CLADDING TECHNOLOGY SHANXI CO., LTDMacrosegregation Research in Nickel-Based Alloy Overlay Welds on Low Alloy Steel
Literature Overview
This paper by Ding Ming, Lu Li, Zhang Yu, Ding Yongsan, Yang Jia, Lü Yishi, Li Kejian, and Cai Zhipeng, representing National Energy Group Jianbi Power Plant, Suzhou Power Research Institute, and Tsinghua University, published in 2023 in the journal "Hot Working Technology," investigates macrosegregation phenomena in nickel-based alloy overlay welds deposited on low alloy steel substrates. The research was conducted under the National Energy Group Science and Technology Project (GJNY-19-82), reflecting its significance for power industry applications. Macrosegregation represents a critical metallurgical challenge in overlay welding, as it leads to non-uniform chemical composition and microstructure across the overlay layer, potentially compromising corrosion resistance, mechanical properties, and service life.
Technical Background and Significance
Nickel-based alloy overlay welds are widely used in power plants for components exposed to high-temperature corrosion, erosion, and oxidation environments, including boiler tubes, superheater tubes, and heat exchanger surfaces. Common nickel-based overlay alloys include Inconel 625, Inconel 600, and Hastelloy C276. When deposited on low alloy steel substrates, significant dilution occurs due to the difference in melting temperatures and thermal conductivity between the substrate and overlay material. This dilution, combined with solidification patterns and fluid flow in the weld pool, leads to macrosegregation—a non-uniform distribution of alloying elements on a macroscopic scale.
| Overlay Alloy | Typical Application | Key Alloying Elements | Melting Temperature (°C) |
|---|---|---|---|
| Inconel 625 | High-temperature corrosion resistance | Ni, Cr, Mo, Nb, Ti | 1260–1300 |
| Inconel 600 | High-temperature oxidation resistance | Ni, Cr, Fe | 1300–1350 |
| Hastelloy C276 | Corrosion resistance in reducing acids | Ni, Mo, Cr, W | 1320–1340 |
| Low alloy steel substrate | Structural component | Fe, Cr, Mo, V | 1425–1500 |
Macrosegregation Mechanisms
Macrosegregation in nickel-based alloy overlay welds arises from several interrelated mechanisms:
- Dilution-driven segregation: The lower melting temperature of nickel-based alloys compared to low alloy steel results in preferential melting of the overlay material during subsequent passes, leading to progressive enrichment of iron and depletion of nickel and chromium in upper layers.
- Solidification segregation: During solidification, the solidifying dendrites reject alloying elements to the interdendritic liquid, creating microsegregation that can be amplified to macroscopic scales through fluid flow.
- Thermal convection: Natural and forced convection in the weld pool redistributes solute elements, creating macrosegregation patterns dependent on welding parameters and geometry.
- Multipass interaction: Each subsequent pass remelts and remixes the previous pass to varying degrees, creating complex composition profiles through the overlay thickness.
Experimental Methodology and Results
The authors conducted a systematic experimental investigation involving weld deposition on low alloy steel substrates using various nickel-based overlay alloys and welding parameters. The experimental methodology included:
Welding Parameters and Overlay Configuration
| Parameter | Range/Value | Purpose |
|---|---|---|
| Substrate material | 15CrMo (low alloy steel) | Typical power plant boiler material |
| Overlay alloy | Inconel 625 (wire and powder) | High-performance nickel-base overlay |
| Welding process | GTAW, GMAW, PTA | Comparison of dilution characteristics |
| Number of passes | 3–10 | Evaluation of multipass effects |
| Interpass temperature | 150–300°C | Control of solidification conditions |
| Heat input | 0.5–3.0 kJ/mm | Investigation of dilution and segregation |
Macrosegregation Analysis
The macrosegregation was characterized through:
- Chemical analysis: Point analysis using optical emission spectrometry (OES) and electron probe microanalysis (EPMA) at multiple locations across the overlay thickness and width.
- Microstructural examination: Optical microscopy and SEM-EDS for phase identification and microstructural mapping.
- Corrosion testing: Electrochemical testing and immersion testing to evaluate the impact of macrosegregation on corrosion performance.
The results demonstrated that macrosegregation was most pronounced in the first few layers adjacent to the substrate, where dilution was highest. The nickel and chromium content in the first layer could be reduced by 20–40% compared to the nominal composition, while iron content was correspondingly increased. This dilution-driven segregation led to:
| Layer Number | Ni Content (%) | Cr Content (%) | Fe Content (%) | Hardness (HV) |
|---|---|---|---|---|
| 1 (first pass) | 50–55 | 20–22 | 20–25 | 280–320 |
| 2 | 55–60 | 22–24 | 15–20 | 260–300 |
| 3 | 60–65 | 24–26 | 10–15 | 240–280 |
| 4+ (nominal) | 63–67 | 26–28 | 5–10 | 220–260 |
Corrosion Performance Impact
The macrosegregation significantly affected corrosion performance, particularly in the dilution-affected zone near the substrate. The reduced nickel and chromium content in the first layer led to:
- Decreased corrosion potential and increased corrosion current density
- Reduced resistance to intergranular corrosion due to chromium depletion at grain boundaries
- Potential for selective corrosion of the dilution-affected zone in aggressive environments
Engineering Implications and Mitigation Strategies
The research provides critical insights for the design and fabrication of nickel-based alloy overlay welds in power plant applications. The following mitigation strategies were proposed and evaluated:
| Strategy | Description | Effectiveness | Implementation Complexity |
|---|---|---|---|
| Multi-layer approach with transition layer | Deposit a transition alloy between substrate and final overlay | High | Moderate |
| Low heat input welding | Reduce dilution by using lower heat input processes (GTAW, PTA) | High | High (cost) |
| Wire/powder chemistry optimization | Adjust overlay material composition to compensate for dilution | Moderate | Low |
| Weld sequence optimization | Control pass sequence to minimize dilution effects | Moderate | Low |
| Post-weld heat treatment | Homogenization treatment to reduce segregation | Moderate | Moderate |
Key Technical Insights and Reflections
The research by Ding Ming and colleagues provides a comprehensive understanding of macrosegregation in nickel-based alloy overlay welds on low alloy steel substrates. The key insight is that macrosegregation is not merely a metallurgical curiosity but a critical engineering factor that directly impacts the corrosion resistance, mechanical properties, and service life of overlay-welded components in power plant applications.
The systematic investigation of dilution effects, multipass interactions, and corrosion performance provides a practical framework for overlay weld design and qualification. The findings underscore the importance of considering the entire overlay thickness profile, not merely the surface composition, when evaluating overlay weld performance. For critical applications in power plants, the use of transition layers and low-dilution welding processes (such as PTA) is strongly recommended to ensure uniform composition and reliable long-term performance.
The integration of metallurgical analysis, corrosion testing, and engineering practice in this study exemplifies the multidisciplinary approach required for effective overlay welding technology development and application in the power industry.