Figure 1. Conventional welding sinusoidal wave with 50 percent positive and 50 percent negative polarities.

Figure 2. Conventional square wave with 50 percent positive and 50 percent negative polarities.

Figure 3. A variable-balanced square wave with 60 percent positive and 40 percent negative polarities.

Preliminary results from a national research program on making shipbuilding in the U.S. more competitive indicate that the program's projected benefits may be exceeded.

The National Shipbuilding Research Program, Advanced Shipbuilding Enterprise (www.nsrp.org) launched its year-long, research and development program in February 2005 to evaluate variable balance, submerged arc welding (SAW), one of the primary processes used in shipbuilding. The primary objective of the research was to exploit technological advancements in SAW power source and consumable design to improve the productivity rates of one-sided welding (OSW), and to reduce the costs of building commercial and naval vessels in U.S. shipyards. The program was called Evaluation of Variable Balance Submerged Arc Welding and Metal-Cored Electrode Technology.

Projected benefits were to include:

• A 40 percent increase in weld completion rates.
• A 20 percent decrease in weld metal requirements and costs.
• An improvement in impact energy and consistency of properties in heat-affected zones.
• And, reduced defects.

So far, so good
The final report has not yet been submitted. However, the primary researcher said one result was that the time needed to complete a welded joint was decreased significantly. The early report indicates that joint-completion rates for 0.5-in. steel were doubled, and the joint completion rates for 1-in. steel increased more than seven fold.

These statistics are the result of investigating advanced submerged arc welding technologies. The investigation included power sources with variable balance, AC waveform control and metal-cored electrodes in a tandem configuration, and compared the results to shipyards' current use of SAW with conventional, direct current (DC) or AC power sources and solid-wire electrodes in a series arc configuration.

The materials tested ranged from DH36, a grade of steel used in commercial applications, to HSLA 65 and HSLA 100 high-strength, low-alloy steels that are used in the U.S. Navy's aircraft carrier programs.

The technology and consumables tested included a three-phase, square wave, AC machine with variablebalance control from Miller Electric Mfg. Co. (www.millerwelds.com), metal-cored electrode compositions from Hobart Brothers (www.hobartbrothers.com), and flux from The Lincoln Electric Co. (www.lincolnelectric.com) and Esab (www.esabna.com)

An interim report on the program hypothesized that the impact toughness of welds on ships was compromised because of the high heat input used to make them. The report said that many shipyards improved their manufacturing productivity in the past by using one-sided, SAW welding stations equipped with one, two or three electrodes. In some instances, shipyards used welding stations with more than three electrodes to gain additional productivity increases. Using those single and multiple electrode stations to butt weld plate blankets by filling the joints of long, thick panel seams in a single run diminished the impact toughness of the welds. The past practice used a flux-filled, contoured, copper backing bar to support the weld pool on the back side of the joint. The flux protected the weld from atmospheric contamination and served to shape the weld back bead profile. A potential resolution that was suggested to improve impact toughness for those welds was to use SAW welding power sources with waveform control technology and metal-cored electrodes. The report said using this combination of technologies would complete the weld without sacrificing quality.

A little physics
Alternating current is a sinusoidal (sine) wave that over time starts at zero, increases to a maximum value, decreases to a minimum value and repeats. Conventional welding sine waves, are split between 50 percent electrode positive polarity and 50 percent electrode negative polarity, so they are balanced, says Darren Begg, manager of welding engineering at BMT Fleet Technology Ltd. (www.fleetech.com). The positive polarity of the waveform puts more heat into the base metal; the negative polarity of the AC waveform puts more heat onto the electrode. This arrangement creates a balance between fusion and penetration, and electrode melt-off rates. The graph of a conventional sine wave looks like the sine wave in Figure 1.

Researchers are testing a threephase, square-wave AC machine with variable-balance control. The graph of a balanced square waveform is shown in Figure 2.

Variable balance allows the time spent in each polarity to be controlled. Reducing the positive polarity dwell-time for a given set of welding parameters reduces the amount of heat generated at the work-piece compared with conventional sine wave output. Less heat transfer promotes higher weld zone cooling rates, resulting in improved impact toughness. Less heat transfer also reduces stresses that cause angular distortion. As the positive polarity of the variable balance waveform is reduced, the effect is longer negative polarity dwell times. This arrangement of polarities promotes higher electrode melt-off and weld metal deposition rates, allowing the joint to be completed more quickly. In addition, the arc produced by square waveform is more stable than an arc produced by a conventional AC sine wave. The result is fewer weld defects. The graph of a variable balanced sine wave with 60 percent positive polarity and 40 percent negative polarity is shown in Figure 3.

Electrodes
Another part of the project involved developing an optimal metal-cored wire that can be used with basic fluxes for depositing high-heat input, one-sided welds in 0.5-in. and 1.0-in., quenched and tempered steel plates in grades of DH36, HSLA-65 and HSLA-100 steels. The metal-cored wire was developed with the help of Hobart Brothers. Welding 0.5-in. and 1-in. HSLA-100 plate is limited to a heat input of 85 kJ/in. to keep the minimum-specified, under-matched weld metal yield strength of 88,000 psi. Variable balance may allow for this heat input limitation to be extended, and allow for higher heat inputs to be used to improve productivity without sacrificing weld zone integrity.