Zhuojin Pipe Fitting Co., LtdMacro-Segregation in Low Alloy Steel Overlay Nickel-Based Alloy 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" in 2023 (Volume 52, Issue 15, pages 55-58), investigates the macro-segregation characteristics and formation mechanisms in overlay joints between low alloy steel and nickel-based alloys. The study was funded by the National Energy Group Science and Technology Project (GJNY-19-82) and conducted by researchers from the National Energy Group Jianbi Power Plant, Suzhou Thermal Power Research Institute, and Tsinghua University.
Technical Background
Overlay welding of nickel-based alloys onto low alloy steel is a common practice in power plant applications, particularly for components subjected to high-temperature corrosion, such as superheater tubes, reheater tubes, and boiler components. The T23 low alloy steel (9Cr-1Mo-V-Nb) is a typical base material, while ERNiCr-3 (Inconel 625) is a common nickel-based overlay material.
Material Properties
| Property | T23 Low Alloy Steel | ERNiCr-3 Nickel Alloy |
|---|---|---|
| Base Composition | 9Cr-1Mo-V-Nb | Ni-21Cr-9Mo-Nb-Ti |
| Liquidus Temperature | ~1420°C | ~1370°C |
| Solidus Temperature | ~1380°C | ~1320°C |
| Thermal Conductivity | 25-30 W/(m·K) | 11-13 W/(m·K) |
| Thermal Expansion | 12-14 × 10⁻⁶/K | 13-14 × 10⁻⁶/K |
| Crystal Structure | BCC (FCC at high T) | FCC |
Macro-Segregation Characteristics
Location of Macro-Segregation
The research reveals that macro-segregation primarily occurs at the weld bead overlap regions, specifically in the weld metal near the fusion line. The segregated region is characterized by:
- High iron content: Significantly higher than the nominal composition of the nickel-based weld metal
- Crystal structure similarity: The segregated region has a crystal structure similar to the low alloy steel base metal (BCC or mixed BCC/FCC)
- Banded morphology: The segregated region appears as a band-like structure near the fusion line
Microstructural Analysis
Using optical microscopy, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD), the researchers characterized the microstructure of the overlay joint:
| Region | Microstructure | Characteristics |
|---|---|---|
| Base Metal | Ferrite + Carbides | Typical T23 microstructure |
| Fusion Line | Mixed structure | Transition zone with complex morphology |
| Segregated Band | BCC + Carbides | High Fe content, similar to base metal |
| Weld Metal | FCC + Laves phase | Typical nickel alloy structure |
| HAZ | Ferrite + Carbides | Altered microstructure due to thermal cycling |
Formation Mechanism
Temperature Differential Theory
The key mechanism for macro-segregation formation is related to the difference in liquidus temperatures between the base metal and the welding material:
- Liquidus temperature difference: The nickel-based welding material has a lower liquidus temperature (~1370°C) than the low alloy steel base metal (~1420°C)
- Local temperature depression: At the weld bead overlap regions, the local molten pool temperature is lower than the liquidus temperature of the base metal
- Base metal melting: The base metal melts and flows into the region where the temperature is below its liquidus temperature
- Rapid solidification: The melted base metal undergoes rapid solidification due to the temperature gradient
- Segregation formation: The rapid solidification leads to the formation of a segregated band with high iron content
Process Parameters and Macro-Segregation
| Parameter | Effect on Macro-Segregation | Recommended Range |
|---|---|---|
| Heat Input | Higher heat input → more segregation | Low heat input (0.5-1.0 kJ/mm) |
| Travel Speed | Slower speed → more dilution | Moderate speed (50-80 mm/min) |
| Electrode Angle | Steeper angle → less penetration | 10-20° from vertical |
| Interpass Temperature | Higher temperature → more dilution | < 200°C |
| Weld Bead Overlap | More overlap → more segregation | Controlled overlap (25-50%) |
Countermeasures
Material Selection
The research suggests that selecting welding materials with liquidus temperatures closer to the base metal can reduce macro-segregation:
| Welding Material | Liquidus Temperature | Compatibility with T23 |
|---|---|---|
| ERNiCr-3 (Inconel 625) | ~1370°C | Moderate (some segregation) |
| ERNiCrMo-3 (Inconel 625 variant) | ~1380°C | Better (less segregation) |
| ERNi-5 (Incoloy 825) | ~1400°C | Good (minimal segregation) |
| ERNiFe-5 (Stellite 6) | ~1420°C | Excellent (no segregation) |
Process Optimization
- Reduce heat input: Lower the welding current and increase travel speed to minimize base metal melting
- Control interpass temperature: Keep the interpass temperature below 200°C to limit dilution
- Optimize weld bead overlap: Use a controlled overlap pattern to minimize the area of potential segregation
- Use multiple thin passes: Deposit multiple thin layers instead of a few thick layers to reduce dilution
- Post-weld treatment: Consider stress relief or solution treatment to improve the microstructure
Engineering Applications
Power Plant Applications
The overlay welding of nickel-based alloys onto low alloy steel is widely used in power plant components:
| Component | Application | Requirements |
|---|---|---|
| Superheater tubes | High-temperature corrosion resistance | Overlay thickness 1-3 mm |
| Reheater tubes | Sulfidation resistance | Overlay thickness 1-2 mm |
| Boiler components | General corrosion protection | Overlay thickness 0.5-2 mm |
| Steam drums | Internal corrosion protection | Overlay thickness 2-5 mm |
Quality Control Considerations
The presence of macro-segregation can affect the performance of overlay-welded components:
- Mechanical properties: The segregated region may have different mechanical properties than the nominal weld metal
- Corrosion resistance: The segregated region may be more susceptible to corrosion
- Creep resistance: The segregated region may have reduced creep resistance at high temperatures
- Fatigue life: The segregated region may be a site for crack initiation
Key Reflections
This research provides valuable insights into the metallurgical behavior of dissimilar metal overlay joints. The finding that macro-segregation is primarily driven by the liquidus temperature difference between the base metal and the welding material has important implications for:
- Material selection: Engineers should consider the liquidus temperature compatibility when selecting welding materials
- Process design: Low heat input and controlled interpass temperature are essential for minimizing macro-segregation
- Quality control: The fusion line region should be carefully inspected for segregation
- Performance prediction: The presence of macro-segregation should be considered when predicting component life
The research also highlights the importance of understanding the fundamental metallurgical mechanisms in welding. For engineers working with dissimilar metal joints, this knowledge is essential for designing reliable and durable components.