CLADDING TECHNOLOGY SHANXI CO., LTDNumerical Simulation of Hot-Wire TIG Root Weld Penetration with Misalignment in L360QS-N08825 Composite Pipe
Literature Overview
This 2021 study published in Hot Working Technology, authored by Han Yongkai, He Yazhang, Tang Deyu, Wang Tianqi, and Liu Jian, investigates the numerical simulation of hot-wire TIG root weld penetration in L360QS-N08825 composite pipes with misalignment defects. The research is funded by the National Natural Science Foundation of China (U1733125), Tianjin Natural Science Foundation Key Project (17JCZDJC38700), and Tianjin Natural Science Foundation General Project (18JCYBJC18700). The collaboration between Tianjin Polytechnic University and CNPC Marine Engineering Key Laboratory addresses a critical fabrication challenge in composite pipe manufacturing for offshore and subsea applications.
Technical Background and Problem Statement
L360QS-N08825 composite pipe consists of a carbon steel outer layer (L360QS, a high-strength low-alloy steel with minimum yield strength of 360 MPa) bonded to an inner layer of Alloy 825 (N08825), a nickel-iron-chromium alloy with excellent resistance to sulfuric acid and hydrochloric acid corrosion. This composite configuration provides the structural strength of carbon steel with the corrosion resistance of Alloy 825, making it ideal for sour service in oil and gas production, particularly in subsea environments governed by API 934 and NACE MR0175/ISO 15156.
The root weld of a composite pipe joint is the most critical weld because it must achieve full penetration through both the outer carbon steel layer and the inner Alloy 825 layer while maintaining the integrity of the metallurgical bond interface. Misalignment between the two pipe ends, which can occur due to manufacturing tolerances, handling damage, or assembly errors, significantly complicates the welding process by creating an uneven gap that must be bridged by the weld metal.
Hot-Wire TIG Process Parameters
Hot-wire TIG welding, also known as hot-wire gas tungsten arc welding, introduces a preheated filler wire directly into the arc zone, providing additional heat input without increasing the arc current. This technique is particularly advantageous for composite pipe welding because it allows precise control of dilution between the carbon steel and Alloy 825 layers.
| Parameter | Typical Range | Function |
|---|---|---|
| Arc current | 80-150 A | Primary heat source |
| Hot-wire current | 30-80 A | Additional heat input |
| Wire feed speed | 2-5 m/min | Controls deposition rate |
| Wire preheat temperature | 300-600 °C | Reduces arc instability |
| Travel speed | 3-8 cm/min | Controls heat input per unit length |
| Shielding gas | 100% Ar or Ar/He mix | Arc stability and penetration |
The numerical simulation employs a coupled thermal-electromagnetic model to predict weld pool geometry, temperature distribution, and penetration depth under various misalignment conditions. The model accounts for the different thermal properties of L360QS and N08825, the electromagnetic forces acting on the liquid metal, and the convective flow within the weld pool driven by electromagnetic, buoyancy, and Marangoni forces.
Simulation Results and Defect Analysis
The simulation reveals that even small misalignments of 0.5-1.0 mm can significantly affect weld penetration, particularly on the side with the larger gap. The asymmetric heat flow caused by misalignment results in incomplete fusion on the upper side of the misalignment and excessive penetration on the lower side. The study identifies critical misalignment thresholds beyond which full penetration cannot be achieved even with optimal process parameters.
| Misalignment (mm) | Penetration Deficiency (%) | Risk Level | Recommended Countermeasure |
|---|---|---|---|
| 0.0-0.5 | <5% | Low | Standard parameters |
| 0.5-1.0 | 5-15% | Medium | Increase hot-wire current |
| 1.0-1.5 | 15-30% | High | Reduce travel speed, increase arc current |
| >1.5 | >30% | Critical | Re-align or use backing bar |
The research also examines the effect of misalignment on dilution control, which is critical for maintaining the corrosion resistance of the Alloy 825 inner layer. Excessive dilution from the carbon steel base metal into the weld metal can reduce the chromium and nickel content below the threshold required for resistance to sulfide stress corrosion cracking (SSC) in accordance with NACE MR0175/ISO 15156.
Engineering Practice and Quality Control Implications
For composite pipe fabrication in accordance with API 934, the root weld must achieve full penetration with no incomplete fusion, undercuts, or excessive dilution. The simulation results provide a basis for establishing acceptance criteria for misalignment during assembly and for developing welding procedure specifications (WPS) that account for realistic assembly tolerances. Non-destructive testing of composite pipe welds typically includes radiographic testing (RT) for volumetric defects, ultrasonic testing (UT) for planar defects, and magnetic particle testing (MT) or dye penetrant testing (PT) for surface defects on the carbon steel side.
The study's findings have direct implications for the design of assembly fixtures and the specification of assembly tolerances in composite pipe manufacturing. Tighter control of misalignment during assembly, potentially through the use of precision alignment tools and pre-weld fit-up verification, can significantly reduce the risk of weld defects and the need for rework. The numerical simulation approach also provides a valuable tool for optimizing welding parameters for specific misalignment conditions encountered during production, reducing the reliance on trial-and-error experimentation.
Study Insights and Outlook
This research demonstrates the value of numerical simulation in addressing complex welding problems that are difficult to solve through experimentation alone. The ability to predict weld penetration under various misalignment conditions provides a powerful tool for process optimization and quality assurance. However, the validation of simulation results against experimental data is essential, and the study would benefit from comparative analysis with physical welds to confirm the accuracy of the predicted penetration profiles. The findings are directly applicable to the fabrication of composite pipes for subsea production systems, where the consequences of weld defects can be catastrophic, and the cost of non-conformance is extremely high.