If you've ever tried to run a root pass with a 7018 rod, you already know the problem. The puddle drops through, the arc blows out, and you're left with a pile of bird crap. The welding rod for open root vs fill and cap passes isn't about preference.
It's about physics.
Open roots demand a fast-freeze rod that can bridge an unsupported gap. That's why E6010 and its cellulosic cousins dominate root passes. Per AWS D1.1, electrode classification determines penetration, slag behavior, and hydrogen levels.
As of 2026, more shops are running hybrid procedures with TIG roots and stick fills. Let's break down the why and how.
Quick Answer
For open root passes, use E6010 or E7010. For fill and cap passes, switch to E7018. E6010 gives deep penetration and fast-freeze slag.
E7018 provides low-hydrogen strength and a smooth final profile. Match the rod to each pass.
Why the Root Pass Rod Isn’t the Right Choice for Your Fill and Cap
A single electrode that works for the root will often cause problems in the fill and cap. Here's why.
The open root wants a fast-freeze rod that solidifies quickly. That rod, usually E6010, digs deep and leaves a thin slag that's easy to remove. But if you keep using 6010 for fill passes, you're introducing high hydrogen levels into the weld.
That's a real risk for hydrogen-induced cracking in thicker sections.
The fill and cap want a low-hydrogen rod like E7018. It deposits smoothly, offers better impact toughness, and produces a clean cap. But 7018 is sluggish on an open root.
It doesn't freeze fast enough, so the puddle falls through the gap.
The solution is simple. Use one rod for the root, then switch rods for the fill and cap passes.
How Open Root Welds Change Your Rod Selection
What an Open Root Joint Actually Demands
An open root joint has a beveled edge with no backing bar. The first pass must bridge a gap, usually 1/8 to 3/16 inch. This means the weld puddle has nothing underneath it.
If the electrode doesn't solidify fast enough, the molten metal falls through. The result is burn-through, excessive drop-through, or poor root reinforcement on the back side. That's why the root pass rod needs a coating that creates a stiff, fast-freezing slag.
Why Fast-Freeze Rods Like E6010 Handle the Unsupported Gap
E6010 is a cellulosic electrode. Its coating produces a strong shielding gas and a digging arc. The arc force literally pushes the molten puddle into the joint, while the fast-freeze slag holds it in place.
This makes it ideal for out-of-position and unsupported welds.
Manufacturer specifications indicate that E6010 is designed for all-position welding, including vertical uphill and downhill. That's a direct result of the fast-freeze characteristics. The rod freezes quickly enough to hold the root, even when you're welding overhead.
Per AWS D1.1, the structural welding code, this electrode group is the standard choice for open roots.
The Typical Welding Sequence: Root → Hot Pass → Fill → Cap
The sequence matters as much as the rod. A typical open root weld in a 1/2-inch plate or 6-inch pipe involves four distinct passes.
| Pass | Purpose | Typical Rod |
|---|---|---|
| Root | Penetrate and fuse the bottom of the joint | E6010 |
| Hot | Burn out any remaining slag and reinforce the root | E6010 or E7018 |
| Fill | Build up the weld to near the surface | E7018 |
| Cap | Final layer with a controlled profile | E7018 |
The hot pass is often skipped by beginners. That's a mistake. The hot pass locks the root, removes slag pockets, and provides a clean platform for the fill passes.
It also helps anneal the root bead, which reduces crack risk.
After the hot pass, the joint is now supported. That's when the fill and cap passes can use a slower, high-deposition rod like E7018.
Your Decision Tree for Choosing the Root Pass Rod
The right root rod depends on three variables. Those are your power source, your base metal, and your welding position. Use this decision tree to narrow it down.
E6010 vs. E6011 vs. E7010: Which One Fits Your Power Source and Pipe Grade?
Start with your machine.
- If your welder only puts out AC, choose E6011. It's the AC-compatible version of the cellulosic family.
- If you have DCEN or DCEP capability, choose E6010. It's the standard for open roots in structural work.
- If you're welding high-strength pipe, such as X60 or X65, choose E7010 or E8010. These offer higher tensile strength while keeping cellulosic handling.
If you're unsure about your base metal, check the WPS. The welding procedure specification from your shop or engineer will list an approved electrode. For code work, that document often follows ASME Section IX.
Don't improvise without it.
Position Matters: Uphill vs. Downhill Root Welding
Position changes the technique, but it doesn't usually change the rod family.
Downhill welding is common on pipe because it's faster. E6010 works well for downhill root passes. The bead stays thin, and the fast-freeze slag keeps up with the travel speed.
Uphill welding is slower but gives better penetration control. E6010 still works. Some welders prefer E7018 for uphill roots when they can run a backing ring, but that's a different joint type.
If you're welding structural steel in position, stick with E6010 for the root and E7018 for everything else.
Choosing the Fill Pass Rod Without Breaking Compatibility
Once the root and hot pass are done, the joint is closed. Now you can switch to a low-hydrogen rod without worrying about the puddle falling through.
E7018 is the default fill and cap choice for carbon steel. It offers 70,000 psi tensile strength, excellent ductility, and low hydrogen content. That last part matters because hydrogen can cause cold cracking in the weld metal and heat-affected zone.
Make sure your E7018 is dry. The low-hydrogen coating absorbs moisture from the air. Per AWS guidelines, electrodes must be stored in a rod oven at 250°F to 300°F.
If you're not sure whether a rod has been exposed to humidity, recondition it or throw it away.
What about E7010 or E8010 for fill and cap? In pipeline welding, those cellulosic rods are sometimes used for all passes. That's allowed by certain procedures.
But for general structural work, the safer choice is E7018, especially on thicker plate or restrained joints.
The compatibility is mostly about technique. Your amperage, travel speed, and interpass cleaning all need to shift when you switch rods. E7018 runs hot and leaves a thicker slag.
You need to clean it thoroughly between passes. Use a chipping hammer and wire brush, or a grinder for stubborn spots.
Picking the Cap Pass Rod for Profile and Toughness
E7018 is the standard cap pass rod for carbon steel. Its low-hydrogen coating produces a smooth, slightly convex bead. That profile sheds water, resists corrosion, and looks clean enough for inspection.
You also get better impact toughness with 7018 compared to cellulosic rods. Charpy V-notch values commonly land around 20 to 60 foot-pounds at -20°F, depending on the specific electrode grade. That matters for structural steel, pressure vessels, and anything in cold service.
Run the cap at a slightly lower amperage than your fill passes. This gives you better control over the final width and contour. If the cap is too hot, you'll get a flat or concave bead that can trap slag along the edges.
For the cap pass, use a stringer bead or a light weave. Keep it tight. A cap that's too wide or too convex looks bad and makes NDT interpretation harder.
When TIG Roots and Stick Fill/Cap Combo Makes Sense
Not every open root has to be welded with a cellulosic stick rod. TIG welding the root, then switching to stick for the fill and cap, is a proven hybrid approach. It's especially common in code pipe work.
TIG gives you perfect control of the root bead. You can see the keyhole open and close. The result is a clean, uniform root that doesn't need grinding or back-gauging.
This is why many ASME Section IX procedures specify GTAW for the root on critical high-pressure lines.
The trade-off is speed and cost. TIG roots are slower than 6010 roots. They demand more skill and a gas lens setup.
But if your procedure calls for it, there's no way around it.
Aggregate reviews of welding procedures report this hybrid approach on X52 through X70 pipe. It's the default on many refinery and chemical plant jobs. If you're working with chrome-moly or stainless alloys, the TIG root plus stick fill combo is often required by the WPS.
Step-by-Step Welding Playbook for Root, Fill, and Cap Passes
Joint Prep and Fit-Up Before You Strike the Arc
Clean the bevel faces down to bright metal. Remove all rust, paint, oil, and moisture. Contaminated base metal causes porosity in the root and slag inclusions between passes.
Set your root gap and root face according to the WPS. A typical gap for 6010 is 1/8 inch. The root face lands around 1/16 to 3/32 inch.
Too small a gap makes it hard to get penetration. Too wide and you'll blow through.
Tack weld the joint at the correct alignment. Use the same rod you'll run for the root. Keep tacks short and grind them to a feather edge so they don't interfere with the arc.
Root Pass Technique: Whip, Pause, and Keystoning
Run the root with a whip and pause motion. Strike the arc, let it dig into the root face, then whip forward slightly to let the puddle freeze. Pause again to deposit more filler.
For pipe, run downhill whenever your procedure allows. It's faster and produces a thinner root bead. For structural plate, you'll often run uphill to get better penetration.
Watch the keyhole at the leading edge of the puddle. If it's too wide, you're burning through. If it's barely visible, you're not getting enough penetration.
Adjust your travel speed and amperage to keep that keyhole consistent.
Hot Pass Timing and Why It's Not Optional
The hot pass is your second pass over the root. It burns out any slag that hardened in the root bead's tight corners. That's why skipping it is so common and so dangerous.
Run the hot pass hot. Use a higher amperage than the root. Move at a steady travel speed so the arc washes the sides of the bevel.
This helps eliminate lack-of-fusion at the root edges.
The hot pass also locks the root in place. It prevents the root bead from cracking as the weld cools and shrinks.
Fill Pass Sequence: Interpass Cleaning and Slag Control
Before every fill pass, clean the previous layer. Chip off the slag, wire brush the bead, and check for any visible defects. This is non-negotiable if you want good fusion between passes.
Weave only within the width of the bevel. Let the bead fill the groove evenly. A good fill pass should finish about 1/16 inch below the plate surface, leaving room for the cap.
Control interpass temperature. For most carbon steel, keep it below 550°F. If the joint gets too hot, you risk grain growth and reduced toughness.
Cap Pass: Weave vs. Stringer Beads for the Final Profile
A single stringer bead works for narrow joints. For wider joints, a light weave is fine. Watch your puddle edges so they tie in cleanly with the plate surface.
Underfill at the toes is a common reason caps fail visual inspection.
Don't overbuild the cap. Excess reinforcement increases stress concentration and makes the weld look sloppy. Keep it flat to slightly convex, just proud of the plate.
If the cap looks pitted or porous, check your electrode storage. Moisture is the number one cause of porosity in 7018 welds. Its low-hydrogen coating must be kept dry.
Per the American Welding Society, store E7018 at 250°F to 300°F in a rod oven. Recondition rods that have been exposed to humidity.
Common Mistakes That Ruin Open Root Welds
The biggest mistake is running the root too cold. A cold 6010 won't penetrate properly. You get a root bead that sits on top of the joint instead of fusing through it.
This fails bend tests every time.
Another mistake is weaving the root pass. The root should be a stringer, not a weave. Weaving an open root leads to a wide, shallow bead that can't support the layers above it.
Skipping the hot pass is a classic error, too. It leaves slag trapped in the root. That slag causes inclusions when you run the fill passes.
Other common problems:
- Pulling the arc away on the whip motion, which creates arc blow and porosity.
- Running too long an arc length, which lets nitrogen contaminate the weld.
- Using 7018 for the root on a tight gap, which causes drop-through.
- Forgetting to preheat on thick or high-carbon steel. Check your WPS for preheat requirements. Many oil field and structural jobs require a minimum preheat of 150°F to avoid hydrogen cracking.
Real-World Scenarios: Pipeline vs. Structural Steel
The rod selection logic stays the same, but the rules differ between the two worlds.
Pipeline welding typically runs downhill with E6010 or E7010 for the root. Fill and cap passes also run downhill with the same electrode, per most pipeline procedures. This is a speed decision.
Miles of pipe can't be welded with 7018 at the same pace.
Structural steel welding is mostly uphill. E6010 for the root, then E7018 for the hot pass, fills, and cap. This combination handles thicker, more restrained joints better than cellulosic rods all the way through.
If you're on a pipeline job, a 7018 fill may not even be allowed by your WPS. If you're on a building or bridge job, a 6010 fill will likely fail hydrogen control requirements.
Know which world you're in before you set your amperage. The welding procedure specification is your map, and the electrode is your tool. Both have to work together.
Amperage and Electrode Diameter Quick Reference for Each Pass
Amperage varies by rod diameter, welding position, and base metal thickness. The table below gives typical starting points for carbon steel. Always adjust to your specific fit-up and machine calibration.
| Pass | Rod | Diameter | Amperage Range |
|---|---|---|---|
| Root | E6010 | 1/8" | 75–110 A |
| Root | E6010 | 5/32" | 110–150 A |
| Hot | E6010 | 1/8" | 90–120 A |
| Hot | E7018 | 1/8" | 110–140 A |
| Fill | E7018 | 1/8" | 120–150 A |
| Fill | E7018 | 5/32" | 180–240 A |
| Cap | E7018 | 1/8" | 110–130 A |
| Cap | E7018 | 5/32" | 160–200 A |
Run the root hot enough to keyhole. Run the cap slightly cooler than your fills. Vertical and overhead positions require dropping amperage about 10 to 15 percent from flat numbers.
WPS and Code Compliance: What You Must Follow
Your welding procedure specification dictates everything. It lists the approved rod, amperage range, preheat, interpass temperature, and position. That document is your contract with the quality department or client.
ASME Section IX governs many pipe and pressure vessel procedures. AWS D1.1 does the same for structural steel. Both require a qualified PQR before the WPS gets written.
You cannot deviate from the WPS and still call the weld code-compliant.
If you swap a rod without approval, the weld fails qualification. Worse, a welded joint can crack months later. Check the WPS first, then pick your electrode.
Final Decision Guide: Which Rod for Which Pass?
Stick to this simple logic for carbon steel:
- Root pass: E6010 for DC machines, E6011 for AC-only, E7010 for higher-strength pipe.
- Hot pass: use the same cellulosic rod or switch to E7018 if the WPS allows.
- Fill and cap: E7018 in most structural work, or the approved cellulosic rod for pipeline procedures.
- TIG root: ER70S-2 or the specified filler, then E7018 for the rest.
That's the whole trick. Match the rod to the position and the joint condition. A root is unsupported, so it needs a fast-freeze rod.
A fill and cap are supported, so they can handle a low-hydrogen rod.
Frequently Asked Questions
Can I use E7018 for an open root?
No. E7018 is a low-hydrogen rod with a sluggish puddle. It won't freeze fast enough to bridge an unsupported gap.
The molten weld metal will drop through, giving you burn-through or a poor backside bead. Use E6010 for the root instead.
What amperage should I run E6010 for the root?
For a 1/8-inch E6010, start around 80 to 100 amps on plate and 70 to 90 amps on pipe. Watch the keyhole. If you're burning through, drop the amps by 5 to 10.
If you see no keyhole, increase amperage or open the gap.
Why does my root pass keep blowing through?
You're running too hot, or your root gap is too wide. Check the gap against the WPS. Most open roots land between 1/8 and 3/16 inch.
Reduce amperage slightly and try a shorter arc length. Also, keep the whip motion controlled rather than yanking the arc away.
What's the difference between E6010 and E7010?
Both are cellulosic rods with fast-freeze slag. E6010 has 60,000 psi tensile strength. E7010 has 70,000 psi tensile strength plus better impact toughness.
Use E7010 for high-strength pipe like X60 or X65, and E6010 for general structural steel.
Before You Strike the Arc: A Quick Checklist
- Verify your WPS lists the rod you're using.
- Confirm the electrode oven is running at 250°F to 300°F for 7018.
- Clean the bevel faces to bare, bright metal.
- Set the root gap and root face per the procedure.
- Check that your travel speed and amperage match the position.
Weld the root with E6010, then switch to E7018 for the fill and cap. That combination has handled open roots on pipelines, bridges, and pressure vessels for decades. It's the right answer for most jobs.
