8.1 In-Service Welding Dynamics, Burn-Through & HIC Risks

8.1 In-Service Welding Dynamics, Burn-Through & HIC Risks

Welding directly onto an operating, pressurized pipeline — installing repair sleeves, reinforcing fittings, or hot-tap branches without shutting down flow — is one of the highest-risk operations in pipeline work. API 1104:2021 Annex B (In-service Welding) covers recommended practices for making repairs and installations on in-service pipelines, defined as those containing crude petroleum, petroleum products, or fuel gases that may be pressurized and/or flowing.

Annex B names two dominant concerns:

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1. Thermal Burn-Through

Burn-through (3.1.6) occurs when the welding arc causes the pipe wall to be breached: the arc heats the wall until its elevated-temperature strength can no longer resist the hoop stress σ = P·D/(2t) from internal pressure, and the weakened wall bulges and ruptures.

The 0.250 in. Threshold

Annex B's guidance is direct: burn-through is unlikely when installing an appurtenance (sleeve or fitting) if the wall thickness is 0.250 in. (6.4 mm) or greater. Below 0.250 in.:

• the risk of burn-through should be specifically considered;
• recognized thermal-analysis computer models (e.g., the Battelle/PRCI thermal models) or another proven method should determine safe heat-input envelopes;
• burn-through becomes more likely when welding directly onto the pipe — as when depositing the buttering layers of a temper-bead sequence or making a weld-deposition repair on remaining wall.

Risk Drivers

1. Excessive heat input (HI = 60·V·A/(1000·S), per 5.3.2.6).
2. Slow travel speed dwelling the arc on one spot.
3. High internal pressure raising hoop stress on the thermally softened wall.
4. Low-flow or stagnant contents that cannot carry heat away from the inside surface.

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2. The Heat-Sink Problem (Hydrogen Cracking)

The same flowing contents that protect against burn-through create the opposite hazard: a powerful heat sink. Liquid or fast-moving gas strips heat from the pipe wall, so in-service welds cool far faster than new-construction welds. The cooling time through the critical 800→500°C transformation range (t8/5) collapses from tens of seconds to a few seconds, and the heat-affected zone transforms to hard, brittle untempered martensite.

The Hydrogen-Cracking Triad

Hydrogen cracking of an in-service weld needs ALL THREE simultaneously:

1. Susceptible microstructure — a high-hardness HAZ (martensite). Annex B controls this through maximum hardness values in Table B.4 (see Section 8.3 of this guide).
2. Diffusible hydrogen — from moisture, contamination, or high-hydrogen (cellulosic) consumables.
3. Tensile stress — residual welding stress plus operating hoop and bending stress.

Eliminate any one leg and hydrogen cracking cannot occur. This is why in-service procedures push toward low-hydrogen processes and controlled thermal cycles.

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3. The Design Tension

The two risks pull opposite directions, and Annex B says so explicitly:

• Preventing burn-through argues for LOW heat input and thicker remaining wall.
• Preventing hydrogen cracking argues for HIGH enough heat input (or preheat, or temper-bead technique) to slow cooling and soften the HAZ.

When the maximum heat input that avoids burn-through is insufficient to protect against hydrogen cracking, Annex B directs alternative precautions — classically a temper-bead deposition sequence (Figure B.1). Preheating is helpful where practicable, but the flowing contents often make effective preheat difficult. Proper fit-up also matters: minimizing stress on the weld reduces the stress leg of the triad.

4. How Annex B Reads

Two structural notes matter for the exam:

• Annex B is written as recommended practice ("should" language) rather than minimum mandatory requirements — it covers recommended welding practices for repairs and appurtenance installation on in-service pipelines. Company specifications routinely convert its guidance into hard requirements.
• Appurtenance vs. direct-on-pipe: the annex separates welding an appurtenance (sleeve, saddle, fitting) onto the carrier pipe from welding directly on the pipe wall (buttering layers of a temper-bead sequence, weld-deposition repair). Burn-through is unlikely for appurtenance work at ≥ 0.250 in. wall; direct-on-pipe deposition raises the risk and demands the thermal-model analysis.
• Additional guidance: the annex points users to API 2201 (safe hot tapping practices) for complementary requirements, and pressure/flow reduction during welding is a standard risk-control measure set by the operating company.

5. Qualified Envelopes and Operating Conditions

Because cooling severity depends on what flows inside the pipe, Annex B qualifications record the pipeline operating conditions — pipe contents and flow rate — for which the procedure applies, and conditions may be grouped into an envelope. Thermal-analysis computer models (the PRCI/Battelle models are the industry reference) predict whether a candidate heat input stays above the hydrogen-cracking floor and below the burn-through ceiling for a given wall thickness, content, and flow rate — but the annex is explicit that models supplement, never replace, procedure qualification under B.2.