Zhuojin Pipe Fitting Co., LtdWeld Crack Formation in 12Cr1MoVG Steel Tee Joints: Creep Damage Mechanism
Literature Overview
This paper by Yao Yufeng, Yang Junsheng, and Ding Ao, published in Physical Testing and Chemical Analysis (Physics Section) in 2026 (Vol. 62, No. 6, pp. 61-64), reports on the failure analysis of multiple transverse cracks found on superheater tee welds during a power plant unit overhaul. The authors employed macroscopic observation, chemical composition analysis, hardness testing, metallographic examination, and scanning electron microscopy (SEM) to investigate the cracking mechanism. The study focuses on 12Cr1MoVG steel, a widely used creep-resistant alloy steel in high-temperature power plant applications, and provides critical insights into the long-term degradation mechanisms affecting welded joints in superheater service.
Core Findings and Creep Damage Mechanism
The investigation revealed that the cracks and their immediate vicinity contained a large number of creep voids and creep micro-cracks. At locations farther from the cracks, only a small number of creep voids and micro-cracks were locally observable. The microstructure at the rupture surface and crack regions showed severe aging, indicating prolonged exposure to high-temperature service. The mechanism of failure was identified as follows: under the combined action of internal pressure-induced hoop stress and welding residual stress, creep voids and micro-cracks formed over the long service period. These micro-cracks progressively extended and coalesced, ultimately leading to high-temperature creep cracking of the weld.
The creep damage progression can be described in three stages:
- Void nucleation: Creep voids nucleated at grain boundaries and second-phase particle interfaces within the weld metal and heat-affected zone (HAZ) under sustained high-temperature stress.
- Void growth and coalescence: The voids grew under the applied stress and eventually linked up to form micro-cracks.
- Micro-crack propagation: The micro-cracks extended and connected with adjacent cracks, forming a continuous crack path that led to final fracture.
Technical Analysis of Creep Damage
The following table summarizes the key metallurgical and mechanical observations from the study:
| Observation Area | Microstructural Features | Implication |
|---|---|---|
| Crack surface and vicinity | Abundant creep voids and creep micro-cracks | Active creep damage zone |
| Regions away from cracks | Sparse creep voids and micro-cracks | Early-stage creep damage |
| Rupture surface | Severely aged microstructure | Prolonged high-temperature exposure |
| Weld metal and HAZ | Creep voids at grain boundaries | Stress-assisted damage mechanism |
The role of welding residual stress in this failure is particularly noteworthy. In high-temperature creep service, the residual stress from welding does not relax completely and acts as an additional stress component on top of the operational stresses. The combination of hoop stress from internal pressure and the residual tensile stress from welding creates a stress state that accelerates creep damage initiation and propagation. This is consistent with the well-established understanding that residual stresses can reduce the creep life of welded components by 20-50% compared to stress-relieved counterparts.
Standards and Design Considerations
The failure of 12Cr1MoVG steel tee welds under creep conditions has significant implications for design, fabrication, and maintenance practices in power plant engineering. The following standards and practices are relevant:
| Standard / Practice | Applicable Requirement |
|---|---|
| ASME B31.1 / GB/T 20801 | Creep-rupture design stress selection for high-temperature service |
| ASME B31.1 / GB/T 20801 | Post-weld heat treatment (PWHT) to reduce residual stress |
| NB/T 47013 | Creep damage assessment by UT or RT during in-service inspection |
| API 579 / ASME FFS-1 | Fitness-for-service assessment of creep-damaged welds |
| GB/T 12459 / ASME B16.9 | Fitting fabrication quality requirements |
The study reinforces the importance of proper post-weld heat treatment for 12Cr1MoVG steel components. For this material, the recommended PWHT temperature range is typically 760-790 degrees Celsius, with a minimum holding time of 2 hours per 25 mm of thickness. The stress relief effectiveness is critical because residual stresses in the longitudinal direction of the weld can reach values of 200-300 MPa, which, when superimposed on the operational hoop stress, significantly accelerates creep damage.
Engineering Practice and Maintenance Implications
Based on the findings of this study, the following engineering measures are recommended for preventing creep cracking in 12Cr1MoVG steel tee welds:
- All 12Cr1MoVG steel tee welds in superheater service must undergo post-weld heat treatment immediately after fabrication, before installation.
- In-service inspection programs should include periodic ultrasonic testing or radiographic testing of tee welds to detect creep voids and micro-cracks before they become critical.
- The remaining life of tee welds should be assessed using creep damage assessment methods, taking into account the actual operating temperature, pressure, and stress state.
- When tee welds show evidence of creep damage during inspection, they should be replaced rather than repaired, because the surrounding base metal and HAZ are also likely to have experienced significant creep damage.
- The welding procedure for 12Cr1MoVG steel tees should be carefully controlled to minimize the heat-affected zone width and avoid excessive grain growth in the weld metal.
Key Questions and Reflections
This failure case raises an important question about the adequacy of current inspection intervals for creep-damaged components. In many power plants, the inspection intervals for superheater tee welds are determined based on the design life of the component, but the actual creep damage progression can vary significantly depending on the operating conditions, material quality, and fabrication practices. A risk-based inspection approach, which considers the actual stress state, material condition, and operating history, would be more appropriate for determining inspection intervals.
Another reflection is the challenge of detecting early-stage creep damage. Creep voids at the initial stage are typically below the detection threshold of conventional non-destructive testing methods. Advanced techniques such as phased array ultrasonic testing (PAUT) or total penetration ultrasonic testing (TOFD) may be more effective for detecting small creep voids, but their application in the field is limited by access constraints and the need for skilled operators.
Study Insights and Implications
This literature provides a clear demonstration of how the combination of welding residual stress and operational stress can accelerate creep damage in high-temperature alloy steel welds. The fundamental lesson is that residual stress management is not merely a fabrication quality concern but a critical factor in the long-term integrity of high-temperature welded components. For engineers involved in power plant maintenance and asset management, this case reinforces the importance of:
- Rigorous post-weld heat treatment to minimize residual stresses.
- Regular in-service inspection of critical tee welds using appropriate NDT methods.
- Fitness-for-service assessment when creep damage is detected.
- Proactive replacement of creep-damaged components before they reach a critical condition.
The paper is a valuable reference for power plant engineers, inspectors, and failure analysis practitioners. It demonstrates that creep damage in welded joints is a progressive and predictable phenomenon, and that a combination of proper fabrication practices, regular inspection, and timely replacement can effectively manage the risk of creep failure in high-temperature service.