Nickel-Based Alloy Surfacing of MSR Tube Sheets

Literature Overview

The paper by Li Zhijie et al. (2000), published in Welding, addresses the surfacing of nickel-based alloys on tube sheets for Moisture Separator Reheater (MSR) units in power plant boilers. MSR tube sheets are critical components in steam turbine cycles that operate under extreme conditions of high temperature, high pressure, and aggressive chemical environments. The surfacing of nickel-based alloys on these tube sheets is essential for ensuring long-term service life and resistance to corrosion and erosion.

Core Technical Approach

The MSR (Moisture Separator Reheater) is a component in coal-fired power plants that separates moisture from wet steam and reheats it before it enters the turbine. The tube sheets in MSR units are subjected to:

• High-temperature steam (typically 350 to 450 degrees Celsius)
• High pressure (typically 10 to 15 MPa)
• Corrosive environments due to the presence of moisture and chemical impurities
• Thermal cycling during startup and shutdown
• Erosion from high-velocity steam flow

The surfacing of nickel-based alloys on the tube sheet surfaces provides a barrier against these aggressive conditions. Nickel-based alloys such as Inconel 625, Hastelloy C-276, or similar compositions offer excellent resistance to corrosion, oxidation, and erosion at elevated temperatures.

Key Technical Considerations

Metallurgical Challenges

The surfacing of nickel-based alloys on steel substrates presents several metallurgical challenges:

1. Thermal expansion mismatch: Nickel-based alloys have different thermal expansion coefficients compared to steel, which can lead to thermal stresses during heating and cooling cycles.
2. Intermetallic compound formation: At the interface between the steel substrate and the nickel-based deposit, brittle intermetallic compounds can form, compromising the bond strength and fatigue resistance.
3. Dilution control: Excessive dilution of the nickel-based alloy with the steel substrate reduces the corrosion resistance of the surfacing layer. Insufficient dilution may lead to poor bonding.
4. Crack susceptibility: Nickel-based alloys are susceptible to hot cracking during welding due to their solidification characteristics and the formation of low-melting-point phases.

Process Selection and Parameters

The selection of surfacing process is critical for achieving the required quality on MSR tube sheets:

• SAW (Submerged Arc Welding): Suitable for thick deposits with high deposition rates. Requires careful flux selection to minimize dilution and prevent contamination.
• GTAW (Tungsten Inert Gas Welding): Provides excellent control over heat input and dilution. Suitable for thin, high-quality deposits but with lower deposition rates.
• PAW (Plasma Arc Welding): Offers a compromise between deposition rate and quality control. Suitable for medium-thickness deposits with good surface finish.
• Oxy-fuel surfacing: Sometimes used for pre-deposition of a transition layer to reduce dilution in subsequent arc surfacing passes.

Typical process parameters for nickel-based alloy surfacing include:

• Preheating temperature: 150 to 250 degrees Celsius (to reduce residual stress and prevent cracking)
• Interpass temperature: Maintained below 250 degrees Celsius
• Post-weld heat treatment: Stress relief at 550 to 650 degrees Celsius for 2 to 4 hours
• Welding current: Optimized to minimize dilution while ensuring complete fusion

Quality Control and Inspection

For MSR tube sheet surfacing, the quality control requirements are stringent due to the critical nature of the application:

• Dimensional inspection: Verification of tube hole alignment and surfacing thickness uniformity
• Visual inspection: Checking for surface defects, undercut, and incomplete coverage
• Penetrant testing (PT): Detection of surface cracks and lack of fusion
• Ultrasonic testing (UT): Detection of subsurface defects and measurement of deposit thickness
• Hardness testing: Verification of deposit hardness and gradient from deposit to substrate
• Metallographic examination: Verification of microstructure, dilution, and interface quality
• Corrosion testing: Verification of corrosion resistance in simulated service conditions
• Hydrostatic testing: Pressure testing to verify structural integrity of the tube sheet

Engineering Practice Implications

The surfacing of nickel-based alloys on MSR tube sheets is a well-established practice in the power generation industry. Key lessons from engineering practice include:

1. Transition layer strategy: For thick deposits, a multi-layer approach with a transition layer (such as a nickel-iron alloy) between the steel substrate and the final nickel-based layer can reduce dilution and improve corrosion resistance.
2. Welding sequence optimization: The welding sequence should be designed to minimize residual stress and distortion. Symmetrical welding patterns and controlled heat input are essential for maintaining tube hole alignment.
3. Post-weld heat treatment: Stress relief heat treatment is essential to reduce residual stresses that can lead to cracking during service. The heat treatment temperature and duration must be carefully controlled to avoid sensitization of the nickel-based alloy.
4. Regular inspection and maintenance: Even with high-quality surfacing, periodic inspection of the tube sheet surfaces is necessary to detect any degradation of the surfacing layer during service.

Study Reflections

This paper, though brief, addresses a critical application in the power generation industry. The surfacing of nickel-based alloys on MSR tube sheets is a mature technology, but the challenges of maintaining quality and reliability over decades of service remain significant.

The key insight from this work is that the selection of surfacing alloy and process must be guided by the specific service conditions of the MSR unit. Different alloys offer different combinations of corrosion resistance, erosion resistance, and mechanical properties, and the optimal choice depends on the specific operating parameters and chemical environment.

From a standards perspective, the surfacing of nickel-based alloys on tube sheets should comply with relevant codes and standards such as ASME Section IX (for welding procedure qualification), ASME B31.3 (for process piping), and applicable manufacturer specifications. The welding procedure qualification should include mechanical property testing, metallographic examination, and corrosion testing to verify that the surfacing process produces deposits with the required properties.