Any specific parameters or dimensions utilized in an actual project should always follow the supplied Welding Procedure Specifications (WPS), as well as applicable codes, such as API 1104, Standard for Welding Pipelines and Related Facilities, and ASME Boiler and Pressure Vessel Code, Section IX.
Why Downhill and EXX10?
SMAW remains the preferred process for situations that require manual field welding because it minimizes equipment requirements and skilled operators can consistently produce sound weld beads. On thinner-wall pipe, downhill welding enables operators to run “hot and fast,” increasing productivity compared to welding uphill, which is required on thicker wall pipe to increase heat input to ensure complete penetration.
With downhill welding, controlling the molten weld pool and prevent the slag from rolling on front of the pool requires a “fast-freezing” EXX10 cellulosic electrode. These electrodes consist of thin coating (10 to 12 percent by weight) that contains around 30 percent cellulose (wood flour) and its associated moisture content. Other ingredients include a sodium-silicate binder, titanium dioxide to create a quick freezing slag, deoxidizers such as ferromanganese and ferrolsilicon, as wel as other elements that vary by manufacturer.
During welding, the heat of the arc melts the cellulose causing it to turn into carbon monoxide, carbon dioxide and large amounts of hydrogen. Carbon dioxide turns into the shielding gas and hydrogen increases arc voltage to create a driving, deeply penetrating arc — a desirable characteristic when welding an open-root joint in field conditions, as well as for melting through rust and dirt in field repair applications. In addition, cellulosic electrodes strike easily, which makes them well suited for tack welds.
EXX10 electrodes produce a weld pool that wets and spreads well, yet sets up fast enough to make this electrode great for downhill welding techniques. The weld bead is flat with coarse ripples and covered with a thin, friable slag layer that is removed easily, helping prevent slag inclusions when making multiple passes. As an interesting side note, the first covered electrode, patented in 1904 by Oscar Kjellberg, was of the cellulosic type.
Power Sources for EXX10
Direct currect electrode positive (DCEP) polarity and more voltage than other electrodes is required for EXX10. Power sources built for running EXX10 electrodes have a high open circuit voltage (OCV), which is voltage at the electrode before the arc is struck. Think of high OCV as a garden hose with the water turned on but the nozzle closed. Good electrical pressure directly relates to positive arc starts. OCVs typically range from 60 to 90 V.
Additionally, power sources for EXX10 electrodes have a good inductor (an inductor resists change in the electric current passing through it). Inductors act as a power reserve to keep the arc established as the operator manipulates the electrode. With their large magnetics and smooth output, direct current welding generators have historically set the standard for EXX10 arc performance. That said, a new generation of inverters has been designed to deliver optimal results for welding with cellulosic electrodes so that field welding applications can benefit from light-weight, portable units. A “cellulosic” operating mode is featured on these inverters that mimics the “drooping” volt/amp curve preferred for pipe welding. They can create a crisper, more forceful and driving arc that benefits open root welding and EXX10 arc characteristics. In addition, they have an adjustable arc force function so operators can tailor the arc to match the application and personal preferences.
The Set Up
Downhill pipe generally requires a 60-deg included angle, or 30-deg bevel. Compared to the 75-deg included angle or 37.5-deg bevel for uphill pipe welding (necessary to reduce slag entrapment potential when using an EXX18 electrode), the narrower angle reduces deposition requirements and improves productivity.
The bevel will terminate in a 1/16-inch or 3/32 inch root face (flat), depending on pipe diameter, to support the heat of the arc. Operators often refer to these sizes as “dime” and “nickel” root face, respectively. Welding pipe requires an open root to ensure complete penetration, so WPSs require a root opening between pipe sections, with the root open typically set the same size as the root face.
Operators may have the flexibility to select whether want to set 1/16 or 3/32-inch root face and root openings, as well as use either a 1/8- or 3/32-inches electrode for the root pass, depending on the WPS. If the application allows, a nickel root face and a 5/32-inch electrode can be chosen because it allows flexibility if the root opening tightens as the pipe heats, cools and contracts. If a 3/32-inch root opening narrows, the operator might have a wide enough opening to push the molten metal through to the backside of the joint, as well as the option to step down to a 1/3-inch electrode. If a 1/16-inch root opening tightens, there is a higher probability that the operator will need to utilize a grinder to open the root opening to ensure penetration.
After the root opening thickness has been set, operators make four or more 1-inch long tack welds at the 12, 3, 6 and 9 o’clock positions to maintain root opening thickness and hold the pipe in place. Keep in mind that the size of the tack weld allowed varies by pipe diameter. Tacks should be ground to bare metal and the ends feathered.
Successful pipe welding demands a good setup: concentrically align the pipe ends and make sure an even root opening around the entire circumference. If the setup is less than perfect, fix it now if at all possible.
The Root Pass: Four Key Adjustments
Make sure to set welding amperage to fall within the WPS and then match personal preferences. General starting points are 80 to 90 A for a 1/8-inch electrode and 105 to 115 A for a 5/32-inch electrode. Strike the arc on a tack weld at the top of the pipe, holding the rod perpendicular to the pipe. When it penetrates through the pipe, the operator will clearly hear the arc, and a small “keyhole” will open behind the electrode. At this point, tilt the electrode and begin traveling toward the bottom of the pipe, holding a 5- to 15-deg drag angle and moving in a straight line (e.g., no weave).
Very little arc light will be visible on the outside of the pipe. Experienced pipe welders know how to read the keyhole and make one of four adjustments to control keyhole size. This should roughly match root opening width. If not keyhole is not seen by the operator, that indicates insufficient penetration. To correct the situation, the operator can do one or more of the following:
- Increase amperage, generally done on the fly by a welder’s helper wither a remote amperage control
- Hold a longer arc, which increases voltage and overall heat input.
- Use more of a drag angle, which pushes more heat back into the joint.
- Reduce travel speed.
If the keyhole is too big, one or more of the following corrections can be made by the operator:
- Reduce amperage.
- Increase travel speed until the keyhole reaches the correct size.
- Decrease arc length to lower voltage and “cool” the weld pool.
- Hold the electrode more perpendicular.
Beginners usually need to put more pressure on the electrode than they think (“bury the rod” is a common instruction). Sometimes the right amount of pressure can cause the rod to bend a little bit, particularly with a smaller diameter electrode and a narrow root opening.
There are two issues operators may encounter on a root pass. One issue is the arc may wander to one side, and this can be caused by a concentricity problem with the electrode coating. In SMAW, the coating crater, or the cup-like formation of the coating that extends beyond the melting core wire, performs the function of concentrating and directing the arc. Concentration and direction of the arc stream is achieved by having a coating crater, somewhat similar to the nozzle on a water hose, directing the flow of the weld metal. When the coating is not concentric to the core wire, the poor arc direction results in inconsistent weld beads, poor shielding and incomplete penetration. The electrode melts off unevenly, leaving a projection on the side where the coating is the heaviest. This condition is typically referred to as “fingernailing.”
Counteracting fingernailing requires pushing the thin side of the electrode further into the groove to direct the arc force into the joint. The second issue, which as a similar solution, is arc blow, where magnetic forces try to push the arc toward the opposite side of the joint and try to create a more even melt-off rate. Arc blow can occur as a result of poor grounding. Be sure the pipe is well-grounded; re-positioning the ground clamp can solve the issue.
Old electrodes may also cause welding problems. Where EXX18 low-hydrogen electrodes will absorb moisture and cause problems, the cellulose in EXX10 electrodes can dry out, leaving insufficient gases for the electrode to perform properly.
Hot Pass
A good root pass will create reinforcement on the inside of the pipe that is flush with the inside. On the exterior, the root pass will leave a convex (humped) weld bead with “wagon tracks” of slag on either side. Grind the bead with a disc grinder to slightly flatten the bead and expose the wagon tracks, as they can entrap slag. Don’t grind the bead too thin, as it needs to support the heat of the hot pass, which will work the slag to the top so that it joins the new slag layer rather than become trapped.
If the WPS allows the flexibility to increase electrode diameter, keep in mind that utilizing a 5/32-inch electrode and running hotter tends to melt out slag better. However, utilizing a 5/32 or 3/16-inch electrode will allow for more weld metal deposition into the groove to fill the groove faster. With more weld metal being used, caution must be taken with larger electrodes to utilize proper technique to avoid discontinuities that can become trapped.
When making the hot pass, a slight weave may be required to fill the joint, and holding a longer arc also assists with widening the pool and increasing the heat input. Otherwise, the electrode does not require much manipulation until reaching the bottom of the joint. Here, when welding pipe in the 5G or 6G position, the pool may tend to sag. If working with a helper, ask them to decrease amperage. In addition, many operators utilize a stepping motion: drag the electrode forward to melt out the slag, step back an electrode diameter to give the front edge of the pool a chance to cool, then move forward and repeat.
If the pool becomes fluid and wants to run ahead of the arc when transitioning from to 2 to 4 o’clock position, there is a misconception that amperage should be decreased. More times than not, the solution is to increase amperage and utilize the additional arc force to push the pool back into the joint. Also, it may be necessary to increase travel speed to stay ahead of the pool.
When transitioning toward the bottom of the pipe, make sure to maintain a drag angle. A significant percentage of weld flaws occur because of poor electrode angle between the 4 and 8 o’clock positions.
After the root pass, keep in mind that the WPS may call for an E7010 or E8010 electrode; regardless of EXX10-type electrode, the technique will be similar. Also, keep in mind that several electrode manufacturers offer EXX10 and EXX10 “plus” electrodes. The “plus” electrodes create a slightly narrower and less fluid arc, so operators prefer them for the root pass. The standard EXX10 electrodes create a slightly more fluid arc, helping wet the sidewalls on the hot pass and spread the pool on the fill and cap passes.
Fill and Cap
For the fill and cap passes, operators will typically step up to the largest electrode allowed, often a 3/16-inch to provide greater deposition and to help create a wider pool. In fact, a cap made in a single pass is typically called a “pool cap.”
For the first still pass, use a weave to ensure tie-in with the pipe wall. Moving the electrode side to side and creating an upside-down U shape is common, as is holding a longer arc than for the previous passes. Combined with a proper drag angle, these techniques prevent the center of the pool from sagging.
One of the most common defects is insufficient fill, so “stripper passes” may need to be added to build up the weld metal so that it is flush or almost flush with the top of the joint. The spots between the 2 and 5 and 7-10 o’clock positions are notorious for having low spots in the center, and adding a stripper pass in this area may be necessary.
The cap pass should bring the weld metal up to the point where the cap is flush to no higher than 1/16 inch above the pipe surface. Without the need to tie-in to the pipe wall, lower currents may be utilized than for the fill pass(es).
Practice Makes Perfect
Welding pipe downhill with cellulosic electrodes is not more difficult than uphill welding, but it does require different techniques. The skills learned for welding uphill simply don’t translate. For instance, the “whip and pause” technique required for uphill EXX10 welding has no place in downhill welding, and the slag systems for basic and rutile electrodes provide completely different characteristics.
At Pennsylvania College of Technology, students spend 80 h on their introduction to downhill pipe welding course. The course provides a good foundation and will let students know if they have an aptitude for the process. However, there’s only one way to gain proficiency: spend time in the booth and practice — downhill.
Source: aws.org
