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Homopolar Welding

Program Description

Homopolar Pulse Welding (HPW) is a one-shot stage welding process for the joining of metals. It is particularly attractive for joining pipe. HPW uses the very high current, low voltage electrical pulse produced by homopolar generator to rapidly resistance heat the interface between abutting pipe ends, producing a full circumference resistance forge weld requiring no filler metal. The weld is complete in only three seconds.

The pipe ends are machined and butted with an initial contact pressure. Two sets of electrodes encircling the pipes are located in a fixed distance from the pipe ends. The distance between the pipe ends in variable and partially determines the cooling rate after welding. The generator rotor is spun to a predetermined speed, and a uniform magnetic field is impressed across it. Brushes contact the rotor, completing the circuit whose maximum resistance is at the interface of the pipe joint. Upset pressure is applied, material is displaced, and the forge weld is made.

The Program

The homopolar Offshore Pipeline Welding Research Program began in February, 1993, funded by a consortium of six oil companies (Shell, Exxon, BP, Texaco, Amoco and Mobil.) CRC-Evans, Parker Kinetic Design (PKD), the Office of Pipeline Safety of the DOT, and the mineral Management Services of the DOI have become members as well. The five year program’s ultimate goal was to produce a prototype pipe welding system suitable for installation on a barge. The first and second year goals were the demonstration of acceptable strength and toughness in different grades and thickness of a 3 in. API 5L line pipe. The fourth year goal was the building of a fixture for and the successful welding of 12 in. schedule 60 carbon steel pipe. The program met it’s technical objectives.

The Weld

Homopolar pulse welding is properly classified as a resistance-forge welding process. The method is similar to other resistance welding methods, with the exception that the energy is delivered in the form of a single high current dc pulse. The method is therefore very fast and does not produce the localized skin heating effects present in ac resistance welding methods. Flash Butt Welding can be used on the same pipe diameters that are being targeted, but flash butt welding takes from one to two minutes per joint and produces extensive melting and a large heat affected zone (HAZ). Homopolar pulse welding requires from two to three seconds and is a solid state welding process.

X-52

Strength. Welds in this material are full strength, with flat tensile specimens failing outside of the weld and heat affected zone at parent metal strength.

Toughness. Charpy V-notched impact toughness in as-welded condition typically 10-30 ft-lb at 0&Mac176;C. Postweld Pulse Normalizing increased impact toughness to >30 ft-lb.

X-60

Strength. Welds full strength at fusion line, but softened in the heat affected zone (fine grained re-crystallized region). Degree of softening controlled by postweld cooling rate.

Toughness. Sponsor specified impact toughness criteria of 32 J (23.6 ft-lb) minimum average value for ½ size specimens at 0&Mac176;C repeatedly exceeded in the as-welded condition.

X-65

Strength. Parent metal strength is routinely achieved in the grade. Higher strength, cleaner steels such as X-65, seem to be more amenable to the process than lower grade, in that values approaching those of the parent metal are routinely and repeatable achieved.