TiC-VC Wear-Resistant Overlay Welding Electrodes for Industrial Abrasive Applications

Literature Overview and Industrial Context

The research by Zou Zengda, Wang Xinhong, Yang Shanglai, Qu Shiyao, and Wang Yufu from Shandong University and Shandong Lunan Fertilizer Plant, published in 2001 in Materials Science and Engineering, presents the development and characterization of TiC-VC composite carbide wear-resistant overlay welding electrodes. This work, supported by the Shandong Provincial Natural Science Foundation (Z2000F02), addresses a critical industrial need: the development of economical and effective overlay solutions for components subjected to severe abrasive wear in chemical and mining applications.

The industrial context is particularly relevant to the fertilizer industry, where equipment such as slurry pumps, valve seats, and mixer shafts are subjected to continuous abrasive wear from solid particles suspended in corrosive slurries. The conventional approach of replacing worn components at frequent intervals results in significant downtime and material costs. Overlay welding with hard carbide-containing electrodes offers a practical solution by restoring or enhancing the wear resistance of existing components, extending their service life by a factor of 3–10 times compared to the base material.

Electrode Composition and Metallurgical Design

The TiC-VC composite carbide system was selected based on a careful analysis of the thermodynamic stability, hardness, and compatibility of various carbide types with steel matrices. Titanium carbide (TiC) has a theoretical hardness of 24 GPa and a melting point of 3140 °C, while vanadium carbide (VC) has a theoretical hardness of 28 GPa and a melting point of 2830 °C. The combination of these two carbides in a single electrode provides a synergistic effect: TiC offers excellent thermal stability and resistance to oxidation, while VC contributes higher hardness and improved wear resistance.

The electrode design incorporates several key metallurgical considerations. The carbon content is deliberately elevated above the eutectic composition to ensure sufficient carbon availability for carbide formation during solidification. The chromium content provides both carbide formation (Cr7C3, Cr23C6) and corrosion resistance through the formation of a passive chromium oxide film on the overlay surface. The nickel addition stabilizes the austenitic matrix, reducing the transformation stress during cooling and improving the toughness of the overlay layer.

Microstructural Characterization and Hardness Analysis

The microstructure of the TiC-VC overlay weld metal exhibits a complex multi-phase morphology consisting of a primary carbide phase (TiC and VC particles), a secondary carbide phase (Cr7C3, Cr23C6), and a metallic matrix that is predominantly martensitic with retained austenite. The TiC and VC particles typically range in size from 5 to 30 μm and are distributed throughout the matrix, providing the primary wear resistance mechanism through load support and resistance to abrasive cutting.

The hardness of the overlay layer is typically in the range of 800–1000 HV, which is significantly higher than the 200–300 HV of typical carbon steel substrates. The hardness distribution across the overlay thickness is not uniform: the surface layers exhibit the highest hardness due to the rapid cooling rate and fine carbide dispersion, while the layers near the fusion boundary show lower hardness due to the higher dilution from the base metal and coarser microstructure.

Wear Mechanism Analysis and Performance Evaluation

The wear resistance of the TiC-VC overlay is governed by two primary mechanisms: abrasive resistance and adhesion resistance. The hard TiC and VC particles resist abrasive wear by providing a load-bearing structure that prevents the cutting and plowing of the overlay surface by abrasive particles. The metallic matrix, primarily martensitic, provides the necessary toughness to prevent the debonding of carbide particles under impact loading.

The adhesion resistance is enhanced by the chromium content in the overlay, which promotes the formation of a protective oxide film on the surface during wear. This oxide film reduces the direct metal-to-metal contact between the overlay and the counterface, thereby minimizing adhesive wear. The retained austenite in the matrix also contributes to adhesion resistance by transforming to martensite under deformation (strain-induced transformation), which increases the local hardness and reduces the tendency for material transfer.

Engineering Application and Process Considerations

The application of TiC-VC overlay electrodes in industrial settings requires careful attention to several process parameters. The welding current should be controlled in the range of 180–250 A for typical 3.2 mm diameter electrodes, with a voltage of 24–28 V and a travel speed of 200–300 mm/min. The interpass temperature should not exceed 200 °C to prevent the coarsening of carbide particles and the transformation of retained austenite to martensite, which would increase the brittleness of the overlay.

A critical process consideration is the management of hydrogen-induced cracking (HIC) in the overlay layer. The high carbon and alloy content of the overlay metal increases its susceptibility to HIC, particularly in the presence of moisture in the electrode coating. The electrode storage and baking procedures must be strictly followed: electrodes should be stored at 100–150 °C and baked at 300–350 °C for 2 hours before use. Additionally, the substrate should be preheated to 150–200 °C to reduce the cooling rate and minimize the formation of martensite with high residual stress.

Study Reflections and Practical Implications

This research demonstrates the effectiveness of composite carbide overlay electrodes in addressing the wear challenges faced by the chemical and mining industries. The TiC-VC system offers a favorable balance of hardness, toughness, and corrosion resistance that is well-suited to the demanding conditions of slurry service and abrasive wear. The relatively low cost of the electrode system compared to advanced overlay methods such as laser cladding or plasma spraying makes it an attractive option for large-scale industrial applications.

However, the research also highlights the importance of understanding the fundamental metallurgy of the overlay process. The wear performance of the overlay is not determined solely by the hardness of the carbide phase but by the complex interaction between the carbide particles, the metallic matrix, and the residual stress state of the overlay. Engineers must consider the entire system—electrode composition, welding parameters, substrate preparation, and post-weld treatment—to achieve optimal wear performance.

The findings of this research have broader implications for the design of wear-resistant overlay systems. The concept of composite carbide reinforcement, where multiple carbide types are combined to achieve synergistic properties, can be extended to other carbide systems such as WC-Co, Cr3C2-NiCr, and Mo2C-Fe. The key principle is to match the carbide type and distribution to the specific wear mechanism encountered in service, whether it be abrasion, adhesion, fatigue, or a combination thereof.