Nickel-Based Alloy Overlay Welding on MSR Tube Sheets

Literature Overview

The paper authored by Li Zhijie, Zhu Hongguang, Yu Jiangtao, and Zou Xuesong from Harbin Boiler Co., Ltd., published in the journal Welding in 2000, addresses the overlay welding of nickel-based alloys onto MSR (Multi-Stage Reheat / Multi-Stage Recovery) tube sheets. Harbin Boiler, as one of China's leading boiler manufacturers, has extensive experience with high-pressure and high-temperature pressure vessel fabrication, making this work particularly valuable for understanding the practical challenges of nickel-based alloy cladding in power generation equipment.

Background and Technical Significance

MSR systems in modern coal-fired power plants operate at elevated temperatures and pressures with aggressive working media that can cause severe corrosion of conventional carbon and low-alloy steels. The tube sheet, as a critical structural component, must simultaneously provide:

• Structural integrity for tube installation and sealing
• Corrosion resistance against the process medium
• Compatibility with the tube material (often nickel-based alloys such as Inconel 625, Monel 400, or Hastelloy C-276)

The overlay welding of nickel-based alloys onto the tube sheet surface provides an economical alternative to fabricating the entire tube sheet from expensive nickel alloys, while ensuring corrosion resistance at the critical interface with the process medium.

Core Technical Content

Material System and Weldability Challenges

The base material for MSR tube sheets is typically a low-alloy steel such as 15CrMoR or 12Cr1MoV, while the overlay layer is a nickel-based alloy. This material combination presents several weldability challenges:

Welding Process Selection

For tube sheet overlay welding, the following processes are most commonly employed:

1. Submerged Arc Welding (SAW) – High deposition rate (8–15 kg/h), suitable for thick overlay layers (10–25 mm total), excellent protection against atmospheric contamination
2. Gas Metal Arc Welding (GMAW) – Good control, moderate deposition rate, suitable for complex geometries near tube holes
3. Electroslag Welding (ESW) – Very high deposition rate, used for large flat areas away from tube hole regions

The overlay layer typically requires a minimum thickness of 6–10 mm to ensure the final composition after dilution achieves adequate corrosion resistance. The first layer (tack weld) is critical as it establishes the bond and influences the dilution ratio for subsequent layers.

Dilution Control Strategy

The dilution rate in nickel-based alloy overlay welding is the most critical parameter affecting final performance. Key strategies include:

• First-layer thickness optimization – A thicker first layer (3–5 mm) reduces the dilution ratio for subsequent layers
• Filler metal selection – Using a more corrosion-resistant filler (e.g., Inconel 625 with higher Nb content) to compensate for dilution
• Multi-layer approach – Typically 3–5 layers are applied, with dilution decreasing from approximately 40–60% in the first layer to 10–20% in the final layer
• Post-overlay composition verification – Chemical analysis of the final surface layer to confirm Ni content ≥ 55 wt% and adequate Cr, Mo, Nb levels

Standards and Inspection Requirements

The fabrication of MSR tube sheets with nickel-based alloy overlays must comply with stringent standards:

Non-destructive testing requirements typically include:

• 100% magnetic particle testing (MT) of the overlay surface
• 100% ultrasonic testing (UT) of the bond line
• Radiographic testing (RT) of a representative percentage of welds
• Hardness survey of the overlay layer

Engineering Practice and Defect Analysis

In practical fabrication, the following defects are most commonly encountered:

1. Cracking in the overlay layer – caused by excessive cooling rate, high carbon content in the weld metal, or inadequate preheating. Preheating to 200–300 °C and maintaining interpass temperature below 250 °C are essential controls.
2. Bond line lack of fusion – occurs when the base material is not adequately melted during the first overlay pass. This is particularly problematic on curved tube sheet surfaces and near tube hole edges where heat dissipation is rapid.
3. Porosity – hydrogen porosity is common in nickel-based alloy welds due to the low hydrogen solubility in solid nickel. Thorough cleaning of the base surface and use of low-hydrogen consumables are critical.
4. Excessive dilution – results in final overlay composition that fails to meet corrosion resistance requirements. This is managed through procedure optimization and composition verification.

Key Reflections

The work by Harbin Boiler engineers reflects deep practical experience with the challenges of nickel-based alloy overlay welding in power generation equipment. Several insights stand out:

• The tube sheet geometry presents unique challenges due to the dense array of tube holes, which act as heat sinks and complicate weld access
• The economic benefit of overlay welding versus full nickel alloy tube sheets is substantial, typically reducing material costs by 60–70%
• The long-term reliability of the overlay depends critically on the bond line quality, which is difficult to inspect but essential for service integrity

This work, though published in 2000, remains highly relevant to modern MSR and supercritical boiler fabrication, where the demands on tube sheet materials continue to increase with rising operating parameters.

Summary

The nickel-based alloy overlay welding of MSR tube sheets represents a mature engineering solution that balances corrosion resistance requirements with economic constraints. The key to successful implementation lies in rigorous control of dilution, careful management of thermal input to prevent cracking, and comprehensive non-destructive testing of the bond line. For engineers involved in power generation equipment fabrication, this literature provides essential guidance on procedure development, defect prevention, and quality assurance for one of the most critical cladding applications in the industry.