Nickel-Based Alloy Overlay Welding on MSR Tube Sheets

Literature Overview

The technical paper published in Welding (2000, No. 6, pp. 36-37) by Li Zhijie and colleagues from Harbin Boiler Works Limited addresses the challenge of nickel-based alloy overlay welding on tube sheets for Moisture Separation and Reheating (MSR) systems in power generation boilers. MSR tube sheets are critical components in ultra-supercritical (USC) boilers where they must withstand extreme thermal cycling, high-pressure steam, and potential corrosion from moisture carryover. The overlay welding of nickel-based alloys provides enhanced corrosion and erosion resistance at critical interfaces where tube-to-tube-sheet joints are subjected to severe service conditions.

Technical Background and Application Context

MSR System Requirements

Moisture Separation and Reheating (MSR) systems are essential components in modern power plants, particularly in ultra-supercritical and supercritical boilers. These systems serve the following functions:

1. Moisture separation: Remove entrained water droplets from steam to prevent erosion of turbine blades
2. Reheating: Restore steam temperature to optimal levels for efficient turbine operation
3. Pressure regulation: Maintain stable pressure conditions for downstream equipment

The tube sheets in MSR systems are subjected to:

• Thermal cycling: Repeated heating and cooling during startup and shutdown
• High pressure: Typically 25-35 MPa steam pressure
• Corrosive environment: Moisture carryover can cause pitting and crevice corrosion
• Erosion: High-velocity steam with entrained particles causes surface erosion

Nickel-Based Alloy Selection

Nickel-based alloys are selected for overlay welding on MSR tube sheets due to their exceptional properties:

For MSR tube sheets, Inconel 625 or similar alloys are typically selected due to their combination of corrosion resistance, thermal stability, and weldability.

Overlay Welding Process Analysis

Process Selection and Parameters

The overlay welding process for MSR tube sheets must be carefully selected to ensure:

1. Minimal dilution: Maintain nickel-based alloy composition for corrosion resistance
2. Low residual stress: Prevent cracking and distortion in thick tube sheets
3. Good bond strength: Ensure reliable attachment to the base material
4. Uniform coverage: Provide consistent protection across the entire surface

Multi-Pass Welding Strategy

For thick overlay layers required on MSR tube sheets, a multi-pass welding strategy is essential:

1. Bonding pass: Establish metallurgical bond between base material and overlay

• Use lower heat input to minimize dilution
• Consider using a transition layer if needed
• Ensure complete fusion and wetting

1. Fill passes: Build up the required overlay thickness

• Maintain controlled interpass temperatures
• Use back-step welding to minimize distortion
• Monitor dilution through composition analysis

1. Cap pass: Final surface layer with optimal composition

• Use lowest heat input settings
• Ensure smooth, uniform surface finish
• Consider post-weld heat treatment if required

Quality Control and Testing

Non-Destructive Testing (NDT)

Mechanical Property Requirements

The overlay weld must meet the following mechanical property requirements:

• Tensile strength: Minimum 550 MPa for Inconel 625 overlays
• Elongation: Minimum 20% to ensure ductility and fatigue resistance
• Hardness: 250-300 HV for Inconel 625, consistent throughout the overlay
• Impact energy: Minimum 50 J at -40°C to ensure low-temperature toughness

Corrosion Testing

Given the critical service environment, the overlay must undergo rigorous corrosion testing:

1. Electrochemical testing: Measure corrosion potential and current density in simulated MSR conditions
2. Potentiodynamic polarization: Determine pitting potential and passivation behavior
3. Immersion testing: Long-duration exposure to moisture and steam conditions
4. Cyclic corrosion testing: Simulate startup/shutdown thermal cycling with corrosion exposure

Engineering Challenges and Solutions

Challenge 1: Dilution Control

Excessive dilution from the base material reduces the corrosion resistance of the overlay. Solutions include:

• Use processes with low heat input (TIG, CMT)
• Apply multiple thin passes rather than few thick passes
• Use filler materials with higher nickel content to compensate for dilution
• Consider using a transition layer with intermediate composition

Challenge 2: Residual Stress Management

Thick overlay layers on tube sheets generate significant residual stresses that can cause distortion or cracking. Mitigation strategies include:

• Preheat the tube sheet to 200-300°C
• Use back-step welding to distribute heat input evenly
• Apply peening after welding to introduce compressive stresses
• Perform post-weld stress relief annealing at 550-650°C

Challenge 3: Interface Bonding

The bond between the nickel-based overlay and the carbon or low-alloy steel tube sheet is critical for structural integrity. Key considerations:

• Ensure complete fusion at the interface
• Avoid intermetallic compound formation that can reduce toughness
• Control cooling rate to prevent brittle phases
• Consider using a transition alloy if direct welding is not feasible

Common Defects and Countermeasures

Key Questions and Reflections

The MSR tube sheet overlay welding application raises several important engineering questions:

1. Long-term performance: How does the overlay perform after thousands of thermal cycles in actual service? Laboratory testing must be supplemented with field experience data.
2. Repair procedures: What are the recommended procedures for repairing damaged overlays in service? Hot work procedures must be carefully controlled to avoid further degradation.
3. Cost-benefit analysis: Given the high cost of nickel-based alloys, what is the optimal overlay thickness that provides adequate protection without excessive material usage?
4. Alternative approaches: Could alternative surface engineering techniques (e.g., thermal spray, cladding) provide comparable performance with different advantages?

Study Insights and Implications

This research highlights the critical role of overlay welding in extending the life and reliability of power generation components. The MSR tube sheet application demonstrates that careful process selection, material matching, and quality control are essential for achieving the required performance in extreme service conditions.

The nickel-based alloy overlay provides a proven solution for corrosion and erosion resistance, but engineers must carefully balance the benefits against the costs and potential challenges of welding dissimilar materials. The multi-pass welding strategy with controlled dilution and residual stress management is essential for achieving reliable, long-lasting overlays.

For future applications, the development of advanced nickel-based alloys with improved weldability and corrosion resistance will continue to expand the capabilities of overlay welding technology. Engineers should stay informed about new alloy developments and process innovations that can further enhance the performance of overlay welded components in critical power generation applications.