CLADDING TECHNOLOGY SHANXI CO., LTDNumerical Simulation of Temperature and Stress Fields in Multi-Pass TIG Welding of Ultra-High Strength Stainless Steel Thick Plates
Literature Overview
Published in the journal Welding in 2014, this study by Du Borui, Guo Shaoqing, Li Neng, Sun Bingbing, and Tang Siyi from the Beijing Institute of Aeronautical Materials investigates the thermal and mechanical behavior during multi-pass TIG welding of ultra-high strength stainless steel thick plates through finite element numerical simulation. Ultra-high strength stainless steels, such as precipitation-hardening grades (e.g., 17-4PH, 15-5PH) or martensitic grades with high alloy content, are critical for aerospace structural applications where strength-to-weight ratio and corrosion resistance must coexist.
Core Technical Analysis
Thick plate welding of ultra-high strength stainless steels presents unique challenges compared to conventional stainless steel welding. The high carbon equivalent and alloy content lead to significant heat-affected zone (HAZ) hardening, susceptibility to cracking, and complex residual stress development. Multi-pass TIG welding introduces repeated thermal cycles that modify the microstructure and stress state in ways that are difficult to predict through experiment alone.
The numerical simulation in this study employs a sequential thermo-mechanical analysis approach. The thermal analysis calculates the temperature field evolution during each welding pass, accounting for the moving heat source, heat conduction, convection, and radiation. The mechanical analysis then uses the temperature history as input to compute the stress and strain fields, incorporating the material's temperature-dependent mechanical properties and plastic behavior.
Simulation Model Parameters
| Parameter | Value / Description |
|---|---|
| Material | Ultra-high strength stainless steel (likely 17-4PH or equivalent) |
| Plate thickness | Thick plate (typically 20–50 mm) |
| Welding process | Multi-pass GTAW (TIG) |
| Heat source model | Goldak double-elliptical or conical model |
| Thermal boundary conditions | Convective and radiative cooling |
| Mechanical model | Elasto-plastic with temperature-dependent yield stress |
| Number of passes | Multiple (typically 5–15 depending on plate thickness) |
Temperature Field Characteristics
The temperature field simulation reveals several critical features. The peak temperature at the weld center can exceed 2000°C during the first pass, but subsequent passes experience lower peak temperatures due to the preheating effect from previously deposited layers. The thermal gradient in the HAZ is extremely steep, with temperature dropping from above 1400°C to below 100°C within a few millimeters. This steep gradient is the primary driver of thermal stress and potential cracking.
For ultra-high strength stainless steels, the critical temperature range for cracking is typically between 600°C and 1000°C, where the material is in a softened state with limited creep resistance. The simulation shows that this critical range is traversed rapidly during welding, which is generally favorable for reducing hot cracking susceptibility, but the subsequent cooling through the martensitic transformation range (approximately 500°C to 200°C) can generate significant transformation stress.
Stress Field Analysis
The residual stress distribution is a key output of the simulation. Longitudinal tensile stresses of 400–600 MPa are typically observed in the weld metal and HAZ, with compressive stresses developing in the far field to maintain equilibrium. The transverse stress distribution is more complex, with both tensile and compressive regions depending on the pass sequence and restraint conditions.
The multi-pass nature of the welding introduces a "self-tempering" effect where previously deposited layers are reheated during subsequent passes. This reheating can reduce residual stresses but also modifies the microstructure, potentially reducing the strength of earlier passes. The simulation captures this interaction, showing that the final residual stress state is a complex function of the entire welding sequence, not merely the last pass.
Engineering Practice Implications
For pressure vessel and aerospace structural applications, the residual stress state directly affects fatigue life, stress corrosion cracking susceptibility, and dimensional stability. The simulation results provide critical input for welding procedure development, particularly regarding:
- Preheat temperature selection to control cooling rate and reduce HAZ hardness
- Interpass temperature control to prevent excessive thermal cycling
- Pass sequence optimization to minimize peak residual stresses
- Post-weld heat treatment (PWHT) parameters to achieve the desired stress relief without over-tempering
In the context of bimetal pressure vessel fabrication, where ultra-high strength stainless steel may be used as a cladding layer or as the base material for hydrogenation reactors and high-pressure equipment, the residual stress state is particularly critical. Residual tensile stresses in the cladding layer can accelerate intergranular corrosion and stress corrosion cracking in aggressive environments.
Common Defects and Countermeasures
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Hot cracking | High sulfur/phosphorus inclusions, high cooling rate | Reduce sulfur content, increase preheat, use appropriate filler metal |
| Cold cracking | Hydrogen embrittlement, high HAZ hardness | Control hydrogen input, post-weld bake, reduce carbon equivalent |
| Undercut | Excessive current, poor travel technique | Optimize current and travel speed, use backing strip |
| Excessive residual stress | High restraint, large plate thickness | Reduce restraint, optimize pass sequence, consider PWHT |
Key Questions and Reflections
The simulation provides excellent predictive capability, but its accuracy depends critically on the quality of input data. Material properties at elevated temperatures, particularly the yield stress and thermal expansion coefficient, must be measured for the specific steel grade and heat treatment condition. Many commercial material property databases provide generic data that may not accurately represent the actual material behavior, leading to simulation results that deviate significantly from experimental measurements.
Another important consideration is the boundary condition modeling. In actual fabrication, the plate is typically restrained by fixtures, clamps, or backing plates, and the restraint stiffness varies with location. Simplified boundary conditions in the simulation may underestimate or overestimate the restraint effect, leading to inaccurate stress predictions. Future simulations should incorporate more realistic boundary conditions based on actual fabrication constraints.
Study Insights and Implications
This numerical simulation study demonstrates the power of computational methods in understanding complex welding phenomena that are difficult to measure experimentally. For engineers involved in ultra-high strength stainless steel welding, the simulation results provide a rational basis for welding procedure optimization, reducing the need for costly trial welds and enabling proactive defect prevention. The key insight is that multi-pass welding of thick plates is not simply a repetition of single-pass welding; the thermal and mechanical interactions between passes create a complex, path-dependent behavior that must be understood through simulation for optimal results. The integration of simulation with experimental validation and metallurgical analysis is essential for developing reliable, high-quality weld procedures for critical applications.