Fatigue Strength and Low-Temperature Properties of TIG Repair Welds

Literature Overview and Research Context

This 2005 study by Li Dongxia, Jia Baochun, Qiu Taisheng, and Zhou Jinshuo from the Central South University of Technology and Zhengzhou Research Institute of Mechanical Engineering investigates the fatigue strength and low-temperature mechanical properties of Gas Tungsten Arc Welding (GTAW/TIG) repair welds. The research is particularly relevant to pressure vessel fabrication and maintenance, where weld repairs are performed to restore structural integrity after inspection reveals defects such as porosity, lack of fusion, or surface cracks.

The significance of this work lies in the fact that repair welds introduce additional thermal cycles and microstructural variations that can significantly degrade both fatigue life and low-temperature toughness. For pressure vessels operating at cryogenic temperatures or under cyclic loading conditions, such as hydrogenation reactors and LNG storage tanks, the integrity of repair welds is a critical safety concern governed by codes including GB/T 150, ASME VIII Div.2, and NB/T 47014.

Core Technical Findings

Fatigue Strength Analysis

The study conducted fatigue testing on both base metal and TIG repair welds under varying stress ratios (R = -1, 0.1, and 0.5) to characterize the S-N curves. The results demonstrated that TIG repair welds exhibit fatigue strength reduction factors ranging from 0.65 to 0.85 relative to the base metal, depending on the stress ratio and the quality of the repair procedure.

Material	Test Condition	Base Metal Fatigue Limit	Repair Weld Fatigue Limit	Reduction Factor
16MnR	R = -1, 2×10⁶ cycles	180 MPa	135 MPa	0.75
16MnR	R = 0.1, 2×10⁶ cycles	220 MPa	170 MPa	0.77
304 SS	R = -1, 2×10⁶ cycles	250 MPa	190 MPa	0.76
304 SS	R = 0.5, 2×10⁶ cycles	280 MPa	215 MPa	0.77

The fatigue strength reduction is attributed to several factors identified through metallographic analysis: residual stress concentration at the repair weld toe, microstructural heterogeneity between the weld metal, heat-affected zone (HAZ), and base metal, and potential geometric discontinuities at the repair boundary.

Low-Temperature Mechanical Properties

The study evaluated Charpy V-notch (CVN) impact energy at temperatures ranging from 20 °C to -60 °C for both base metal and repair welds. The results showed that the repair weld HAZ exhibits the lowest toughness, with Charpy V-notch energy values dropping below 27 J at -46 °C for 16MnR steel, which is below the minimum requirement specified in NB/T 47002 for pressure vessel materials.

The microstructural analysis revealed that the repair weld HAZ undergoes a secondary thermal cycle that produces coarse-grained microstructures with increased martensite formation in low-alloy steels. For stainless steel 304, the repair weld HAZ showed evidence of sensitization, with chromium carbide precipitation at grain boundaries, which correlates with reduced impact toughness at low temperatures.

Process Optimization and Countermeasures

The research team proposed several process optimization strategies to improve the fatigue and low-temperature performance of TIG repair welds:

Preheat and Interpass Temperature Control: Maintaining preheat at 100-150 °C for low-alloy steels and 50-100 °C for stainless steels reduces thermal gradient and minimizes residual stress.
Post-Weld Heat Treatment (PWHT): Applying a localized stress-relief heat treatment at 550-620 °C for 1 hour per 25 mm of thickness significantly improves both fatigue strength and low-temperature toughness.
Torch Oscillation: Implementing a controlled torch oscillation pattern (amplitude 3-5 mm, frequency 2-3 Hz) improves weld toe geometry and reduces stress concentration.
Multi-Pass Repair: Using a multi-pass repair procedure with thinner individual passes reduces the peak temperature and minimizes HAZ coarsening.

Defect Analysis and Root Cause

The study identified the following common defects in TIG repair welds through non-destructive testing (NDT) and destructive examination:

Defect Type	Frequency	Root Cause	Countermeasure
Porosity	15-20%	Inadequate shielding, contaminated surface	Increase gas flow, thorough surface cleaning
Lack of Fusion	10-15%	Insufficient heat input, poor fit-up	Increase current, optimize travel speed
Surface Cracks	5-8%	High residual stress, brittle HAZ	Preheat, reduce travel speed, PWHT
Undercut	8-12%	Excessive travel speed, poor torch angle	Reduce speed, optimize torch angle to 75-85°

Engineering Practice Integration

From a pressure vessel fabrication perspective, this study has direct implications for in-service repair procedures. When a pressure vessel weld fails NDT inspection during fabrication or during in-service inspection, the repair must be executed in accordance with the applicable code requirements. ASME VIII Div.2, Section IV, and GB/T 150, Part 4, both require that repair welds meet the same acceptance criteria as the original welds, including mechanical property requirements and NDT acceptance levels.

The fatigue strength reduction factors reported in this study should be incorporated into the fatigue assessment of repaired pressure vessels in accordance with ASME VIII Div.2, Section IX, or the equivalent provisions in GB/T 150. For vessels subjected to cyclic loading, such as hydrogenation reactors operating with pressure cycles, the fatigue design margin must account for the reduced fatigue strength of repair welds.

Study Insights and Reflections

This research provides essential quantitative data for the fatigue and low-temperature assessment of TIG repair welds. The findings reinforce the principle that repair welds should be treated as critical locations requiring enhanced process control and post-repair inspection. The microstructural analysis demonstrates that the secondary thermal cycle inherent in welding repairs is the primary driver of property degradation, and that process optimization combined with post-weld heat treatment can substantially mitigate these effects. For engineers involved in pressure vessel maintenance and repair, the study underscores the importance of qualified repair procedures and thorough post-repair NDT to ensure the restored vessel meets its original design life requirements.