising Technique
Friction welding is considered a solid-state process, as no melting of material occurs. There is a fusion line, as compared to a typical fusion zone, and the heat-affected zone (HAZ) is relatively small compared with arc processes.

As a result of this process, the weld strength is equal to or greater than the base material. Problems from inclusions and porosity are eliminated, since there is no liquid state during the friction process.

However, the process is sensitive to carbon equivalent (CE), with an increase in hardness resulting from an increase in CE. Hardness values on HY-80 have been reported as high as 463 VHN. The hardness values do not hamper the use of the process for submarine rescue applications where the attachments will have a limited service life.

While the commercial application was a success, the technical challenge of using this process offshore were many. Development of the friction welding process to use half-inch diameter studs underwater required a research effort that basically started from scratch.