1. Hydrogen Introduction During Welding
1.1 Welding Flux and Moisture Control
The submerged arc welding process used in forming LSAW welded pipes relies on flux coverage for shielding and weld bath stability. If the welding flux or electrode absorbs ambient humidity - especially at uncovered storage sections - hydrogen forms in the melt pool and becomes diffusible hydrogen inside weld and HAZ. Moisture control must start long before pipe forming. Flux drying prior to welding and shielding the welding groove from industrial contaminants are among the first process variables linked to HIC failures.
1.2 Contamination Sources: Oil, Water, and Handling Residue
External contamination - for instance, residual crane hoist oil, plate handling moisture, or open groove surface film - is a common indirect hydrogen source during welding. These hydrogen sources later diffuse into longitudinal weld seams.
1.3 Pre-Heating and Inter-Pass Thermal Sequencing
Pre-heating larger wall thickness steel plates prior to LSAW manufacturing serves a dual role: lowering the cooling rate to avoid quench-hardened HAZ structures, and enabling hydrogen to diffuse out during welding rather than locking inside the weld root. Inter-pass thermal sequencing ensures heat is evenly retained, reducing hydrogen concentration variance across weld direction.
Table 1: Welding Variable vs Hydrogen Contribution
| Welding Variable | Hydrogen Contribution Level | Crack Risk Increase | Control Method |
|---|---|---|---|
| Undried Flux | High | Very High | Oven baking 300–350 °C |
| Groove Oil Film | Medium | High | Industrial solvent cleaning |
| Low Pre-Heat | Medium | High | 150–250 °C pre-heating |
| Rapid Cooling | Indirect | Very High | Insulation cooling control |
Table 2: Recommended Flux Drying Parameters
| Flux Condition | Drying Temperature | Drying Time | Expected Hydrogen Reduction |
|---|---|---|---|
| Ambient Humid Flux | 300 °C | 2–4 h | 80–90% reduction |
| Cold Winter Storage Flux | 350 °C | 4–6 h | 90% or more |
2. Thermal and Bead Geometry Effects
2.1 Weld Bead Penetration and Root Shape
2.1.1 Deep Penetration vs Hydrogen-Pressure Channeling
The weld root is a preferred hydrogen accumulation location in longitudinal LSAW bead geometry. Excessively deep penetration may push hydrogen to upper fusion lines, while insufficient penetration forms stress concentration voids at roots. Root morphology must balance penetration and stress transition smoothness.
2.1.2 Root Notch Effect
If the root geometry shows acute notch edges or bead collapse zones, stress-concentration accelerates hydrogen-induced brittle nucleation. HIC failures in industrial samples often show root crack extension rather than mid-wall crack origins.
2.2 Cooling Gradient and Thermal Differential
Hydrogen diffusibility is temperature sensitive. Cooling gradients in large diameter LSAW pipes trap hydrogen earlier at colder seams. Managing large weld-line insulation helps maintain even hydrogen diffusibility prior to final solidification.
3. Stress Contribution from Welding Runs
3.1 Residual Stress from Longitudinal Welding
3.1.1 Tensile Stress Along Seam Direction
Long weld runs on LSAW pipes create directional tensile residual stress aligned with longitudinal seams. Residual stress alone may be sufficient to activate HIC if diffusible hydrogen threshold is reached.
3.1.2 Multi-Pass Accumulated Stress
LSAW pipes often require multi-pass welding - each pass can accumulate stress and trap more diffusible hydrogen if thermal sequencing, flux drying, or inter-pass cooling control is not rigorously managed.
3.2 Forming Stress Added to Welding Stress
Steel forming stress from plate bending plus longitudinal weld residual stress often jointly create peak tensile stress zones at seam lines - common initiation points for HIC in LSAW chains.
4. Industrial Process Recommendations
4.1 Hydrogen Removal Post Welding
Thermal hydrogen out-baking, or "de-hydrogenation baking," after the final weld pass is a well-documented industrial mitigation step for LSAW hydrogen pipelines - especially when large wall thickness introduces longer hydrogen diffusion latency.
4.2 Process Control Checklist for LSAW Weld Runs
A basic industrial process checklist includes: flux dryness confirmation → groove cleaning → pre-heat verification → inter-pass insulation → root geometry compliance → post-weld hydrogen baking → hardness gradient map → final weld imaging scan.
4.3 Practical Outcomes from Process Adjustment
In many industrial production lines, once flux dryness, oil-film contamination, and weld-notch geometry issues are corrected, similar hydrogen cracking defects drastically decline in final pipe reviews.
