The end of every weld pass contains the highest-risk point in the entire joint. As the welder breaks the arc, the weld pool contracts from a large, hot, fluid mass down to the last small pocket of molten metal at the crater. That crater solidifies last, under the maximum shrinkage strain, with the least support from surrounding structure. Centerline solidification cracks begin at craters — and from there, they propagate.
AWS D1.1:2025 addresses crater fill as a workmanship requirement, not an essential variable — which means it doesn't change your WPS qualification requirements if you adjust technique. But it is absolutely a code requirement, and failure to fill craters before breaking the arc is a rejectable condition under the code's visual acceptance criteria. Understanding what the code requires, why craters crack, and how to build crater fill into your production QC program is basic structural welding hygiene.
What Makes Craters Crack
Weld cracking follows the path of least resistance, and craters offer the worst combination of conditions: high temperature, high shrinkage stress, and the narrowest cross-section. The metallurgy is straightforward. As the weld pool solidifies, low-melting-point compounds — sulfides, phosphides, and other impurities — are pushed to the solidification front. The crater, being the last zone to freeze, accumulates these segregates at the centerline. That boundary, already stressed by thermal contraction, is the crack initiation site.
This mechanism — hot cracking or solidification cracking — is distinct from hydrogen-induced cold cracking, though both can originate at or near the arc termination zone. Crater cracking is predominantly a hot cracking phenomenon driven by solidification behavior, not hydrogen diffusion. That matters because the mitigation strategies are different: preheating addresses cold cracking, but crater fill technique addresses hot cracking at the termination point.
On high-strength steels or restrained joints, an unfilled crater can also initiate delayed cracking after the weld cools, particularly if hydrogen is present. The combination of a geometric stress riser (the unfilled crater) and residual hydrogen makes termination points disproportionately dangerous in heavy fabrication.
AWS D1.1:2025 Requirements for Arc Termination
The code requires that each weld pass be terminated by filling the crater to the full cross-section of the weld before breaking the arc. This is a workmanship requirement that applies regardless of welding process — SMAW, GMAW, FCAW, SAW, and GTAW all have the same obligation.
In practice, "filling the crater" means the welder must pause at the end of the pass, reduce travel speed, and add weld metal into the crater until it is level with the surrounding weld crown or slightly above. The exact technique varies by process:
- SMAW: Pause and reduce travel speed at the termination point, or use a back-step technique to overlap the final bead with the crater zone before breaking.
- GMAW/FCAW: The wire feed speed and voltage must support a brief pause to fill — triggering the arc stop while the puddle is still over the crater. Some wire feeders have crater-fill modes that reduce wire feed speed and current for a set interval before extinguishing the arc.
- SAW: Automatic SAW is particularly prone to leaving craters because the travel mechanism stops sharply. Run-off tabs (discussed below) are the standard solution for SAW at weld ends.
- GTAW: Arc out slowly while adding filler; post-flow shielding gas must be maintained until the crater solidifies to prevent porosity.
Run-Off Tabs and Their Code Status
Run-off tabs (also called weld tabs or runout tabs) are extensions temporarily attached at the start and end of a groove weld to move craters and poor-quality start beads off the finished structural joint. AWS D1.1:2025 specifies when run-off tabs are required and how to handle them after welding. (Rule library based on AWS D1.1:2025; verify against your governing edition.)
When required by the engineer of record or owner specification, run-off tabs must be removed after welding is complete and the ends ground flush to the base metal. The standard prohibits simply cutting the tabs off with a torch and leaving a rough surface — the cut and grind requirement produces a surface compatible with the code's visual acceptance criteria.
Run-off tabs are effective for butt joints and long seam welds where the tab can be machined or field-cut after welding. They are less practical for connection joints in erected structures. For fillet welds, run-off tabs are less commonly specified because fillet ends are typically handled through end return requirements rather than tab removal.
The important distinction: run-off tabs address the crater problem at the structural weld end by physically relocating the termination point. They do not address the quality of arc starts, mid-pass arc restarts after interruption, or intermediate craters at stop points within the weld length.