Welding Process
Applications
- Fabrication
Welding Benefits
- Reduced Welding Cost
- Energy Efficiency
- Improved Weld Quality
- Reduced Downtime
FIRST SUB ARC TECHNOLOGY BREAKTHROUGH IN DECADES GUARANTEES PRODUCTIVITY INCREASE IN MANY APPLICATIONS
People naturally tend to resist change, but those whose applications require submerged arc welding (SAW) on butt or groove joints with solid wire (single or tandem arc) now have a strong incentive to embrace change: an almost unbelievable productivity increase.
As shown in Fig. 1, welding with variable balance AC squarewave SAW -a new technology patented by Miller Electric Mfg. Co.-instead of DC electrode positive (DCEP) increases deposition rates by up to 59 percent using exactly the same variables. As shown in Fig. 2, deposition rates can be increased by 20 percent or more by switching from an AWS EM1 solid wire to Hobart Brother's Tri-Mark brand Metalloy® EM12KS (AWS A5.17 EC1) tubular (metal cored) wire. Combining the two produces even better deposition rates, as well as eliminates many of the problems associated with current SAW technology.
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Fig. 1 |
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Fig. 2 - Deposition Rates |
New AC Technology
A variable balance AC squarewave SAW output, produced by Miller's Summit Arc 1000 power source, occurs by coordinating the timing of when the SCRs fire. This essentially produces separate pulses of DC electrode positive (DCEP) and DC electrode negative (DCEN) welding power. By coordinating their constant voltage (CV) outputs with a microprocessor control, this power source creates a "hybrid" AC welding process.
Unlike conventional AC SAW power sources that produce a balanced sine wave output (50 percent EP/50 percent EN), this new technology produces a squarewave output with variable control of EP and EN dwell time (see Fig. 3).
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Fig. 3 - Balance Control |
The EP portion of the cycle preheats the plate more than the electrode. This provides deeper penetration and good wetting at the toes of the weld. The EN portion of the cycle preheats the electrode more than the plate. This melts the electrode faster for better deposition.
Thus far, the variable balance combinations that offer the most promise for increasing deposition rates generally favor higher EN values. They are, by percent, 30EP/70EN, 34EP/66EN and 40EP/60EN. However, the micrograph shown in Fig. 4 proves that weld beads made with all types of EP/EN values and metal cored wire have almost identical penetration profiles as those made with DCEP and solid wire.
According to ASME Section IX, most weld beads cannot exceed a width of 1/2 in. All the beads in Fig. 4 meet that requirement. This means that this new technology/wire combination can be easily substituted for existing applications. And while all the beads look the same, there is one critical difference: those made with variable balance AC squarewave technology and metal cored wire were put in much faster because of superior deposition rates and faster travel speeds.
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Fig. 4 - Sub Arc Bead |
Take the example of railcar manufacturer making a butt weld on 40 ft.-long, 1/4 in.-thick plate with a 1/8 in. diameter electrode. Using variable balance AC squarewave technology, it achieved a travel speed of 65 in./min. This is faster than tandem arc SAW (two arcs in series) without any of the problems, initial cost, power use or consumable consumption of a two-arc system.
When bead width is not a consideration, variable balance AC squarewave technology reduces the amount of passes required. One fabricator in the offshore market working with 1-1/2 in. plate switched from DCEP to variable balance AC squarewave technology and reduced the number of passes from 19 to 12 while maintaining every other variable (wire type, flux, groove angle, travel speed and volts/amps). Another bridge fabricator reduced the number of passes from 31 to 24.
Natural Synergy
When working with SAW, the welding engineer's challenge is to produce a weld with the ideal penetration profile at the highest deposition rate possible while avoiding base metal dilution and warping. A DCEP SAW output, the most common output for single wire welding on butt or groove joints, permits focusing on just one of these variables. With the ability to tailor how much heat it directs into the plate and into the electrode, variable balance AC squarewave SAW technology allows engineers to emphasize two of these three primary attributes. Variable balance AC squarewave enables:
* Good penetration with increased deposition
* Good penetration with lower heat input
* Increased deposition with lower heat input
Metal cored wire and SAW have a natural synergy that further enable emphasizes this flexibility. Metal cored wire consists of an outer sheath of solid metal wrapped around a core of metal powders. Because of their granular nature, the metal powders conduct electricity poorly. As a result, the welding current concentrates entirely on the outer sheath (with solid wires, the wire's entire cross section carries the current).
As shown in Fig. 5 and Fig. 6, concentrating the current in a smaller area produces higher current densities, which in turn melts the wire faster. The high amperages used for SAW further enhance current density; they are up to two times higher for SAW than GMAW. Referring to Fig. 2, notice how the deposition rate differences between metal cored and solid wire become more pronounced as amperage increases.
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Fig. 5 - Current Density |
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Fig. 6 - Current Density |
The synergy between metal cored wire and a variable balance AC squarewave output occurs because the EN portion of the cycle focuses substantially more current on the electrode than DCEP. Greater current densities require increasing wire feed speed to match the faster burn-off rate. This maintains amperage levels and increases deposition rates and travel speed.
It is important to remember that the EP portion of the variable balance AC squarewave cycle provides sufficient penetration. For example, one manufacturer making a butt weld on 1/2-in. plate completely eliminated all prep work and still achieved 100 percent penetration. As a side note, metal cored wires have a broader arc cone that bridges gaps and overcomes poor fit-up much better than the narrower arc cone of solid wire.
Less Heat, Same Filling
The synergy between metal cored wire and variable balance AC squarewave technology also occurs when lowering heat input. For example, the deposition rate achieved with solid wire/DCEP can be maintained, yet the weld can be made with less heat directed into the plate because of the current density/EN dwell time advantage. Less heat input helps when making welds on plate sensitive to warping, on heat treatable materials (e.g., 4130, 4140), in thinner applications sensitive to burn-through (10 gauge to 1/4 in.) and in applications situations where base metal dilution and subsequent loss of alloy properties in the weld area have made the weld susceptible to cracking.
Power piping applications may especially benefit. When pipe welding, most fabricators run a GTAW root and a GMAW or FCAW hot pass to put sufficient weld metal in the joint to absorb the heat of an SAW fill pass. Now, because lower heat input can eliminate burn-through, it may be possible to SAW weld after a single root pass (note that metal cored wire minimize burn-through because it lacks the deep finger of penetration that characterizes solid wire). After the first SAW pass, the fabricator can finish the joint by shifting to the high deposition/faster travel speed advantages of metal cored wire/adjustable balance AC squarewave technology.
For SAW welding, it is important to note that the cost for large diameter metal cored wires (approximately $.85 to $1.00 per pound) approaches that of solid wires. Given that its other advantages (highlighted in Fig. 7) dramatically increase productivity, this more than compensates for any slight cost premium.
[Fig. 7 - Advantages of metal cored wire over solid wire in SAW]
* 20 percent deposition rate increase using same size wire
* Faster travel speeds
* Reduce or eliminate plate preparation
* Bridge poor fit-up better
* Less susceptible to burn-through
* Easier to feed
* Overall weld costs reduced
No Arc Blow
Concerns about filler metal costs may be a moot point anyway because variable balance AC squarewave technology can lower filler metal use by up to 42 percent.
Anyone familiar with DC SAW knows about arc blow, which occurs when the DC welding current creates a magnetic force that pulls the arc from side-to-side of the joint. This causes problems with weld quality. If arc blow occurs in the middle of a weldment, engineers specify an included angle large enough minimize its effects. To compensate for arc blow at the end of a joint, an 8 to 10 in. run-off tab is often necessary. Both solutions waste filler metal, flux and time.
Because the variable balance AC squarewave does not magnetize the weldment like DC SAW, may joints may be designed with smaller included angles and shorter run-off tabs. To quantify filler metal savings, Miller engineers welded sections of 1.25 in.-thick plate with included angles of 50, 40 and 30 degrees using variable balance AC squarewave technology. Arc blow was not a factor.
As shown in Fig. 8, reducing the included angle by 20 degrees lowers filler metal use from 2.73 to 1.57 lb. per foot, or 42 percent. Assuming a cost of $1.00 per pound for filler metal, the new technology potentially lowered filler metal cost by $1.16 per foot of weldment. It also reduced flux use, labor time (less preparation, arc-on time) and heat input (fewer passes lowers the interpass temperature, which in turn minimizes waiting time between passes).
Fig. 8
Tandem Substitute
The combination of metal cored wire and variable balance AC squarewave SAW technology is so productive than it can also compete with twin wire (two arcs from one modified contact tip) or tandem arc (two torches) technology on butt and groove joints (not fillet welds, however).
For example, one manufacturer working with 7/16-in. steel plate was beveling both sides of the plate, and, using tandem arcs, made one pass on the back side and two passes on the front side to complete the joint. Using a metal cored wire and single variable balance AC squarewave power source, the manufacturer eliminated the bevel on the back side and complete the joint by welding with just one pass on each side. This produced $65,000 annual savings from reduced prep work alone, or a payback period of less than three months.
Given the complexity a multiple torch system, the fact that it cost $10,000 more than a single variable balance AC squarewave power source and the lower productivity of single wire DCEP, manufacturers looking to improve productivity and lower overhead costs need to examine new SAW technology.
