Groove Weld Strength Troubleshooting

Why Calculated Strength Is an Upper Bound

The calculator assumes the effective throat you entered is fused metal across the weld length. Anything that removes fusion area, notches the toe, or seeds gas in the root reduces actual capacity — sometimes to a fraction of φRn — without changing how “pretty” the cap looks.

1. Incomplete Fusion (Sidewall or Interpass)

How to spot it: Cold shelving against the bevel face, lack of wetting at the sidewall, or a straight dark line on a ground section / RT / UT. Cap may look fine while the sidewall never merged.

What causes it: Wrong work angle (arc not directed into the bevel), heat too low for the thickness, travel too fast to wet the wall, slag left between passes, mill scale or rust on groove faces, or weaving that skims the face without dwelling on the sidewall.

Fix the technique:

• Point the arc into the root of the bevel, not down the plate centerline.
• Raise heat into the qualified WPS range before blaming filler class.
• Chip / brush to bright metal between passes — slag lines are fake fusion.
• Grind groove faces to clean metal before the root; grit left under the puddle is fusion insurance that fails.

2. Incomplete / Lack of Penetration

How to spot it: Root does not show on the back side when open-root is required; remaining land or sharp root notch on cross-section; RT/UT shows an unfused root. Cap height is irrelevant here.

What causes it: Root opening too tight, land too thick, amps/volts too low for root pass, travel too fast, no backgouging when the procedure requires it, backing omitted when the WPS expects it, or starting with fill settings on a root that still needs keyhole control.

Fix the technique: Open the root gap to the WPS (see prep technique), thin the land if it is blocking the keyhole, slow down and feed enough heat for root penetration, and backgouge / grind to bright sound metal before the first fill when the procedure demands full penetration from both sides.

3. Undercut at the Toe

How to spot it: A sharp notch melted into the plate at the weld toe — common on the high-heat side of the final pass. Inspectors fail undercut that exceeds code limits even when throat looks full.

What causes it / fixes: Excess amps or voltage, travel too fast to fill the melted toe, long arc, or weave that skips the toes. Drop heat into range, slow enough to wash and fill, pause briefly at each toe, shorten arc length. For general undercut visual ID across fillet and groove work, see Common Weld Defects: Porosity, Undercut & Overlap — this page stays focused on why undercut punches a hole in assumed strength (section loss + fatigue raiser).

4. Porosity in the Root Pass

How to spot it: Pinholes or worm tracks on a ground root, sponge-like fracture of a test coupon, or RT indications at the root land. Surface foam on a cold fill pass after a porous root does not “heal” the root.

What causes it / fixes: Oil, paint, rust, wet electrodes, mill scale in the groove, drafts stealing shielding, clogged nozzles, moisture in flux-core / stick, or contaminating backing. Clean to bright metal, verify gas CFH, block drafts, dry consumables, and grind out porous root — do not bury it. General porosity causes/fixes are covered in Common Weld Defects; on grooves, root porosity is uniquely expensive because it sits exactly where te starts.

Closing the Gap Between Calculated and Actual Strength

1. Re-verify CJP vs PJP and te on the print before blaming the welder.
2. Inspect the root first — fill and cap cannot invent missing te.
3. Repair with grind-out to sound metal, then requalified passes — cold stacking over defects preserves the weak path.
4. Recheck capacity in the calculator after any intentional PJP change so the file matches the metal.