Finite Element Simulation of TIG Welding Temperature Field on Austenitic Stainless Steel 0Cr18Ni9

Literature Overview

The study by Liu Xiangyu (2016), published in the Journal of Chengde Petroleum College, presents a finite element analysis (FEA) of the thermal field during TIG welding of 0Cr18Ni9 austenitic stainless steel. The work was conducted on the ANSYS platform and addresses a fundamental question in welding engineering: how do process parameters—welding current, arc voltage, and travel speed—govern the thermal cycle and temperature distribution during welding? For engineers involved in stainless steel pipe fabrication, particularly those working with ASME B31.3 or API 5L piping systems, understanding the thermal behaviour of austenitic stainless steels is critical for controlling microstructure, avoiding sensitisation, and ensuring long-term corrosion resistance.

Core Technical Findings

The author established both a mathematical model and a physical model for the TIG welding process, implementing them through ANSYS thermal analysis. The key findings are summarised below.

The study concludes that welding current and arc voltage directly determine the heat input to the workpiece, thereby exerting the most significant influence on the peak temperature of the thermal cycle. Travel speed, by contrast, mainly governs the peak temperature in the fusion zone and the heat-affected zone but has limited effect on the base metal at distances remote from the weld path.

Interpretation of Technical Points

Heat Input and Thermal Cycle Control

The heat input equation for TIG welding can be expressed as Q = (η × U × I) / v, where η is the thermal efficiency, U is arc voltage, I is welding current, and v is travel speed. The study implicitly confirms that current and voltage are multiplicative contributors to heat input, while speed is inversely proportional. For 0Cr18Ni9 (equivalent to 304 stainless steel), the sensitisation temperature window of approximately 450–850 °C is of paramount concern. Prolonged exposure within this range promotes chromium carbide precipitation at grain boundaries, leading to intergranular corrosion susceptibility.

Travel Speed as a Thermal Control Lever

The finding that travel speed has limited influence on base metal temperature at remote locations is physically intuitive: the heat source is localised, and the thermal gradient decays rapidly with distance. However, for thin-walled pipe applications (such as ERW pipe or thin-wall butt-weld fittings), even modest changes in travel speed can shift the HAZ width and the cooling rate at 800–500 °C (t8/5), which directly governs grain growth and potential phase transformations.

Engineering Implications for Pipe Welding

In the context of stainless steel pipe welding under ASME B31.3, the peak temperature and cooling rate are not merely academic parameters—they dictate whether post-weld heat treatment (PWHT) is required, whether sensitisation occurs, and whether the weld meets the Charpy impact requirements for low-temperature service. The FEA approach demonstrated in this paper provides a rational basis for optimising TIG parameters before physical trials, reducing trial-and-error costs.

Integration with Engineering Practice

In my experience with stainless steel pipe fabrication, the FEA results align with practical observations: increasing current from 120 A to 180 A at constant voltage and speed can raise the peak temperature by several hundred degrees, potentially pushing the HAZ into the sensitisation range. Conversely, reducing travel speed from 120 mm/min to 60 mm/min at constant current and voltage effectively doubles the heat input per unit length, with a disproportionate effect on the HAZ width.

Key Questions and Reflections

The study raises several questions worth further investigation. First, the model assumes a simplified heat source distribution (likely a Gaussian or double-ellipsoidal model), whereas real TIG arcs exhibit complex plasma flow patterns influenced by gas shielding, nozzle geometry, and electrode condition. Second, the study does not address residual stress development, which is a critical concern in pipe fitting fabrication where distortion control is essential for assembly fit-up. Third, the thermal model does not incorporate phase transformation effects, which, although less pronounced in austenitic stainless steels than in ferritic or martensitic grades, still influence the final properties of the weld metal.

Study Insights and Implications

This paper serves as a valuable reference for engineers seeking to establish a rational, simulation-based approach to TIG parameter selection for austenitic stainless steels. The clear delineation of parameter effects—current and voltage governing heat input magnitude, speed governing thermal cycle shape—provides a practical framework for process optimisation. For pipe and fitting manufacturers, the ability to predict thermal cycles through FEA before physical welding trials can significantly reduce scrap rates and improve first-pass quality. The methodology also lends itself to extension: by coupling the thermal model with a mechanical model, one can predict residual stresses and distortions, thereby enabling virtual weld procedure qualification. The study's primary limitation lies in its focus on temperature alone, without addressing the metallurgical consequences of the predicted thermal cycles. Nevertheless, as a foundational thermal analysis, it provides the essential input data for subsequent microstructure and property predictions. Engineers should treat the FEA results as a starting point for physical validation, not as a replacement for weld procedure qualification testing under applicable codes such as ASME Section IX or EN ISO 15614.