Microstructure and Wear Resistance of Mo-Cr-Fe-B Alloy Overlay Deposits

Literature Overview and Research Significance

The paper by Li Zihao, Pan Yingjun, Ke Deqing, Huang Liao, and Chang Zhimin from Wuhan University of Science and Technology, published in Journal of Wuhan University of Science and Technology (2018, Vol. 41, No. 4, pp. 252-256), presents a study on the microstructure and wear resistance of a self-developed Mo-Cr-Fe-B system flux-cored wire overlay on Q235 steel substrate. This research is funded by the State Key Laboratory of Refractory Materials and Metallurgy and the Key Laboratory of Ferrous Metallurgy and Resources Utilization, both jointly established at the provincial-ministerial level. The work addresses the challenge of developing cost-effective, high-performance wear-resistant overlays using readily available alloying elements.

Microstructural Characterization

The overlay was characterized using optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), micro-hardness testing, and pin-on-disk wear testing. The resulting microstructure consists of multiple boride phases, including Mo2FeB2, M3B2 (where M = Mo, Cr, Fe), Fe2B, and Fe(Cr, Mo) solid solution. This multiphase microstructure is characteristic of boride-containing wear-resistant alloys and is responsible for the exceptional hardness achieved.

The maximum hardness achieved is 980 HV0.5, which is exceptionally high for an iron-based overlay system. This level of hardness is comparable to cobalt-based and nickel-based cermets, which are significantly more expensive.

Interface Analysis and Reaction Mechanism

A critical finding of this study is the observation of elemental diffusion at the overlay-substrate interface. The researchers documented the reaction process during overlay formation, which involves:

1. Melting of the flux-cored wire and substrate surface
2. Chemical reactions between the molten pool and substrate
3. Formation of intermetallic phases at the interface
4. Diffusion of elements across the interface boundary

The good metallurgical bond between the overlay and the Q235 steel substrate is confirmed by the absence of unmelted particles, porosity, or delamination at the interface. The elemental diffusion observed at the interface suggests a degree of mutual solubility that contributes to the bond strength.

Wear Performance and Engineering Relevance

The wear resistance of the overlay was evaluated using a pin-on-disk tribometer. The results demonstrate good wear resistance, which is attributed to the combination of hard boride phases dispersed in a ductile matrix. This microstructural architecture is analogous to the "hard phase + tough matrix" design philosophy that has been successful in cemented carbides and cermets.

For engineering applications, the Mo-Cr-Fe-B overlay system offers several advantages:

• Cost effectiveness: The use of Mo, Cr, Fe, and B avoids expensive elements such as Co and Ni
• High hardness: 980 HV0.5 exceeds most conventional iron-based overlays
• Good bond strength: Confirmed metallurgical bond to carbon steel substrates
• Versatility: Flux-cored wire format allows use with standard GMAW equipment

Key Questions and Reflections

The boride phase composition is complex, with multiple boride types present. Understanding the relative contribution of each phase to wear resistance would require more detailed quantitative analysis, such as phase fraction measurement using image analysis or Rietveld refinement. The presence of Fe2B, which is known to be brittle, raises a question about the overall toughness of the overlay. In applications where impact loading is present, the brittleness of Fe2B could lead to spalling or cracking.

Another consideration is the influence of welding parameters on the phase composition. The study does not appear to systematically vary welding parameters, which could shift the phase balance toward or away from specific boride types. In production applications, parameter control would be essential to maintain consistent microstructure and performance.

Summary and Reference Value

This study demonstrates that a self-developed Mo-Cr-Fe-B flux-cored wire can produce overlay deposits with exceptional hardness (980 HV0.5) and good wear resistance on Q235 steel substrates. The multiphase boride microstructure provides a cost-effective alternative to nickel-based and cobalt-based wear-resistant overlays. Engineers seeking economical solutions for wear protection in applications such as mining equipment, material handling systems, and industrial components should consider this alloy system as a viable option. The interface diffusion analysis provides valuable insight into the bonding mechanism, which is critical for ensuring long-term service reliability.