Weld Overlay Repair of 600MW Steam Turbine Lower Outer Casing Bearing Seat

Introduction and Engineering Significance

Large steam turbines in power generation plants are critical assets with high replacement costs and significant operational impact when taken offline for repair. The 600MW steam turbine represents a major power generation unit, and its low-pressure outer casing is a large cast component subjected to thermal cycling, mechanical vibration, and occasional impact loading. The bearing seat area, where the rotor bearing housing is mounted, is particularly critical as any deformation or damage in this area can affect rotor alignment and bearing performance. This 2007 study by Liu Hua and Li Zhihong from Sichuan Engineering Vocational and Technical College addresses the weld overlay repair of the lower outer casing bearing seat of a 600MW steam turbine.

The repair of such large cast components presents unique challenges related to residual stress management, distortion control, and metallurgical compatibility between the overlay weld metal and the base casting material.

Base Material and Defect Characteristics

The low-pressure outer casing of a 600MW steam turbine is typically fabricated from low-carbon steel casting (such as ASTM A216 WCB or equivalent Chinese grades like ZG230-450). The bearing seat area may suffer from:

• Abrasive wear from vibration-induced micro-movement of the bearing housing
• Impact damage from improper assembly or disassembly procedures
• Thermal fatigue cracking from repeated start-stop cycles
• Corrosion pitting from moisture ingress during shutdown periods

The defect geometry and depth determine the repair approach. Shallow surface damage (less than 2 mm) can be repaired with simple weld overlay, while deeper defects may require material removal followed by multi-pass overlay rebuilding.

Weld Overlay Repair Procedure

The repair procedure for the bearing seat weld overlay must be carefully planned to minimize distortion and ensure metallurgical integrity:

The selection of overlay material depends on the specific requirements of the bearing seat application. If the bearing seat requires corrosion resistance (for example, in areas where moisture may accumulate), a stainless steel overlay such as 309L or 310L is appropriate. If the primary concern is wear resistance, a medium-carbon steel overlay with controlled hardness may be more suitable.

Distortion Control and Stress Management

One of the most critical aspects of repairing large cast components is the control of welding-induced distortion. The lower outer casing of a 600MW steam turbine is a large, thin-walled component that is highly susceptible to distortion from welding heat input. The following measures are essential:

1. Symmetrical welding sequence: Welds should be applied in a balanced pattern to minimize differential thermal expansion. For a circumferential bearing seat repair, the weld should be applied in short segments with a symmetrical pattern around the circumference.
2. Backing plate and拘束 (restraint): Temporary backing plates and mechanical restraints can be used to minimize local distortion. However, excessive restraint can lead to high residual stresses and cracking, so a balance must be struck.
3. Controlled heat input: Low heat input per pass (typically 1.0–1.5 kJ/mm for GTAW) minimizes the heat-affected zone size and reduces distortion.
4. Post-weld stress relief: A controlled PWHT cycle at 550–620°C is essential to relieve residual stresses that could otherwise lead to delayed cracking or dimensional instability during subsequent machining and service.

Quality Assurance and Inspection

The quality of the weld overlay repair on a turbine casing bearing seat is critical to the safety and reliability of the entire power generation unit. The following inspection regime is recommended:

• Visual inspection (VT): 100% inspection of all weld surfaces for surface defects
• Magnetic particle testing (MT): 100% inspection of weld surfaces and HAZ for surface-breaking defects
• Ultrasonic testing (UT): Spot check or 100% inspection of weld zones for subsurface defects, depending on the criticality assessment
• Hardness testing: Verify that the overlay hardness is within the specified range and that the HAZ has not been excessively hardened or softened
• Dimensional inspection: After machining, verify that the bearing seat geometry meets the original design tolerances (typically ±0.02 mm for flatness and ±0.05 mm for concentricity)

Study Insights

The repair of large turbine components through weld overlay is a challenging but well-established practice in the power generation industry. The 2007 study by Liu Hua and Li Zhihong contributes to the body of knowledge on practical repair techniques for critical rotating machinery. The emphasis on distortion control, metallurgical compatibility, and comprehensive quality assurance reflects the high stakes involved in power generation equipment repair. Modern practices have evolved to include advanced techniques such as robotic weld overlay with precise heat input control, real-time monitoring of welding parameters, and advanced NDT methods such as phased array ultrasonic testing (PAUT) for more sensitive defect detection. However, the fundamental principles described in this study—careful preheating, controlled heat input, appropriate material selection, and thorough post-weld inspection—remain the cornerstone of successful weld overlay repair for critical power generation components.