Preparation of Wear-Resistant Alloy Powder Blocks Using WO3 and Carbon Nanomixed Powder for Overlay Welding

Literature Overview

This publication by Chen Chunhuan, Zhao Xiujuan, and Yang Dexin from the Department of Materials Science and Engineering at Dalian Railway Institute (2003) addresses a fundamental challenge in overlay welding consumable development: the preparation of a novel wear-resistant alloy powder block incorporating tungsten trioxide (WO3) and carbon nanomixed powder. The work was supported by the Liaoning Provincial Natural Science Foundation (Grant No. 9910300503) and was published in the journal Rare Metals and Cemented Carbides. The research targets the creation of a consolidated welding consumable form—specifically a powder block (also termed a welding strip or electrode block)—that can be used in strip-cladding or submerged arc welding (SAW) processes to deposit hard, wear-resistant overlay layers.

The motivation behind this work is clear from an engineering standpoint. Conventional carbide-forming welding consumables rely on Cr3C2, WC, or TiC as primary hard phases. While these provide good abrasion resistance, they often suffer from limited thermal stability above 600 °C, poor spalling resistance under impact loading, and limited ductility in the overlay matrix. The introduction of WO3 and carbon nanomixed powder offers a novel approach: WO3 can decompose during the welding thermal cycle to form W2C or WC hard phases in situ, while the nanoscale carbon serves as a carbon source and nucleation agent for fine carbide precipitation.

Core Technical Points

Powder Block Fabrication

The powder block (also called a welding strip or electrode) is a consolidated consumable form that combines the advantages of strip cladding—high deposition rate, low dilution, and good bond strength—with the compositional flexibility of powder metallurgy. The fabrication process typically involves:

1. Powder mixing and homogenization — WO3 powder, carbon nanomixed powder, and base alloy powder (typically an austenitic or martensitic stainless steel matrix) are blended in controlled ratios.
2. Cold or hot pressing — The mixed powder is consolidated into a strip or block form using cold isostatic pressing (CIP) or hot pressing under vacuum or inert atmosphere.
3. Sintering — The pressed block is sintered at 1100–1300 °C for 2–4 hours to achieve sufficient mechanical integrity while preserving the nanoscale carbon distribution.

In-Situ Carbide Formation Mechanism

During the welding thermal cycle, WO3 undergoes reduction reactions with carbon and the molten pool metals:

• WO3 + 3C → W2C + 3CO (at temperatures above 1400 °C)
• W2C + C → WC (further carburization)
• W + 3C → WC (direct formation)

The nanoscale carbon particles serve a dual function: they act as a reducing agent for WO3 and as heterogeneous nucleation sites for carbide precipitation in the solidifying overlay. This results in a finer and more uniformly distributed carbide network compared to conventional consumables where carbides are pre-formed and can coarsen during the welding thermal cycle.

Microstructure and Properties of the Overlay

The resulting overlay typically exhibits:

• Hardness: 800–1000 HV0.3, significantly higher than conventional Cr3C2-based overlays (600–750 HV)
• Carbide morphology: Fine, rod-like or spherical W2C and WC particles (1–5 μm) dispersed in an austenitic or martensitic matrix
• Toughness: Improved spalling resistance due to the ductile austenitic matrix accommodating carbide deformation
• Thermal stability: Retention of hardness above 800 °C, superior to WC-based overlays which soften rapidly above 600 °C

Engineering Practice Considerations

From a practical standpoint, the use of powder blocks in SAW overlay welding introduces several process challenges that must be addressed:

Dilution control: The dilution ratio between the base metal and the overlay must be carefully managed. For wear-resistant applications, dilution should be kept below 15–20% to preserve the carbide content in the final overlay. This is achieved through:

• Using a multi-pass welding strategy with the first pass having lower heat input
• Selecting appropriate welding parameters (lower current, higher travel speed)
• Using a backing layer of higher alloy content before depositing the final wear layer

Bond strength: The bond strength between the overlay and the base metal is critical. For powder block consumables, the bond strength should meet or exceed the minimum requirements specified in GB/T 150 (for pressure vessel applications) or API 934 (for general overlay welding qualification). Typical acceptable bond strength values are:

• Tensile bond strength: ≥ 350 MPa (or exceeding the yield strength of the base metal)
• Shear bond strength: ≥ 200 MPa

Common defects and countermeasures:

Study Insights and Reflections

This research represents an early but significant exploration of in-situ carbide formation in welding consumables. The concept of using metal oxides as precursors for hard phase formation during the welding thermal cycle is elegant in its simplicity—it eliminates the need for pre-formed carbide particles, which are expensive and can be difficult to distribute uniformly. The use of nanoscale carbon as both a reducing agent and nucleation agent is particularly ingenious, as it leverages the high reactivity of nanoscale particles to promote fine carbide precipitation.

However, from a decades-long engineering perspective, several questions remain for practical implementation. The long-term stability of W2C and WC phases under cyclic thermal loading (as encountered in thermal cycling applications) needs further investigation. The cost-effectiveness of WO3-based consumables compared to conventional WC or Cr3C2 consumables must be evaluated on a per-unit-deposited-volume basis. Additionally, the welding process parameters for powder blocks incorporating WO3 may differ significantly from those for conventional powder blocks, and systematic welding procedure qualification (WPQ) studies are essential before industrial deployment.

The work also raises an important philosophical point in consumable development: rather than simply adding harder phases to existing consumables, one can design the chemistry such that the desired phases form in situ during the welding process. This approach offers superior compositional control and microstructural refinement, and it represents a paradigm shift from "what we add" to "what forms."

In summary, this research provides a promising foundation for developing next-generation wear-resistant overlay consumables. The in-situ formation of tungsten carbides from WO3 and carbon nanomixed powder offers superior hardness, thermal stability, and potentially improved toughness compared to conventional consumables. However, the transition from laboratory-scale demonstration to industrial-scale application requires rigorous process development, welding procedure qualification, and long-term service validation under actual operating conditions. The principles established in this work—namely, the use of oxide precursors and nanoscale carbon for in-situ carbide formation—remain highly relevant to contemporary research in advanced welding consumables and additive manufacturing feedstock development.