Preparation and Wear Resistance of Cobalt-Based Tungsten Carbide Overlay Cladding on H13 Die Steel

Overview of the Study

This 2019 paper, published in Forging Technology, investigates the preparation and wear resistance of cobalt-based tungsten carbide (WC) overlay cladding layers on H13 hot work die steel. The authors, Zhou Yanxia, Hong Feng, Wang Huajun, and Li Ainong from Hubei Water Conservancy and Hydropower Vocational College and Wuhan University of Technology, employed powder plasma arc welding to deposit WC-reinforced cobalt-based overlay layers with varying WC content on H13 substrate. The study was supported by the National Natural Science Foundation of China.

Material Design and Experimental Configuration

The study designed four different overlay formulations varying the WC content to investigate the effect of reinforcing phase concentration on overlay layer properties. The cobalt-based alloy matrix provides excellent hot hardness, corrosion resistance, and thermal stability, while the WC particles serve as hard reinforcing phases that significantly enhance wear resistance.

The powder plasma arc welding process was selected for its ability to produce overlay layers with low dilution rates, good metallurgical bonding, and uniform composition. The plasma arc provides a concentrated heat source with high energy density, enabling precise control of the melting and solidification process.

Microstructure and Compositional Analysis

Metallographic examination and scanning electron microscopy (SEM) were employed to analyze the microstructure and morphology of the overlay layers. Energy-dispersive X-ray spectroscopy (EDS) was used to analyze the chemical composition of the overlay layers and the interface regions.

The microstructure of the cobalt-based WC overlay layers is characterized by a complex phase composition:

• Matrix phase: Cobalt-based austenite or martensite, depending on the cooling rate and alloy composition
• Reinforcing phase: Tungsten carbide (WC) particles, which may partially dissolve or transform during the welding process
• Secondary phases: Chromium carbides and other intermetallic compounds formed from alloying elements

The metallurgical bonding between the overlay layer and the H13 substrate was confirmed to be good, indicating effective fusion and interdiffusion at the interface. The dilution rate was controlled within acceptable limits to maintain the desired overlay layer composition and properties.

Mechanical Properties and Wear Performance

The microhardness measurements and wear testing results reveal clear trends with respect to WC content:

The results demonstrate that the overlay layer hardness increases significantly with WC content up to 30 wt%, after which the improvement plateaus or slightly decreases. The wear resistance improvement follows a similar trend, with the 30 wt% WC formulation achieving the best wear resistance, approximately 10 times that of the H13 base steel.

The slight decrease in hardness and wear resistance at 40 wt% WC content may be attributed to several factors:

1. Particle agglomeration: Excessive WC content may lead to uneven distribution and agglomeration of carbide particles, creating stress concentration sites.
2. Matrix brittleness: High WC content may embrittle the cobalt matrix, reducing its ability to accommodate deformation and resist crack propagation.
3. Phase transformation: At high WC concentrations, the thermodynamic stability of the WC phase may be affected, leading to partial decomposition or transformation into lower-hardness phases.

Engineering Application and Design Guidelines

The findings of this study have direct implications for the design of overlay cladding systems for hot work die applications. The optimal WC content of 30 wt% provides a clear design guideline for maximizing wear resistance while maintaining adequate toughness and crack resistance.

Key design considerations for practical applications include:

1. Thermal cycling resistance: Hot work dies experience repeated heating and cooling cycles, and the overlay layer must maintain its properties under thermal fatigue conditions.
2. Residual stress management: The mismatch in thermal expansion coefficients between the overlay layer and the substrate can generate significant residual stresses, potentially leading to cracking or delamination.
3. Surface finish requirements: The overlay layer surface finish affects the material flow and ejection in forging operations, and may require post-weld machining.
4. Process repeatability: Consistent overlay layer properties require precise control of welding parameters, powder feed rate, and travel speed.

The powder plasma arc welding process offers several advantages for this application, including low dilution, good bonding strength, and the ability to deposit thick overlay layers in a single pass. However, the process also has limitations, including equipment cost, powder handling requirements, and the need for inert gas shielding to prevent oxidation.

Study Insights and Practical Implications

This research provides valuable insights into the design of cobalt-based overlay cladding systems for hot work die applications. The systematic investigation of WC content effects establishes clear guidelines for formulation optimization, and the characterization of microstructure-property relationships provides a scientific basis for understanding the wear mechanisms involved.

The 10-fold improvement in wear resistance achieved with the 30 wt% WC formulation represents a substantial enhancement in die service life, which translates directly into economic benefits through reduced die replacement frequency and lower production downtime. For industries that rely heavily on hot forging operations, such as automotive, aerospace, and power generation, the economic impact of such improvements can be significant.

The study also highlights the importance of balancing hardness and toughness in overlay layer design. While higher WC content increases hardness, it may compromise toughness and crack resistance, which are critical for hot work die applications where impact loading and thermal cycling are present. The optimal formulation must therefore consider the specific service conditions and loading scenarios of the application.

This research contributes to the advancement of surface engineering technologies for hot work dies and demonstrates the effectiveness of powder plasma arc welding as a process for depositing high-performance overlay layers. The systematic approach to formulation design and property characterization provides a template for similar studies on other overlay systems and application scenarios, and the findings offer practical guidelines for engineers involved in die design and surface engineering.