Development of Cemented Carbide Composite Hardfacing Electrodes

Literature Overview

This 1990 paper by Zou Xumei from Zigong Cemented Carbide Works, published in New Technology and New Process, presents a novel approach to hardfacing electrode manufacturing using cemented carbide powder. The paper describes the use of recycled powder from cemented carbide production and mechanically crushed waste cemented carbide to manufacture composite hardfacing electrodes. The key innovation is the use of a composite electrode structure that overcomes the porosity problem inherent in tubular hardfacing electrodes.

Cemented carbide is one of the hardest engineering materials, with hardness values exceeding 90 HRA. Its application in hardfacing overlays provides exceptional wear resistance for components subjected to severe abrasive or erosive conditions. However, the incorporation of cemented carbide particles into a weld deposit presents significant metallurgical challenges, including particle settling, porosity formation, and dilution softening.

Technical Challenges of Cemented Carbide Hardfacing

The use of cemented carbide in hardfacing overlays is limited by several technical challenges:

1. Particle settling: Cemented carbide particles are denser than the molten weld metal and tend to settle to the bottom of the weld pool during solidification. This results in a non-uniform distribution of hard particles and reduced hardness at the surface.
2. Porosity: The high melting point of cemented carbide particles can trap gas in the weld pool, leading to porosity in the overlay. This is particularly problematic in tubular electrodes, where the powder filling is packed in a tube and the porosity rate can be as high as 20–30 percent.
3. Dilution softening: The base metal dilutes the overlay during welding, reducing the hardness of the near-surface region. This effect is more pronounced with hard particles that do not mix with the molten metal.
4. Brittleness: The cemented carbide particles themselves are extremely hard but brittle. If the matrix material surrounding the particles is not sufficiently tough, the overlay is prone to cracking and spalling.

Composite Electrode Design

The composite electrode design proposed in this paper addresses these challenges through a novel electrode structure. Instead of using a single type of filler, the electrode combines cemented carbide powder with a suitable matrix alloy in a configuration that promotes uniform particle distribution and minimizes porosity.

The key innovation is the use of a composite structure rather than a tubular structure. In a tubular electrode, the powder filling is packed in a steel tube, and the powder particles can settle during storage and handling, leading to non-uniform distribution. The composite electrode design incorporates the cemented carbide powder directly into the electrode composition, ensuring a more uniform distribution throughout the weld deposit.

Manufacturing Process

The manufacturing process for the composite electrode involves the following steps:

1. Powder preparation: Cemented carbide powder is obtained from recycled production waste or mechanically crushed waste cemented carbide. The powder is sieved to a uniform particle size distribution, typically in the range of 50–200 micrometers.
2. Matrix alloy preparation: A suitable matrix alloy is prepared to provide the necessary toughness and weldability. The matrix alloy typically contains iron, nickel, chromium, and cobalt, with the specific composition optimized for the intended application.
3. Composite electrode fabrication: The cemented carbide powder and matrix alloy are combined in a specific ratio and fabricated into an electrode. The fabrication process must ensure uniform distribution of the carbide particles throughout the electrode.
4. Electrode coating: The electrode is coated with a suitable flux coating to stabilize the arc and protect the weld from atmospheric contamination. The coating composition is optimized for the specific electrode composition and welding parameters.
5. Quality control: The electrode is tested for density, porosity, and mechanical properties to ensure it meets the required specifications.

Performance Characteristics

The composite electrode offers several performance advantages over conventional tubular electrodes:

• Reduced porosity: The composite structure eliminates the porosity problem associated with tubular electrodes, resulting in a denser overlay with improved mechanical properties.
• Uniform particle distribution: The cemented carbide particles are uniformly distributed throughout the overlay, providing consistent hardness and wear resistance.
• Improved weldability: The composite electrode can be welded with standard SMAW equipment, making it accessible to a wide range of applications.
• Cost effectiveness: The use of recycled cemented carbide powder significantly reduces the material cost of the electrode.

The hardness of the overlay achieved with this electrode is typically in the range of 65–75 HRC, depending on the cemented carbide content and the welding parameters. This hardness level provides excellent resistance to abrasive wear and is suitable for applications such as mining equipment, crusher components, and other wear-critical parts.

Engineering Applications

The composite hardfacing electrode is suitable for a wide range of applications where abrasive or erosive wear is the primary failure mechanism. Some typical applications include:

The electrode can be applied using standard SMAW welding equipment, making it accessible to maintenance shops with limited welding resources. The welding parameters are similar to those used for conventional hardfacing electrodes, with the exception that the current may need to be slightly reduced to prevent overheating of the cemented carbide particles.

Study Insights

This paper represents an important contribution to the field of hardfacing technology. The use of recycled cemented carbide powder is not only technically innovative but also environmentally sustainable. The utilization of production waste and mechanically crushed scrap reduces the environmental impact of cemented carbide manufacturing and provides a cost-effective source of hard particles for hardfacing electrodes.

The solution of the porosity problem is particularly significant. Porosity in hardfacing overlays is a common defect that can severely compromise the wear resistance and structural integrity of the overlay. The composite electrode design eliminates this defect by ensuring a uniform distribution of the cemented carbide particles and promoting complete fusion during welding.

The simplicity of the manufacturing process is another important advantage. The electrode can be manufactured using standard electrode manufacturing equipment, without the need for specialized machinery or processes. This makes the technology accessible to a wide range of manufacturers and reduces the cost of the electrode.

The paper also demonstrates the importance of waste utilization in materials engineering. The cemented carbide production process generates significant quantities of waste powder that would otherwise be discarded. By incorporating this waste into a hardfacing electrode, the paper demonstrates a circular economy approach that reduces waste and creates value from by-products.

The work by Zou Xumei is a valuable reference for engineers developing hardfacing solutions for abrasive wear applications. The composite electrode concept can be extended to other hard particle systems, including tungsten carbide, silicon carbide, and boron carbide, providing a platform for the development of advanced hardfacing technologies.

The paper also highlights the importance of process innovation in welding technology. The solution of the porosity problem through electrode design rather than process modification demonstrates a fundamental principle in engineering: the most effective solutions are often those that address the root cause of the problem rather than its symptoms.