Embedded M23C6 Carbides and Wear Resistance in Open-Arc Overlay Welding Alloys

Literature Overview

This study, published in Powder Metallurgy Materials and Engineering (Volume 18, Issue 4, 2013, pp. 579-584), investigates the microstructure and wear resistance of wear-resistant alloys produced by self-shielded open-arc welding using metal-powder flux-cored wire. The research was conducted by researchers at Xiangtan University's School of Mechanical Engineering and supported by multiple funding sources including the National Natural Science Foundation of China (51271158). The study focuses on a specific compositional range of Cr-B-C alloy overlays and examines how carbon content influences the formation of embedded M₂₃C₆ carbide structures and their contribution to wear performance.

Core Technical Content

Alloy Composition and Processing

The overlay alloys are produced using a self-shielded open-arc welding process with metal-powder flux-cored wire. The composition range investigated is:

This composition places the alloy in the medium-chromium iron-based hardfacing category, with boron added as a carbide former to promote the formation of hard boride and carbide phases. The relatively high carbon content (up to 4.7%) is characteristic of high-performance hardfacing alloys designed for severe abrasive wear conditions.

Microstructural Evolution with Carbon Content

The study reveals a clear progression of microstructural features as carbon content increases:

1. Low carbon range: Cellular α-Fe matrix structure is dominant, with relatively fine eutectic colonies. The M₂₃C₆ phase is present but not yet organized into distinct morphologies.
2. Medium carbon range: The cellular α-Fe matrix gradually disappears as carbon content increases. Eutectic quantity decreases, and ordered clusters of hard phases begin to appear. These clusters increase in size with increasing carbon.
3. High carbon range: Well-defined, regularly arranged globular hard phases dominate the microstructure. Backscattered electron (BSE) analysis reveals these globular structures to be dual-phase composite organizations.

The Embedded M₂₃C6 Composite Structure

The most significant microstructural finding is the characterization of the globular hard phases as dual-phase composite structures:

• Internal core: Embedded hexagonal M₂₃C₆ carbide with microhardness of 1300-1748 HV₁.₀
• External shell: M₆C-type carbide with microhardness above 1000 HV₁.₀

This core-shell morphology is metallurgically significant because it represents a hierarchical hard phase structure where the harder M₂₃C₆ core is protected by a slightly less hard but more ductile M₆C shell. This configuration can enhance wear resistance by combining the extreme hardness of the M₂₃C₆ core with the mechanical integrity of the M₆C shell, reducing the likelihood of carbide fracture and pull-out during abrasive contact.

Hardness and Wear Performance

The non-monotonic relationship between carbon content and wear performance is explained by competing mechanisms:

• At lower carbon levels, increasing carbon promotes more hard phase formation, directly enhancing wear resistance.
• At higher carbon levels, excessive carbide formation leads to matrix embrittlement, increased residual stress, and potential microcracking, which can reduce overall wear resistance despite the higher carbide volume fraction.

Wear Mechanism Analysis

The wet sand abrasion test results demonstrate excellent wear resistance for the open-arc overlay alloys. Surface wear morphology analysis identifies micro-cutting as the primary wear mechanism, which is characteristic of hardfacing alloys where the hard carbide phases resist penetration by abrasive particles, resulting in material removal primarily through cutting and ploughing rather than adhesive or fatigue mechanisms.

Engineering Practice Implications

Composition Design Guidelines

The findings provide practical guidance for alloy design in medium-chromium hardfacing applications:

1. Optimal carbon range: The transition from increasing to decreasing wear resistance at approximately 63.6 HRC suggests an optimal carbon content around 3.0-3.5% for this alloy system. Engineers should target this range for maximum wear performance.
2. Boron addition: The consistent 1.3-1.5% B content across all compositions suggests that boron level is not the critical variable in this study. However, boron plays an important role in promoting the formation of the composite carbide structures, and maintaining this level is essential for achieving the desired microstructure.
3. Chromium level: The 11-13% Cr range provides sufficient chromium for M₂₃C₆ formation while maintaining adequate matrix toughness. Higher chromium levels would promote M₇C₃ formation instead, which may be beneficial for different service conditions.

Process Considerations for Open-Arc Welding

The open-arc (self-shielded) welding process offers advantages for field application but introduces specific challenges:

• Atmospheric sensitivity: Open-arc welding exposes the molten pool to atmospheric nitrogen and oxygen, which can cause nitride formation and oxide inclusions. The flux in the wire provides partial protection, but the overlay may contain higher levels of inclusions compared to gas-shielded processes.
• Heat input control: The lower heat input of open-arc welding compared to submerged arc or gas-shielded processes promotes faster cooling, which can enhance carbide refinement but may increase residual stress.
• Dilution: The self-shielded process typically has moderate dilution, which must be controlled through proper base material preparation and welding parameter selection.

Study Insights and Reflections

The discovery of the dual-phase composite carbide structure—M₂₃C₆ core embedded within M₆C shell—represents a significant microstructural insight with implications for hardfacing alloy design. This hierarchical structure is not merely a curiosity but a functional design feature that combines the extreme hardness of M₂₃C₆ with the mechanical robustness of M₆C. For engineers developing new hardfacing consumables, this finding suggests that compositions should be optimized not just for maximum carbide hardness but for the formation of these composite structures, which may provide superior wear resistance through a combination of hardness and fracture resistance. The non-monotonic wear performance with carbon content also serves as a reminder that "more hard phase is not always better"—the matrix must maintain sufficient toughness to support the hard phases without cracking or spalling under service loads.