Embedded M23C6 Carbides and Wear Resistance in Open-Arc Overlay Alloy

Literature Overview

This paper published in Powder Metallurgy Materials and Engineering (Vol. 18, No. 4, 2013) by Gong Jianxun and colleagues from Xiangtan University investigates the microstructure and wear resistance of open-arc overlay alloys containing embedded M₂₃C₆-type carbides, deposited using a metal-powder flux-cored wire with self-shielded open-arc welding. The research is supported by the National Natural Science Foundation of China (51271158) and multiple provincial funding sources, reflecting the significance of this work in the field of hardfacing technology.

Core Technical Content

The researchers prepared overlay alloys with a composition range of 11–13% Cr, 1.6–4.7% C, 0.4–0.5% Si, and 1.3–1.5% B (mass fraction) using metal-powder flux-cored wire self-shielded open-arc welding. The carbon content was varied systematically to study its influence on microstructure evolution and wear performance.

Microstructural Evolution with Carbon Content

The most significant finding of this study is the identification of a unique composite carbide structure. As carbon content increases, the cellular α-Fe matrix gradually disappears and the eutectic volume fraction decreases. A novel feature emerges: regularly arranged nodular hard phases appear and grow in size. Backscattered electron analysis reveals that these nodular structures are actually dual-phase composite microstructures, with an inner core of embedded hexagonal M₂₃C₆ (microhardness 1300–1748 HV₁.₀) surrounded by an outer shell of M₆C-type carbide (microhardness above 1000 HV₁.₀).

Hardness and Wear Performance

The hardness of the overlay alloy exhibits a non-monotonic relationship with carbon content. It increases from 58.8 HRC to a peak of 63.6 HRC at intermediate carbon levels, then decreases to 60.6 HRC at the highest carbon content. This behavior correlates with the evolution of the nodular composite carbide structure. At intermediate carbon levels, the nodular M₂₃C₆/M₆C composite phases are present in optimal size and quantity, providing maximum reinforcement. At higher carbon levels, the excessive growth of these nodules may lead to coalescence and the formation of microcracks at the interfaces, reducing overall hardness.

The wear resistance follows a similar trend, increasing continuously to a maximum and then slightly declining. Wet sand abrasive wear testing confirmed the excellent wear performance of the optimized alloy, and surface wear morphology analysis identified micro-cutting as the primary wear mechanism.

Carbide Morphology and Properties

The embedded M₂₃C₆ hexagonal carbide core is particularly notable for its high microhardness (1300–1748 HV₁.₀), which is significantly higher than the surrounding M₆C shell (above 1000 HV₁.₀). This dual-phase composite structure provides a synergistic reinforcement effect: the hard M₂₃C₆ core resists micro-cutting and abrasion, while the M₆C shell provides a transition zone that improves the bonding strength between the hard phase and the matrix, reducing the risk of phase pullout under wear loading.

Engineering Practice Implications

The identification of the embedded M₂₃C₆/M₆C composite carbide structure provides a new design criterion for hardfacing alloy development. Rather than simply maximizing the total carbide volume fraction, engineers should target the formation of composite carbide structures where the hardest phase is encapsulated within a slightly softer but still hard matrix. This approach can improve both wear resistance and the effective utilization of the hard phase.

For practical implementation, the open-arc self-shielded welding process used in this study is well-suited for field repair and fabrication applications where shielding gas equipment is not available. The metal-powder flux-cored wire format allows for precise compositional control, enabling the targeted production of the optimal carbon content range that yields the maximum hardness of 63.6 HRC and peak wear resistance.

The wet sand abrasive wear testing methodology used in this study is relevant to many industrial applications, including cement mill liners, mining equipment, and agricultural machinery. The identification of micro-cutting as the dominant wear mechanism suggests that the overlay alloy is particularly well-suited for sliding and plowing wear conditions, where the hard carbide phases effectively resist material removal by cutting.

Key Questions and Reflections

The study raises an important question regarding the stability of the embedded M₂₃C₆ phase under thermal cycling conditions. M₂₃C₆ is known to be thermodynamically stable at high temperatures but may undergo phase transformations or coarsening during prolonged service at elevated temperatures. For applications involving thermal cycling, such as exhaust system components or heat exchanger elements, the long-term stability of the composite carbide structure should be evaluated.

Additionally, the effect of boron content on the composite carbide formation is not fully explored. Boron is known to form hard borides that can influence the carbide morphology and distribution. A systematic study of boron content variation in conjunction with carbon content would provide a more comprehensive understanding of the compositional design space for optimizing the embedded M₂₃C₆/M₆C composite structure.

Study Insights and Implications

This research contributes a significant microstructural insight to the field of hardfacing technology: the formation of dual-phase composite carbide nodules with an embedded M₂₃C₆ core and M₆C shell provides superior wear resistance compared to single-phase carbide structures. The non-monotonic relationship between carbon content and wear performance underscores the importance of optimizing the carbide morphology rather than simply maximizing the hard phase volume fraction. For engineers developing new hardfacing alloys, this study suggests that the compositional window should be carefully targeted to promote the formation of these composite carbide structures, with the intermediate carbon content range representing the optimal design space for maximum hardness and wear resistance.