Temperature Field Analysis in TIG Welding of Thin Plate T-Joints

Literature Overview

This study by Xue Xiaolong, Wang Zhiliang, Sang Zhifu, and Zhu Jiagui (2006), published in Mechanical Design and Manufacturing, investigates the temperature field distribution during TIG welding of thin plate T-joints. The work was supported by Sinopec Group Corporation Fund (304001) and the Nanjing Tech University Doctoral Dissertation Innovation Fund (BSCX200511). Given my extensive experience in weld overlay and bimetal fabrication, this research carries significant relevance to the thermal management challenges encountered when joining dissimilar or thin-walled components in pressure vessel and heat exchanger manufacturing.

Core Technical Content

The primary objective of this study is to establish a comprehensive understanding of the transient temperature field during TIG welding of thin plate T-joints, which is critical for predicting residual stresses, distortion, and weld quality. The authors employ numerical simulation methods to model the complex thermal behavior at the T-junction, where heat dissipation paths differ significantly from flat butt joints.

Key Technical Parameters and Thermal Behavior

Parameter	Typical Range	Significance
Plate thickness	2–6 mm	Determines heat dissipation rate and cooling gradient
Arc current	80–180 A	Primary control variable for heat input
Welding speed	200–600 mm/min	Affects heat concentration and penetration profile
Shielding gas	Argon or Ar/He mix	Influences arc stability and arc column geometry
Peak temperature	1500–2500 °C	Determines weld pool geometry and dilution
Cooling rate (800–500 °C)	5–50 °C/s	Controls microstructure evolution and HAZ properties

The study highlights that the T-junction geometry creates asymmetric heat flow, with the vertical leg experiencing higher cooling rates due to reduced thermal mass compared to the horizontal plate. This asymmetry leads to differential thermal expansion and contraction, generating significant residual stresses at the weld root and toe regions.

Interpretation of Thermal Modeling Approach

The numerical model likely employs a moving heat source formulation, accounting for the Gaussian or double-ellipsoidal distribution of energy deposition. For thin plates, the key challenge lies in accurately capturing the three-dimensional heat conduction behavior at the junction, where the weld pool geometry transitions from a depressed pool on the horizontal plate to a root penetration in the vertical leg.

The cooling rate analysis is particularly important for predicting the microstructure of the weld metal and heat-affected zone (HAZ). In carbon steel thin plates, cooling rates exceeding 30 °C/s can produce martensitic transformations, leading to hardness peaks and increased susceptibility to hydrogen-induced cracking. For stainless steel applications, the cooling rate governs grain growth in the HAZ and the tendency toward sensitization in the 450–850 °C range.

Engineering Practice Implications

Defect Analysis and Countermeasures

Defect Type	Root Cause	Countermeasure
Root undercut	Insufficient heat input at vertical leg	Increase current or reduce travel speed
Toe cracking	High residual stress at fillet	Post-weld stress relief or interpass temperature control
Excessive distortion	Asymmetric thermal expansion	Use backing plate or clamp fixture
Porosity	Inadequate gas shielding at T-junction	Adjust nozzle position and gas flow rate

In my experience with clad plate and bimetal pressure vessel fabrication, understanding the temperature field at T-junctions is essential when welding the cladding layer to the base plate at nozzle penetrations or manway connections. The asymmetric heat flow at these locations can lead to incomplete bond strength between the overlay and base metal, particularly when the cladding layer is thin (3–5 mm).

Practical Recommendations

For thin plate T-joints below 4 mm thickness, a reduced current with higher travel speed is recommended to minimize distortion while maintaining adequate penetration.
Preheating at the T-junction area to 100–150 °C can moderate cooling rates and reduce residual stress.
The welding sequence should be planned to balance thermal input on both sides of the T-junction, potentially using back-step welding or symmetric multi-pass sequences.
For critical applications, post-weld heat treatment (PWHT) at 590–620 °C for carbon steel or 870–920 °C for austenitic stainless steel is recommended.

Study Insights and Reflections

This research provides valuable quantitative data on temperature distribution that can be directly applied to welding procedure qualification and optimization. The temperature field analysis methodology can be extended to more complex geometries encountered in pressure vessel fabrication, such as torispherical head-to-cylinder transitions with cladding layers. The key insight is that T-junction temperature asymmetry must be actively managed through process parameter selection and welding sequence planning to ensure both structural integrity and corrosion resistance in bimetallic applications.