Increasing Lifespan and Reliability of Hot Section Turbine Components Through Technical Expertise and Materials Engineering
Introduction: Maintenance Challenges and Extending Gas Turbine Lifespan
Power plants—especially gas turbine units, which are at the core of energy production—play a vital role in providing stable and reliable energy. The reliability of these units directly impacts the stability of the electrical grid and helps prevent costly outages. Given the extremely harsh operating conditions, including very high temperatures (often above 900°C) and significant mechanical and thermal stresses, key hot section turbine components such as the Inner Casing gradually experience wear, microstructural changes, and degradation after thousands of operating hours. This wear can lead to reduced efficiency, increased risk of catastrophic failures, and unscheduled downtimes.
Paya Mavad Technology Company, with a profound understanding of these challenges and relying on advanced technical knowledge in metallurgy and surface engineering, has successfully completed a complex refurbishment and life-extension project of the Inner Casing in one of the units at Qaen Power Plant. This achievement has greatly contributed to the safe and economical continuous operation of the unit.
Challenge Description: Inspection of the Inner Casing at Qaen Power Plant
The Inner Casing in question, after long operating hours (over 100,000 equivalent operating hours), had sustained multiple concerning damages. Initial detailed inspections carried out by Paya Materials’ expert technical team revealed various dimensions of these damages:
- Multiple Cracks: Cracks were identified in sensitive, highly stressed areas such as the hub. These cracks, especially under cyclic operational conditions (start-up and shutdown cycles), had the potential to grow to critical sizes, threatening the structural integrity of the entire assembly.
- Corrosion and Wear: Observation of hot corrosion (caused by the combustion atmosphere and fuel impurities) and severe wear in various parts, particularly on heat shields that protect the main body and seal rings which play a crucial role in maintaining turbine efficiency. These phenomena led to thickness reduction, dimensional changes, and performance decline of these components.
- Surface Degradation and Deformation: Besides corrosion, overall surface degradation of the Inner Casing and deformation caused by long-term mechanical and thermal stresses (creep phenomenon) were evident. These deformations could cause misalignment of parts, increased clearances, and consequently increased vibrations and decreased efficiency.
- Microstructural Damage: Metallurgical examinations showed that the main microscopic degradation mechanisms included creep (permanent deformation under stress at high temperature), oxidation, hot corrosion, and thermal fatigue (resulting from heating and cooling cycles). These mechanisms significantly weakened the mechanical and metallurgical properties of the base alloy, namely Inconel 617 superalloy. Specifically, the continuous network-like precipitation of carbides along grain boundaries, while potentially increasing strength at low temperatures, drastically reduced alloy ductility and created easy paths for crack propagation, ultimately greatly increasing the risk of brittle and sudden failure.
The combination of these damages was a serious warning signal for the safe and economical continued operation of the turbine. It could lead to reduced efficiency, increased unwanted vibrations, and in the worst case, sudden and costly failure of the unit.
Paya mavad Solution: Reconstruction Based on Materials Science and Precise Engineering
Fully understanding the technical complexities and adopting a comprehensive, science-based approach, Paya Materials designed and successfully executed a precise, multi-stage refurbishment process for the Inner Casing:
Accurate Assessment and Damage Analysis:
- Technical Inspections: This stage included detailed visual inspections by trained experts, advanced non-destructive testing (NDT) such as penetrant testing (PT) to detect even very fine surface cracks invisible to the naked eye, precise ultrasonic thickness measurements at multiple critical points to assess material loss due to corrosion and wear, and complete dimensional control using precise measuring equipment to determine the extent of deformation and distortion relative to the original design dimensions.
- Metallurgical Analysis: The core of the diagnostic process was microstructural examination (metallography). By sampling specific areas and preparing samples meticulously, the microstructure of the Inconel 617 alloy after prolonged service was examined under optical and electron microscopes. These examinations enabled accurate identification of the existing phases (such as carbides), their morphology (shape and size), and distribution (especially along grain boundaries), providing deep insight into the active degradation mechanisms in the component.
- Mechanical Testing: To quantitatively evaluate the residual mechanical properties and the effects of refurbishment processes, key mechanical tests were performed. Room temperature tensile tests provided important data on yield strength, ultimate strength, and ductility (% elongation). More importantly, stress-rupture tests at elevated temperatures (simulating turbine operating conditions) were conducted to carefully assess the creep behavior and rupture resistance of the component under constant load at high temperature, both before and after heat treatment.
Rehabilitation and Rejuvenation Process:
- Specialized Heat Treatment:Based on the precise results of metallurgical and mechanical analyses, multi-stage heat treatment cycles—including solution annealing and aging—were meticulously designed and executed in advanced furnaces with controlled atmospheres. The purpose of solution annealing was to dissolve coarse and continuous carbide precipitates along grain boundaries in the austenitic matrix, which significantly increased ductility and reduced the risk of brittle fracture. During the aging stage, with careful control of temperature and time, beneficial carbide precipitates were re-formed as fine, dispersed particles along grain boundaries and partially within grains to improve creep strength and resistance to grain boundary sliding at elevated temperatures without severely compromising ductility. Precise control of heating and cooling rates during these processes was essential to achieve the desired microstructure.
- Advanced Repair and Welding:Detected cracks and areas with severe thickness reduction or other damages were repaired using specialized welding techniques suitable for nickel-based superalloys such as TIG/GTAW welding (which offers high process control). Consumable materials (filler metals) with chemical compositions exactly matching or compatible with the base alloy (Inconel 617) were employed. All welding and repair operations were performed under strict supervision and according to approved procedures (WPS) and international standards for repair and refurbishment.
- Continuous Quality Control:At all stages of the refurbishment process—including after each welding and heat treatment step—non-destructive tests (especially penetrant testing on welded zones) and dimensional inspections were repeated to ensure that repair quality met the required standards and that all component dimensions remained within the allowable design tolerances. These quality controls guaranteed full compliance with technical requirements and quality standards.
Results and Achievements: Return of the Inner Casing to Operational Cycle
The precise and scientific execution of the refurbishment process by the Paya Mavad team led to outstanding results and the complete success of the Inner Casing refurbishment project at the Qaen power plant:
- Complete Defect Resolution:All identified cracks and areas damaged by corrosion and wear were successfully repaired and refurbished in full compliance with the highest standards, restoring the component to a highly satisfactory structural condition.
- Improved Mechanical Properties:The results of mechanical tests performed after the completion of the refurbishment and final heat treatment showed a significant improvement in key mechanical properties. Notably, there was a remarkable increase in ductility (%Elongation) along with the retention or even enhancement of strength (Yield Strength, Ultimate Tensile Strength). This clearly indicates the success of the metallurgical rejuvenation process and the recovery of the alloy’s desirable properties. This enhancement improves the component’s resistance to degradation mechanisms.
- Extended Remaining Life:One of the most important achievements of this project was the significant extension of the component’s remaining service life. Based on precise technical assessments and calculations derived from accelerated failure testing and using reputable life estimation models (such as Robinson’s life fraction rule), the remaining life of the refurbished inner casing under normal operating conditions and in accordance with guidelines was estimated to be about 3 years (approximately 24,000 working hours). This accomplishment enabled the safe and economical reuse of this critical and high-value component for another substantial operational period.
- Quality Assurance:All final inspection stages and quality control tests confirmed the very high quality of the refurbishment process and full compliance of the final component with all technical requirements, design drawings, and relevant industry standards.
Conclusion: Paya Mavad Expertise in Service of the Power Industry
The successful and scientific refurbishment of the Qaen power plant inner casing stands as a prominent example of the capabilities and deep expertise of Paya Mavad Technology Company in key areas such as surface engineering, advanced materials science, detailed failure mechanism analysis, and the development of innovative engineering solutions for extending service life, reliability, and safety of sensitive and strategic power plant equipment.
This project not only resulted in significant cost savings for the client by avoiding the need to purchase a new imported component, but also contributed greatly to the reliability and continuity of sustainable electricity production at the Qaen power plant by returning this vital component to the operational cycle.
Relying on cutting-edge knowledge, extensive experience, and a commitment to quality, Paya Mavad remains ever-ready to offer specialized refurbishment and optimization services for hot turbine components and other industrial equipment at the highest quality and in accordance with global standards.
