Microstructure and Abrasive Wear Performance of Overlay Alloys Containing Inherent Carbide Particles

Literature Overview

This 2010 study by Tang Wenbo, Guo Yungang, Zhang Yawei, and Wang Hongrui from the School of Materials Science and Engineering at Zhengzhou University, published in the Transactions of the Welding Institute of China, examines overlay alloys that inherently contain carbide particles formed during the welding process itself, rather than through external addition of hard particles. The research is significant because it addresses a practical approach to achieving high wear resistance without the complexity and cost of adding exogenous carbide particles, which can introduce porosity, incomplete melting, and bonding defects in the overlay.

Core Technical Findings

Inherent Carbide Formation Mechanism

The study investigates how specific combinations of C, Cr, Mo, and V in the overlay alloy composition lead to in-situ carbide precipitation during solidification and subsequent cooling. The key alloys examined include:

Carbide Morphology and Distribution

The study reveals that inherent carbide particles exhibit a distinct morphology compared to exogenously added carbides. Inherent carbides form dendritic or network structures along grain boundaries, with sizes typically in the 2–10 μm range. This morphology provides superior load-bearing capability compared to the isolated spherical particles of exogenous additions, because the interconnected carbide network distributes contact stresses more effectively during abrasion.

Abrasive Wear Testing Results

ASTM G65 dry sand rubber-wheel abrasion tests at 10 N load demonstrated that Alloy D achieved the highest wear resistance, with a wear index 72% higher than the baseline Alloy A. The wear mechanism transitions from abrasive grooving in low-carbide alloys to a combination of micro-plowing and carbide fracture in high-carbide alloys. The optimal carbide volume fraction for maximum wear resistance was identified at approximately 35–45 vol%, beyond which excessive brittleness leads to catastrophic spalling.

Thermal Stability

An important finding is the thermal stability of inherent carbides. Unlike exogenous carbide particles that may partially dissolve or sinter during service at elevated temperatures, inherent carbides maintain their morphology and distribution through repeated thermal cycles up to 500°C. This makes them particularly suitable for pressure vessel internals exposed to thermal cycling, such as superheater tubes and hydrogenation reactor internals.

Engineering Practice Implications

For pressure vessel engineers, the concept of inherent carbide formation offers a practical pathway to enhanced wear resistance without the process complications of adding external hard particles. Key considerations include:

1. Consumable selection – Wire or electrode consumables with carefully balanced C, Cr, Mo, V, and W content can be procured from commercial suppliers, avoiding the need for custom powder blending.
2. Process window – The inherent carbide formation is sensitive to cooling rate; slower cooling (as in thick-section SAW overlay) promotes coarser carbides, while faster cooling (as in GMAW or PTA) produces finer distributions. Process selection must be matched to the desired carbide morphology.
3. Standards compliance – The overlay alloys described can be qualified under NB/T 47014 or ASME IX with appropriate qualification testing, as they use conventional welding consumable forms.
4. Application matching – Alloy D type compositions are particularly suitable for pressure vessel internals in coal-water slurry systems, cement kilns, and mining equipment where abrasive wear is the dominant degradation mechanism.

Key Reflections

The elegance of the inherent carbide approach lies in its simplicity and process compatibility. Unlike cermet overlays requiring exotic powders and specialized equipment, inherent carbide alloys can be deposited using standard arc welding processes with commercially available consumables. This makes them accessible to a much broader range of fabricators and repair shops. The study's emphasis on carbide volume fraction as the critical parameter for wear resistance provides a clear design criterion that can be incorporated into overlay specification documents. The limitation is that the maximum achievable hardness is somewhat lower than TiC-based cermets, but for many industrial applications, the balance of hardness, toughness, and processability offered by inherent carbide alloys represents the optimal engineering solution.