Microstructure and Abrasive Wear Performance of Overlay Alloy with Endogenous Carbide Particles

Literature Overview

This research by Tang Wenbo, Guo Yungang, Zhang Yawei, and Wang Hongrui from the School of Materials Science and Engineering, Zhengzhou University, published in the Journal of Welding (2010, Vol. 31, No. 8, pp. 73-76), presents a novel overlay alloy containing endogenously formed composite carbide particles. The study employs SMAW (shielded metal arc welding) on Q235 steel substrates to develop an in-situ reinforced overlay alloy, characterized by spectroscopy, hardness testing, optical microscopy, SEM, and EDAX analysis, with abrasive wear testing conducted on a pin-on-disk tribometer. The classification TG455 confirms its relevance to overlay welding materials and processes.

Core Technical Findings

The developed overlay alloy exhibits a unique microstructure consisting of mixed martensite (low-carbon and high-carbon variants in approximately equal proportions) with a small amount of retained austenite, reinforced by uniformly dispersed endogenous composite carbide particles of the (NbCrTi)C type. The key performance indicators are summarized below:

In-Situ Reinforcement Mechanism

The concept of endogenous (in-situ) carbide formation is a critical innovation in this work. Unlike conventional hardfacing alloys that rely on externally added hard particles (e.g., pre-formed WC or Cr₃C₂), this approach allows carbides to precipitate directly from the weld metal during solidification and subsequent cooling. The multi-element nature of the carbide phase—incorporating Nb, Cr, and Ti—provides several advantages:

1. Thermodynamic stability: Multi-element carbides generally have higher melting points and greater thermal stability than binary carbides, making them resistant to dissolution and coarsening during welding thermal cycles.
2. Compositional flexibility: The presence of multiple carbide-forming elements allows the carbide composition to self-adjust based on local carbon activity and cooling rate, providing a degree of self-optimization.
3. Reduced interface stress: In-situ formed particles have better lattice matching with the matrix compared to exogenous additions, reducing interfacial residual stress and improving bonding.

The mixed martensite microstructure is particularly noteworthy. The coexistence of low-carbon martensite (providing toughness) and high-carbon martensite (providing hardness) creates a synergistic effect where the tough phase supports and bridges the hard phase, preventing catastrophic crack propagation. The retained austenite serves as an additional toughening mechanism through transformation-induced plasticity (TRIP) during deformation.

Wear Mechanism Analysis

The superior wear resistance (3.6 times that of D707) can be attributed to the following mechanisms:

• Abrasive resistance: The hard (NbCrTi)C particles (estimated hardness >2000 HV) resist micro-cutting and micro-plowing by abrasive particles.
• Matrix support: The tough martensitic matrix prevents particle pull-out and provides a ductile backing that accommodates plastic deformation without fracture.
• Uniform distribution: The dispersed carbide particles ensure consistent wear resistance across the overlay surface, avoiding localized wear tracks.
• Local agglomeration concern: The study notes local agglomeration of carbide particles, which creates potential weak points where particle clusters may debond collectively under severe loading. This is a critical finding for process optimization.

Process Parameters and Metallurgical Control

Achieving the desired microstructure requires careful control of welding parameters and consumable composition. The following table summarizes the critical process variables:

The dilution rate from the Q235 substrate is a critical factor. Excessive dilution introduces ferrite into the overlay, reducing hardness and altering the carbide precipitation behavior. Multi-pass welding with controlled interpass temperatures helps manage this issue.

Engineering Practice and Application Considerations

This technology is particularly relevant for applications requiring high abrasion resistance with moderate impact loading, such as:

• Mining equipment components (bucket teeth, conveyor rollers)
• Cement mill grinding rings and liners
• Paper machine rollers and guides
• Pipeline components at slurry flow sections

The comparison with D707 (a standard WC-based hardfacing electrode per GB/T standards) provides a benchmark for engineers familiar with conventional hardfacing solutions. The 3.6× improvement in wear resistance translates directly to extended service intervals and reduced maintenance costs, which is a compelling economic argument for adoption.

Key Questions and Reflections

The local agglomeration of carbide particles raises important questions about process reproducibility and quality control. In industrial settings, maintaining uniform carbide distribution across large overlay areas requires consistent welding parameters and consumable quality. Non-destructive testing methods such as ultrasonic testing (UT) or magnetic particle inspection (MT) may be employed to detect subsurface defects, but direct assessment of carbide distribution typically requires destructive sampling and metallographic examination.

Another consideration is the weldability of this overlay alloy to dissimilar substrates. While tested on Q235 steel, practical applications may involve overlaying on stainless steel, alloy steel, or cast iron substrates, each presenting different dilution and metallurgical challenges. The thermal expansion mismatch between the overlay and substrate must be carefully evaluated to prevent spalling under thermal cycling conditions.

Study Insights and Implications

This study demonstrates the viability of in-situ composite reinforcement as an alternative to conventional exogenous particle addition in hardfacing alloys. The multi-element carbide approach offers compositional flexibility and improved thermodynamic stability, while the mixed martensite matrix provides an optimal hardness-toughness balance. For engineers involved in wear-resistant component design, this work highlights the importance of considering both the reinforcement phase and the matrix microstructure as an integrated system. The concept of designing the matrix to "support and bridge" the hard phase is a fundamental principle that should guide all hardfacing alloy development. Future work should focus on optimizing carbide distribution uniformity, evaluating long-term wear behavior under complex loading conditions, and extending the technology to automated welding processes for industrial-scale production.