Microstructure Evolution and Tribological Performance of WC-Reinforced Nickel-Based Overlay Welds at Different Preheat Temperatures

Literature Overview

This paper, published in Surface Technology in 2024 (Vol. 53, No. 9, pp. 127-136), investigates the influence of preheat temperature on the microstructure evolution and tribological behavior of WC particle-reinforced nickel-based alloy overlay welds deposited on 42CrMo steel substrates via plasma arc surfacing. The study is supported by the National Key R&D Program of China (2021YFB3702003) and authored by researchers from Liaoning University of Science and Technology. The work establishes a comprehensive mapping relationship between process parameters, microstructure, mechanical properties, and wear resistance, which is of direct relevance to engineers designing hardfacing overlays for wear-critical components in oil and gas, mining, and power generation industries.

Core Technical Findings

The researchers conducted plasma arc surfacing experiments at two preheat temperatures: 200°C and 400°C. Characterization was performed using XRD, SEM, microhardness testing, and pin-on-disk tribometry. The overlay welds exhibited a complex multi-phase composition including γ-Ni/Fe solid solution, WC, W₂C, M₇C₃, M₂₃C₆, Ni₂W₄C, and Cr₃C₂. The key findings can be summarized as follows:

The critical insight is that higher preheat temperature promotes interface reactions at the WC particle boundaries, driving the precipitation of secondary carbides over a larger area. This mechanism effectively counteracts the gravity-driven settling of WC particles during solidification, leading to a more homogeneous distribution of hard phases throughout the overlay weld.

Interpretation of Technical Mechanisms

WC Particle Settling and Its Consequences

In plasma arc surfacing of WC-reinforced nickel-based alloys, the WC particles (density approximately 15.6 g/cm³) are significantly denser than the molten Ni-based matrix (density approximately 8.9 g/cm³). During solidification, the large density mismatch creates a buoyancy-driven settling tendency. At low preheat temperatures (200°C), the solidification rate is relatively high, and the viscosity of the molten pool is insufficient to retain the WC particles in their intended positions. This results in particle clustering at the top surface and depletion in the lower regions near the fusion line, creating a non-uniform hard phase distribution that severely compromises wear resistance.

Role of Secondary Carbide Precipitation

At 400°C preheat, the extended solidification time allows for more complete interfacial reactions between WC particles and the molten Ni-Cr matrix. The reaction products include W₂C, M₇C₃, M₂₃C₆, and Ni₂W₄C, which form a network of secondary carbides that effectively "anchor" the primary WC particles in place. This phenomenon can be understood through the lens of thermodynamic driving force and kinetic accessibility: higher temperatures provide the activation energy necessary for diffusion-controlled carbide formation reactions at the WC/matrix interface.

Wear Mechanism Transition

The wear behavior at 400°C preheat shows a dramatic improvement because the uniform distribution of hard carbide phases creates a continuous load-bearing network. During sliding contact, the uniformly distributed WC and secondary carbides share the applied load, preventing localized plastic deformation and material removal. The wear mechanism transitions from abrasive ploughing and material removal (dominant at 200°C) to a more balanced combination of micro-cutting by hard carbides and matrix protection (dominant at 400°C).

Process-Structure-Property Mapping

Process Window Optimization

Based on the findings, the following process recommendations can be extracted for engineering practice:

• Preheat temperature: 400°C or higher is strongly recommended for WC-reinforced Ni-based overlays
• Interpass temperature maintenance: Should be maintained above 350°C throughout multi-pass deposition
• Heat input control: Higher preheat extends solidification time, which is beneficial for carbide precipitation but must be balanced against substrate distortion concerns
• Powder feed rate: Should be optimized to maintain adequate dilution control while ensuring complete WC particle engulfment

Dilution and Fusion Line Considerations

A critical aspect not explicitly quantified in this paper but essential for practice is the dilution rate at the fusion boundary. In WC-reinforced overlays, excessive dilution can lead to carbide dissolution and softening at the fusion zone. The 400°C preheat condition, while beneficial for microstructure uniformity, may increase dilution due to the larger molten pool volume. Engineers must carefully balance these competing effects through appropriate layer thickness design and multi-pass strategies.

Engineering Practice Integration

Application Scenarios

The findings of this study have direct applicability to several engineering scenarios in the piping and equipment industry:

1. Valve seat and trim hardfacing: High-pressure valve components in oil and gas service where WC-reinforced Ni-based overlays provide superior erosion-corrosion resistance
2. Cylinder liner remanufacturing: Engine components requiring dimensional restoration with enhanced wear life
3. Pipe end preparation: Hardfacing of pipe ends for coupling applications where galling and wear are concerns
4. Pump impeller and casing repair: Restoration of worn surfaces with improved tribological performance

Quality Control Implications

For production environments implementing these overlays, the following QC checkpoints should be established:

Key Questions and Reflections

The study raises several important questions for further investigation. First, the optimal preheat temperature appears to be 400°C, but what happens at higher temperatures such as 500°C or 600°C? Would further improvements in microstructure uniformity be observed, or would substrate softening and excessive dilution become dominant concerns? Second, the paper does not address the effect of layer thickness on the observed phenomena. In practice, multi-layer overlays are common, and the thermal cycling between layers may further influence carbide precipitation and particle distribution. Third, the wear testing appears to be conducted under dry sliding conditions, which may not fully represent the erosive-corrosive environments encountered in actual service.

From a metallurgical perspective, the observation that secondary carbide precipitation effectively mitigates WC particle settling is particularly elegant. It suggests that the strategy for achieving uniform hard phase distribution need not rely solely on process parameter optimization but can leverage thermodynamic and kinetic mechanisms inherent to the alloy system. This insight opens possibilities for alloy design approaches that deliberately promote secondary carbide formation to improve overlay performance.

Study Insights and Implications

This research contributes significantly to the understanding of process-microstructure-property relationships in WC-reinforced Ni-based overlay welds. The quantification of wear rate reduction (51.89%) provides a concrete engineering benefit that can be incorporated into life-cycle cost analyses for component repair and maintenance strategies. For piping engineers and equipment maintenance specialists, the practical takeaway is clear: investing in proper preheat procedures for hardfacing operations yields substantial improvements in service life and reliability.

The study also underscores the importance of fundamental metallurgical understanding in process development. Rather than relying on empirical trial-and-error, the mechanistic explanation of how preheat temperature influences carbide precipitation and particle distribution provides a framework for systematic process optimization. This approach aligns with modern quality management philosophies such as PDCA (Plan-Do-Check-Act), where understanding the underlying mechanisms enables more effective control of critical process parameters. Future work should extend these findings to include multi-pass deposition sequences, different substrate materials, and accelerated corrosion-wear testing to provide more comprehensive guidance for industrial applications.