Zhuojin Pipe Fitting Co., LtdEffect of Welding Current on Microstructure and Properties of Nickel-Based Niobium Composite Surfacing Layer
Research Background and Objectives
The study by Geng Yanchao, Deng Dewei, Tian Xin, and Sun Qi, published in Physical Testing and Chemical Analysis (Physical Section) in 2019, investigates the influence of welding current on the microstructure and properties of a nickel-based alloy surfacing layer reinforced with in-situ formed NbC particles. The surfacing was performed on Z2C-N18-10 stainless steel substrate using plasma surfacing technology with niobium powder additions. This research is part of a larger project supported by the National 973 Program and the National Natural Science Foundation of China, reflecting the significance of developing advanced surfacing materials for demanding industrial applications.
Experimental Design and Process Parameters
The authors systematically varied the welding current from 110 A to 170 A while maintaining other parameters constant, and analyzed the resulting microstructure, hardness, and wear resistance of the surfacing deposits. The experimental design and key results are summarized in the following table.
| Welding Current | Microstructure | Hardness (HV0.3) | Wear Resistance | Surface Quality |
|---|---|---|---|---|
| 110 A | NbC particles present but coarser; poor bead formation | 509 HV0.3 (highest) | Worst | Poor bead shape and surface finish |
| 140 A | Optimal NbC particle precipitation; fine and uniform | Moderate (between 110 A and 170 A) | Best | Good bead formation and surface quality |
| 170 A | NbC particles dissolved or coarsened; matrix softened | Significantly reduced | Reduced compared to 140 A | Acceptable but with some defects |
The results clearly demonstrate that there is an optimal welding current range for achieving the best combination of microstructure, hardness, and wear resistance. At 110 A, the heat input is insufficient to achieve proper melting and mixing of the niobium powder with the nickel-based alloy, resulting in poor bead formation and non-uniform NbC particle distribution. At 170 A, the excessive heat input causes partial dissolution of the NbC particles and softening of the matrix, reducing both hardness and wear resistance. At 140 A, the heat input is sufficient to achieve complete melting and uniform mixing while preserving the NbC particles, resulting in the best overall performance.
Microstructural Evolution with Welding Current
The microstructural evolution with welding current can be understood through the lens of thermodynamics and kinetics of carbide formation and dissolution. At low current (110 A), the heat input is insufficient to fully dissolve and homogenize the niobium powder, leading to non-uniform distribution of NbC particles and poor bead formation. The NbC particles that do form tend to be coarser and more irregularly distributed, which is detrimental to wear resistance. The high hardness value at 110 A (509 HV0.3) is likely due to the presence of coarse, undissolved niobium particles or NbC particles that were not fully integrated into the matrix, which can artificially inflate the hardness measurement without providing uniform wear resistance.
At optimal current (140 A), the heat input is sufficient to achieve complete melting and homogeneous mixing of the niobium powder with the nickel-based alloy. The NbC particles precipitate during solidification in a fine and uniform distribution, providing effective wear resistance through a combination of particle hardness and matrix support. The microstructure at this current level represents an optimal balance between particle size, distribution, and matrix properties.
At high current (170 A), the excessive heat input causes partial dissolution of the NbC particles during the melting stage, reducing the volume fraction of the hard phase. The remaining particles may also coarsen due to Ostwald ripening during the extended time at elevated temperature. The matrix is also softened due to the higher carbon and alloy content in solution, leading to a significant reduction in hardness. The combination of reduced hard phase volume and softened matrix results in the lowest wear resistance among the three current levels tested.
Wear Mechanism Analysis
The wear resistance of the surfacing layer is governed by the interaction between the NbC particles and the matrix during the abrasive wear process. At 140 A, the fine and uniformly distributed NbC particles provide effective resistance to abrasive particle penetration and sliding. The particles act as load-bearing elements that support the matrix and prevent plastic deformation. The matrix, being a nickel-based alloy, provides good toughness and resistance to crack propagation, ensuring that the particles remain firmly anchored during wear. At 110 A, the non-uniform distribution of NbC particles creates weak regions in the matrix where wear can initiate and propagate more easily. At 170 A, the reduced volume fraction of NbC particles and the softened matrix lead to accelerated matrix wear and premature particle pull-out.
Process Optimization and Quality Control
The study provides valuable guidance for the optimization of plasma surfacing parameters for nickel-based niobium composite surfacing layers. The optimal welding current of 140 A represents a process window that should be maintained in production applications. Key quality control parameters include:
- Welding current: maintain within ±10% of the optimal value (126-154 A)
- Travel speed: must be synchronized with welding current to maintain consistent heat input
- Powder feed rate: must be optimized to achieve the desired NbC particle volume fraction
- Gas flow rate: must be sufficient to protect the molten pool and deposited layer from atmospheric contamination
For production applications, real-time monitoring of welding parameters and periodic microstructural and hardness verification are essential to ensure consistent quality. The use of non-destructive testing methods such as ultrasonic testing (UT) or eddy current testing (ET) can be employed to detect subsurface defects such as porosity and lack of fusion without damaging the surfacing layer.
Engineering Applications and Study Insights
The nickel-based niobium composite surfacing layer developed in this study is particularly suitable for applications requiring both high wear resistance and good corrosion resistance, such as components in the oil and gas industry, chemical processing equipment, and marine applications. The nickel-based matrix provides inherent corrosion resistance, while the NbC particles provide exceptional wear resistance. For pipe and pipe fitting applications, such as valve seats and pump impellers in corrosive and abrasive service, this surfacing technology offers a viable solution.
The study underscores the critical importance of welding current optimization in plasma surfacing operations. Even a deviation of 30 A from the optimal current can significantly affect the microstructure and properties of the surfacing layer. Engineers must carefully characterize and document the process window for each surfacing application, and implement robust process control measures to maintain consistent quality in production. The systematic approach adopted in this study, varying one parameter at a time while maintaining others constant, is a sound methodology that should be adopted in all surfacing process development programs.