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 2014 by researchers from the Beijing Institute of Aeronautical Materials, this study employs finite element analysis (FEA) to simulate the thermal and mechanical behavior during multi-pass TIG welding of ultra-high strength stainless steel thick plates. Ultra-high strength stainless steels, such as 17-4PH (AISI 630) or precipitation-hardened grades with yield strengths exceeding 1000 MPa, present unique welding challenges due to their sensitivity to thermal cycles and susceptibility to cracking.
Core Technical Analysis
The numerical simulation employs coupled thermo-mechanical analysis, solving the heat equation with moving heat source followed by elastic-plastic stress analysis with temperature-dependent material properties. The Johnson-Cook constitutive model is used to describe the material behavior under the complex thermomechanical loading conditions encountered during welding.
Material Properties and Thermal Parameters
| Property | Value | Temperature Dependence |
|---|---|---|
| Yield strength (RT) | 1100–1400 MPa | Decreases above 400 °C |
| Thermal conductivity | 15–18 W/(m·K) | Increases with temperature |
| Specific heat | 500 J/(kg·K) | Increases above 500 °C |
| Thermal expansion | 16 × 10⁻⁶/K | Nonlinear above 600 °C |
| Melting point | ~1400 °C | — |
| Plate thickness | 20–40 mm | — |
Thermal Cycle Characteristics in Multi-Pass Welding
The simulation reveals several critical thermal features:
- Peak temperature: 1450–1550 °C at the weld pool center, decreasing radially
- Heat affected zone width: 8–15 mm per pass depending on heat input
- Cooling rate (t8/5): 3–8 seconds from 800 °C to 500 °C in the HAZ
- Interpass temperature effect: Each subsequent pass raises the base temperature, reducing thermal gradients
- Thermal cycling: Regions near the weld experience 5–8 thermal cycles during multi-pass welding
Residual Stress Distribution
| Location | Peak Residual Stress (MPa) | Stress State |
|---|---|---|
| Weld centerline | 900–1100 (tensile) | Near yield strength |
| HAZ | 600–800 (tensile) | Elastically stressed |
| Base metal (far field) | 50–150 (compressive) | Elastic range |
| Plate edge | 200–400 (tensile) | Constraint-related |
| Plate surface vs. mid-thickness | Surface: higher tensile | Constraint gradient |
Process Optimization Based on Simulation Results
| Optimization Strategy | Implementation | Expected Benefit |
|---|---|---|
| Back-step welding | Divide weld into segments, alternate direction | Reduce longitudinal residual stress by 30–40% |
| Tack welding sequence | Pre-apply tacks at calculated intervals | Control distortion within 1.5 mm/m |
| Interpass temperature control | Maintain 150–250 °C between passes | Reduce cracking susceptibility |
| Weld sequence optimization | Center-out or symmetric sequence | Minimize angular distortion |
| Post-weld stress relief | PWHT at 620 °C for 2 h | Reduce residual stress by 60–70% |
Engineering Practice and Standards Compliance
For ultra-high strength stainless steel pressure vessels governed by ASME VIII Div.2 or GB/T 150, the residual stress predictions from this simulation are essential for:
- Fitness-for-service assessments under Part 5 of ASME Div.2
- Hydrostatic test pressure calculation accounting for residual stress
- Fatigue analysis per ASME Div.2 Part 5 using stress concentration factors
- Crack propagation assessment for flaw tolerance evaluation
- PWHT qualification per NB/T 47014 and ASME IX
The simulation confirms that without PWHT, residual stresses in the weld zone approach or exceed the yield strength of the material at room temperature. This creates a condition where any additional loading—thermal, mechanical, or pressure-induced—will cause immediate plastic deformation in the weld region, potentially initiating cracking in the heat-affected zone.
Study Insights and Reflections
The numerical simulation approach provides engineers with predictive capability before committing to expensive multi-pass welding trials on expensive ultra-high strength materials. The key insight is that the thermal-mechanical coupling creates a complex residual stress state that cannot be predicted from single-pass data alone. Each additional pass modifies the stress field of previously deposited material, creating a cumulative effect that must be accounted for in process design. For pressure vessel fabrication, this work underscores the necessity of PWHT for ultra-high strength stainless steels and provides quantitative justification for stress relief parameters in the WPS.