CLADDING TECHNOLOGY SHANXI CO., LTDNumerical Simulation of Temperature Field in Deep Penetration TIG Water-Cooled Welding of Low-Alloy Steel
Literature Overview
This study by Huang Yifei and colleagues from Tianjin University, published in the Acta Metallurgica Sinica in 2017 under the National Natural Science Foundation of China grants 51275342 and 51405334, addresses a critical and underexplored welding configuration: deep penetration gas tungsten arc welding (GTAW/TIG) combined with internal water cooling applied to low-alloy steel plates. The research establishes a three-dimensional finite element thermal model to simulate the transient temperature field during welding and investigates how the internal water cooling channel influences heat distribution, cooling rate, and the resulting solidification microstructure. This work is particularly significant for pressure vessel and heat exchanger fabrication where thin-wall or high-thickness components require both deep penetration efficiency and controlled thermal input.
Core Technical Approach
The authors developed a coupled heat-transfer model that accounts for the convective heat removal by the circulating coolant flowing through a channel machined into the base plate. The key modeling assumptions and parameters include:
| Parameter | Typical Value / Range |
|---|---|
| Base material | Low-alloy steel (e.g., 15CrMo, 12Cr1MoV equivalent) |
| Welding current | 150–300 A |
| Arc voltage | 18–28 V |
| Travel speed | 50–150 mm/min |
| Coolant flow rate | 2–10 L/min |
| Coolant inlet temperature | 15–25 °C |
| Plate thickness | 8–25 mm |
| Element size near weld pool | 0.5–1.0 mm |
| Time step | 0.01–0.05 s |
The model incorporates latent heat effects through an apparent specific heat function, surface radiation and convection losses, and a convective boundary condition at the water-cooled channel wall. The heat transfer coefficient at the coolant interface is calculated using a Dittus-Boelter-type correlation, which depends on the Reynolds number, Prandtl number, and hydraulic diameter of the cooling channel.
Interpretation of Key Findings
Temperature Gradient and Cooling Rate
The numerical results reveal that the introduction of internal water cooling dramatically increases the local cooling rate at the fusion line. Without water cooling, the cooling rate from 800 °C to 500 °C (t800-500) typically falls in the range of 1–5 °C/s for medium-thickness plates. With water cooling activated at a flow rate of 5 L/min, this rate can increase to 20–50 °C/s locally at the fusion boundary. This has profound implications for microstructure formation: higher cooling rates promote the formation of martensite and bainite in the heat-affected zone (HAZ), particularly in low-alloy steels with higher carbon equivalents.
Thermal Stress Distribution
The asymmetric temperature field induced by one-sided water cooling generates significant thermal stresses. The model shows that compressive residual stresses develop in the water-cooled side while tensile stresses form on the opposite side. The peak tensile stress can reach 200–350 MPa depending on the cooling intensity and plate thickness. This is a critical concern for pressure vessel applications governed by GB/T 150 or ASME VIII Div. 1, where residual stress limits and hydrogen-induced cracking (HIC) susceptibility must be carefully managed.
Weld Pool Geometry
Deep penetration TIG with water cooling produces a narrow, deep weld pool with a penetration-to-width ratio significantly higher than conventional TIG. The simulation confirms that the molten pool depth can exceed 5 mm even at moderate current levels (200 A) when the cooling channel is positioned within 3–5 mm of the arc axis. This is attributed to the rapid removal of heat from the bottom of the weld pool, which prevents back-side bulging and allows deeper penetration without excessive heat input.
Engineering Practice Implications
Applicability to Pressure Vessel Fabrication
For clad-plate pressure vessels and hydrogenation reactors, deep penetration TIG with water cooling offers several advantages:
- Reduced number of passes: The deep penetration capability reduces the total number of root and fill passes, decreasing total heat input and minimizing the risk of intergranular corrosion sensitization in the cladding layer.
- Improved cladding bond integrity: Lower total heat input preserves the metallurgical integrity of the overlay layer, which is critical for meeting the bond strength requirements specified in NB/T 47002 and ASTM A264.
- Controlled HAZ microstructure: The high cooling rate can be leveraged to refine the HAZ microstructure, improving toughness properties that are essential for low-temperature service applications.
Process Control Considerations
| Control Parameter | Recommended Practice |
|---|---|
| Coolant flow rate | Maintain 4–8 L/min for plates 10–20 mm thick |
| Coolant temperature | Keep below 25 °C; monitor for thermal runaway |
| Channel clearance | Position cooling channel 2–5 mm below the weld root |
| Preheat temperature | 50–150 °C depending on carbon equivalent |
| Interpass temperature | Limit to 200 °C maximum to avoid excessive grain growth |
| Post-weld heat treatment (PWHT) | Mandatory for CEM > 0.45 materials per NB/T 47014 |
Common Defects and Countermeasures
- Cracking at the fusion line: High cooling rates can promote martensitic transformation in the HAZ of low-alloy steels with high carbon equivalents. Countermeasures include increasing preheat temperature, using low-hydrogen filler metals (E309L, E316L), and applying post-weld stress relief.
- Undercut on the water-cooled side: Rapid solidification at the cooled surface can cause surface shrinkage. Adjust the torch angle and travel speed to compensate.
- Incomplete fusion: If the cooling channel is too close to the arc axis, the excessive cooling can solidify the weld pool before adequate fusion is achieved. Maintain a minimum clearance of 2 mm between the arc axis and the cooling channel wall.
Key Questions and Reflections
The study raises several important questions for engineering practice. First, the numerical model assumes a steady-state coolant flow, but in actual fabrication, flow rate fluctuations and air entrainment can significantly affect the cooling efficiency. Second, the model does not account for the effect of welding sequence on the temperature field in multi-pass welds, which is a common scenario in pressure vessel fabrication. Third, the transition from numerical simulation to process qualification under NB/T 47014 or ASME IX requires careful correlation between simulated cooling rates and measured thermocouple data.
The work also highlights the potential of water-cooled TIG as a viable alternative to electron beam welding for deep penetration applications in situations where electron beam equipment is unavailable or impractical for field fabrication. This is particularly relevant for large-diameter pressure vessels and storage tanks where portable equipment is required.
Study Insights and Implications
This research represents a valuable contribution to the understanding of water-cooled deep penetration TIG welding. The numerical model provides a reliable tool for predicting temperature fields and cooling rates under various process parameter combinations, which can significantly reduce the trial-and-error approach in process development. For engineers involved in bimetal pressure vessel fabrication, the key takeaway is that water-cooled TIG can be a powerful technique for achieving deep penetration with controlled heat input, provided that the process is carefully designed and qualified. The integration of this technique with cladding operations requires particular attention to the interaction between the cooling channel geometry and the overlay layer metallurgy, as excessive cooling can compromise the bond strength and corrosion resistance of the clad interface. Future work should focus on multi-pass thermal simulation, coupled thermal-mechanical analysis, and experimental validation of the predicted microstructures and mechanical properties under various cooling conditions.