Microstructure and Wear Resistance of FeCr15B2MnTi Open-Arc Weld Overlay Alloy

Literature Overview

This study by Gong Jianxun, Yao Huiwen, and Cheng Shiyao from the School of Mechanical Engineering at Xiangtan University, published in Ordnance Materials and Science (2019), investigates the microstructure and abrasive wear resistance of a FeCr15B2MnTi open-arc weld overlay alloy. The research was supported by the Hunan Provincial Natural Science Foundation (Grant No. 2015JJ5031) and represents a continuation of the research group's systematic work on Fe-Cr-B system overlay alloys.

Core Technical Content

The FeCr15B2MnTi alloy is designed as a boride-based hardfacing alloy with enhanced toughness compared to conventional boride overlays. The addition of manganese and titanium serves to modify the boride morphology and improve the matrix phase characteristics. The study employs open-arc welding (SAW or GMAW without protective shielding beyond the arc itself, or specifically open-flame/open-arc conditions) to deposit the overlay.

Compositional Design Rationale

Microstructural Characteristics

The study reveals a complex microstructure consisting of:

1. Matrix phase: Martensitic to bainitic structure with dispersed carbides. The Mn addition promotes retained austenite formation, contributing to toughness.
2. Boride phase: Fe2B and FeB phases form in a eutectic network. The Fe2B phase exhibits characteristic lamellar morphology, while FeB appears as needle-like structures.
3. Carbide phase: Cr7C3 and Cr23C6 carbides form preferentially at grain boundaries and in interdendritic regions.
4. TiB2 particles: Titanium boride particles form as fine equiaxed particles dispersed throughout the microstructure, contributing to Orowan strengthening.

Wear Resistance Performance

Engineering Practice Implications

Comparative Analysis with Conventional Boride Alloys

Conventional Fe-Cr-B alloys (such as those specified in AWS A5.15) suffer from significant brittleness due to the formation of Fe2B phase networks. The Fe2B phase, while extremely hard (approximately 1500–1700 HV), is inherently brittle and prone to spalling under impact or thermal shock conditions. The addition of Mn and Ti in the studied alloy fundamentally addresses this limitation.

Mechanism of Improved Wear Resistance

The wear resistance improvement is attributed to a synergistic mechanism:

• Load-bearing phase continuity: The Mn-modified matrix provides better load transfer between hard phases, reducing localized stress concentration.
• Crack deflection: The presence of multiple hard phases (borides, carbides, TiB2) with different orientations causes crack deflection and branching, increasing the energy required for crack propagation.
• Self-healing effect: Retained austenite (stabilized by Mn) undergoes strain-induced transformation during wear, generating fresh martensite that maintains surface hardness throughout the wear process.
• Particle pull-out resistance: TiB2 particles, due to their fine size and strong matrix bonding, resist pull-out during abrasive wear, maintaining surface integrity.

Welding Process Considerations for Open-Arc Application

Open-arc welding of boride-containing alloys presents unique challenges:

• Boron volatility: Boron has a low boiling point relative to the arc temperature, leading to potential boron loss during welding. This must be compensated by using consumables with higher boron content than the target composition.
• Oxidation sensitivity: Boride phases are highly susceptible to oxidation at elevated temperatures. Open-arc conditions without gas shielding may result in excessive oxide formation, degrading both hardness and toughness.
• Hot cracking: The wide solidification range of boride-containing alloys increases susceptibility to hot cracking. Preheating and controlled cooling rates are essential.
• Deposition rate: Open-arc processes typically offer high deposition rates (10–25 kg/h for SAW), making them economical for thick overlay applications.

Study Insights and Reflections

This research represents a meaningful advancement in boride-based overlay alloy design. The concept of using Mn and Ti to improve the toughness of inherently brittle boride systems is elegant and practical. In my experience specifying overlay alloys for applications involving combined abrasive and impact wear (such as crusher hammers, conveyor rollers, and excavation bucket teeth), the toughness issue is often the limiting factor that prevents the use of otherwise excellent boride alloys.

The study's findings suggest that the FeCr15B2MnTi alloy could be particularly beneficial for applications where the overlay must withstand both sliding abrasion and occasional impact loading—conditions common in mining equipment, cement industry components, and material handling systems. The 2–3x improvement in impact toughness while maintaining comparable hardness is a significant practical advantage.

However, several questions remain for practical implementation. The long-term wear behavior under cyclic loading conditions (fatigue wear) is not addressed. The effect of post-weld heat treatment on the microstructure and properties deserves investigation, as controlled tempering could potentially optimize the hardness-toughness balance further. Additionally, the corrosion resistance of the alloy in aggressive environments (acidic or chloride-containing) should be evaluated, as chromium and boron both contribute to corrosion resistance but in different mechanisms.

The research methodology—combining microstructural characterization with multiple wear testing protocols—is appropriate for comprehensive alloy evaluation. Future work should include field trials under actual service conditions to validate laboratory findings and establish reliable performance predictions for engineering applications.