Numerical Simulation of Pulsed TIG Welding for 06Cr18Ni11Ti Stainless Steel Thin Plate

Literature Overview

The paper by Huang Wenxiang, Zhang Chun, Li Hui, and Meng Lei, published in Hot Working Technology (Vol. 51, No. 3, 2022, pp. 131–133), addresses a persistent engineering challenge: the control of welding distortion in thin austenitic stainless steel plates. The authors focus on 06Cr18Ni11Ti (equivalent to UNS S32168 / 321H), a titanium-stabilized austenitic stainless steel widely used in marine, chemical, and nuclear applications. The study employs pulsed TIG welding for T-joint configuration and combines metallographic analysis with finite element numerical simulation using a generalized double-ellipsoid heat source model.

Core Technical Content

Austenitic stainless steels are notorious for their high thermal expansion coefficient (approximately 17×10⁻⁶ /°C, comparable to aluminum) and low thermal conductivity (approximately 16 W/m·K), which together create severe distortion tendencies in thin-plate welding. The 06Cr18Ni11Ti alloy, with its Ti stabilization against intergranular sensitization, is particularly important for high-temperature and corrosive service environments where distortion control is critical for dimensional accuracy.

The study investigates the following key aspects:

• Microstructure of the weld zone: The molten pool exhibits fine austenite grains with a worm-like (vermicular) and dendritic morphology. This is attributed to the rapid solidification rates achieved with pulsed TIG, which promotes high nucleation density and suppresses grain growth.
• HAZ microstructure: The heat-affected zone shows no significant grain coarsening relative to the base metal, indicating that the thermal input from pulsed TIG is sufficiently controlled to avoid excessive recrystallization and growth.
• Residual stress distribution: After cooling, residual stresses are concentrated primarily in the weld zone, with minimal extension into the base metal.
• Distortion pattern: The overall deformation is minimal, manifesting primarily as z-direction (vertical) contraction of the weld and vertical plate, along with angular distortion about the x-axis (weld direction).

Process Parameters and Simulation Setup

The generalized double-ellipsoid heat source model is a well-established approach for simulating moving heat sources in welding processes. It captures both the keyhole effect (deep penetration) and the surface spreading effect (wide weld bead) by representing the heat source as two ellipsoids—one in front of the arc center and one behind.

The double-ellipsoid model parameters (a, b, c, d, f, q) are calibrated against experimental weld geometry to ensure accurate prediction of temperature fields, stress distributions, and distortion.

Microstructural Analysis

The worm-like and dendritic austenite morphology in the weld metal is significant from a mechanical properties perspective. Fine, elongated grains improve toughness and reduce the risk of cracking during service. The absence of significant HAZ grain growth is particularly important because:

• Large grains in the HAZ reduce toughness and increase susceptibility to cracking.
• In austenitic stainless steels, grain boundaries are sites for sensitization (chromium carbide precipitation), so minimizing grain coarsening also reduces sensitization risk.
• The Ti stabilization in 06Cr18Ni11Ti preferentially forms TiC rather than Cr₂₃C₆, but this is only effective if the thermal cycle does not exceed certain temperature thresholds.

Residual Stress and Distortion Analysis

The simulation results reveal several important features:

1. Stress concentration in the weld zone: The residual stress distribution is localized, which is favorable because it minimizes the risk of stress-driven cracking in the base metal. The peak residual stresses are tensile in the weld and compressive in the adjacent HAZ.
2. Angular distortion: The primary distortion mode is angular deformation about the weld axis, which is characteristic of T-joints in thin plates. This occurs because the vertical plate experiences asymmetric heating and cooling relative to the horizontal plate.
3. z-direction contraction: The weld and vertical plate contract in the vertical direction due to the asymmetric thermal profile. This contraction can be compensated through fixture design or post-weld machining.

Engineering Practice Implications

For engineers working with thin austenitic stainless steel components, the following recommendations emerge:

• Pulsed TIG is preferred over continuous TIG for thin plates because it allows precise control of heat input. The duty cycle and peak current can be adjusted to maintain a stable arc while minimizing total energy input.
• Interpass temperature control is critical. For multi-pass welds, the interpass temperature should be maintained below 150°C to prevent sensitization and excessive distortion.
• Weld sequence planning should follow a symmetric pattern to balance thermal input on both sides of the joint.
• Post-weld stress relief may be necessary for high-stress applications. A solution heat treatment at 1050–1100°C followed by rapid cooling can relieve residual stresses and restore full austenitic properties.

Study Insights and Reflections

This study exemplifies the power of combining experimental metallography with numerical simulation to understand and predict welding outcomes. The generalized double-ellipsoid heat source model, while not capturing every physical detail of the welding process, provides sufficiently accurate predictions for engineering purposes. The key insight is that pulsed TIG welding, when properly parameterized, can achieve excellent microstructural quality and minimal distortion in thin austenitic stainless steel plates—making it a viable alternative to more expensive processes such as laser welding or electron beam welding for certain applications. The finding that residual stresses are localized in the weld zone is particularly encouraging, as it suggests that the base metal retains its full mechanical integrity and corrosion resistance.