Microstructure and Wear Resistance of Boride-Strengthened Iron-Based Overlay Alloys

Literature Overview

This paper by Zong Lin and Ning Jianrong from Shenyang University of Chemical Technology, published in Welding Technology (Vol. 41, No. 8, pp. 13-15, 2012), investigates the effect of boron addition on the microstructure and wear resistance of iron-based overlay alloys produced by plasma arc welding. The study compares boron-free Fe-Cr-C overlays with boron-containing variants, identifying an optimal boron content of 4.5 wt% that maximizes wear resistance through specific boride phase formation.

Core Technical Findings

The microstructural analysis reveals that boron addition fundamentally alters the phase composition and morphology of the overlay alloy:

The boride phases form with distinctive morphologies, particularly the dandelion-like M23(C,B)6 structure that provides extensive surface coverage with high hardness. The combination of hard boride phases distributed within a tough martensitic-austenitic matrix creates a composite-like structure that achieves superior wear resistance.

Mechanism Analysis

The wear resistance improvement follows a non-monotonic trend with boron content, first increasing then decreasing. At the optimal 4.5 wt% boron, the boride phases are sufficiently numerous and well-distributed to provide effective abrasion resistance without creating excessive brittleness. Beyond this optimum, excessive boride formation may reduce matrix continuity and promote intergranular fracture, diminishing overall wear performance.

The plasma arc welding process is well-suited for this application due to its high energy density, which promotes rapid solidification and fine microstructure formation. The dilution characteristics of plasma arc welding also allow precise control of the deposited composition, which is essential for achieving the optimal boron content.

Engineering Practice Implications

For engineers designing overlay systems for severe abrasive service, this study provides quantitative guidance on boron content optimization. The six-fold improvement in wear resistance represents a significant practical benefit, potentially extending component service life substantially. The boride-strengthened system is particularly relevant for applications involving hard, angular abrasive particles where micro-cutting is the dominant wear mechanism.

The dandelion-like morphology of M23(C,B)6 is particularly effective for wear resistance because it provides three-dimensional reinforcement of the matrix while maintaining interconnectivity. This morphology is more effective than isolated blocky particles because it creates a continuous network that resists particle penetration and material removal.

Key Questions and Reflections

The study does not address the effect of boron on weldability, which is a practical concern given that boron can promote hot cracking in weld metals. The long-term stability of the boride phases under thermal cycling is also unexplored, yet many industrial applications involve temperature variations that may affect phase stability. Additionally, the comparison is limited to dry sliding wear, whereas practical applications often involve lubricated or mixed-mode wear conditions where the performance advantage may differ.

Study Insights and Implications

This paper demonstrates that strategic boron addition can dramatically enhance the wear resistance of iron-based overlay alloys, with the optimal content identified through systematic experimentation. The key insight is that boron does not merely add hardness but fundamentally changes the phase morphology to create a more effective wear-resistant structure. For engineers working with boride-strengthened overlays, the 4.5 wt% boron content represents a practical target that balances wear resistance with mechanical integrity. The plasma arc welding process enables the precise composition control necessary to achieve this optimum, making this approach commercially viable for demanding industrial applications.