Zhuojin Pipe Fitting Co., LtdEffect of Laser Remelting Scanning Speed on Microstructure and Wear Resistance of Co-Based Surfacing Layer
Literature Overview
The paper authored by Hong Yongchang from Anhui University of Technology, published in China Mechanical Engineering (2004, Vol. 15, No. 20, pp. 1876–1879), investigates the influence of laser remelting scanning speed on the microstructure and wear resistance of a Co-based alloy surfacing layer deposited on Q345 steel substrate. The work addresses a critical engineering challenge in tribological surface engineering: how to optimize post-deposition thermal processing parameters to enhance the functional performance of hardfacing overlays. The study employs surface surfacing followed by laser remelting as a two-step approach, examining two distinct scanning speeds and their effects on grain refinement, hardness distribution, and abrasive wear behavior.
Core Technical Content
The research methodology follows a systematic approach: first, a Co-based alloy layer is deposited onto Q345 steel via conventional surfacing techniques, establishing a baseline microstructure. Subsequently, surface laser remelting is applied at two different scanning speeds to modify the near-surface microstructure. The key findings demonstrate that laser remelting significantly refines the microstructure of the surfacing layer, and that increasing the scanning speed leads to further grain refinement, more uniform and dense microstructure, and higher hardness values.
The underlying metallurgical mechanism is rooted in the rapid solidification induced by the laser beam. During laser remelting, the extremely high cooling rates (typically on the order of 10^3 to 10^6 K/s) suppress the growth of primary carbides and promote the formation of fine, uniformly distributed secondary phases within the binder matrix. The scanning speed directly controls the heat input per unit length, thereby governing the solidification rate and the resulting microstructural features.
Interpretation of Key Technical Parameters
| Parameter | Effect on Microstructure | Effect on Wear Resistance |
|---|---|---|
| Lower scanning speed | Coarser grains, larger carbides | Moderate hardness, lower wear resistance |
| Higher scanning speed | Finer grains, denser microstructure | Higher hardness, superior wear resistance |
| Laser power (constant) | Controls melt pool depth | Affects dilution and alloy retention |
| Melt pool aspect ratio | Influences columnar grain orientation | Determines crack susceptibility |
The relationship between scanning speed and solidification rate can be understood through the following thermodynamic considerations. At higher scanning speeds, the thermal input per unit length decreases, resulting in steeper temperature gradients and faster solidification rates. This promotes heterogeneous nucleation at the original solid-liquid interface and suppresses lateral grain growth, yielding a finer equiaxed microstructure with more uniformly dispersed hard carbide particles.
Connection with Engineering Practice
In the context of pipe fitting and valve component repair, Co-based hardfacing layers are widely employed for components subjected to severe sliding and abrasive wear, such as valve seats, pump impellers, and compressor components. The laser remelting technique offers a practical solution for post-weld microstructural optimization without requiring additional alloying elements or complex multi-pass welding strategies.
From a standards perspective, the performance of Co-based surfacing layers should be evaluated against relevant specifications such as ASTM A276 for cobalt-based castings, AWS A5.16 for cobalt-base welding consumables, and ISO 2553 for surface engineering terminology. The wear resistance improvement achieved through laser remelting directly correlates with extended service life in applications governed by API 6D for line pipe and pipe fittings, where surface durability is a critical design consideration.
Process Optimization and Defect Analysis
The laser remelting process introduces specific quality concerns that must be addressed in engineering practice:
- Cracking susceptibility – Rapid solidification can increase the likelihood of microcracking, particularly if the scanning speed is too high and the melt pool becomes excessively thin.
- Porosity – Inadequate shielding during laser remelting may lead to gas inclusion, especially in Co-based alloys containing significant carbon content.
- Dilution control – The remelting depth must be carefully controlled to avoid excessive substrate dilution, which would degrade the alloy composition and reduce hardness.
- Residual stress – The rapid thermal cycling during laser remelting generates significant residual stresses that may require subsequent stress-relief annealing.
The optimal scanning speed represents a balance between achieving maximum grain refinement and maintaining adequate melt pool stability. In practice, a scanning speed in the range of 100–300 mm/min typically yields the best combination of microstructural refinement and defect-free processing for Co-based overlays on carbon steel substrates.
Study Insights and Engineering Implications
This research provides valuable insight into the post-deposition processing of hardfacing overlays, demonstrating that microstructural optimization through laser remelting is a viable and effective strategy for enhancing wear performance. For engineers working on pipe fitting repair and rehabilitation, this approach offers a minimally invasive method to restore or enhance surface properties without replacing entire components. The findings also underscore the importance of process parameter optimization in surface engineering applications, where small changes in processing conditions can yield significant improvements in functional performance.