CLADDING TECHNOLOGY SHANXI CO., LTDCladding Repair of T91 Steel for Ultra-Supercritical Turbine Rotors
Literature Overview
This study addresses the challenges of cladding repair on T91 steel components used in ultra-supercritical (USC) steam turbine rotors. T91 steel, a martensitic 9 percent chromium-1 percent molybdenum vanadium steel, is widely used for high-temperature components in USC power plants operating at steam temperatures exceeding 600 degrees Celsius. The extreme operating conditions, combined with thermal cycling and mechanical loading, lead to degradation of the material properties, necessitating periodic inspection and repair. Cladding repair is a critical technique for restoring the dimensional integrity and surface quality of T91 components while maintaining the mechanical and metallurgical properties required for continued service.
Core Technical Points
Material Characteristics and Repair Challenges
T91 steel is characterized by a tempered martensitic microstructure stabilized by fine carbide precipitates of M23C6, MX (MC), and Laves phase. The material exhibits excellent high-temperature strength, creep resistance, and oxidation resistance, but is susceptible to temper embrittlement, creep cavitation, and intergranular cracking under prolonged high-temperature service. The repair of T91 components requires careful attention to these degradation mechanisms, as the repair process can either exacerbate or mitigate them depending on the process parameters and post-weld heat treatment.
| Property | T91 Steel (Base) | Repair Overlay | Acceptance Criteria |
|---|---|---|---|
| Yield strength at 20°C | 415–550 MPa | 400–500 MPa | Within 10% of base |
| Yield strength at 600°C | 200–280 MPa | 190–260 MPa | Within 10% of base |
| Elongation at 20°C | 12–18% | 10–16% | Minimum 10% |
| Hardness | 220–280 HV | 210–270 HV | Within 15% of base |
| Chromium content | 8.5–9.5% | 8.0–10.0% | Within specification |
| Nickel content | 0.2–0.5% | 0.1–0.6% | Within specification |
Repair Process Selection
The selection of the repair process is critical for maintaining the mechanical properties of T91 steel. The study evaluates several processes including gas tungsten arc welding (GTAW), gas metal arc welding (GMAW), plasma transferred arc (PTA) welding, and laser cladding. Each process has distinct advantages and limitations for T91 repair applications.
GTAW is the most commonly used process for T91 repair due to its precise heat input control and excellent weld quality. The process produces narrow weld beads with minimal heat-affected zone (HAZ), which is essential for maintaining the mechanical properties of the base material. However, GTAW has a relatively low deposition rate, making it less suitable for large-area repairs.
GMAW offers higher deposition rates than GTAW but produces wider weld beads with greater heat input. The higher heat input can lead to coarsening of the martensitic microstructure in the HAZ, reducing the high-temperature strength of the repair area. GMAW is acceptable for surface repairs where the depth of the repair is less than 3 mm, but is not recommended for deep repairs or critical structural components.
PTA welding provides a good balance between deposition rate and heat input control, making it suitable for medium-sized repairs on T91 components. The process produces overlays with fine microstructures and low dilution, but requires careful control of the powder feed rate and travel speed to maintain consistent overlay quality.
Laser cladding offers the lowest heat input and dilution of all the evaluated processes, producing overlays with microstructures closely resembling the base material. However, the limited interaction depth of the laser beam restricts the repair depth to less than 2 mm, and the equipment cost is significantly higher than conventional welding methods.
Post-Weld Heat Treatment
The post-weld heat treatment (PWHT) is the most critical step in T91 repair, as it determines the final mechanical properties and long-term performance of the repair area. The standard PWHT for T91 steel consists of tempering at 740 to 760 degrees Celsius for 2 to 4 hours, followed by furnace cooling to below 400 degrees Celsius to avoid the temper embrittlement range (370 to 570 degrees Celsius). The PWHT must be performed within 4 hours of completing the last weld pass to prevent the formation of untempered martensite in the HAZ.
The study emphasizes that the PWHT temperature and duration must be carefully controlled to achieve the desired microstructure without promoting excessive grain growth or carbide coarsening. Temper embrittlement is a particular concern for T91 steel, as the material is susceptible to embrittlement when held in the range of 370 to 570 degrees Celsius for extended periods. The cooling rate through this range must be controlled to minimize the risk of temper embrittlement, typically by furnace cooling at a rate not exceeding 5 degrees Celsius per hour.
Inspection and Quality Control
The inspection of T91 repairs requires a comprehensive approach that includes visual inspection, magnetic particle testing (MT), ultrasonic testing (UT), and radiographic testing (RT). MT is applied to detect surface and near-surface cracks, particularly at the weld edges and in the HAZ. UT is used to assess the bond quality and detect internal defects such as lack of fusion and porosity. RT provides a comprehensive assessment of the internal quality of the repair, detecting defects that may not be accessible to UT.
| Inspection Method | Coverage | Defect Detection | Acceptance Criteria |
|---|---|---|---|
| Visual inspection | 100% | Surface defects, undercut, excess weld | No visible defects |
| MT | 100% | Surface/near-surface cracks | No indications exceeding 0.5 mm |
| UT | 100% | Internal defects, lack of fusion | No indications exceeding 1.5 mm FBH |
| RT | 100% | Internal defects, porosity, inclusions | No indications exceeding ASME Section V limits |
| Hardness test | 100% | HAZ softening/hardening | Within 15% of base material |
| Impact test | Sample | Ductility verification | Minimum 27 J at 20°C |
Engineering Practice Considerations
Filler Material Selection
The selection of filler material for T91 repair is critical, as the filler composition must match the base material to ensure compatibility of the weld metal and HAZ microstructures. The study recommends using filler materials with compositions closely matching T91 steel, such as ER911 or equivalent grades. The filler material should contain appropriate amounts of vanadium and niobium to promote the formation of MX carbides, which contribute to the high-temperature strength of the repair area.
The carbon content of the filler material must be carefully controlled, as excessive carbon can promote the formation of M23C6 carbides at grain boundaries, reducing the creep resistance of the repair area. The study recommends a maximum carbon content of 0.10 percent in the filler material, with a preferred range of 0.06 to 0.08 percent.
Repair Procedure Development
The development of a repair procedure for T91 steel requires careful consideration of the heat input, interpass temperature, and welding sequence. The heat input should be limited to 1.5 to 3.0 kJ/mm to minimize the HAZ width and prevent excessive grain growth. The interpass temperature should be maintained between 150 and 300 degrees Celsius to prevent the formation of untempered martensite while avoiding excessive cooling rates that can promote cracking.
The welding sequence should be designed to minimize the residual stresses and distortion of the repaired component. For large repairs, a multi-pass approach with alternating weld directions is recommended to distribute the heat input evenly and reduce the risk of cracking. The final pass should be deposited with the lowest possible heat input to minimize the HAZ width and ensure a smooth transition from the repair to the base material.
Study Insights and Reflections
The study underscores the complexity of T91 steel repair in ultra-supercritical turbine applications. The material's exceptional high-temperature properties are achieved through a carefully balanced microstructure of tempered martensite with fine carbide precipitates, and any repair process must preserve this microstructure to ensure the long-term performance of the component. The post-weld heat treatment is the most critical factor in achieving acceptable repair quality, and the PWHT parameters must be carefully optimized for each specific repair application.
The study also highlights the importance of process qualification and operator training for T91 repair applications. The narrow process window for T91 welding, characterized by low heat input, controlled interpass temperatures, and precise PWHT, requires highly skilled operators and well-documented procedures. Any deviation from the qualified procedure can result in unacceptable degradation of the repair area, potentially compromising the safety and reliability of the turbine rotor.
From a practical standpoint, the study recommends a risk-based approach to T91 repair, where the severity of the damage, the location of the repair, and the remaining service life of the component are considered in determining the repair strategy. For critical components such as turbine rotors, the study advocates for conservative repair approaches that prioritize the preservation of the base material properties over the minimization of repair costs.
Summary
The cladding repair of T91 steel for ultra-supercritical turbine rotors is a technically demanding process that requires careful attention to material compatibility, process parameters, and post-weld heat treatment. The success of the repair depends on the ability to reproduce the tempered martensitic microstructure of the base material in the weld metal and HAZ, which requires precise control of the heat input, interpass temperature, and PWHT parameters. Engineers involved in T91 repair applications should adopt a risk-based approach that considers the severity of the damage, the location of the repair, and the remaining service life of the component. The study confirms that T91 repair is achievable and reliable when the qualified procedures are followed and the inspection criteria are met, but emphasizes that the process window is narrow and requires experienced operators and rigorous quality control.