Microstructure and Properties of Mo-Cr-Fe-B Alloy Overlay Surfacing

Literature Overview

The paper by Li Zihao and colleagues from Wuhan University of Science and Technology, published in the Journal of Wuhan University of Science and Technology in 2018, investigates the microstructure, phase composition, hardness distribution, and wear resistance of a Mo-Cr-Fe-B alloy overlay deposited on Q235 steel substrate using self-made flux-cored wire. The study also examines the reaction process during overlay formation at the interface between the deposit and the base metal.

Phase Composition and Microstructure

The overlay microstructure is characterized by a complex multiphase composition, which is typical of boron-containing alloy systems. The identified phases include Mo2FeB2, M3B2 (where M represents Mo, Cr, and Fe), Fe2B, and Fe(Cr, Mo) solid solution. The formation of these phases can be understood through the thermodynamic stability of various borides in the Mo-Cr-Fe-B system.

The highest hardness achieved in the overlay reaches 980 HV0.5, which is exceptionally high and attributable to the hard boride phases, particularly the M3B2 phase. This hardness level is comparable to that of ceramic materials and far exceeds that of conventional surfacing alloys, making it suitable for severe abrasion applications.

Interface Characterization and Bonding Quality

The study reports good bonding between the overlay and the steel substrate, with elemental diffusion observed at the interface. This diffusion zone is critical for ensuring mechanical integrity of the overlay system. The diffusion of alloying elements such as molybdenum and chromium from the overlay into the base metal, and iron from the base metal into the overlay, creates a gradual compositional transition that reduces the risk of interfacial cracking during thermal cycling or mechanical loading.

The reaction process during overlay formation involves several sequential steps. Initially, the arc heat melts the flux-cored wire and the surface of the base metal, creating a molten pool with a composition that is a mixture of the wire alloy and the base metal. As the molten pool cools, the high-temperature phases form first, followed by the lower-temperature phases. The boride phases, which are thermodynamically stable but kinetically sluggish, form during the later stages of solidification, resulting in their characteristic interdendritic and grain boundary distributions.

Wear Performance and Mechanism

The wear resistance of the Mo-Cr-Fe-B overlay is reported to be good, which is consistent with the high hardness achieved. The wear mechanism in such hard boride-containing overlays typically involves the fracture and pullout of hard boride particles from the relatively softer matrix, followed by abrasive wear of the exposed matrix. The high volume fraction and fine dispersion of hard boride phases help to resist the initial stages of abrasive wear, while the matrix provides the necessary toughness to accommodate the stresses generated during the wear process.

The self-made flux-cored wire approach offers advantages in terms of compositional flexibility and cost control. By adjusting the relative proportions of molybdenum, chromium, iron, and boron in the wire core powder, the overlay properties can be tailored for specific applications. The flux component in the wire provides self-shielding and slag protection, making the process suitable for field applications where external shielding gas is not available.

Engineering Practice Implications

The overlay hardness of 980 HV0.5 positions this material for applications involving severe abrasion, such as mining equipment, material handling chutes, and grinding mill liners. However, the brittleness associated with high boride content must be considered, as impact loading or thermal shock could cause spalling or cracking of the overlay. The good interfacial bonding reported in this study is essential for preventing overlay delamination under service conditions.

Quality control for this type of overlay should include hardness mapping to verify uniform composition, microstructural examination to confirm the expected phase distribution, and interfacial bond strength testing to ensure adequate adhesion. The self-made wire production process also requires quality control of the powder mixing, wire drawing, and flux composition to ensure batch-to-batch consistency.

This research contributes valuable knowledge about the Mo-Cr-Fe-B alloy system for overlay applications. The combination of high hardness, good wear resistance, and reasonable bonding strength makes this system a promising candidate for severe abrasion applications, provided that the brittleness limitations are understood and managed through appropriate design and process control.