Crack Analysis of Tee Weld Joint in a Power Plant

Literature Overview

The 2019 paper by Wang Hao, Song Li, Yuan Baozi, Ju Guangyu, Yang Xirui, and Deng Hui, published in Physical Testing and Chemical Analysis (Physical Section), presents a comprehensive failure analysis of repeated cracking at a tee weld joint in a main steam pipe at a power plant. The research team from China Datang Corporation Science and Technology Research Institute (Central China Branch) and Datang Luoyang Cogeneration Co., Ltd. employed a multi-method approach including non-destructive testing, hardness testing, metallographic examination, piping stress analysis, thrust force analysis, and finite element analysis. The findings reveal a complex failure mechanism involving excessive bending stress, stress concentration, microstructural aging, and creep cracking.

Core Technical Findings

The investigation identified a combination of factors that led to the repeated cracking of the tee weld joint:

The maximum stress at the weld joint exceeded the high-temperature creep strength of the weld material. This, combined with the microstructural aging that reduced the mechanical properties of the weld joint, initiated creep cracking. The cracks then propagated under the combined action of thermal cycling, mechanical loading, and sustained creep stress.

Interpretation of Technical Points

Creep cracking in welded joints at elevated temperatures is a well-documented failure mode in power plant piping systems. The mechanism involves the nucleation and growth of voids at grain boundaries, particularly at the heat-affected zone (HAZ) and weld metal where microstructural changes are most pronounced. The creep strength of the material decreases with increasing temperature and time, and the presence of stress concentrators accelerates the creep process.

The microstructural aging observed in the weld joint is attributed to the long-term exposure to high-temperature steam conditions. In the HAZ, the tempering of the martensitic microstructure leads to the coarsening of carbide precipitates, which reduces the strength and ductility of the material. The weld metal, depending on the filler metal composition, may also undergo aging, with precipitate coarsening and phase transformations that degrade mechanical properties.

The pipe layout deficiency identified in this case is a critical finding. In power plant piping systems, the tee junction is a point of geometric discontinuity where the main pipe intersects with a branch pipe. The abrupt change in cross-section creates a stress concentration factor that can exceed 2.0, depending on the geometry and the type of loading. When combined with thermal expansion mismatches and mechanical loads from pipe weight and pressure, the stress at the weld joint can reach levels that exceed the material's creep strength.

The finite element analysis (FEA) provided quantitative evidence of the stress concentration at the weld toe. The analysis revealed that the maximum von Mises stress at the weld joint was significantly higher than the nominal stress in the pipe wall, confirming the role of stress concentration in initiating the cracking. The thrust force analysis further demonstrated that the pipe layout subjected the tee weld joint to significant bending moments, which were not adequately accommodated by the pipe support arrangement.

Integration with Engineering Practice

This case study provides a valuable lesson for power plant piping engineers regarding the importance of pipe layout optimization and weld joint integrity assessment. The failure analysis methodology — combining non-destructive testing, metallographic examination, stress analysis, and finite element modeling — represents a best practice approach for investigating weld joint failures in critical piping systems.

For piping design, the following recommendations emerge from this case:

The case also highlights the importance of considering the combined effects of thermal cycling, mechanical loading, and sustained creep in the design and assessment of high-temperature piping systems. The piping stress analysis should include both the thermal expansion loads and the mechanical loads, and the results should be evaluated against the material's allowable stress at the operating temperature, as specified in ASME B31.1 or B31.3.

The metallographic examination revealed microstructural aging, which is a time-dependent degradation mechanism that cannot be detected by non-destructive testing alone. This underscores the importance of periodic destructive testing of spare weld coupons or representative weld samples to assess the long-term degradation of the material properties. The results of such testing should be used to update the remaining life assessment of the piping system.

Key Questions and Reflections

The repeated cracking at the same weld joint raises questions about the effectiveness of the repair strategy. If the same welding procedure and material were used for the repair, the same failure mode would be expected to recur. The repair should have addressed the root cause — the excessive bending stress — rather than merely replacing the cracked weld. This may require modifying the pipe layout, adding supports, or installing stress relief devices.

The study does not address the role of hydrogen-induced cracking or stress corrosion cracking (SCC) in the failure mechanism. In high-temperature steam environments, SCC can initiate at the weld toe or HAZ, particularly if the material is susceptible to sulfide stress cracking or chloride stress corrosion cracking. The fracture surface analysis should be examined for evidence of SCC, such as intergranular cracking with secondary branching, to rule out this failure mode.

Furthermore, the effect of weld defects — such as porosity, lack of fusion, or slag inclusion — on the crack initiation and propagation has not been discussed. Even small weld defects can act as stress concentrators and initiate cracks under sustained creep loading. The non-destructive testing results should be carefully evaluated to determine whether any weld defects were present at the time of failure.

Study Insights and Implications

This failure analysis case study provides a comprehensive demonstration of the multi-disciplinary approach required for investigating weld joint failures in critical power plant piping systems. The combination of experimental characterization, numerical analysis, and engineering judgment is essential for identifying the root cause and implementing effective corrective measures.

For piping engineers, the key lessons are: first, pipe layout design must account for the combined effects of thermal expansion, mechanical loading, and sustained creep; second, weld joint integrity must be assessed over the entire service life, considering time-dependent degradation mechanisms; and third, failure analysis should be conducted systematically, combining multiple investigation methods to build a complete picture of the failure mechanism.

The case also underscores the importance of a proactive approach to piping system integrity management. Rather than waiting for failures to occur, engineers should implement regular inspection programs, remaining life assessments, and predictive maintenance strategies to identify and address potential failure modes before they result in unplanned shutdowns or safety incidents. This proactive approach, guided by the lessons learned from failure analysis, is essential for ensuring the safe and reliable operation of power plant piping systems.