Microstructure and Wear Resistance of FeCr15B2MnTi Open-Arc Surfacing Alloy A Technical Study Note

Literature Overview

This paper by Gong Jianxun, Yao Huiwen, and Cheng Shiyao from Xiangtan University, published in Ordnance Materials and Science (2019, Vol. 42, No. 4, pp. 82-86), presents a comprehensive investigation of the FeCr15B2MnTi surfacing alloy deposited using self-shielded flux-cored wire open-arc welding (also known as self-protected arc surfacing). Supported by the Hunan Provincial Natural Science Foundation (2015JJ5031), this research extends the earlier work on Fe-C-V-B systems by introducing titanium as a fourth alloying element and exploring the effects of high boron content on microstructure, crack resistance, and wear behavior. The characterization toolkit includes XRD, OM, SEM, and energy-dispersive spectrometry (EDS), providing a multi-scale understanding of the alloy system.

Core Technical Findings

The FeCr15B2MnTi alloy represents a high-boron, high-chromium surfacing system where titanium serves as a grain refiner and phase modifier. The key findings can be organized into three interconnected themes: primary phase formation, crack resistance improvement through Ti addition, and wear mechanism identification.

Primary Phase Characterization

The alloy precipitates a large volume fraction (over 80%) of white primary phases with microhardness values ranging from 927.3 HV0.1 to 1739 HV0.1. These phases are identified as M2B-type compounds, specifically M23(C,B)6, which belong to the carbon-boron hard phase family. The extremely high hardness of these phases (up to 1739 HV0.1) indicates that they are likely vanadium- or titanium-enriched borides, possibly TiB2 or mixed (Ti, V)B2, which are known to exhibit hardness values in the 2000-3000 HV range in pure form. The reduction to 1739 HV0.1 in the alloy matrix suggests solid-solution effects and compositional variations within the phase.

The Role of Titanium in Crack Resistance

Perhaps the most practically significant finding is the demonstration that increasing titanium content reduces the brittle eutectic (α-Fe + M3(C,B)) and transforms the weld from a cracked condition to a crack-free condition. This is a critical engineering insight because high-boron surfacing alloys are notoriously prone to hot cracking due to:

• The formation of low-melting-point Fe-B eutectics at grain boundaries (melting point approximately 1200°C for Fe-FeB eutectic)
• The high thermal contraction of boride phases relative to the iron matrix
• The limited plasticity of the M3(C,B) phase at elevated temperatures

Titanium addresses these issues through multiple mechanisms:

1. Nucleation effect: TiC particles act as heterogeneous nucleation sites for the primary M2B phase, refining the microstructure and reducing grain boundary area where cracking can initiate
2. Eutectic modification: Ti preferentially forms TiB2 rather than FeB, shifting the eutectic composition and reducing the volume fraction of low-melting-point phases
3. Strain accommodation: The presence of discrete TiC particles can deflect crack paths and provide local strain relief through particle debonding rather than matrix cracking

Wear Mechanism Analysis

The wear behavior of FeCr15B2MnTi is characterized by two mechanisms: micro-cutting (micro-plowing) and micro-delamination, with micro-cutting being the dominant mechanism. This is consistent with the very high hardness of the primary phases, which effectively resist abrasive particle penetration. The micro-delamination component suggests that, despite the high volume fraction of hard phases, some degree of matrix softening or interface weakening exists under sustained abrasive loading.

The dominance of micro-cutting over micro-delamination is favorable for long-term service because micro-cutting produces predictable, gradual material loss, whereas delamination can lead to sudden, catastrophic material removal. However, the presence of any delamination component indicates that further optimization of the matrix-hard phase interface may be beneficial.

Process Analysis and Standards Context

The use of self-shielded flux-cored wire open-arc welding for this application represents a pragmatic choice for field repair and maintenance operations. Unlike submerged arc welding, which requires shielding gas infrastructure and is limited to shop conditions, open-arc welding with self-shielded flux-cored wire can be performed outdoors, in windy conditions, and on large components that cannot be moved to a workshop. This aligns with SY/T and GB/T standards for field welding procedures in the oil and gas and mining industries.

Process Parameters and Their Effects

Engineering Practice Integration

The FeCr15B2MnTi alloy system finds direct application in:

• Mining equipment: Excavator bucket teeth, conveyor rollers, and crusher liners where high abrasion resistance is required
• Cement industry: Kiln liners, mill balls, and slide surfaces subject to both abrasion and moderate thermal cycling
• Agricultural machinery: Plow shares and tillage implements exposed to abrasive soils

From a quality assurance perspective, the following inspection criteria should be established:

1. Visual inspection (VT): No visible cracks, porosity, or undercut exceeding 0.5 mm depth
2. Hardness testing: Vickers hardness of 900-1700 HV0.1 for hard phases; overall surface hardness >800 HV0.1
3. Macrographic examination: Sound weld profile, no incomplete fusion or lack of penetration
4. Metallographic examination: Confirm >80% volume fraction of primary hard phases; verify absence of hot cracks

Key Questions and Reflections

The relationship between TiC precipitation quantity and the reduction of primary phase volume fraction is particularly intriguing. The paper notes that as TiC first-precipitation phases increase in number, the primary M2B phase volume fraction decreases. This suggests a competitive nucleation phenomenon where TiC and M2B compete for the available boron and carbon in the liquid. Understanding this competition is essential for optimizing the Ti content to achieve the desired balance between hard phase volume fraction and crack resistance.

A critical concern from a metallurgical engineering perspective is the long-term stability of the M23(C,B)6 phase. Borides are known to be susceptible to oxidation at elevated temperatures, and the M2B phase may undergo phase transformation during thermal cycling service. The paper does not address this aspect, which represents a gap in the understanding of long-term performance.

Another consideration is the weldability of this high-boron, high-chromium alloy in multi-pass applications. The high carbon and boron content creates significant hot cracking susceptibility in subsequent passes, particularly at the root of the next layer. Interpass temperature control and possibly the use of a transition layer between the base metal and the surfacing alloy may be necessary.

Study Insights and Practical Recommendations

This research contributes significantly to the understanding of high-boron surfacing alloys by demonstrating that titanium addition can resolve the fundamental crack resistance problem that has limited the industrial adoption of such alloys. The practical implication is that FeCr15B2MnTi-type alloys can now be confidently specified for field applications where extreme abrasion resistance is required, provided that appropriate welding procedures are followed.

For engineers specifying this alloy system, the following guidelines should be observed:

• Titanium content should be optimized to balance crack resistance against hard phase volume fraction, with a recommended range of 0.5-1.5 wt% Ti
• Preheating to 100-150°C is advisable to reduce cooling rates and minimize thermal stresses, though excessive preheating (>250°C) should be avoided to prevent softening of the hard phases
• Post-weld stress relief at 550-600°C for 1-2 hours can further reduce residual stresses without significantly affecting the hard phase stability
• Quality assurance should include hardness mapping across the weld cross-section to verify uniform microstructure and the absence of soft zones

The work of Gong Jianxun and colleagues represents a systematic approach to surfacing alloy development that bridges fundamental metallurgical understanding with practical engineering requirements. The integration of phase diagram analysis, microstructural characterization, and mechanical property evaluation provides a model methodology for future alloy development programs. The findings are directly applicable to the design of hardfacing consumables for the mining, cement, and agricultural industries, where the combination of high abrasion resistance and crack-free welds is essential for reliable long-term service.