What's inside
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Slag inclusions are bits of flux trapped inside a weld or between passes. They weaken the joint and can cause a structural weld to fail inspection, even when the bead looks sound from the outside. Prevention comes down to joint access, suitable settings, controlled bead placement, and thorough cleaning between passes. Follow the project’s welding procedure specification (WPS) and inspection requirements; general settings are not a substitute for them.
Why slag gets trapped
Stick electrodes leave a slag covering over the weld metal. That covering should rise to the surface and be removed after each pass. It can instead become trapped along the weld toe, in a narrow groove, or between passes if the puddle runs ahead of the slag, the arc is too short, or the joint is difficult to reach. A wide weave, low heat input, and poor electrode angle can also leave ridges that the next pass fails to melt.
Slag inclusions are not the same as surface slag that chips off after welding. A bead can look clean on top while concealing trapped slag below. On structural work, do not assume appearance alone proves soundness; use the specified inspection method.
Prepare the joint for access and fusion
Remove rust, paint, oil, moisture, and loose scale from the weld area. Clean the groove faces and at least the adjacent area the electrode and holder need to reach. Use a grinder or wire brush suited to the steel and the job; a brush removes light residue but will not reliably remove heavy scale or a sharp groove-edge defect.
Check that the groove angle and root opening match the drawing or WPS. A tight groove may prevent the electrode from reaching the root and make it hard to remove slag between passes. If fit-up is wrong, grinding, repositioning, or correcting the joint is better than trying to bury the problem with extra weld metal. Do not change a specified bevel or root gap without approval.
Choose the electrode and settings
Use the electrode classification and diameter specified for the job. Low-hydrogen electrodes such as E7018 require dry storage and handling as directed by the manufacturer or WPS; moisture control helps prevent hydrogen-related cracking, though it does not by itself prevent slag inclusions. Keep electrodes clean and in good condition. A damaged coating or damp, contaminated electrode can make arc behavior erratic.
Set amperage within the electrode maker’s range and the approved procedure. As a rough shop reference, a 1/8-inch (3.2 mm) E7018 commonly runs around 90–130 amps, but position, joint design, and machine polarity affect the suitable setting. Too little current can leave a sluggish puddle and poor sidewall fusion; too much can make the puddle difficult to control and increase undercut or spatter. Adjust in small steps on scrap of similar thickness, then follow the WPS for production.
| Condition | What you may see | Practical correction |
|---|---|---|
| Amperage too low | Sticking, a cold or ropey bead, slag that crowds the puddle | Increase within the approved range; shorten the arc and check travel speed |
| Amperage too high | Wide, fluid puddle, undercut, excessive spatter | Reduce current within the approved range; improve control and travel angle |
| Pass not cleaned | Slag line or hard ridge before the next pass | Chip and brush to clean metal, including toes and groove corners |
| Access or angle is poor | One groove wall is missed or slag is pushed ahead | Improve access or reposition; use a suitable electrode angle and bead sequence |
Control the puddle and bead
Maintain a short, steady arc. A long arc is harder to control and can produce a broad, unstable puddle that fails to fuse cleanly at the groove walls. Hold the electrode so the arc reaches the joint face rather than merely washing metal over it. A slight drag angle is common with many stick electrodes, but the correct angle depends on electrode type, position, and procedure.
Use stringer beads or a modest weave rather than an unnecessarily wide sweep. Wide weaving can leave slag at the edges, especially in vertical or overhead work. Keep travel speed steady: moving too quickly may leave inadequate fusion, while moving too slowly can create a large puddle that overtakes the slag. Watch the leading edge of the puddle and both groove walls, not just the bead’s center.
Clean every pass before the next
After each pass, let the weld cool enough to handle safely, then remove all visible slag with a chipping hammer and wire brush. Pay particular attention to the toes, starts and stops, and corners of the groove. If slag is stubborn, use a grinder carefully to expose clean metal; avoid gouging the weld or reducing required thickness. A chipping hammer and weld-cleaning wire brush are inexpensive essentials, but they do not replace careful inspection.
Before restarting, grind or otherwise prepare a crater or irregular stop when the procedure calls for it. Restart slightly ahead of the previous bead, establish the arc, and blend back through the stop so the new metal ties in without leaving a pocket. If you cannot remove slag from a narrow area, do not cover it with another pass.
Inspect and correct defects
Inspect the cleaned pass under good light before adding another. Look for dark lines, sharp slag-filled grooves, cold laps, undercut, and unfused edges. A visible defect should be removed to sound metal and rewelded using the approved repair procedure—not hidden beneath a cap pass. On a structural joint, report suspected internal defects and follow the project’s inspection requirements, which may call for visual testing, magnetic-particle testing, or ultrasonic testing.
Wear a suitable helmet, gloves, and protective clothing while welding, and use eye protection while chipping or grinding. If you are learning a procedure, practice on matching scrap and have a qualified person assess your technique before welding a load-bearing member. The cheapest useful setup is often a sound chipping hammer, brush, and grinder you already own; buy additional equipment only when access, inspection requirements, or production volume justify it.