Zhuojin Pipe Fitting Co., LtdCause Analysis of T91 Steel Pipe Leakage in Secondary Superheater
Literature Overview
This case study documents the failure investigation of a T91 (9Cr-1Mo-V-Nb) alloy steel pipe in the secondary superheater of a power boiler. T91 steel is widely used in ultra-supercritical (USC) power plants operating at steam temperatures above 600 °C due to its superior creep strength and oxidation resistance compared to traditional 9Cr-1Mo steels. However, T91 is also known for its susceptibility to creep rupture, stress corrosion cracking, and intergranular degradation under prolonged high-temperature service. The failure analysis provides critical lessons for condition monitoring, inspection protocols, and lifecycle management of T91 components in power generation applications.
Failure Mechanism Investigation
Operating Conditions and Service History
The failed pipe segment was located in the secondary superheater section, operating at a nominal steam temperature of 605 °C and pressure of 25.4 MPa. The component had accumulated approximately 45,000 hours of service, with frequent load cycling due to grid demand variations. The pipe specification was ASTM A335 P91 with a wall thickness of 14 mm and outer diameter of 48 mm.
| Parameter | Design Value | Actual Operating Value |
|---|---|---|
| Steam temperature (°C) | 600 | 605–615 (peak) |
| Steam pressure (MPa) | 25.4 | 25.4 |
| Wall thickness (mm) | 14 | 13.2 (measured at failure) |
| Service hours (h) | — | ~45,000 |
| Load cycling frequency | — | ~15 cycles/week |
Metallographic and Microstructural Examination
Cross-sectional metallographic analysis of the failed pipe revealed a multi-stage degradation pattern. The inner wall exhibited a distinct creep damage zone extending approximately 2 mm from the bore surface, characterized by:
- Intergranular void formation along prior austenite grain boundaries, with void density increasing from 5% near the inner surface to 25% at the outer edge of the creep zone
- Carbide coarsening and spheroidization of the originally dispersed M₂₃C₆ and MX-type carbides, indicating microstructural coarsening at the service temperature
- Oxide scale formation on the inner surface with a thickness of 0.15–0.20 mm, consisting primarily of Fe₃O₄ with minor Cr₂O₃ inclusions
The outer wall showed evidence of thermal fatigue cracking, with micro-cracks originating at the external oxide scale and propagating inward over a depth of 0.5–0.8 mm. This is attributed to the thermal cycling experienced during frequent load variations.
Fractography Analysis
Scanning electron microscopy (SEM) of the fracture surface revealed a mixed-mode failure pattern. The fracture initiated from the inner wall creep damage zone and propagated through a combination of:
- Creep rupture zone (inner): Characterized by intergranular void coalescence, leaving a characteristic "carpet-like" appearance with elongated voids aligned with the hoop stress direction
- Transitional zone: Mixed intergranular and transgranular fracture, indicating the transition from creep-dominated to overload-dominated failure
- Final overload zone (outer): Ductile dimple fracture with micro-void coalescence, indicating final separation under excessive stress
The fracture path was predominantly intergranular in the first 60% of the wall thickness, confirming that creep damage was the primary failure mechanism, with thermal fatigue and wall thinning acting as contributing factors.
Contributing Factors and Root Cause Analysis
Primary Cause: Creep Rupture
The dominant failure mechanism was creep rupture at the inner wall, driven by the combination of sustained high temperature, hoop stress from internal pressure, and axial stress from thermal expansion. At 605–615 °C, T91 steel operates in the upper range of its design creep life, where small deviations in temperature or stress can significantly accelerate creep damage accumulation. The Larson-Miller parameter (LMP) calculation indicates that the actual operating conditions correspond to an LMP of approximately 26,000–26,500, placing the component at the lower end of its expected creep life.
Contributing Factor 1: Wall Thinning
The measured wall thickness of 13.2 mm at the failure location, compared to the nominal 14 mm, represents a 5.7% reduction. This thinning is attributed to a combination of internal oxidation and possible erosion-corrosion from the steam flow. The wall thinning increased the hoop stress by approximately 6%, accelerating creep damage progression.
Contributing Factor 2: Thermal Fatigue
The frequent load cycling (approximately 15 cycles per week) imposed thermal stresses on the pipe wall. The temperature differential between the inner and outer wall surfaces during load ramps creates a thermal gradient that generates alternating stresses. Over 45,000 hours of service with thousands of thermal cycles, this contributed to the formation of external thermal fatigue cracks that reduced the effective load-bearing cross-section.
Contributing Factor 3: Microstructural Degradation
The coarsening of carbide precipitates and intergranular void formation represent irreversible microstructural changes that reduce the creep strength of T91 steel over time. The original fine dispersion of MX-type (Nb, V)C carbides, which provide solid solution strengthening and inhibit grain boundary sliding, becomes less effective as these particles coarsen and coalesce.
Engineering Recommendations and Prevention Measures
Inspection Protocol Enhancement
Based on this failure analysis, the following inspection enhancements are recommended for T91 superheater tubes:
- Ultrasonic wall thickness measurement at every support location and at 300 mm intervals, with a minimum remaining thickness criterion of 90% of nominal
- Eddy current testing for detection of internal creep damage, using dual-frequency techniques to differentiate between surface and subsurface indications
- Radiographic testing of weld joints every 10,000 hours to detect internal creep cavitation
- In-situ strain measurement at critical locations using vibrating wire or optical fiber sensors to monitor creep strain accumulation
Operational Controls
| Control Measure | Specification | Rationale |
|---|---|---|
| Maximum steam temperature | ≤ 600 °C | Reduce creep damage rate by ~40% compared to 615 °C |
| Load ramp rate | ≤ 2 °C/min | Minimize thermal fatigue stress |
| Minimum load | ≥ 50% MCR | Avoid low-load operation that promotes creep |
| Inspection interval | Every 10,000 hours | Detect degradation before critical stage |
Material and Design Considerations
For future replacements or upgrades, engineers should consider:
- Improved T91 variants with higher niobium and vanadium content (e.g., T92/9Cr-0.5Mo-1.5Cr-0.4Nb-0.15V) that offer superior creep strength at 600–620 °C
- Alternative materials such as Sanicro 25 or modified 9Cr-1Mo steels for sections experiencing the most severe conditions
- Design modifications to reduce thermal stress concentrations at supports and bends by incorporating flexible expansion joints or repositioning support locations
Study Insights and Reflections
This failure case underscores the critical importance of lifecycle management for advanced alloy components in power generation. T91 steel, despite its excellent creep properties, is not immune to degradation when operated at the upper limits of its design envelope. The combination of creep, thermal fatigue, and oxidation creates a synergistic degradation mechanism that is more severe than any single mechanism acting alone. Engineers must adopt a holistic approach to component integrity management, integrating material science knowledge, operational data, and advanced inspection techniques.
The case also highlights the importance of understanding the "why" behind failure mechanisms rather than simply applying corrective actions. Simply replacing the failed tube without addressing the root causes—temperature excursions, aggressive cycling, and inadequate inspection—would inevitably lead to repeat failures. A systematic approach combining root cause analysis, preventive maintenance, and continuous monitoring is essential for maximizing the service life and reliability of T91 components in USC power plants.