Penetration Prediction Based on Variable-Speed GTAW Welding Temperature Field Analytical Model

Literature Overview and Core Objective

This study presents an analytical model for predicting weld penetration in gas tungsten arc welding (GTAW) processes where the welding current is varied during the welding cycle. The research addresses a critical challenge in overlay welding and cladding applications: the need to precisely control penetration depth while maintaining surface quality and minimizing dilution. In bimetal pressure vessel fabrication, where overlay layers are applied to carbon steel or low-alloy steel base metals, controlling the interaction between the overlay layer and the base metal is essential for achieving the required bond strength without compromising the mechanical integrity of the substrate. The variable-speed GTAW technique allows operators to dynamically adjust the welding current throughout the bead, which directly influences the thermal input, the shape of the weld pool, and ultimately the penetration profile.

Interpretation of the Temperature Field Model

The analytical model derives the temperature distribution within the weld pool by solving the heat conduction equation under the boundary conditions imposed by the moving arc and the variable current input. The model treats the welding arc as a heat source with a Gaussian distribution, where the heat flux is modulated by the time-varying welding current. Key assumptions include the rigid body approximation for the workpiece, constant thermophysical properties within a defined temperature range, and neglect of convective heat transfer at the surface in the initial formulation. The model integrates the effect of welding speed variation on the thermal gradient and the resulting penetration depth.

The penetration prediction is based on the isotherm criterion, where the penetration depth is defined as the distance from the surface to the point where the temperature equals the melting point of the base metal. By varying the welding current as a function of time or position along the weld, the model captures the dynamic response of the weld pool geometry. This is particularly relevant for overlay welding where multi-pass procedures require each pass to achieve a specific penetration to ensure metallurgical bonding between successive layers.

Connection with Engineering Practice

In the fabrication of clad-plate pressure vessels per GB/T 150 and NB/T 47002, GTAW is commonly employed for the first pass of overlay welding to establish initial bonding. The variable-current approach described in this study offers a practical method to reduce the dilution rate in the first pass, which is critical when overlaying stainless steel grades such as 304 or 316 onto carbon steel substrates. High dilution in the first pass can lead to sensitization of the stainless steel overlay, resulting in intergranular corrosion susceptibility that violates the requirements of intergranular corrosion testing per ASTM A263.

The model also has direct implications for the design of welding procedure specifications (WPS) per NB/T 47014 and ASME IX. By understanding how current variation affects penetration, engineers can optimize the WPS parameters to achieve the minimum required penetration while limiting dilution. For hydrogenation reactors where nickel-based alloys such as Inconel 625 are overlaid on low-alloy steel, the control of dilution is even more critical because excessive dilution introduces carbon into the nickel-based overlay, which can precipitate carbides and reduce corrosion resistance in hydrogen service.

Key Technical Insights and Reflections

The analytical approach provides a theoretical framework that complements the empirical methods traditionally used in welding procedure qualification. The model's predictive capability allows engineers to perform virtual trials before committing to physical coupon tests, thereby reducing the cost and time associated with WPS qualification. However, the model's accuracy depends on the validity of the assumptions regarding thermophysical properties, which vary significantly between carbon steel, stainless steel, and nickel-based alloys. In practice, calibration with experimental measurements is recommended before applying the model to production welding.

A notable insight is that the variable-current technique can be used to achieve a "soft" start and "soft" stop of the weld bead, reducing defects such as undercuts and cold shuts at the weld terminations. This is particularly beneficial for overlay welding where the surface quality of the overlay layer is often inspected by magnetic particle testing (MT) or liquid penetrant testing (PT) per JB/T 4730. The model also highlights the importance of the current ramp rate, which affects the thermal cycling experienced by the weld zone and can influence the residual stress distribution in the overlay layer.

Summary

This study provides a valuable analytical tool for predicting weld penetration under variable-current GTAW conditions, directly applicable to overlay welding and cladding processes in bimetal pressure vessel fabrication. The model's integration of dynamic current variation into the temperature field solution offers a pathway for optimizing dilution control, improving bond strength, and enhancing the corrosion resistance of overlay layers. Engineers should consider incorporating such analytical models into their welding procedure development workflow, complementing traditional empirical methods with theoretical predictions to achieve more reliable and efficient cladding processes.