Analysis of Crack Causes in a Power Plant Tee Welding Joint

Literature Overview

The paper by Wang Hao, Song Li, Yuan Baozi, Ju Guangyu, Yang Xirui, and Deng Hui, published in Physical Testing and Chemical Analysis (Physical Section) in 2019 (Vol. 55, No. 7, pp. 501–505), presents a comprehensive failure analysis of a tee welding joint in the main steam pipeline of a power plant. The research was conducted by the Central China Branch of China Datang Group Science and Technology Research Institute Co., Ltd. and Datang Luoyang Cogeneration Co., Ltd. The tee welding joint experienced repeated cracking, and the investigation employed non-destructive testing, hardness testing, metallographic examination, piping stress analysis, piping thrust analysis, and finite element analysis to identify the root cause.

Core Technical Content

The failure analysis follows a systematic methodology consistent with established failure analysis practices. The main steam pipeline tee experienced multiple cracking events at the welding joint connecting the tee to the upper connecting pipe. The investigation revealed that the root cause was a combination of excessive bending stress, stress concentration, and microstructural aging at the crack location.

Investigation Methodology

Root Cause Analysis

The investigation identified three contributing factors to the cracking:

1. Unreasonable piping layout near the tee: The piping arrangement around the tee was not optimized, leading to excessive bending moments and stresses at the welding joint.
2. Excessive bending stress and stress concentration: The maximum stress at the welding joint exceeded the high-temperature creep strength of the material, creating conditions favorable for crack initiation and propagation.
3. Microstructural aging: The microstructure at the crack location had undergone aging, resulting in degraded mechanical properties and reduced resistance to creep cracking.

Process and Standards Analysis

The failure analysis and subsequent corrective actions must comply with several relevant standards:

The high-temperature service environment of the main steam pipeline places the welding joint under sustained thermal stress, making it susceptible to creep cracking. The creep strength of the material at the operating temperature is a critical parameter in the design and assessment of such components. The ASME B31.1 code provides specific requirements for creep-rupture design and stress analysis in high-temperature service, including the use of creep-rupture curves and minimum wall thickness requirements.

Engineering Practice Integration

Stress Analysis and Piping Layout Optimization

The finding that the piping layout was unreasonable provides a clear corrective action. In practice, the following measures should be implemented:

1. Piping stress analysis per ASME B31.1 or B31.3 to identify high-stress areas and verify that stresses are within allowable limits.
2. Layout optimization to minimize bending moments at the tee welding joint, including the addition of support structures, expansion loops, or flexible connectors.
3. Thrust load mitigation through the use of thrust blocks, anchors, or spring supports to reduce the load on the tee.

Material and Welding Considerations

The microstructural aging observed at the crack location suggests that the welding heat-affected zone (HAZ) may have been subjected to excessive thermal cycling or prolonged exposure to high temperatures. The following measures should be considered:

• Welding procedure qualification to ensure that the welding parameters (heat input, interpass temperature, preheat) are optimized to minimize HAZ aging.
• Post-weld heat treatment (PWHT) per ASME B31.1 to relieve residual stresses and improve the microstructure of the HAZ.
• Material selection to ensure that the base metal and weld metal have adequate creep resistance at the operating temperature.

Inspection and Monitoring

For existing installations, the following inspection and monitoring practices should be implemented:

• Regular NDT inspections (RT, UT, or PAUT) at the tee welding joint to detect early-stage cracking.
• Hardness mapping to identify areas of microstructural degradation.
• Metallographic examination of suspect areas to assess the extent of aging and other microstructural changes.
• Stress monitoring using strain gauges or fiber optic sensors to track stress levels over time.

Corrective Actions

Based on the root cause analysis, the following corrective actions are recommended:

1. Redesign the piping layout to reduce bending stresses at the tee welding joint.
2. Add or modify support structures to reduce thrust loads on the tee.
3. Replace the affected tee welding joint with a new fitting using improved welding procedures and materials.
4. Implement a monitoring program to track the performance of the corrected installation and prevent recurrence.

Key Questions and Reflections

The failure analysis presented in this paper follows a rigorous methodology and provides clear identification of the root cause. However, several questions remain regarding the broader implications of this failure. First, the paper does not provide detailed information on the welding procedure, heat input, and PWHT parameters, which are critical factors in the development of HAZ microstructure and creep resistance. Second, the paper does not discuss the fatigue life of the welding joint under cyclic loading, which may be a contributing factor in the cracking mechanism. Third, the paper does not address the potential for similar failures at other tee welding joints in the same system, which warrants a systematic review of all similar components.

The finding that the maximum stress exceeded the high-temperature creep strength raises questions about the original design and construction of the piping system. If the stress analysis was performed during the design phase, the discrepancy between the calculated and actual stresses suggests that the design assumptions may have been inaccurate, or that the piping layout was modified after the design was completed. This highlights the importance of maintaining accurate as-built documentation and performing stress analysis after any piping modifications.

Study Insights and Implications

This paper provides a valuable case study in the failure analysis of tee welding joints in high-temperature power piping. The systematic approach to investigation—combining NDT, hardness testing, metallographic examination, stress analysis, and FEA—demonstrates the importance of a multi-faceted approach to failure analysis. The identification of unreasonable piping layout as the primary cause underscores the critical role of piping stress analysis and layout optimization in preventing failures in high-temperature service.

For pipe fitting and welding engineers, this case study reinforces several key principles:

• Piping layout is a critical design parameter that directly affects the stress state and service life of fittings and welding joints.
• Microstructural aging is a significant concern in high-temperature service, and welding procedures must be optimized to minimize HAZ degradation.
• Comprehensive failure analysis requires the integration of multiple investigation methods to identify all contributing factors.
• Preventive maintenance and monitoring are essential for detecting early-stage degradation and preventing catastrophic failures.

The corrective actions recommended in this study—layout redesign, support modification, and joint replacement—are standard practices in the industry, but their successful implementation requires careful engineering judgment and adherence to applicable codes and standards. The case study also highlights the importance of communication between design engineers, welding engineers, and operations personnel to ensure that design intent is maintained throughout the lifecycle of the piping system. This failure analysis serves as a reminder that even well-designed and constructed systems can develop problems over time, and that a proactive approach to inspection, monitoring, and maintenance is essential for ensuring the long-term reliability and safety of power piping systems.