Zhuojin Pipe Fitting Co., LtdMacro Segregation in Low Alloy Steel to Nickel-Based Alloy Overlay Joints
Literature Overview
This paper by Ding Ming, Lu Li, Zhang Yu, Ding Yongsan, Yang Jia, Lü Yishi, Li Kejian, and Cai Zhipeng, published in Hot Working Technology (热加工工艺) in 2023, Volume 52, Issue 15, investigates the macro segregation phenomenon in overlay weld joints where ERNiCr-3 nickel-based alloy is deposited onto T23 low alloy steel. The research, funded by the National Energy Group of China (Project No. GJNY-19-82), employs optical microscopy, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) to characterize the microstructure at the weld interface and elucidate the formation mechanism of macro segregation.
Technical Context and Industrial Relevance
The overlay welding of nickel-based alloys onto low alloy steels is a common practice in power generation and petrochemical industries for corrosion and wear protection. T23 (9Cr-1Mo-V) is a widely used ferritic heat-resistant steel for high-pressure boiler tubes and supercritical steam pipes. ERNiCr-3 (Inconel 625 equivalent) is a nickel-chromium-molybdenum alloy with excellent corrosion resistance in oxidizing and reducing environments.
The challenge lies in the significant metallurgical mismatch between these materials:
| Property | T23 Low Alloy Steel | ERNiCr-3 Nickel Alloy |
|---|---|---|
| Liquidus temperature | ~1420°C | ~1320°C |
| Solidus temperature | ~1390°C | ~1260°C |
| Thermal expansion coefficient | ~12×10⁻⁶/°C | ~13×10⁻⁶/°C |
| Crystal structure | BCC (ferrite) | FCC (austenite) |
| Density | ~7.8 g/cm³ | ~8.4 g/cm³ |
| Thermal conductivity | ~26 W/m·K | ~11 W/m·K |
The difference in liquidus temperatures (approximately 100°C) is the root cause of the macro segregation phenomenon identified in this study.
Macro Segregation Formation Mechanism
The authors identified that macro segregation occurs primarily at weld bead overlap regions, where a stripe-like structure with high iron content and a crystal structure matching the base metal (BCC ferrite) forms near the fusion line. The formation mechanism is explained as follows:
- Temperature gradient: At bead overlap regions, the local pool temperature is lower than in single-pass regions due to the cooling effect of the already-solidified adjacent bead.
- Sub-liquidus condition: The local pool temperature at the overlap region may fall below the base metal liquidus temperature but above the weld metal liquidus temperature.
- Base metal melting: Molten base metal (T23 steel) flows into the overlap region due to convection and surface tension forces.
- Rapid solidification: Because the local temperature is below the base metal liquidus, the molten base metal undergoes rapid solidification, forming a stripe-like structure with base metal composition and crystal structure.
- Composition segregation: The rapid solidification traps a composition that is significantly different from the surrounding weld metal, creating macro segregation.
This mechanism is fundamentally a consequence of the liquidus temperature difference between the base metal and the weld metal. When the base metal liquidus is higher than the weld metal liquidus, the base metal can melt and flow into regions where the pool temperature is below its own liquidus, leading to rapid solidification and segregation.
Microstructural Characterization
The authors employed a multi-scale characterization approach:
| Technique | Information Obtained | Key Finding |
|---|---|---|
| Optical microscopy (OM) | Overall microstructure, segregation morphology | Stripe-like segregation at bead overlaps |
| Scanning electron microscopy (SEM) | Elemental distribution, phase identification | High Fe content in segregated regions |
| Energy-dispersive X-ray spectroscopy (EDS) | Quantitative elemental analysis | Fe enrichment in segregated stripes |
| Electron backscatter diffraction (EBSD) | Crystal orientation, phase identification | BCC structure matching base metal |
The EBSD analysis confirmed that the segregated regions have a BCC crystal structure identical to the T23 base metal, not the FCC structure of the ERNiCr-3 weld metal. This is direct evidence that the segregated material is rapidly solidified base metal, not a modified weld metal composition.
Countermeasures and Process Optimization
The authors propose two primary strategies for reducing macro segregation:
- Selecting weld consumables with liquidus temperatures closer to the base metal: By reducing the liquidus temperature difference, the driving force for base metal melting and flow is minimized.
- Reducing welding heat input: Lower heat input reduces the extent of base metal melting and the volume of the molten pool, thereby limiting the opportunity for segregation to occur.
Additional measures that can be considered include:
- Multi-pass welding with controlled overlap: Minimizing the overlap area between adjacent beads reduces the number of segregation-prone regions.
- Welding sequence optimization: Arranging the welding sequence to ensure uniform thermal history across the overlay surface.
- Preheating: Moderate preheating can raise the base metal temperature, reducing the effective liquidus temperature difference.
- Post-weld heat treatment: Solution treatment and aging can homogenize the segregated regions, though complete elimination of segregation may not be achievable.
Engineering Practice Implications
For power plant applications involving T23 steel components overlaid with nickel-based alloys, macro segregation has several implications:
- Mechanical properties: Segregated regions may have different mechanical properties than the surrounding weld metal, potentially creating weak links.
- Corrosion resistance: The BCC ferrite structure in segregated regions is less corrosion resistant than the FCC nickel alloy matrix, potentially creating preferential corrosion sites.
- Fatigue performance: Microstructural inhomogeneity can serve as fatigue crack initiation sites under cyclic loading.
- Service life: In high-temperature service, segregation can accelerate creep damage and reduce component life.
Quality control procedures should include:
| Inspection Method | Purpose | Acceptance Criteria |
|---|---|---|
| Visual inspection (VT) | Surface defects | No visible segregation |
| Dye penetrant testing (PT) | Surface cracks | No linear indications |
| Ultrasonic testing (UT) | Internal defects | No indications exceeding threshold |
| Hardness mapping | Microstructural inhomogeneity | Hardness variation within specified range |
| Metallographic examination | Segregation assessment | Segregation area < specified limit |
Study Insights and Reflections
This paper provides a clear and well-supported explanation of macro segregation formation in dissimilar material overlay welds. The identification of liquidus temperature difference as the root cause is a fundamental insight that has broader applicability beyond this specific material combination.
From a practical standpoint, the implications for power plant maintenance and repair are significant. Many existing components with nickel-based overlays may contain macro segregation that has not been previously recognized. The authors' findings suggest that a systematic review of overlay welding procedures for dissimilar material combinations is warranted, with particular attention to the liquidus temperature compatibility of base and weld metals.
The research also highlights the importance of multi-scale characterization in understanding welding phenomena. The combination of OM, SEM, EDS, and EBSD provided complementary information that would not have been obtainable from any single technique. This approach should be standard practice in welding research and quality investigation.
One area for further investigation is the long-term behavior of macro segregated regions under service conditions. While the paper establishes the formation mechanism, the effect of segregation on creep, fatigue, and corrosion resistance under actual operating conditions remains to be fully characterized.