Domestication Trial Study of Cobalt-Based Wear-Resistant Cemented Carbide Arc Welding Overlay

Literature Overview

This study, published in the journal "Welding Machine" in 2010 by Ma Ming, Li Yinan, and Li Chunguang from Harbin Boiler Works Co., Ltd., investigates the domestication of cobalt-based wear-resistant cemented carbide arc welding overlay materials. The research addresses a critical supply chain issue: the dependence on imported cobalt-based hardfacing electrodes and wires for wear-resistant overlay applications in power generation equipment. The study aims to develop domestically produced welding consumables that match or exceed the performance of imported products while reducing costs and ensuring supply security.

Core Technical Content

Cobalt-based alloy systems are widely used for wear-resistant overlay applications due to their exceptional combination of high hardness, excellent hot hardness, good corrosion resistance, and superior toughness. Unlike iron-based or nickel-based hardfacing alloys, cobalt-based alloys maintain their hardness at elevated temperatures (up to 600–800°C) due to the solid solution strengthening effect of carbide-forming elements such as chromium, tungsten, molybdenum, and vanadium. In the power generation industry, cobalt-based overlays are used on boiler components such as air preheater tubes, furnace burners, and slag spouts, where the combination of high temperature, abrasive wear, and corrosive environment demands the most demanding overlay materials.

The study focused on developing a cobalt-based cemented carbide welding consumable that could be applied by shielded metal arc welding (SMAW) or flux-cored arc welding (FCAW) to deposit wear-resistant overlay layers on boiler components. The key challenge was to formulate a consumable that achieves the desired hardness (800–1000 HV) while maintaining sufficient toughness to resist cracking during welding and in service. The cemented carbide component, typically WC or Cr₃C₂, provides the primary wear resistance, while the cobalt-based matrix ensures good bonding and thermal stability.

The development process involved multiple iterations of alloy composition optimization, consumable design, and welding procedure qualification. The researchers systematically varied the cobalt, chromium, tungsten, and carbon content to identify the optimal composition for the target application. The flux coating composition was also carefully designed to promote proper arc stability, slag formation, and microstructure refinement. The final consumable design incorporated a tubular electrode geometry that provided consistent arc characteristics and uniform deposition.

Microstructural Characterization and Performance Testing

The microstructure of the domestic cobalt-based overlay consists of a solid solution matrix of Co, Cr, and W with dispersed primary and secondary carbide phases. The primary carbides are WC particles retained from the consumable, while the secondary carbides (Co₃W, Co₂W₄C, Co₆W₆C) form during solidification. The distribution and morphology of these carbides directly influence the wear resistance and fracture behavior of the overlay.

The wear testing was conducted using standardized abrasive wear tests (ASTM G65) and field trials on actual boiler components. The results showed that the domestic consumable achieved wear resistance comparable to the imported product, with a wear rate reduction of 80–85% compared to the uncladded base material. The hot hardness retention at 600°C was also equivalent to the imported product, confirming the suitability of the domestic consumable for high-temperature wear applications.

The mechanical property testing included hardness profiling across the overlay layer, microstructural examination of the weld interface, and bond strength testing. The hardness gradient from the overlay surface to the base metal was found to be gradual, with the transition zone extending approximately 0.5–1.0 mm into the base metal. This gradual transition is beneficial for stress distribution and reduces the risk of overlay spalling under cyclic loading.

Engineering Practice and Qualification

The qualification of the domestic cobalt-based overlay consumable followed the requirements of NB/T 47014 for welding procedure qualification. The qualification process included welding procedure specification (WPS) development, qualification welds, destructive testing (hardness, microstructure, bond strength), and non-destructive testing (MT and PT). The results confirmed that the domestic consumable meets all acceptance criteria specified in the applicable standards.

For boiler component applications, the overlay thickness is typically 3–5 mm for air preheater tubes and 5–10 mm for furnace burners and slag spouts. The multi-pass welding strategy involves a transition layer of nickel-based or austenitic alloy followed by multiple passes of the cobalt-based hardfacing material. The interpass temperature is controlled to remain below 250°C to prevent excessive softening of previously deposited layers. A post-weld stress relief treatment at 650–700°C for 2 hours is recommended to reduce residual stresses and minimize the risk of delayed cracking.

Key Questions and Reflections

The successful domestication of cobalt-based wear-resistant overlay consumables is a significant achievement from both a technical and strategic perspective. However, several questions remain regarding the long-term performance and reliability of the domestic product. First, the field performance data from extended service trials should be collected and analyzed to confirm that the laboratory and qualification test results translate to reliable performance in actual service conditions. Second, the consistency of the domestic product across different production batches should be monitored to ensure that the performance characteristics remain within the specified ranges.

The study also raises broader questions about the domestication of other specialized welding consumables for power generation and petrochemical applications. The same approach—systematic composition optimization, consumable design, qualification testing, and field validation—can be applied to nickel-based superalloy overlays, titanium-based overlays, and other specialized hardfacing materials. The strategic importance of reducing dependence on imported consumables cannot be overstated, particularly for critical infrastructure such as power plants and refineries where supply security is essential.

Study Insights and Implications

This research demonstrates the feasibility and value of domesticating specialized welding consumables that are currently dependent on imports. The systematic approach to alloy development, consumable design, and qualification testing provides a replicable framework for other specialized welding applications. The economic benefits of domestic production, with a 30–40% cost reduction, are substantial and contribute to the overall competitiveness of the power generation equipment manufacturing industry. Engineers and procurement specialists should consider the domestic consumable as a viable alternative to imported products for cobalt-based overlay applications, provided that the qualification and quality assurance requirements are rigorously met. The broader implications extend to the development of a self-sufficient supply chain for specialized welding consumables, which is essential for the long-term sustainability and security of China's heavy industry sector.