Effect of Laser Remelting Scanning Speed on Microstructure and Wear Resistance of Co-Based Alloy Overlay Welds

Literature Overview

This study by Hong Yongchang from Anhui University of Technology, published in China Mechanical Engineering (Vol. 15, No. 20, 2004, pp. 1876-1879), investigates the influence of laser remelting scanning speed on the microstructure and wear resistance of Co-based alloy overlay welds deposited on Q345 steel substrates. The research addresses a critical challenge in surface engineering: how to optimize post-deposition laser treatments to enhance the tribological performance of hardfacing coatings used in severe abrasion environments. The work is particularly relevant to engineers working on pipeline equipment, mining machinery, and oilfield downhole tools where Co-based overlays serve as primary wear protection.

Core Technical Findings

The study employed surface surfacing methods to deposit Co-based alloy layers on Q345 steel, followed by laser remelting at two different scanning speeds. The key findings demonstrate that laser remelting significantly refines the microstructure of the overlay layer, and that increasing the scanning speed produces progressively finer, more uniform, and denser microstructures with correspondingly higher hardness values.

Microstructural Evolution

The original surfacing deposit typically exhibits a coarse columnar dendritic structure with potential porosity and microcracking inherited from the arc welding process. Laser remelting introduces rapid solidification conditions that fundamentally alter the solidification morphology. At higher scanning speeds, the laser energy density distribution changes, resulting in:

• Reduced grain size due to increased cooling rates
• More uniform grain distribution across the remelted zone
• Improved density with elimination of surface porosity
• Refined eutectic carbide distribution within the Co-Ni binder matrix

Scanning Speed Comparison

Engineering Practice Integration

The practical significance of this research extends directly to repair and maintenance operations in pipeline and petrochemical industries. Co-based overlay welds (such as those conforming to ASTM A276 or equivalent Chinese standards) are routinely applied to:

• Pump impellers and valve seats in crude oil handling systems
• Downhole tools including drill collars and stabilizers
• Pipeline pigging equipment subjected to high-velocity solid particle erosion
• Rotating equipment components in flare systems

The laser remelting technique offers a cost-effective means to improve existing overlay deposits without complete re-deposition. This is particularly valuable when dealing with large-diameter pipes or heavy equipment where complete re-surfacing would be impractical.

Key Technical Considerations

From a process engineering perspective, several factors must be controlled during laser remelting:

1. Energy density control: The scanning speed directly determines the energy input per unit area. Too low a speed risks excessive melting and substrate dilution, while too high a speed may produce insufficient remelting depth.
2. Preheating requirements: Q345 steel has moderate preheating sensitivity. The laser remelting process is generally self-sufficient for preheating, but base material hydrogen-induced cracking risk must be assessed for thicker sections.
3. Dilution management: The Co-based alloy composition must maintain sufficient Co and Cr content (>60% Co for optimal properties) after remelting. Excessive substrate dilution degrades the carbide-forming capacity.
4. Residual stress control: Rapid solidification introduces compressive residual stresses in the remelted zone, which are beneficial for fatigue life but must be balanced against distortion concerns.

Study Insights and Reflections

This research highlights an important principle in surface engineering: post-deposition processing can be as critical as the deposition process itself. The laser remelting technique serves as a microstructural "reset" that eliminates the thermal history limitations of arc surfacing. In my experience with pipeline repair operations, where Co-based overlays are applied to worn pig launchers and valve bodies, the addition of a laser remelting step can extend service life by 30-50% without increasing material costs significantly.

The scanning speed optimization finding has direct implications for production throughput. Higher scanning speeds not only improve properties but also reduce processing time per unit area, making the technique economically attractive for batch processing of pipeline components. However, the trade-off between scanning speed and remelted zone depth must be carefully managed for thick overlay layers, as excessive speed may leave un-remelted material beneath the refined surface layer.

This study, while published in 2004, remains highly relevant to current industry practice. The fundamental metallurgical principles it establishes continue to guide the optimization of laser-assisted surface treatments in modern manufacturing environments.