Arc Strikes in Welding: Small Sparks With Serious Metallurgical Consequences
An arc strike — also called a stray arc — is one of the most underestimated imperfections in welding practice. Defined under ISO 6520-1 as local surface damage caused when an arc is initiated outside the intended weld joint, an arc strike is often dismissed as a minor cosmetic blemish. In reality, it can create a metallurgical time bomb: a brittle, hydrogen-susceptible, crack-prone zone that may sit undetected until it drives a fatigue crack or brittle fracture in service.
ISO standards classify welding as a special process — meaning quality cannot be fully inspected into existence after the fact. If metallurgical damage is introduced during welding, it is already embedded in the component. This philosophy applies with particular force to arc strikes: the damage is done in an instant, often by a welder who does not even realise they have introduced a defect. Understanding arc strike metallurgy, recognising which base materials are most vulnerable, and knowing how codes require remediation are essential competencies for any welder, welding inspector, or welding engineer.
This article explains exactly what happens metallurgically when an arc strikes outside the joint, why certain steels are far more sensitive than others, what the relevant codes require, and how a properly documented repair sequence should be carried out. Two original engineering diagrams are included to illustrate the microstructural and thermal mechanisms involved.
What Exactly Happens During an Arc Strike
When an electrode, TIG torch, or any live welding conductor makes brief unintended contact with the base metal surface away from the joint, an arc is established. The plasma column of a welding arc routinely exceeds 5000 °C at its core. Even though the contact duration is typically a fraction of a second, the energy density delivered to the tiny surface contact area is enormous — comparable to a spot-welding operation on an uncontrolled, non-procedure-qualified basis.
The sequence of events is as follows. The electrode touch point melts a small volume of base metal, creating a shallow crater. The molten pool superheats to temperatures well above the steel’s liquidus. The surrounding solid base metal acts as a massive heat sink and quenches the molten zone at an extraordinarily high cooling rate — often orders of magnitude faster than the cooling rate experienced by the weld metal in the joint itself, which at least benefits from multi-pass heating, pre-heat, and controlled interpass temperature.
This combination of peak temperature and rapid quench creates the worst possible conditions for several metallurgical damage mechanisms simultaneously. Unlike a proper weld, there is no welding procedure specification, no preheat, no post-weld heat treatment, and no controlled heat input — all the protections that a qualified WPS provides are absent.
Mechanism 1: Martensite Formation and Local Hardening
In carbon and low-alloy steels, the steel transforms to austenite in the heat-affected zone. If the subsequent cooling rate is fast enough to suppress diffusion-controlled transformations (ferrite, pearlite, bainite), the austenite transforms martensitically to a hard, body-centred tetragonal structure. Martensite is extremely hard — in high-carbon steels hardness can exceed 700 HV — and, critically, it is brittle and highly susceptible to cracking.
The governing parameter is the steel’s carbon equivalent (CE). The higher the CE, the lower the critical cooling rate needed to form martensite, and the harder and more brittle that martensite will be. The Pcm formula (IIW) is often used for higher-strength steels to quantify cracking susceptibility. Even steels with a CE of 0.35–0.40 — considered moderate — can develop dangerously hard martensitic zones under the extreme cooling rates associated with an arc strike.
Mechanism 2: Hydrogen-Induced Cold Cracking
Welding environments always contain a potential source of diffusible hydrogen — from electrode coatings, moisture, surface contamination, or shielding gas impurity. Martensite is an ideal hydrogen trap. Diffusible hydrogen atoms migrating through the steel lattice accumulate at the hard martensitic zone created by the arc strike, progressively building up local stress intensity. Cracking may initiate minutes, hours, or even days after the arc strike event — this is delayed hydrogen cracking (also called cold cracking or HAC).
The insidious nature of delayed cracking is that visual inspection immediately after the arc strike may show nothing. A component can be inspected, accepted, and dispatched — and crack in service or during post-weld heat treatment.
Mechanism 3: Microcracking from Thermal Stress
The extremely rapid thermal expansion at the arc strike point, followed by equally rapid contraction during quenching, generates severe local tensile stresses. In brittle martensitic microstructures these stresses can exceed the fracture toughness of the material and produce microcracking with no applied external load whatsoever. These thermally induced cracks may be surface-breaking and detectable by MT or PT, or they may be subsurface and only detectable by UT.
Mechanism 4: Stress Concentration and Fatigue Damage
The crater, pitting, and surface roughening left by an arc strike acts as a notch. In fatigue design, a notch reduces the effective fatigue strength of the joint by a factor related to the stress concentration factor Kt. Even a very small arc strike crater — visually almost invisible — can dramatically reduce the number of cycles to crack initiation under cyclic loading. In pressure vessels, pipework, and structural members subject to vibration or pressure pulsation, this is a significant structural integrity concern.
Steel-by-Steel Sensitivity to Arc Strike Damage
Not all steels respond equally to arc strike damage. The key material variables are carbon content, alloy content (which governs hardenability), heat treatment condition, and any pre-existing hydrogen content. The table below summarises sensitivity by material family.
| Steel Family | Typical CE Range | Arc Strike Risk | Primary Failure Mode | PWHT Likely? |
|---|---|---|---|---|
| Mild / low-carbon (<0.2% C) | 0.25 – 0.38 | Moderate | Hardening, notch, corrosion initiation | No |
| Medium-carbon structural (0.2–0.35% C) | 0.38 – 0.55 | High | Martensite, microcracks, hydrogen cracking | Possible |
| High-carbon / tool steels (>0.35% C) | > 0.55 | Very High | Martensite, brittle fracture, immediate cracking | Yes |
| Low-alloy HSLA (S355NL, A572) | 0.38 – 0.50 | High | HAZ hardening, delayed hydrogen cracking | Possible |
| Quenched & Tempered (S690, A514) | 0.45 – 0.65 | Very High | Destroys heat treatment, brittle local zone | Yes (if repair) |
| Cr-Mo steels (P11, P22, P91) | 0.50 – 0.80+ | Very High | Martensite, hydrogen cracking, PWHT mandated | Yes — mandatory |
| Austenitic stainless (304, 316) | N/A | Moderate | Hot cracking, sensitisation, corrosion loss | No (solution anneal if severe) |
| Duplex stainless (2205, 2507) | N/A | Moderate–High | Phase imbalance, sigma, corrosion | Solution anneal if significant |
| Nickel alloys / Inconel | N/A | Moderate | Hot cracking, grain boundary damage | Depends on alloy |
A Closer Look at Chromium-Molybdenum Steels
Among the most vulnerable materials in industrial fabrication are the Cr-Mo steels used in high-temperature pressure service — P11 (1.25Cr-0.5Mo), P22 (2.25Cr-1Mo), and especially P91 (9Cr-1Mo-V). These steels have very high hardenability. The martensite formed by an arc strike in P91 is particularly brittle and requires a full post-weld heat treatment cycle (typically 730–760 °C for P91) to temper it back to acceptable toughness. An arc strike on P91 that is ground out and left without PWHT will have a local hardness that may exceed 450 HV — far above the 250 HV maximum typically specified for P91 weld metal — and represents a genuine risk of in-service cracking under thermal cycling.
Arc Strikes on Austenitic Stainless Steel
Although austenitic stainless steels such as 304L and 316L do not undergo the austenite-to-martensite transformation, arc strikes still cause serious problems. The intense local heating and rapid cooling can drive a sensitisation reaction: chromium carbides precipitate at grain boundaries, depleting the surrounding matrix of chromium below the approximately 10.5 wt% minimum needed for passivity. This creates a susceptible zone for intergranular corrosion attack — the phenomenon known as weld decay. In aggressive environments (chloride-bearing, acidic, or oxidising), the arc strike area will become an active corrosion site even after grinding.
Additionally, the very rapid solidification of the small melt pool in austenitic stainless steels during an arc strike promotes hot cracking (solidification cracking), particularly if the composition is near a fully austenitic solidification mode (low Creq/Nieq ratio). For duplex stainless steels, as discussed in the duplex stainless steel welding guide, an arc strike disrupts the ferrite-austenite phase balance and can locally produce sigma phase or excessive ferrite depending on the thermal cycle, both of which impair corrosion resistance and toughness.
What ISO and ASME Codes Actually Require
Neither ISO nor ASME allow arc strikes outside the weld zone to remain unremediated. The relevant code requirements, while expressed in various documents, are consistent in their intent:
- AWS D1.1 (Structural Welding — Steel): Arc strikes outside the weld area shall be avoided. Any arc strikes that occur shall be ground smooth and inspected by MT. Hard spots exceeding 35 HRC (approximately 340 HV) in high-strength steels require further evaluation.
- ASME Section VIII Div. 1 / Div. 2: The Code does not explicitly define arc strike remediation in a single clause but the requirements flow from the material specification, the WPS, and the applicable NDE requirements. Hard spots, cracks, and reduction in section thickness below the minimum required thickness all constitute code violations requiring documented repair.
- EN ISO 5817: Arc strikes are not assigned a specific acceptance category within the quality levels B, C, or D — instead they are simply not permitted. Any arc strike must be removed.
- NACE MR0175 / ISO 15156 (sour service): In sour service applications, hard zones above 250 HV10 are not acceptable in any location — including arc strike remnants. This standard has zero tolerance for hardness exceedances.
Step-by-Step Repair Procedure for Arc Strikes
The following sequence represents the minimum acceptable practice for arc strike remediation on carbon and low-alloy steels in code-governed fabrication. The procedure should be adapted based on material grade, code of construction, and component criticality.
- Identify and document. Mark the arc strike location with a paint marker. Record location, approximate size, and material specification on the NCR or inspection record. Photograph before any grinding commences.
- Grind to sound metal. Use a die grinder or angle grinder with a fine-grit abrasive disc to remove all visible damage: melt pool, crater, heat tint, and any visible cracks. The removal must blend smoothly and gradually into the surrounding surface — no sharp edges, steps, or abrupt transitions that would create new stress concentrations. Keep the grinding tool moving; do not dwell in one spot.
- Check remaining thickness. Use an ultrasonic thickness gauge to confirm that the ground area has not removed metal below the minimum required wall thickness. If it has, a weld repair is mandatory regardless of the NDT result.
- Surface NDT. Apply Magnetic Particle Testing (MT) to ferritic steels or Liquid Penetrant Testing (PT) to austenitic steels, titanium, and non-magnetic alloys. For high-carbon and high-alloy steels with a known hydrogen risk, consider delaying NDT by at least 24–48 hours after grinding to allow any diffusible hydrogen to redistribute and potential delayed cracks to develop. Re-examine the area by MT/PT after the hold period.
- Hardness survey (where required by code or material spec). Use a portable Vickers or Brinell hardness tester to check the hardness of the ground area. Compare against the code or material specification maximum — typically 250 HV10 for sour service, 350 HV for general structural, or the specific limit in the applicable WPS. Excessive hardness without cracking may still require weld repair and PWHT to temper the martensitic zone.
- Qualified weld repair (if cracks found or thickness lost). Carry out the repair under a qualified WPS using low-hydrogen consumables matching the base metal chemistry. Apply preheat per the WPS or per the CE-based preheat calculation. Control interpass temperature. Use the TIG process for precision repairs on P-materials or where minimum heat input is critical.
- Post-weld heat treatment. Apply PWHT in accordance with the applicable code, the material specification, or the repair WPS — whichever is most stringent. For Cr-Mo materials such as P91, P22, or P11, PWHT is mandatory regardless of repair thickness. For carbon steels above a code-defined thickness threshold, PWHT is code-mandatory even for minor repairs.
- Final NDT and documentation. Re-examine the repaired area by MT or PT. Issue a final inspection certificate recording the original arc strike report, repair WPS number, preheat and PWHT records, and NDT reports. Update the as-built weld map accordingly.
Prevention: Eliminating Arc Strikes at Source
Prevention is always preferable to remediation. The following measures are standard practice in controlled fabrication environments:
Welder Training and Discipline
Welders must be trained to initiate arcs only within the joint groove or on run-off tabs provided for that purpose. Scratch-starting outside the joint — even as a “quick test” before beginning a pass — is not acceptable on code-governed work. This discipline should be explicitly addressed in the project-specific welding procedure and in the welder performance qualification training package.
Use of Run-On / Run-Off Tabs
Where practicable, provide weld run-on and run-off tabs at the start and end of joints, particularly for structural and pressure-retaining butt welds. Arc initiation and termination on the tabs keeps any potential arc strike damage outside the parent material. Tabs are removed and discarded after welding.
Equipment Condition
Faulty or worn welding return cables, inadequate clamping of the work return, and loose connections at the welding machine all promote arc blowout — the uncontrolled momentary discharge between the cable connection and the workpiece that produces arc burns on the base metal surface well away from the joint. Inspect and maintain welding return circuits regularly.
High-Frequency Arc Starting (TIG)
In GTAW (TIG) welding, using high-frequency arc starting eliminates the need for any contact between the tungsten and the base metal, removing one common mechanism for accidental arc strikes during arc establishment.
Carbon Equivalent and Preheat — How They Relate to Arc Strike Risk
The same carbon equivalent formula used to determine preheat requirements for welding also predicts the severity of arc strike damage. A higher CE means a higher hardenability, a greater tendency to form martensite under rapid cooling, and a harder, more crack-prone microstructure at the arc strike location.
Use the WeldFabWorld Carbon Equivalent Calculator to compute CE and Pcm for your specific material chemistry and determine both welding preheat and the appropriate level of arc strike repair caution.
Arc Strikes and Sour Service / NACE Applications
In pipelines, pressure vessels, and valves designed to handle sour service environments (H2S-containing fluids per NACE MR0175 / ISO 15156), the requirements for arc strike control are the most stringent of any application. ISO 15156-2 sets a maximum hardness limit of 250 HV10 in the weld and HAZ of carbon and low-alloy steels. This limit exists because H2S promotes hydrogen evolution at the steel surface, and hard zones — particularly martensite — are acutely susceptible to Sulphide Stress Cracking (SSC) and Hydrogen-Induced Cracking (HIC).
An arc strike on a sour service component that produces a local hardness zone above 250 HV10 — even without any visible crack — is an automatic rejection requiring documented weld repair. In practice, any arc strike on P-number 1 (carbon steel) sour service components with a carbon content above approximately 0.15% is likely to produce a hardness exceedance and will require repair and re-qualification by hardness survey. The corrosion mechanisms involved are detailed separately in the WeldFabWorld corrosion guide.
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