Numerical Simulation of Multi-Pass TIG Welding Temperature and Stress Fields in Ultra-High Strength Stainless Steel Thick Plate

Literature Overview

This study, published in Welding in 2014 by researchers from the Beijing Institute of Aeronautical Materials, presents a numerical simulation of multi-pass TIG welding of 15 mm thick Ferriun S53 ultra-high strength stainless steel using the SYSWELD software platform. The research addresses a critical engineering challenge: predicting and controlling residual stresses and distortions in thick-section weldments made from advanced high-strength materials. The simulation approach provides insights that complement experimental measurements and guide welding procedure optimization.

Simulation Methodology

The study established a comprehensive material property database for Ferriun S53 steel, including thermal conductivity, specific heat, elastic modulus, yield strength, and thermal expansion coefficient as functions of temperature. The SYSWELD software was employed with a double-ellipsoid heat source model to represent the heat distribution during TIG welding. The simulation was conducted under free-fixturing conditions, which represents the most realistic scenario for understanding the true deformation behavior of the weldment.

Key Results and Analysis

The temperature field simulation results demonstrate realistic thermal behavior, with peak temperatures in the weld zone reaching values consistent with the melting point of the material. The phase distribution analysis revealed that partial martensitic transformation occurs in the weld zone, which is a critical finding for understanding the mechanical properties of the weld. The formation of martensite in the weld metal and heat-affected zone (HAZ) introduces high hardness but also increases susceptibility to cracking and reduces ductility.

The residual stress distribution shows that the maximum residual stresses are concentrated in the weld zone and the adjacent fusion boundary region. This is consistent with the fundamental principle that residual stresses develop due to differential thermal expansion and contraction during welding. The stress magnitude in the weld zone typically approaches the yield strength of the material at room temperature, creating a state of self-equilibrium.

The deformation analysis showed that the simulated deformation trends are consistent with actual observations, validating the simulation model. The primary deformation modes include angular distortion at the weld seam and longitudinal shrinkage along the weld length. These deformations are governed by the thermal gradients and the restraint conditions imposed on the weldment.

Engineering Implications for Welding Practice

The simulation results have direct implications for welding procedure design. The prediction of partial martensitic transformation in the weld zone indicates that post-weld heat treatment (PWHT) is essential to relieve residual stresses and transform brittle martensite into more ductile phases. The residual stress distribution pattern suggests that stress-relieving treatments should target the weld zone and HAZ, where the highest stresses are concentrated.

For thick-section welding of ultra-high strength stainless steels, the multi-pass strategy must be carefully designed to minimize the thermal cycles experienced by the base metal. Each subsequent pass acts as a tempering treatment for the previous pass, but excessive thermal cycling can lead to microstructural degradation. The simulation provides a tool for optimizing the number of passes, the welding sequence, and the interpass temperature to achieve the best combination of mechanical properties and dimensional accuracy.

From a quality assurance perspective, the simulation can be used to predict critical areas for non-destructive testing. Regions with high residual stress and potential martensitic transformation are more susceptible to cracking, and should receive priority inspection using methods such as magnetic particle testing (MT) and ultrasonic testing (UT).

Study Insights and Reflections

This study demonstrates the value of numerical simulation as a complementary tool to experimental welding research, particularly for complex multi-pass welding of thick-section advanced materials. The ability to predict temperature fields, phase transformations, residual stresses, and deformations in a single integrated simulation provides comprehensive insights that would be difficult and expensive to obtain through experimentation alone. However, the accuracy of the simulation depends critically on the quality of the material property database, and engineers must validate simulation results against experimental data before using them for design decisions. The finding of partial martensitic transformation in the weld zone underscores the need for careful control of cooling rates and the importance of PWHT in ensuring the long-term performance of weldments made from ultra-high strength stainless steels.