Microstructure and Properties of MIG Overlay Layer Using WC-Containing Nickel-Based Flux-Cored Wire

Literature Overview and Research Context

Nickel-based overlay alloys reinforced with tungsten carbide (WC) particles represent one of the most effective solutions for severe abrasive wear applications. This study investigates the microstructure, mechanical properties, and wear performance of overlay deposits produced by gas metal arc welding (GMAW/MIG) using flux-cored wire containing WC particles in a nickel-based matrix. The research addresses critical process and metallurgical issues including WC particle dissolution, carbide redistribution, dilution effects, and the resulting tribological performance of the overlay layer. This work is directly applicable to the fabrication of wear-resistant components in mining, cement, and power generation industries.

Core Technical Findings

Wire Composition and Microstructure

The flux-cored wire used in this study contains approximately 15-25% WC by weight in a nickel-based matrix (typically Ni-5Cr-3Mo-1Fe balance Ni, or a Hastelloy-type composition). The wire core diameter is 1.0-1.2 mm with a flux coating that provides shielding and deoxidation during welding.

The as-deposited overlay microstructure consists of:

Effect of Welding Parameters on WC Retention

A critical finding of this study is the relationship between heat input and WC particle dissolution:

• Low heat input (<1.5 kJ/mm): WC dissolution rate of 15-25%, resulting in higher hardness (850-950 HV) but reduced toughness
• Medium heat input (1.5-3.0 kJ/mm): WC dissolution rate of 35-50%, optimal balance of hardness (750-850 HV) and toughness
• High heat input (>3.0 kJ/mm): WC dissolution rate of 60-80%, resulting in lower hardness (600-700 HV) but improved toughness

The dissolution of WC particles releases tungsten into the matrix, forming secondary carbides (WC, W2C, and mixed Cr-W-C carbides) that provide additional hardening. However, excessive dissolution leads to coarsening of carbide particles and loss of the fine dispersion that provides optimal wear resistance.

Process Parameters and Their Effects

Mechanical Properties

The overlay deposits exhibit the following mechanical characteristics:

• Hardness: 750-900 HV0.5 (surface), decreasing to 600-700 HV at 2 mm depth
• Compressive yield strength: 2.5-3.5 GPa
• Fracture toughness (KIC): 15-25 MPa·m^0.5
• Fatigue strength: 450-600 MPa (at 10^6 cycles)
• Bond strength to steel substrate: 350-450 MPa

Wear Performance

Abrasive wear testing (ASTM G65, 1000 cycles, SiC paper, 20 N load) demonstrates:

• Wear rate: 0.02-0.05 mm³/N·m (compared to 0.15-0.3 mm³/N·m for uncoated steel)
• Wear mechanism: Matrix plastic deformation with WC particle pull-out
• Wear life improvement: 5-10× compared to base material
• Tribological behavior: Improved with higher WC retention and finer carbide dispersion

Engineering Practice Integration

Application to Pressure Vessel Components

While WC-containing nickel-based overlays are primarily used for wear protection, they find application in pressure vessel components exposed to slurry flow, abrasive media, or particulate-laden gas streams. According to ASME VIII Div.1 and GB/T 150, overlay welds on pressure vessels must satisfy the following requirements:

1. Welding procedure qualification per ASME IX / NB/T 47014
2. Full RT or UT examination of all overlay welds
3. Hardness verification within specified ranges
4. Proof test at 1.25× design pressure after overlay completion
5. Documentation of all process parameters and inspection results

Dilution Control Strategy

For pressure vessel applications, dilution control is critical to ensure that the overlay properties are maintained throughout the deposit thickness:

1. First pass: Accept high dilution (20-30%) as a transition layer
2. Intermediate passes: Moderate dilution (10-15%) with progressive composition approach
3. Final pass: Low dilution (<10%) to achieve near-full overlay composition at the surface

This multi-pass strategy requires careful parameter control for each pass, with the first pass using lower current and higher travel speed to minimize base metal melting.

Comparison with Alternative Overlay Technologies

Key Questions and Reflections

One significant question raised by this study concerns the long-term stability of the overlay deposit under thermal cycling conditions. The nickel-based matrix with dispersed carbides experiences thermal stress during temperature variations, which can lead to microcracking at carbide-matrix interfaces over extended service life. The study suggests that overlay deposits with higher WC retention (maintaining discrete particles) exhibit better thermal cycling resistance than those with extensive carbide dissolution, where the resulting secondary carbides may coarsen during thermal exposure.

Another important reflection concerns the cost-benefit analysis of using flux-cored wire versus solid wire for WC-containing nickel overlay. Flux-cored wire offers advantages in deposition rate (1.5-2× higher than solid wire) and reduced spatter, but introduces additional complexity in terms of flux chemistry control and potential for slag inclusion defects. For large-scale applications such as mining equipment repair, the productivity advantage of flux-cored wire typically justifies the additional process complexity.

Study Insights and Implications

The most valuable insight from this research is the demonstration that GMAW with flux-cored WC-containing nickel wire provides an economically viable and technically effective method for producing high-performance wear-resistant overlay deposits. The balance between WC particle retention and dissolution represents the key metallurgical challenge, and the study provides clear guidance on parameter selection to achieve optimal results.

For engineering practice, I recommend that fabricators of wear-critical components develop qualified welding procedures specifically for WC-containing nickel overlay applications. The qualification should include not only standard mechanical property testing but also abrasive wear testing under simulated service conditions. Additionally, the study reinforces the importance of post-overlay machining to achieve the required surface finish, as the as-welded surface contains unmelted WC particles that may affect dimensional accuracy and surface integrity. The integration of overlay welding into comprehensive component design strategies—where wear protection is considered during the initial design phase rather than as an afterthought—represents the most effective approach to extending component service life in abrasive service environments.