Zhuojin Pipe Fitting Co., LtdResidual Stress and Its Elimination in T-Joints After TIG Repair Welding
Literature Overview
The research by Lv Yishi et al. (2012), published in Welding Journal (Vol. 33, No. 9, pp. 37-40), investigates the residual stress state in P355NL1 steel T-joints following TIG repair welding, and evaluates the effectiveness of subsequent heat treatment for stress relief. This work is funded by the National Natural Science Foundation of China and the Ministry of Education Doctoral Program, reflecting its academic rigor. The study employs a combination of experimental measurement (infrared thermography, blind hole method) and finite element simulation to provide a comprehensive understanding of the stress evolution during and after repair welding.
Core Technical Content
Material and Test Configuration
P355NL1 is a normalized fine-grained structural steel with minimum yield strength of 355 MPa, designed for low-temperature service. The T-joint configuration represents a common structural arrangement in pressure vessels and piping systems, where weld repair is frequently required due to fabrication defects or service damage.
Experimental Methodology
The study employs a multi-method approach:
| Method | Purpose | Key Parameters |
|---|---|---|
| Infrared thermography | Temperature field measurement during welding | Spatial resolution, temperature range |
| Finite element simulation | Predictive temperature and stress field analysis | SYSWELD or similar software |
| Blind hole method | Direct residual stress measurement | Hole diameter, strain gauge configuration |
| Indirect calculation | Stress field estimation from temperature data | Elastic-plastic constitutive model |
The blind hole method, while destructive, provides the most reliable direct measurement of residual stress at specific points. The infrared thermography provides full-field temperature data during welding, which serves as input for finite element models.
Residual Stress Findings
The study reveals a complex stress evolution pattern:
| Stage | Weld Center Stress | Repair Zone Stress | Overall Stress Level |
|---|---|---|---|
| After MAG welding | High tensile | N/A | High |
| After TIG repair | Reduced (lowered) | Increased (elevated) | Moderate |
| After heat treatment | Significantly reduced | Significantly reduced | Low (residual remains) |
The key finding is that TIG repair welding, while reducing the original weld center stress, introduces new stress concentrations in the repair zone itself. This is a critical insight for engineers: repair welding is not a simple "healing" process but rather a stress redistribution that may create new problem areas.
Heat Treatment Effectiveness
Post-weld heat treatment (PWHT) proves effective for stress relief:
- Elimination rate: Removes the majority of repair-induced residual stresses
- Residual stress: Some stress remains even after heat treatment, likely due to constrained cooling and non-uniform temperature distribution
- Temperature profile: Uniform heating and slow cooling are essential for maximum stress relief
The finite element simulation results correlate well with experimental measurements, validating the predictive capability of the modeling approach. This validation is important for process planning, as it allows engineers to predict stress states and optimize heat treatment parameters before actual production.
Engineering Practice Integration
Repair Welding Procedure Design
Based on the study findings, the following procedure design principles are recommended for TIG repair welding:
- Pre-repair assessment: Measure existing residual stress distribution to identify high-stress regions
- Repair geometry optimization: Minimize repair area to reduce new stress introduction
- Welding sequence planning: Use multiple short welds rather than continuous long welds to distribute heat input
- Parameter control: Use lower current and slower travel speed to minimize thermal cycle severity
- Post-repair stress measurement: Verify stress state after repair to confirm effectiveness
Heat Treatment Optimization
| Parameter | Recommended Value | Rationale |
|---|---|---|
| Heating temperature | 580-620 °C | Below Ac1 to avoid phase transformation |
| Heating rate | ≤10 °C/min | Prevent thermal shock and distortion |
| Holding time | 1-2 hours per 25 mm thickness | Ensure uniform temperature throughout |
| Cooling rate | ≤10 °C/min | Slow cooling to avoid re-introduction of stress |
| Cooling medium | Furnace cool or still air | Avoid quenching effects |
Stress Measurement Protocol
For critical applications, the following measurement protocol is recommended:
- Pre-weld: Establish baseline stress state using blind hole method or ultrasonic methods
- Post-weld: Measure stress at weld center, HAZ, and repair zone boundaries
- Post-heat treatment: Verify stress reduction and confirm uniformity
- Acceptance criteria: Residual stress should be below 50% of yield strength for critical components
FMEA Application to Repair Welding
A Failure Mode and Effects Analysis (FMEA) for TIG repair welding identifies the following critical risk factors:
| Failure Mode | Severity | Occurrence | Detection | RPN | Countermeasure |
|---|---|---|---|---|---|
| Excessive stress concentration | 9 | 5 | 3 | 135 | Limit repair area, use multi-pass technique |
| Cracking during repair | 10 | 4 | 2 | 80 | Preheat, control cooling rate, use low-hydrogen filler |
| Incomplete stress relief | 8 | 4 | 2 | 64 | Optimize heat treatment parameters, verify with measurement |
| Distortion during heat treatment | 7 | 3 | 4 | 84 | Use fixtures, control heating rate, monitor dimensional changes |
Key Reflections and Implications
This study makes a critical contribution to the understanding of repair welding as a stress redistribution process rather than a simple defect elimination technique. The finding that TIG repair welding reduces original weld stress while increasing stress in the repair zone has profound implications for engineering practice: repair welding must be followed by comprehensive stress relief to ensure structural integrity.
The validation of finite element simulation against experimental measurements provides engineers with a powerful predictive tool for process planning. In modern practice, this capability allows for virtual optimization of repair procedures and heat treatment parameters before actual implementation, reducing trial-and-error costs and improving first-time quality.
For pressure vessel and piping applications governed by codes such as ASME Section VIII or GB 150, the residual stress state after repair is a critical acceptance criterion. The study's demonstration that heat treatment can effectively eliminate most repair-induced stresses provides a clear pathway for achieving code compliance. However, the persistence of some residual stress even after heat treatment suggests that for the most critical applications, additional measures such as shot peening or vibration stress relief may be necessary to achieve near-zero residual stress states.