Arc blow is one of those field conditions that derails a good welder on a perfectly set-up SMAW procedure. The arc drifts — forward in the direction of travel, or backward against it — the bead goes crooked, spatter increases, and in severe cases you get porosity or lack of fusion at the weld toe. It is almost always a DC phenomenon, most common on thick structural plate at the ends of long joints or in corner conditions.
Understanding what causes arc blow, how to respond during production, and how to document or adjust your WPS is part of solid quality management practice on any structural fab floor.
What Arc Blow Actually Is
The welding arc generates a magnetic field. In direct current welding, the base metal itself can retain or guide that field in ways that pull the arc off-center. Ferromagnetic steel — which includes every common structural grade (A36, A572, A992, A913) — concentrates magnetic flux, particularly at discontinuities, corners, and thickness changes.
When the magnetic path around the arc is asymmetric, the arc deflects toward the path of least magnetic reluctance. Two types result:
Forward arc blow: The arc deflects in the direction of travel. The puddle gets ahead of the arc, resulting in underfill, undercut, or burn-through risk on the leading edge. Often described as the arc "running away" from the welder.
Back arc blow: The arc deflects opposite to the direction of travel. The puddle piles up behind the arc, increasing the risk of lack of fusion on the side walls and trapped slag in multi-pass welds.
Both types are most severe at the beginning and end of a weld joint, where the magnetic field path changes abruptly.
Field Conditions That Trigger It
Arc blow is not random. Certain joint configurations and production setups are predictable triggers:
Work lead placement. The single biggest variable a shop controls. When the work lead (ground clamp) is clamped to one end of a long structural member, the return current path is asymmetric along the length of the joint. The welder experiences severe back arc blow welding away from the ground and forward arc blow welding toward it. The fix is often simply moving the ground clamp closer to the active weld point, or clamping at both ends of the member and using a weld cable splitter.
Magnetized base metal. Material that has been handled with magnetic lifting equipment or stored near DC current sources can retain residual magnetism. Check with a compass or a magnetic field meter — a compass needle that deflects from the joint axis is a reliable indicator. Degaussing with AC coils or a reducing welding run from end to end can reduce residual magnetism.
Corner and edge conditions. Column base plates, box sections welded from inside, and stiffener corners all create magnetic paths that concentrate at the weld terminus. Ending the weld in a corner is the worst scenario. Where possible, run-off tabs extend the joint past the corner so the welder can crater-fill in a magnetically neutral zone.
High-amperage DC on thick plate. Higher amperage generates a stronger arc magnetic field. Heavy-section column connections welded with large-diameter E7018 electrodes at 200+ amps are more susceptible than light-gauge work.
Corrective Actions
Most arc blow correction happens at the welder level without requiring a WPS change:
- Move the work lead closer to the active weld, or use multiple grounds balanced along the joint length
- Shorten arc length — a shorter arc is less susceptible to deflection and compensates for mild arc blow
- Use a backstep technique — welding in short increments in the opposite direction from overall joint progress can equalize the magnetic field
- Angle the electrode into the direction of arc blow — if the arc deflects forward, angle the electrode slightly in the forward direction to compensate
- Weld toward the ground clamp rather than away from it when possible
If these measures do not resolve the condition, the next corrective step involves a WPS-level decision.
When Arc Blow Forces a WPS Change
Switching from DCEP to AC is the definitive correction for arc blow on SMAW. Under AC, the arc extinguishes and re-establishes every half-cycle (100–120 times per second on 50/60 Hz power), which prevents the sustained unidirectional magnetic field that causes deflection. AC-capable SMAW power sources are standard in most structural shops.
However, current type and polarity is an essential variable under AWS D1.1:2025 Table 6.6. A welder cannot independently switch from DCEP to AC to solve arc blow — the WPS must authorize AC operation for that electrode and joint configuration. If your existing WPS specifies DCEP only, you have two options:
- Obtain an existing WPS that qualifies the same electrode in AC
- Revise the WPS to add AC authorization (which may require PQR support, depending on whether the change falls within the qualified range or constitutes an essential variable change)
Note that not all SMAW electrodes run equally well on AC. E7018 low-hydrogen electrodes are designed for both DCEP and AC — most manufacturers' data sheets confirm this. Verify your specific electrode classification and review the current type and polarity essential variable requirements before authorizing an AC change.
CWI Documentation and Inspection Response
If arc blow is severe enough that the welder stops or changes technique, the CWI should document the condition in the production weld record:
- Joint ID and pass range where arc blow was observed
- Nature of the deflection (forward/back) and severity
- Corrective action taken (work lead relocated, electrode angle adjusted, switched to AC)
- Confirmation that welding resumed within WPS parameter ranges
- Visual inspection result after the affected passes
If there is any question about whether arc blow caused a defect — porosity, lack of fusion, or irregular bead profile — require NDE before the joint is covered by subsequent passes or paint. Magnetic particle testing (MT) or ultrasonic testing (UT) can evaluate the affected region.
Arc blow–induced defects that are missed at the pass level are nearly impossible to repair economically after the joint is complete. The production welding parameter log guidelines cover how to structure inspection records so that non-conformances like arc blow are tied to specific passes, not just logged as a general comment.
Documenting WPS Controls for Arc-Blow-Prone Joints
If your shop regularly welds joint configurations known to cause arc blow — long field splices, base plate connections on heavy columns, or interior corners on box sections — consider adding work lead positioning requirements to the WPS or to the project-specific Inspection and Test Plan (ITP). This is not an AWS D1.1 requirement, but it is good practice and demonstrates positive quality control in an AISC audit.
Specific language might read: "For joints exceeding 48 in. in length, work leads shall be attached at both ends of the joint. Lead arrangement shall be verified by the CWI before welding commences on each joint."
This kind of prescriptive language in an ITP prevents arc blow from being treated as a random nuisance and makes the corrective action standard operating procedure.
Essential Variable Cross-Check Before Production
Before starting any SMAW procedure on thick structural members or corner conditions, confirm:
- Work lead placement is documented or specified in the ITP
- The WPS authorizes AC current if switching may be needed as a corrective
- The production parameter log has a field for non-conformance notes (arc blow is a non-conformance worth tracking)
- The CWI knows to require NDE if arc blow severity warrants it
Arc blow is a well-understood physical phenomenon — not a mystery, and not a quality failure by itself. The quality failure is an uncontrolled response that drifts outside WPS parameters without documentation. A CWI who recognizes arc blow early, documents the correction, and confirms the weld result keeps the procedure under control.
Rule library based on AWS D1.1:2025; verify against your governing edition.
Keep arc blow corrections, production weld records, and WPS revisions in one audit-ready place with WPS Welding.