Zhuojin Pipe Fitting Co., LtdZhanjiang Power Plant Turbine Casing Splitting Line Overlay Repair Welding
Literature Overview
This paper, published in Hot Working Technology (2006, Vol. 35, No. 3, pp. 27-28) by Jin Yongqiang from Guangdong Zhanjiang Power Co., Ltd., documents the field repair of the high-pressure and medium-pressure outer casing splitting line deformation defect on the 1# turbine unit at Zhanjiang Power Plant during the 1999 major overhaul. The repair employed an overlay sealing strip method using manual TIG welding (GTAW) with nickel-based consumable wire, incorporating pre-weld low-temperature preheating and post-weld weld hammering as key process elements. This case study provides valuable practical insights into field repair welding of large turbine components under real operational constraints.
Repair Challenge and Process Strategy Development
The deformation defect at the turbine casing splitting line presents a complex engineering challenge that requires balancing multiple competing requirements: weldability of the dissimilar metal interface, residual stress management, distortion control, and field accessibility constraints. The following table summarizes the key process elements and their technical rationale:
| Process Element | Specification | Technical Rationale |
|---|---|---|
| Welding Process | Manual TIG (GTAW) | Precise heat input control, low dilution, suitable for thin overlay |
| Consumable Wire | Nickel-based alloy | Thermal expansion compatibility, good weldability, corrosion resistance |
| Preheating | Low-temperature preheating | Reduce thermal gradient, minimize residual stress, prevent cracking |
| Post-weld Treatment | Weld hammering | Relax residual stress, improve weld ductility, reduce distortion |
| Repair Method | Overlay sealing strip | Restore sealing integrity, compensate for deformation gap |
The selection of nickel-based consumable wire is particularly significant from a metallurgical perspective. Nickel alloys exhibit thermal expansion coefficients closer to that of turbine casing materials than iron-based alloys, reducing the risk of thermal mismatch cracking during subsequent operational thermal cycling. Additionally, nickel-based welds demonstrate excellent resistance to oxidation and corrosion at elevated temperatures, which is critical for the long-term integrity of the turbine casing sealing interface.
Welding Process Implementation and Quality Control
The cold overlay welding process, as described in this case study, represents a practical approach to field repair welding where extensive preheating facilities are unavailable. The term "cold welding" in this context refers to the minimal preheating approach, which is feasible when using nickel-based consumables that have excellent weldability characteristics and low susceptibility to hydrogen-induced cracking.
The weld hammering technique applied after welding serves multiple purposes: it plastically deforms the weld metal to relieve compressive residual stresses that would otherwise cause distortion, it refines the weld grain structure through dynamic recrystallization, and it improves the fatigue resistance of the weld by introducing beneficial compressive surface stresses. The hammering must be performed while the weld is still at elevated temperature, typically between 300-500°C, to be effective without causing cold cracking.
For turbine casing repair applications, the following quality control measures are essential:
- Visual inspection of the weld surface for porosity, undercuts, and incomplete fusion
- Penetrant testing (PT) or magnetic particle testing (MT) for surface and near-surface defect detection
- Dimensional verification of the repaired area to ensure proper sealing gap closure
- Leak testing to confirm the integrity of the repaired sealing interface
The successful implementation of this repair process, as documented in the paper, demonstrates that careful process planning and execution can achieve reliable results even under challenging field conditions with limited equipment and facilities.
Engineering Practice Lessons and Risk Assessment
This case study provides several valuable lessons for engineering practice in power plant maintenance welding. The first lesson is the importance of comprehensive process planning that addresses not only the welding operation itself but also the entire repair sequence, including preheating, welding, post-weld treatment, and quality verification. The second lesson is the value of selecting consumable materials that offer metallurgical compatibility with the base material, even when the base material is not explicitly specified for that application.
From a risk assessment perspective, the key risks in turbine casing repair welding include:
| Risk Category | Specific Risk | Mitigation Measure |
|---|---|---|
| Weld cracking | Cold cracking due to hydrogen | Low-temperature preheating, nickel-based consumables |
| Distortion | Excessive deformation affecting fit-up | Weld hammering, controlled heat input |
| Incomplete repair | Residual deformation after repair | Overlay sealing strip method |
| Service failure | Weld degradation during operation | Quality verification, leak testing |
The use of the overlay sealing strip method, rather than direct welding of the deformed joint, represents a conservative and reliable approach that avoids the need to precisely match the deformed geometry with weld metal deposition. This method effectively creates a new sealing interface that compensates for the deformation without requiring complex fit-up procedures.
Study Insights and Practical Implications
This field repair case study highlights the importance of practical engineering judgment in welding process selection. The decision to use manual TIG welding with nickel-based wire, rather than automated processes or iron-based consumables, reflects a careful consideration of field constraints, metallurgical requirements, and quality objectives. This type of practical knowledge, gained through real-world experience, is often as valuable as academic research for guiding field repair decisions.
The success of this repair in 1999, with the unit subsequently returning to service, validates the process approach and provides confidence for similar repairs on other turbine units. However, the paper's limited scope—focusing primarily on process description rather than detailed metallurgical analysis or long-term performance tracking—suggests opportunities for future research into the microstructural evolution and degradation mechanisms of nickel-based overlay welds under prolonged turbine operating conditions.
Summary and Outlook
The Zhanjiang Power Plant turbine casing repair case study demonstrates a successful field repair approach using manual TIG welding with nickel-based consumables, low-temperature preheating, and weld hammering to address splitting line deformation defects. The overlay sealing strip method provides a practical solution that avoids complex fit-up procedures while restoring sealing integrity. This case study underscores the importance of careful process planning, appropriate consumable selection, and thorough quality verification in field repair welding of critical power generation components. Future improvements could include detailed metallurgical characterization of the repair welds, long-term performance monitoring, and development of standardized repair procedures for similar turbine casing defects.