Zhuojin Pipe Fitting Co., LtdMicrostructure and Properties of Ni-Based Cladding Layers under TIG Arc Overlay Welding Conditions
Literature Overview
The paper by Zhang Guangcheng, Gao Jun, Zhu Ziyue, and Li Jihong, published in Hot Working Technology (2022, Vol. 51, No. 24), investigates the microstructure and properties of Ni-based cladding layers deposited by TIG (Tungsten Inert Gas) arc overlay welding on Q345 low-carbon steel. The study was funded by the National Natural Science Foundation of China (No. 101-413519043) and the Xi'an Science and Technology Plan Project (No. 21XJZZ0057). The research addresses the challenge of reducing the dilution effect of the base metal on the Ni-based cladding layer by designing three sets of Ni-based flux-cored wires with different Fe content, and depositing a three-layer structure consisting of transition layers and an Inconel 625 top layer.
Experimental Design and Methodology
The study employed a systematic approach to optimize the Ni-based cladding layer properties. Three different flux-cored wires with varying Fe content were designed to serve as transition layers between the Q345 base metal and the Inconel 625 top layer. The TIG deposition method was selected for its low dilution rate and precise control over the welding parameters. The characterization methods included optical microscopy (OM), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and microhardness testing.
| Layer | Material | Function | Fe Content (approx.) |
|---|---|---|---|
| Base metal | Q345 | Structural support | ~0.2% |
| Transition layer 1 | Ni-based flux-cored wire (high Fe) | Gradual composition transition | Higher |
| Transition layer 2 | Ni-based flux-cored wire (medium Fe) | Further composition transition | Medium |
| Transition layer 3 | Ni-based flux-cored wire (low Fe) | Near-pure Ni transition | Lower |
| Top layer | Inconel 625 | Final functional layer | ~3-4% |
Microstructural Analysis
The microstructural analysis revealed several important features of the Ni-based cladding layers:
Crystal Morphology Variation
Under the influence of solidification temperature, different crystal morphologies formed from the interface to the top of the cladding layer. Near the interface, where the cooling rate is highest due to the thermal sink effect of the base metal, columnar grains grow perpendicular to the fusion boundary. Moving toward the top of the cladding layer, where the cooling rate decreases, equiaxed grains become more prevalent. This gradient in crystal morphology is a direct consequence of the varying thermal conditions during multi-pass deposition.
Phase Composition
The cladding layer is primarily composed of three phases:
- γ-Ni matrix (FCC structure): The primary phase, providing the base matrix for the cladding layer. The FCC structure contributes to the excellent ductility and corrosion resistance of the Ni-based alloy.
- Laves phase (MoNi4): A brittle intermetallic compound that forms due to the presence of Mo and Ni. The Laves phase provides additional hardness but can reduce toughness if present in excessive amounts.
- MC carbide: A carbide phase formed by the interaction of carbon with elements such as Cr, Mo, and W. The MC carbide contributes to wear resistance but can also act as crack initiation sites if present in large quantities.
Elemental Distribution
EDS analysis revealed significant microsegregation of alloying elements within the solid solution. The distribution pattern is as follows:
| Element | Location of Enrichment | Mechanism |
|---|---|---|
| Ni, Cr, Fe | Within grains (intragranular) | Lower partition coefficient to grain boundaries |
| Mo, Nb | At grain boundaries (intergranular) | Higher segregation tendency during solidification |
This microsegregation pattern has important implications for the mechanical and corrosion properties of the cladding layer. The enrichment of Mo and Nb at grain boundaries can promote the formation of intermetallic phases and carbides, which can affect the grain boundary strength and corrosion resistance.
Effect of Fe Content on Microsegregation
An important finding is that increasing the Fe content in the transition layers reduces the degree of elemental microsegregation. As the Fe content increases, the Laves phase content decreases, and the Laves phase morphology becomes more irregular. This suggests that Fe acts as a diluent that moderates the segregation tendency of other alloying elements, potentially improving the overall homogeneity of the cladding layer.
Mechanical Properties
The microhardness results demonstrate the effectiveness of the multi-layer cladding approach:
| Region | Average Microhardness | Notes |
|---|---|---|
| Q345 base metal | ~200 HV0.2 | Reference baseline |
| Transition layer (upper) | ~180-200 HV0.2 | Slightly lower due to repeated heat input |
| Inconel 625 top layer | >200 HV0.2 | Enhanced by solid solution strengthening and second-phase strengthening |
The Inconel 625 top layer achieves an average hardness exceeding 200 HV0.2, which is higher than the Q345 base metal. This enhancement is attributed to two main strengthening mechanisms:
- Solid solution strengthening: The dissolution of Cr, Mo, Nb, and other alloying elements in the γ-Ni matrix creates lattice distortions that impede dislocation motion, increasing the yield strength and hardness.
- Second-phase strengthening: The Laves phase and MC carbide particles act as obstacles to dislocation motion, providing additional strengthening through the Orowan bypass mechanism.
The slightly lower hardness in the upper transition layer is attributed to the repeated heat input from subsequent passes. Each subsequent pass re-heats the previously deposited layer, promoting grain coarsening and phase transformation that can reduce hardness. This is a common phenomenon in multi-pass welding and must be accounted for in the design of multi-layer cladding systems.
Engineering Practice Implications
The study provides several important insights for engineers designing Ni-based cladding systems:
- Gradient transition layers are essential: The use of transition layers with gradually decreasing Fe content effectively reduces the dilution effect and creates a more homogeneous cladding layer. This approach is particularly important when cladding dissimilar materials with large composition differences.
- TIG welding is suitable for high-quality cladding: The TIG process provides the low dilution rate and precise control necessary for depositing high-quality Ni-based cladding layers. The process is particularly suitable for applications where surface quality and composition control are critical.
- Microsegregation must be managed: The segregation of Mo and Nb to grain boundaries can affect the long-term performance of the cladding layer. Engineers should consider post-weld heat treatment to homogenize the microstructure and reduce segregation.
- Hardness is not the only criterion: While the microhardness results demonstrate the effectiveness of the cladding, engineers should also consider other properties such as toughness, fatigue resistance, and corrosion resistance when evaluating the overall performance of the cladding system.
Key Questions and Reflections
The study raises several questions that warrant further investigation. First, the effect of the number of transition layers on the overall cladding performance is not fully explored. While three transition layers were used in this study, the optimal number may vary depending on the specific application and the required dilution rate. A systematic study of the relationship between the number of transition layers and the final cladding properties would be valuable.
Second, the study focuses primarily on microstructure and hardness but does not extensively address the mechanical properties such as tensile strength, elongation, and fatigue resistance. For engineering applications, these properties are equally important, and a more comprehensive mechanical characterization would enhance the practical value of the study.
Third, the corrosion resistance of the multi-layer cladding system is not evaluated. Since the primary motivation for Ni-based cladding is often corrosion resistance, the corrosion performance of the transition layers and the overall cladding system should be assessed under relevant corrosion conditions.
Study Insights and Implications
This paper is a well-conducted study that demonstrates the effectiveness of a multi-layer Ni-based cladding approach for improving the surface properties of Q345 steel. The systematic design of transition layers with varying Fe content is a practical and elegant solution to the dilution problem that plagues Ni-based cladding on low-carbon steel. The detailed microstructural analysis, including the characterization of phase composition, elemental distribution, and microsegregation patterns, provides valuable insights into the metallurgical behavior of Ni-based cladding layers. The study is particularly relevant for engineers working in the field of surface engineering, where the development of high-performance cladding systems for industrial components is a critical challenge. The findings contribute to the understanding of how to design multi-layer cladding systems that balance dilution control, microstructural homogeneity, and mechanical performance.