6.3 Distortion Control, Arc Strikes, Tack Welds & Weld Repair Rules

Distortion Control, Arc Strikes, Tack Welds & Weld Repair Rules

Structural steel welding introduces intense, localized thermal energy into restrained metallic members. As the weld pool solidifies and cools from roughly 2,800°F [1,540°C] down to ambient temperatures, volumetric shrinkage generates severe residual stresses and angular distortion.

AWS D1.1/D1.1M:2025 Clause 7 (Clause 7.17, 7.20, 7.25, 7.28, and 7.30) defines mandatory fabrication procedures to control distortion, govern tack welding, prevent arc strike metallurgical damage, handle weld extension tabs, and control weld repair operations.

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Distortion and Shrinkage Control (Clause 7.20)

Welding distortion manifests in three primary modes: transverse shrinkage, longitudinal shrinkage, and angular distortion (warping). Clause 7.20 mandates that fabricators prepare and execute welding sequences that minimize distortion and residual stress.

+-------------------------------------------------------------------------+
|                    DISTORTION CONTROL TECHNIQUES                        |
+-------------------------------------------------------------------------+
| 1. BALANCED SEQUENCING:   Welding simultaneously or alternately on      |
|                           opposite sides of the member's neutral axis.  |
| 2. BACKSTEP WELDING:      Depositing short weld increments in the       |
|                           direction opposite to general weld progress.  |
| 3. PRE-SETTING JOINTS:    Angling plates opposite to expected angular   |
|                           shrinkage before clamping and welding.        |
| 4. WELD PROGRESSION:      Welding from rigid interior restraint toward  |
|                           free, unrestrained member ends.               |
+-------------------------------------------------------------------------+

Thermal & Mechanical Straightening Limits (Clause 7.20.2)

When structural members warp beyond permissible dimensional tolerances (Clause 7.22), fabricators may correct distortion using mechanical force or controlled heat application:

Critical Caution: Exceeding 1,100°F [595°C] on quenched and tempered steels destroys their heat-treated martensitic microstructure, drastically reducing yield strength and fracture toughness. Furthermore, accelerated cooling with water or compressed air is strictly prohibited until the heated steel cools naturally below 600°F [315°C].

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Tack Welds & Temporary Welds (Clause 7.17)

Tack welds are small, intermittent welds used to position and hold components in alignment prior to final structural welding. Because tack welds experience extreme cooling rates due to their small volume relative to the massive surrounding base metal, they represent a prime origin site for hydrogen-induced root cracks.

+-------------------------------------------------------------------------+
|                      TACK WELD WORKMANSHIP RULES                        |
+-------------------------------------------------------------------------+
|                                                                         |
|   FEATHERED END              TACK WELD BEAD             FEATHERED END   |
|       /-----------------------------------------------------\           |
|      /                                                       \          |
|  ---+                                                         +---      |
|  ==================== BASE METAL ROOT FACE =======================      |
|                                                                         |
|  - Must be deposited by QUALIFIED WELDERS                               |
|  - Must use a QUALIFIED WPS and MANDATORY PREHEAT                       |
|  - Ends must be FEATHERED (taper-ground) to ensure root pass remelting   |
+-------------------------------------------------------------------------+

1. Incorporated Tack Welds (Clause 7.17.1)

Tack welds that remain in the joint and become part of the final weld must satisfy strict criteria:

• Must be made by qualified welders using approved WPSs and low-hydrogen electrodes.
• The base metal must be preheated to the full minimum preheat temperature required by the WPS (Table 5.11).
• Prior to depositing the final root pass, all slag must be removed, cracked tacks must be excavated, and the ends of tack welds must be feathered (ground to a gentle taper) to ensure complete fusion.

2. Temporary Welds & Bridge Tacks (Clause 7.17.2)

Temporary welds (such as lifting lugs, strongbacks, or alignment bridge tacks) used solely for assembly must be removed. In statically loaded structures, temporary welds must be removed and ground flush when required by the Engineer. In cyclically loaded structures, all temporary welds must be removed, ground flush with base metal, and inspected using Magnetic Particle Testing (MT).

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Arc Strikes: Damage Mechanism & Code Rules (Clause 7.28)

An arc strike occurs when a welder accidentally or intentionally strikes the welding electrode against the base metal outside the designated weld joint area.

+-------------------------------------------------------------------------+
|                      METALLURGY OF AN ARC STRIKE                        |
+-------------------------------------------------------------------------+
|                                                                         |
|        ELECTRODE CONTACT                                                |
|               |                                                         |
|               v                                                         |
|        +-------------+  <-- Molten spot reaches ~2,800°F [1,540°C]       |
|        | ARC STRIKE  |                                                  |
|        +-------------+                                                  |
|        | HARD BRITTLE|  <-- Rapid self-quench by cold surrounding plate  |
|        | MARTENSITE  |      produces hard micro-structure (> 400 HV)    |
|        +-------------+                                                  |
|        | MICRO-CRACKS|  <-- High localized tensile stress causes        |
|        +-------------+      fatigue and brittle crack initiation        |
|                                                                         |
+-------------------------------------------------------------------------+

Code Requirements for Arc Strike Remediation (Clause 7.28)

1. Prevention: Arc strikes outside the area of permanent welds shall be strictly avoided in all fabrication.
2. Mechanical Fairing: Where arc strikes occur, they must be ground smooth to a contoured radius to eliminate sharp notches and gouges.
3. Inspection in Cyclically Loaded Connections: In members subjected to cyclic tensile stress, ground arc strike locations must be inspected using Magnetic Particle Testing (MT). If micro-cracks or excessive hardness are suspected, the area shall be acid-etched with a 10% nital solution to confirm complete removal of the untempered martensitic heat-affected zone.

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Weld Extension Tabs (Run-off Tabs) (Clause 7.30)

2025 change: The 2020 edition required weld tabs "where practical," which was persistently argued about on jobsites. The 2025 edition replaces that vague wording with Clause 7.30.2, Weld Tab Exemptions, an explicit list of the cases in which weld tabs are not required. If a question asks whether tabs were mandatory for a particular joint, check the exemption list rather than reasoning about practicality.

Because the initiation and termination of an arc invariably contain transient instabilities (such as crater porosity, incomplete fusion, or undersized throats), weld extension tabs (run-on/run-off tabs) provide sacrificial run-off zones outside the net structural cross-section.

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Technique for Plug and Slot Welds (Clause 7.24)

Clause 4.11 sets the geometry of plug and slot welds; Clause 7.24 sets how they are actually deposited, and the two are tested separately.

• Flat position: Deposit the weld with the axis of the hole or slot approximately vertical (that is, welding down into the hole in the flat position). Peening the deposit is prohibited.
• Vertical position: Start the root pass at the bottom of the hole and build up in thin layers, keeping the arc on the base metal at the perimeter so the weld metal fuses to the walls of the hole rather than bridging them.
• Overhead position: Deposit the root pass around the perimeter where the joint faces meet, clean each pass thoroughly before the next, and build up in thin layers. Overhead plug and slot welds are the most likely to trap slag at the perimeter.
• Cleaning between passes is explicit in Clause 7.24 for the vertical and overhead cases; slag left at the wall of the hole is the classic cause of a plug weld that macroetches with sidewall lack of fusion.
• Filling depth is still governed by Clause 4.11: full depth for material up to and including 5/8 in. [16 mm], and at least half the thickness but not less than 5/8 in. for thicker material.

Remember also that plug and slot welds are prohibited on members subject to cyclic tensile stress (Clause 4.19), so a Clause 7.24 technique question set in a crane-runway or bridge-girder context usually has a prior design violation baked into it.

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Postweld Heat Treatment for Stress Relief (Clause 7.8)

Where the contract documents require it, or where the Engineer approves it, welded assemblies may be stress relieved by postweld heat treatment (PWHT) under Clause 7.8. PWHT is not a discretionary shop practice: the 2025 edition states that when PWHT is performed it shall be specified in the contract documents or approved by the Engineer, and shall be carried out under a WPS that was either qualified with PWHT or is prequalified and meets the Clause 5 PWHT requirements (Clause 5.9).

• Written procedure required. PWHT must follow a written procedure covering heating rate, holding temperature, holding time, cooling rate, and heating method.
• Holding time. Minimum holding time is taken from Table 7.2, based on the thickness of the thickest part.
• Alternate schedules. Table 7.3 permits alternate lower-temperature, longer-time stress-relief schedules where the standard holding temperature is not practical.
• 2025 change — furnace entry temperature. The initial maximum furnace temperature at which the assembly may be placed in the furnace was raised from 600°F to 800°F in the 2025 edition. If your study material still says 600°F, it is written to the 2020 code.
• Quenched and tempered steels. PWHT is not applied to Q&T steels without specific engineering evaluation, because the stress-relief temperature can approach or exceed the original tempering temperature and degrade the heat treatment.

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Weld Repair Procedures & Cavity Excavation (Clause 7.25)

When non-destructive testing reveals rejectable discontinuities (such as cracks, lack of fusion, or excessive porosity), repairs must follow standardized protocols under Clause 7.25:

+-------------------------------------------------------------------------+
|                       WELD REPAIR EXECUTION FLOW                        |
+-------------------------------------------------------------------------+
| 1. DEFECT EXCAVATION:   Remove defect by gouging (CAC-A) or grinding.   |
| 2. CAVITY PREPARATION:  Fair excavation with minimum 20° groove angle   |
|                         and generous blend radii (no sharp roots).      |
| 3. NDT CONFIRMATION:    Inspect cavity with MT/PT to confirm complete   |
|                         removal of crack tips prior to rewelding.       |
| 4. QUALIFIED REPAIR:    Weld using qualified repair WPS and full        |
|                         prescribed preheat temperature.                 |
| 5. FINAL RE-INSPECTION: Re-examine repaired area with original NDT.     |
+-------------------------------------------------------------------------+

High-Strength Steel Repair Limitations

In high-strength quenched and tempered steels (e.g., ASTM A514/A517), consecutive weld repairs at the same joint location are strictly limited (typically to a maximum of two repair cycles). Repeated heat cycles degrade the fine-grained heat-affected zone, resulting in local grain coarsening, embrittlement, and micro-fissuring.

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Worked Example: Crane Runway Repair & Remediation

Problem Statement

A CWI is inspecting the fabrication of a welded heavy plate girder intended for a cyclically loaded industrial overhead crane runway (tension flange is ASTM A572 Grade 50). Visual and magnetic particle inspection reveals two nonconformances: (1) an accidental arc strike located on the bottom tension flange plate 4 in. [100 mm] away from the web-to-flange weld, and (2) weld extension tabs remaining on a CJP transverse flange butt joint. What corrective procedures must the inspector mandate under AWS D1.1?

Step-by-Step Code Evaluation

1. Remediate the Arc Strike (Clause 7.28):
   • Because the member is in a cyclically loaded tension connection, the arc strike creates a severe fatigue hazard.
   • The contractor must grind the arc strike smooth, creating a fair, contoured radius with grinding marks parallel to the longitudinal tensile stress.
   • Following grinding, the inspector must perform Magnetic Particle Testing (MT) over the ground area to verify that all quench-induced micro-cracks have been fully removed.
   • If required by specification, acid etching with 10% nital shall verify the removal of the hard martensitic zone.
2. Remediate the Weld Extension Tabs (Clause 7.30):
   • In cyclically loaded structures, weld extension tabs shall not remain.
   • The contractor must thermally cut or mechanical cut the tabs, then grind the weld ends completely flush with the edges of the flange plates (flush within $1/32\text{ in.}$ [1 mm]).
   • Grinding marks must be oriented parallel to the direction of applied stress (along the longitudinal axis of the flange) to avoid introducing transverse notch stress risers.