Zhuojin Pipe Fitting Co., LtdEffect of Plasma Surfacing Parameters on Ni60 Alloy Microstructure and Microhardness
Literature Overview
This study by Ge Yanliu et al. (2011), published in China Surface Engineering, investigates the influence of plasma arc surfacing parameters—specifically welding current and powder feeding rate—on the microstructure and microhardness of Ni60 alloy coatings deposited on Z2CN18-10 stainless steel substrates. The research was supported by the National Natural Science Foundation of China (Grant No. 11072045) and conducted jointly by Dalian University of Technology and Shenyang Blower Works Group. The work is particularly relevant to engineers working on repair and overlay applications for stainless steel components in aggressive service environments.
Core Technical Findings
The researchers employed scanning electron microscopy (SEM), electron probe microanalysis (EPMA), X-ray diffraction (XRD), and microhardness testing to characterize the surfacing layers. The key findings can be summarized as follows:
| Parameter | Test Condition | Result |
|---|---|---|
| Welding current | 110 A | Microhardness of surfacing layer: 630 HV |
| Welding current | Increased from 110 A | Hardness of surfacing layer and HAZ gradient both decreased |
| Powder feeding rate | 6 g/min | Directional elongated and isolated block-like borides appeared in the mid-region |
| Powder feeding rate | Decreased | Needle-like structures near the fusion line became coarser |
The surfacing layer exhibited a pronounced hardness improvement compared to the Z2CN18-10 substrate. A distinct hardness transition and elemental diffusion zone were observed at the fusion region, which is a critical observation for engineers evaluating the mechanical integrity of the overlay.
Microstructural Analysis
The surfacing layer displayed a clear structural gradient consisting of four distinct zones:
- Fusion zone — characterized by partial melting and mixing of substrate and filler material.
- Near-fusion zone dendritic region — exhibiting needle-like, strip-like, and small-flower-shaped (rosette) eutectic structures.
- Near-surface equiaxed grain region — with block-like and lamellar morphologies.
A critical observation was that increasing the welding current did not eliminate the small-flower-shaped eutectic structure in the near-surface region; instead, its volume fraction increased. This indicates that higher heat input promotes eutectic phase formation rather than suppressing it, which has implications for the wear resistance and corrosion resistance of the overlay. When the powder feeding rate was reduced, the needle-like structures near the fusion line coarsened significantly, suggesting that lower powder feeding rates lead to slower cooling rates and larger grain sizes in the dilution zone.
Engineering Practice Implications
For practical applications in the repair of stainless steel piping and fittings, the following process guidelines can be derived:
- Optimal current range: 110 A provides the highest microhardness (630 HV) for Ni60 plasma surfacing on Z2CN18-10. Higher currents should be avoided unless a thicker deposit is required, as they reduce hardness and flatten the hardness gradient.
- Powder feeding rate control: A rate of 6 g/min produces directional borides in the mid-layer, which may contribute to wear resistance but could also be a source of brittleness. Engineers must balance feed rate against desired microstructure.
- Dilution management: The hardness gradient at the fusion zone is a direct indicator of dilution. Lower current and higher powder feeding rates reduce dilution and maintain a steeper hardness gradient, which is beneficial for maintaining the functional properties of the overlay.
The presence of borides is a material-specific feature of Ni60 alloy and warrants careful attention. While borides contribute to high hardness, excessive or coarse boride formation can reduce toughness and increase susceptibility to cracking during thermal cycling or mechanical loading.
Study Insights and Reflections
This study reinforces a fundamental principle in surfacing technology: the cooling rate and dilution ratio, both controlled by welding parameters, are the primary determinants of overlay microstructure and properties. The observation that the small-flower-shaped eutectic structure persists even at higher currents is particularly instructive—it suggests that the Ni60 alloy system has a strong thermodynamic driving force for eutectic phase formation that cannot be easily suppressed by increasing heat input. This has direct implications for the design of overlay schemes in high-temperature or high-stress applications where microstructural stability is paramount.
For pipe repair applications, the hardness transition zone at the fusion boundary represents a potential weak link. The elemental diffusion observed in this region means that the actual composition at the interface may differ from both the substrate and the bulk overlay, potentially creating zones of reduced corrosion resistance or increased susceptibility to hydrogen-induced cracking. Engineers should consider post-weld heat treatment or multi-pass surfacing strategies to mitigate these concerns.