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, pp. 579–584) by Gong Jianxun and colleagues from Xiangtan University, investigates the microstructure and wear resistance of an open-arc overlay alloy produced using self-shielded metal-powder flux-cored wire. The overlay alloy contains 11–13% Cr, 1.6–4.7% C, 0.4–0.5% Si, and 1.3–1.5% B (mass fraction). The work is supported by the National Natural Science Foundation of China (51271158) and other provincial funding sources, and provides valuable insights into the microstructural evolution and wear mechanisms of high-carbon, high-chromium overlay alloys.

Core Technical Findings

The study systematically varied the carbon content within the specified range and examined the resulting microstructure, hardness, and wear resistance. The key findings are summarized below:

Microstructural Evolution with Carbon Content

The microstructural evolution follows a clear pattern as carbon content increases:

1. Low carbon (1.6%): The overlay consists primarily of a cellular α-Fe matrix with eutectic regions containing carbides. The α-Fe phase provides toughness but contributes less to wear resistance.
2. Medium carbon (~3.0%): The cellular α-Fe matrix gradually disappears as eutectic content increases. Globular hard phases begin to form in regular arrangements, with increasing size.
3. High carbon (4.7%): The cellular α-Fe matrix is completely replaced by eutectic structure. Large, regularly arranged globular hard phases dominate the microstructure.

Dual-Phase Composite Hard Phase

A particularly significant finding is the identification of a dual-phase composite hard phase within the globular structures. Backscattered electron analysis revealed that these globular phases consist of:

• Internal phase: Embedded hexagonal M₂₃C₆ carbide with microhardness of 1,300–1,748 HV₁.₀
• External phase: M₆C-type carbide with microhardness above 1,000 HV₁.₀

This dual-phase composite structure is unusual and provides a unique combination of hardness and potential toughness, as the M₆C outer layer may provide some crack arrest capability while the M₂₃C₆ core provides maximum hardness.

Wear Mechanism Analysis

The wet sand abrasion test results indicate excellent wear resistance for the open-arc overlay alloy. Surface wear morphology analysis reveals that micro-cutting is the primary wear mechanism, characterized by:

• Groove formation along the sliding direction
• Plastic deformation of the softer matrix phase
• Ploughing and cutting by hard abrasive particles
• Limited adhesive transfer

The micro-cutting mechanism is typical of two-body or three-body abrasive wear, where hard abrasive particles (such as sand grains) cut into the overlay surface. The high hardness of the M₂₃C₆ and M₆C carbides provides excellent resistance to this cutting action.

Process Analysis

Open-Arc Self-Shielded Metal-Powder Flux-Cored Wire Process

The open-arc self-shielded metal-powder flux-cored wire (FCAW-S with metal powder core) process is characterized by:

The metal powder core provides additional alloying elements that enrich the overlay composition beyond what would be achieved with a solid wire. The self-shielded nature eliminates the need for external shielding gas, making the process suitable for field applications. However, the open arc configuration results in higher dilution with the base metal, which must be managed through composition design and multi-layer deposition.

Boron Addition Effects

The addition of 1.3–1.5% B is a notable feature of this overlay alloy. Boron in iron-based alloys promotes the formation of borides (Fe₂₃B₆, Fe₅B₂) and influences the carbide morphology. The presence of boron may contribute to the formation of the dual-phase composite hard phase observed in the study, as boron can segregate to phase boundaries and influence the relative stability of M₂₃C₆ versus M₆C carbides.

Engineering Practice Integration

Application Suitability

The open-arc overlay alloy described in this study is suitable for applications involving:

• Abrasive wear: Sand, coal, ore, and mineral processing equipment
• Two-body abrasion: Sliding surfaces with hard counterfaces
• Three-body abrasion: Surfaces exposed to loose abrasive particles
• Mild corrosive environments: The high chromium content provides moderate corrosion resistance

The self-shielded nature of the process makes it particularly suitable for:

• Field repair of large equipment where gas supply is unavailable
• Maintenance of mining and construction equipment in remote locations
• Repair of large structures such as chutes, hoppers, and transfer points

Quality Control and Inspection

The following quality control measures are recommended for open-arc overlay welding:

1. Visual inspection: Check for surface cracks, porosity, and incomplete fusion.
2. Hardness testing: Measure surface hardness at multiple locations to verify uniformity and target values (60–64 HRC).
3. Dilution analysis: Perform chemical analysis of the overlay layer to verify that dilution with base metal does not exceed acceptable limits.
4. Wear testing: Conduct representative wear tests on coupon samples to validate field performance.
5. Bond strength testing: Perform tensile or shear bond strength tests to verify metallurgical bonding between overlay and substrate.

Study Insights and Reflections

The discovery of the dual-phase composite hard phase (embedded M₂₃C₆ core with M₆C outer layer) is a significant contribution to the understanding of high-carbon, high-chromium overlay microstructures. This composite structure suggests that the solidification sequence involves initial formation of M₆C carbides, followed by internal nucleation of M₂₃C₆ within the M₆C matrix during cooling. The resulting composite provides a combination of hardness (from M₂₃C₆) and potential toughness (from the M₆C outer layer), which may contribute to the observed wear resistance.

The non-monotonic relationship between carbon content and wear resistance (increasing then decreasing) highlights the importance of optimizing carbon content for specific applications. The peak hardness of 63.6 HRC at approximately 3.0% carbon represents a good balance between carbide volume fraction and matrix continuity. Excessive carbon content leads to large, isolated hard phases that may be more susceptible to fracture during abrasive wear, explaining the slight decrease in wear resistance at 4.7% carbon.

The micro-cutting wear mechanism identified in this study is consistent with the primary wear mode in many industrial applications involving loose abrasive particles. Engineers designing overlay systems for such applications should focus on maximizing the hardness and volume fraction of the hard phase while maintaining adequate matrix continuity to prevent catastrophic failure.

Reference Value and Outlook

This paper provides valuable insights into the microstructural design of high-carbon, high-chromium overlay alloys for abrasive wear resistance. The identification of the dual-phase composite hard phase opens new avenues for microstructure optimization through controlled solidification and heat treatment. Future research could explore the effects of other alloying elements (such as tungsten, vanadium, or titanium) on the formation and stability of the dual-phase composite structure, as well as the influence of heat treatment on the relative proportions of M₂₃C₆ and M₆C phases. The work also underscores the importance of systematic composition optimization in overlay alloy development, as small changes in carbon content can have significant effects on microstructure and performance.