T91 Steel Pipe Leak Cause Analysis in Secondary Superheater

Incident Description and Background

The literature documents a failure investigation of a T91 alloy steel tube in the secondary superheater section of a power plant boiler. T91, a 9 percent chromium-1 percent molybdenum martensitic heat-resistant alloy, is widely used in ultra-supercritical (USC) boilers due to its excellent high-temperature creep strength and oxidation resistance. However, field experience has shown that T91 tubes are susceptible to several degradation mechanisms that can lead to premature failure.

The secondary superheater operates at steam temperatures exceeding 566 degrees Celsius and pressures above 25 MPa, creating a demanding environment for the tube material. The failure occurred after approximately 30,000 hours of service, which is well below the expected design life of 200,000 hours for this component.

Failure Mechanism Analysis

The investigation employed a combination of macroscopic examination, metallographic analysis, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and hardness mapping. The analysis revealed that the primary failure mechanism was a combination of creep damage and oxidation-assisted cracking.

The microstructure of the failed tube exhibited significant intergranular creep cavitation in the heat-affected zone (HAZ) of a previous weld repair. The creep cavities were concentrated along prior austenite grain boundaries and were aligned with the circumferential stress direction. EDS analysis of the cavities revealed the presence of oxide phases containing iron, chromium, and oxygen, indicating that oxidation had accelerated the cavitation process.

The hardness mapping revealed that the HAZ hardness was significantly higher than the base metal, indicating over-aging of the martensitic microstructure. In T91 steel, the desired microstructure is a tempered martensite with finely dispersed MX carbides (vanadium and niobium carbides). Over-aging causes the coarsening of these fine carbides, which reduces the precipitation hardening effect and accelerates creep damage accumulation.

Contributing Factors and Countermeasures

The root cause analysis identified three contributing factors: (1) excessive weld heat input during a previous repair weld, which caused over-aging of the HAZ; (2) inadequate post-weld heat treatment (PWHT) temperature control, resulting in incomplete recovery of the tempered martensite; and (3) localized overheating due to poor water-side heat transfer, possibly caused by deposit buildup on the tube inner surface.

The literature recommends the following countermeasures: strict control of weld heat input to below 15 kJ/mm for T91 repairs, PWHT at 760 degrees Celsius with a holding time of 2 hours per 25 mm of thickness, and regular inspection of the HAZ region using eddy current testing or ultrasonic testing. Additionally, monitoring of the tube inner surface condition through endoscopic inspection can detect deposit buildup before it leads to overheating.

The study also emphasizes the importance of material traceability and the need to verify the chemical composition of T91 tubes, particularly the nitrogen content (typically 0.03 to 0.10 percent) and vanadium content (0.15 to 0.30 percent), which are critical for maintaining creep strength at elevated temperatures.

In conclusion, the T91 superheater tube failure was caused by oxidation-assisted creep damage in an over-aged weld HAZ, triggered by excessive weld heat input and inadequate PWHT. This case underscores the critical importance of strict welding procedure qualification, post-weld heat treatment control, and periodic in-service inspection for T91 components operating in ultra-supercritical service conditions.