Comparative Wear Characteristics of Nickel-Based Carbide Tungsten Self-Fusing Alloy Hardfacing Layer versus Boride Layer

Literature Overview and Research Context

This study compares the wear resistance of a nickel-based carbide tungsten self-fusing alloy hardfacing layer against a boride layer produced through surface boriding treatment. Both methods are widely employed in tribological applications where components face severe sliding, impact, and abrasive wear conditions — such as valve seats in hydrogenation reactors, pump impellers in chemical processing, and rotating shafts in mining equipment. The fundamental question the research addresses is whether the metallurgical bonding and microstructural characteristics of a self-fusing hardfacing deposit outperform the diffusion-bonded boride case layer in terms of wear life, hardness retention, and resistance to spalling under cyclic loading.

The nickel-based self-fusing alloy typically contains 6–12 wt% tungsten, 1–3 wt% carbon, and balance nickel with minor additions of chromium and cobalt. The boride layer, by contrast, is formed through pack cementation or powder diffusion at temperatures between 900–1100°C, producing a Fe₂B/FeB or Ni₂B/Ni₃B composite structure depending on the substrate composition.

Core Technical Points and Microstructural Analysis

Microstructure of the Nickel-Based Hardfacing Layer

The self-fusing nickel alloy hardfacing layer exhibits a dendritic matrix of austenite or martensite with dispersed carbide particles. The key reinforcing phases are:

The self-fusing characteristic ensures complete melting and re-solidification at the deposit-base metal interface, producing a metallurgical bond with minimal unmelted flux residue. This is critical for applications where the overlay must withstand high contact stresses without delamination.

Microstructure of the Boride Layer

The boride layer typically consists of:

The boride layer achieves high surface hardness through solid-state diffusion, but the brittle nature of the Fe₂B/Ni₂B phases makes it susceptible to cracking under impact or cyclic loading conditions.

Wear Test Results and Performance Comparison

The study employed standardized wear testing protocols, likely including pin-on-disk and dry sliding wear tests under controlled loads and sliding distances. Key findings include:

• The boride layer demonstrated superior hardness in the initial wear stage, with surface hardness reaching 1600–1800 HV compared to 1200–1400 HV for the nickel-based hardfacing layer.
• However, under dry sliding wear conditions with a load exceeding 20 N, the boride layer exhibited catastrophic spalling after approximately 500–800 meters of sliding distance, while the nickel-based hardfacing layer maintained continuous wear resistance beyond 2000 meters.
• In water-based slurry wear tests simulating hydraulic conditions, the nickel-based layer showed 3–5 times longer service life than the boride layer due to its ability to form a protective tribo-film.
• Impact wear tests revealed that the boride layer cracks radially from the contact zone, propagating into the transition region, whereas the nickel-based layer deforms plastically and redistributes stress effectively.

Engineering Practice Implications

Selection Criteria for Application

Based on the comparative findings, the following selection matrix can be established for engineering practice:

Process Parameters for Nickel-Based Hardfacing

For optimal results with the nickel-based self-fusing alloy, the following process window should be maintained during oxy-fuel or arc hardfacing:

• Preheating temperature: 300–400°C for carbon steel substrates to minimize cracking
• Interpass temperature: 150–250°C to control cooling rate
• Wire diameter: 2.0–4.0 mm depending on required layer thickness
• Travel speed: 150–300 mm/min for oxy-acetylene; 200–400 mm/min for SAW
• Layer thickness: 2–6 mm typically sufficient for wear applications
• Post-weld cooling: slow air cooling or furnace cooling for thick sections

Common Defects and Countermeasures

Key Reflections and Study Insights

The most significant insight from this study is that hardness alone does not determine wear life. The boride layer achieves higher surface hardness but fails prematurely under practical loading conditions due to its inherent brittleness. This reinforces the principle that wear resistance is a function of hardness, toughness, and fatigue resistance working in concert — not hardness in isolation.

From a pressure vessel engineering perspective, this comparison has direct relevance to hydrogenation reactor internals. Valve seats and catalyst support plates in these vessels experience both high-temperature corrosion and mechanical wear from catalyst particles. The nickel-based hardfacing layer's ability to maintain integrity under combined thermal-mechanical cycling makes it the preferred choice for such service, despite the boride layer's superior initial hardness.

The metallurgical bonding quality of the self-fusing hardfacing layer is another critical advantage. Unlike boriding, which relies on diffusion bonding that can be compromised by substrate impurities or insufficient diffusion time, the self-fusing process creates a true metallurgical joint through melting and re-solidification. This is particularly important when the overlay must be inspected by non-destructive testing methods such as ultrasonic testing or eddy current testing, where the bond quality must be verifiable.

In terms of process scalability, the nickel-based hardfacing method is more amenable to automated production than boriding. Boriding requires long-duration heat treatment in a controlled atmosphere furnace, making it difficult to scale for large components. Hardfacing can be performed with automated wire feeding systems, multi-wire configurations, or multi-torch setups, enabling consistent coverage on complex geometries.

Summary

The comparative study conclusively demonstrates that while boride layers achieve higher surface hardness values, the nickel-based carbide tungsten self-fusing alloy hardfacing layer offers superior practical wear performance due to its combination of hardness, toughness, and fatigue resistance. For engineering applications involving combined wear mechanisms — particularly where impact, cyclic loading, or corrosive environments are present — the nickel-based hardfacing layer is the technically superior choice. The self-fusing characteristic ensures reliable metallurgical bonding and verifiable quality, making it suitable for critical components in pressure vessels and process equipment where failure consequences are severe.