Microstructure Analysis and Wear Resistance of MIG Melting Injection WC-Enhanced Wear-Resistant Cladding Layers

Literature Overview

This study, published in the Journal of Shenyang Ligong University in 2011 by Liu Aiguo from the School of Materials Science and Engineering, Shenyang Ligong University, investigates the microstructure evolution and tribological performance of tungsten carbide (WC) enhanced wear-resistant overlay layers produced via gas metal arc welding (GMAW/MIG) with a melting-injection technique. The research addresses a critical industrial challenge: how to deposit hardfacing coatings with superior abrasion resistance using commercially available MIG equipment rather than relying exclusively on specialized processes such as plasma transferred arc (PTA) cladding or laser cladding. The work bridges the gap between laboratory-scale wear testing and field-applicable manufacturing methods, making it particularly relevant for engineers working on coal handling equipment, mining machinery, and heavy-duty wear components where cost-effective hardfacing solutions are demanded.

Core Technical Content

The fundamental principle of the melting-injection method involves directing molten tungsten carbide particles into the arc zone during MIG welding so that they are captured in the weld pool and distributed throughout the overlay layer. Unlike conventional powder feeding in GMAW where the powder is pre-mixed with the wire, the melting-injection approach allows for higher WC content and more uniform dispersion. The resulting microstructure typically consists of a martensitic or bainitic matrix with dispersed WC particles, and in some cases, secondary carbides such as Fe₃W₃C, Fe₂W₄C, and Fe₇W₆C form at the interface between the WC particles and the iron-based matrix. These secondary carbides can either enhance or degrade the wear resistance depending on their morphology and distribution.

The key technical parameters governing the process include the arc current, voltage, travel speed, shielding gas composition (typically Ar or Ar/CO₂ mixtures), wire feed rate, and the injection rate and particle size of the WC powder. The melting-injection technique requires precise control over the interaction between the arc plasma and the injected particles to ensure complete melting or controlled dissolution of the WC. Incomplete melting results in unmelted WC cores that can act as crack initiation sites, while excessive melting leads to the formation of brittle iron-tungsten carbides that compromise toughness.

Microstructure Interpretation

The overlay layer microstructure can be analyzed using the following framework:

Microstructural Feature	Formation Mechanism	Effect on Wear Resistance
Martensitic matrix	Rapid cooling from arc welding	Provides baseline hardness and strength
Dispersed WC particles	Direct injection into weld pool	Primary abrasion resistance mechanism
Fe₃W₃C / Fe₂W₄C	Reaction between molten Fe and WC	Moderate hardness contribution
Fe₇W₆C	Low-temperature reaction product	Can reduce toughness if excessive
Grain boundaries with carbide films	Segregation during solidification	Potential crack initiation sites
Columnar dendrites	Directional solidification from substrate	Influences crack propagation direction

The hardness of the overlay layer is primarily governed by the volume fraction and distribution of WC particles. Typical hardness values for WC-enhanced MIG overlay layers range from 600 to 900 HV depending on the WC content (typically 20-60 wt%) and the welding parameters employed. The wear resistance improvement over the base carbon steel substrate can be as high as 5 to 10 times, depending on the sliding conditions and counterface material.

Engineering Practice and Process Control

In practical applications, the following process considerations are critical for successful WC-enhanced MIG overlay welding:

Preheating and interpass temperature control: Substrates such as low-carbon steel (Q235, Q345) and low-alloy steel (16Mn, 15CrMo) typically require preheating to 150-300°C to reduce residual stress and prevent cold cracking. The interpass temperature should be maintained below 250°C to preserve the hardness of the overlay layer.
Multi-pass deposition strategy: A single pass rarely achieves sufficient overlay thickness for industrial wear applications. Multi-pass welding (typically 3-8 passes) is employed, with the first pass serving as a transition layer to dilute carbon from the substrate and the subsequent passes building up the full WC-enhanced composition.
Post-weld treatment: Stress-relief annealing at 550-650°C for 1-2 hours can reduce residual stresses without significantly reducing hardness. However, prolonged annealing or higher temperatures may cause carbide coarsening and hardness loss.
Substrate compatibility: The dilution effect from the base metal must be carefully managed. For high-carbon steels or cast irons, a dedicated transition layer with a nickel-based or austenitic filler is recommended before applying the WC-enhanced overlay.

Common Defects and Countermeasures

Defect Type	Root Cause	Countermeasure
Cracking in overlay	High residual stress, hydrogen embrittlement	Preheat, reduce travel speed, post-weld stress relief
Poor bonding	Surface contamination, insufficient penetration	Thorough surface preparation, increase current
WC particle agglomeration	Uneven injection rate	Optimize injection nozzle geometry and powder flow rate
Excessive dilution	Low travel speed, high current	Adjust parameters, use transition layer
Pore formation	Moisture in shielding gas, surface oil	Dry gas supply, thorough cleaning

Key Reflections and Study Insights

This research is significant because it demonstrates that advanced wear-resistant overlay layers with properties approaching those of PTA or laser cladding can be achieved using conventional MIG equipment with relatively modest modifications. This has profound implications for small and medium-sized enterprises that lack access to expensive specialized cladding equipment. The melting-injection concept essentially converts a standard GMAW system into a hybrid welding-powder injection process, offering a cost-effective pathway to high-performance surface engineering.

However, from an engineering quality assurance perspective, several challenges remain. The uniformity of WC distribution across multiple passes is difficult to guarantee without real-time monitoring. The dilution effect varies with each pass and depends on the geometry of the preceding weld bead, making process parameter optimization a multi-variable problem. Furthermore, the long-term wear performance under cyclic loading conditions (fatigue wear) has not been extensively characterized in this type of study, which is a concern for applications involving impact loading such as excavator buckets or crusher jaws.

For pressure vessel engineers, the relevance of this work lies in the potential application of WC-enhanced overlay layers on wear-critical components such as pump casings, valve seats, and agitator shafts used in chemical processing. The key consideration is ensuring that the overlay does not compromise the structural integrity of the pressure-containing component. Bond strength testing in accordance with NB/T 47014 or equivalent standards would be essential before approving such a process for pressure equipment application.

The study underscores the importance of understanding the fundamental metallurgy of the overlay process. Without knowledge of the phase transformations occurring during solidification and cooling, engineers cannot rationally select welding parameters or predict the final properties of the cladding layer. This reinforces the principle that surface engineering is not merely a process selection exercise but requires deep materials science understanding combined with practical manufacturing experience.