Defect Analysis and Countermeasures for NCu30 Overlay Welding on 16Mn Substrate

Literature Overview

The paper by Zhang Julli, Fu Lin, Wang Lijun, Zhang Juan, Li Yanan, and Zhao Xiuli (China Chemical Equipment, 2025, Vol. 27, No. 4, pp. 11-14) presents a detailed defect analysis and countermeasure study for overlay welding NCu30 (a copper-nickel alloy) onto a 16Mn low-alloy steel substrate. The study identifies porosity and cracking as the primary defects, analyzes their root causes, and proposes process modifications including interpass temperature control, transition layer welding, and enhanced environmental protection.

Material Characteristics and Weldability Challenges

NCu30 is a copper-nickel alloy (approximately 30% Ni, balance Cu) with excellent corrosion resistance, particularly in marine and chemical environments. However, its welding presents unique challenges due to the inherent properties of copper:

The combination of high thermal conductivity, high linear expansion coefficient, and high chemical activity in copper makes it particularly susceptible to porosity and cracking during welding.

Defect Analysis

Porosity

Porosity in NCu30 overlay welds is primarily caused by:

1. Hydrogen entrapment — Copper has high hydrogen solubility, and the rapid cooling rate associated with copper's high thermal conductivity prevents hydrogen from escaping the molten pool.
2. Oxide inclusion — Copper's high chemical activity leads to oxidation during welding, forming Cu₂O inclusions that can act as nucleation sites for porosity.
3. Insufficient shielding — Inadequate gas protection allows atmospheric contamination and oxide formation.

Cracking

Cracking in NCu30 welds can occur through several mechanisms:

1. Hot cracking — Due to low melting point eutectics at grain boundaries and high thermal stresses from differential thermal expansion.
2. Cold cracking — Due to hydrogen embrittlement in the HAZ of the 16Mn substrate.
3. Intergranular cracking — Due to segregation of low melting point elements at grain boundaries in the copper-rich regions.

Process Countermeasures

Based on the defect analysis, the following process modifications are recommended:

Transition Layer Strategy

The use of an ENi-1 (nickel electrode) transition layer is a critical process modification. The rationale is:

1. Composition gradient — The nickel layer creates a gradual transition in composition from the steel substrate to the copper-nickel overlay, reducing thermal stress and cracking susceptibility.
2. Improved weldability — Nickel has better weldability than copper and provides a more forgiving welding environment.
3. Reduced dilution — The nickel layer reduces the dilution of the final NCu30 overlay by the steel substrate, maintaining the intended alloy composition.

The recommended welding sequence is:

1. Preheat 16Mn substrate to 100-150°C.
2. Deposit 1-2 passes of ENi-1 transition layer.
3. Maintain interpass temperature at 60-100°C.
4. Deposit NCu30 overlay using appropriate electrode or wire.
5. Apply post-weld heat treatment if required by specification.

Quality Control Procedures

For NCu30 overlay welding, the following quality control procedures are recommended:

1. Pre-weld inspection — Verify substrate cleanliness, preheat temperature, and electrode condition.
2. In-process monitoring — Monitor interpass temperature, gas flow rate, and welding parameters.
3. Post-weld NDT — Perform visual inspection, dye penetrant testing (PT), and ultrasonic testing (UT) to detect surface and subsurface defects.
4. Hardness testing — Verify overlay hardness and hardness gradient across the weld.
5. Corrosion testing — Conduct salt spray testing or immersion testing to verify corrosion resistance.

Engineering Practice Implications

This study is particularly relevant for chemical processing equipment where corrosion-resistant overlays are applied to carbon steel or low-alloy steel substrates. The NCu30 alloy is commonly used in:

• Heat exchanger tubes and tube sheets
• Condensers and coolers
• Piping systems for corrosive fluids
• Valve bodies and internals

The key lesson from this study is that the welding of copper-nickel alloys requires careful attention to thermal management, environmental control, and process sequencing. The interpass temperature control of 60-100°C is a critical parameter that is often overlooked in standard welding procedures. The transition layer strategy using ENi-1 electrode is a practical and effective solution to the weldability challenges posed by the composition mismatch between copper-nickel alloys and steel substrates.

Study Insights and Reflections

This 2025 study represents a timely contribution to the field of dissimilar metal welding, particularly for copper-nickel alloy overlays. The detailed defect analysis and systematic approach to countermeasure development provide a valuable reference for engineers dealing with similar welding challenges. The emphasis on interpass temperature control and transition layer welding reflects a sophisticated understanding of welding metallurgy and process control. In my experience with overlay welding for corrosion-resistant applications, the challenges identified in this study — particularly porosity and cracking in copper alloys — are common and require careful process development and rigorous quality control. The study's recommendations provide a solid foundation for developing welding procedures for NCu30 and similar copper-nickel alloys on steel substrates.