Boride-Strengthened Iron-Based Overlay Alloys Microstructure and Properties Research

Overview and Background

Boride phases, particularly Fe2B and FeB, are recognized as effective strengthening phases in iron-based overlay alloys for wear-resistant applications. The addition of boron (B) to iron-based overlay consumables promotes the formation of hard boride phases that significantly enhance abrasive wear resistance. This study examines the microstructural characteristics, mechanical properties, and tribological behavior of boride-strengthened iron-based overlay alloys, with particular attention to the role of boron content, welding process effects, and phase stability.

Boride-strengthened overlays are particularly attractive for applications where moderate hardness (900–1200 HV) combined with good toughness is required—such as in mining, aggregate processing, and material handling equipment where impact loading is present alongside abrasive wear.

Core Technical Points

Boride Phase Formation and Characteristics

The boride phases that form in iron-based overlay alloys depend on boron content and cooling conditions:

Boride Phase	Hardness (HV)	Crystal Structure	Formation Conditions	Stability
Fe2B	1200–1500	Orthorhombic	Lower B content (0.3–0.8%)	Stable at room temperature
FeB	1600–1800	Orthorhombic	Higher B content (>0.8%)	Metastable; decomposes at >250 °C
Fe2B + FeB mixture	1400–1700	Mixed	Intermediate B content	FeB decomposes to Fe2B + Fe upon heating

The decomposition reaction of FeB is thermodynamically favorable:

FeB → Fe2B + Fe (occurs above approximately 250 °C)

This decomposition is critical for understanding the long-term stability of boride-strengthened overlays in service conditions involving elevated temperatures.

Microstructural Evolution

The microstructure of boride-strengthened iron-based overlays typically exhibits:

Dendritic primary borides: Fe2B or FeB appearing as elongated or blocky particles aligned with the solidification direction
Eutectic borides: Fine Fe2B/Fe or FeB/Fe eutectic in the interdendritic regions
Matrix phases: Ferrite, pearlite, or martensite depending on cooling rate and carbon content
Carbide phases: M7C3 or M23C6 may coexist with borides in multi-carbide systems

The cooling rate has a decisive effect on the resulting microstructure:

Cooling Rate (°C/s)	Dominant Boride Phase	Matrix Type	Overall Hardness (HV30)
1–5 (SAW)	Fe2B dominant	Pearlite + ferrite	800–950
5–20 (FCAW)	Fe2B with some FeB	Mixed ferrite + pearlite	900–1100
20–100 (GTAW/PTA)	FeB + Fe2B mixture	Martensitic	1000–1200
>100 (Laser cladding)	FeB dominant	Predominantly martensitic	1100–1300

Mechanical Properties and Wear Performance

The mechanical properties of boride-strengthened overlays are characterized by:

Hardness: Typically 900–1300 HV, depending on boride volume fraction and distribution
Toughness: KIC values of 8–15 MPa·m½, generally lower than martensitic overlays but higher than pure carbide overlays
Compressive strength: 2500–3500 MPa, indicating good resistance to indentation and crushing loads
Wear resistance: 2–4× improvement over unalloyed steel in dry sliding abrasion tests

The wear resistance improvement correlates strongly with boride volume fraction:

Boron Content (wt%)	Boride Volume Fraction (%)	Hardness (HV30)	Wear Rate Reduction (%)
0.0%	0%	400–500	Baseline
0.3%	5–10%	700–850	50–60%
0.6%	15–25%	900–1050	70–80%
1.0%	25–40%	1050–1200	80–90%
1.5%	40–55%	1150–1300	85–92%
2.0%	50–65%	1200–1350	88–95%

Beyond approximately 1.5% B, the marginal improvement in wear resistance diminishes, and the risk of excessive brittleness increases significantly.

Process and Application Considerations

Welding Process Selection for Boride Overlays

The welding process must be selected to achieve the desired boride phase distribution and matrix microstructure:

Process	Advantages for Boride Overlays	Limitations
SAW	High deposition rate; suitable for thick overlays	Coarse microstructure; Fe2B dominant (lower hardness)
FCAW	Good balance of rate and microstructure	Moderate boride refinement
GTAW	Fine microstructure; good control	Low deposition rate; limited thickness
PTA	Excellent microstructure control; high rate	Higher equipment cost
Laser cladding	Finest microstructure; minimal dilution	Limited to thin overlays; high equipment cost
Oxy-fuel	Simple equipment; no electrical infrastructure needed	Poor quality control; inconsistent results

For boride-strengthened overlays, FCAW and PTA are generally recommended as they provide an optimal balance of microstructure quality, productivity, and cost.

Dilution Effects on Boride Formation

Base metal dilution significantly affects boride phase formation because the base metal typically contains no boron:

Process	Dilution (%)	Effective B Content Reduction	Impact on Microstructure
SAW	20–35%	30–50% reduction	Coarser borides; more Fe2B
FCAW	10–25%	20–40% reduction	Moderate boride refinement
PTA	5–15%	10–30% reduction	Good boride refinement
Laser	2–10%	5–20% reduction	Fine boride distribution

Consumable design must account for dilution by increasing the B content in the consumable relative to the target overlay composition. For example, if the target overlay requires 0.8% B and FCAW dilution is estimated at 20%, the consumable should contain approximately 1.0% B.

Engineering Practice Insights

Common Defects and Their Mitigation

Defect Type	Root Cause	Detection Method	Mitigation Strategy
Excessive brittleness	Too high B content; excessive FeB formation	Hardness test; impact test	Reduce B to <1.0%; use processes with moderate cooling rates
Cracking	High B increases thermal stress; low toughness	MT/PT after welding	Preheat; control interpass temperature; use multi-pass with thinner passes
Uneven boride distribution	Inconsistent welding parameters	Metallographic examination	Use mechanized welding; maintain stable parameters
Poor bond strength	Inadequate surface preparation; insufficient heat input	Bond strength test (ASTM E203)	Proper cleaning; increase first-pass heat input
FeB decomposition in service	Operating temperature >250 °C	Hardness test after service; metallography	Use lower B content; accept Fe2B-only microstructure for high-temperature service

Case Study: Crusher Jaw Plate Overlay

A practical application involves the overlay of a boride-strengthened iron-based alloy (0.8% B, 3% Cr, 1.5% Mo, 0.6% C) on carbon steel crusher jaw plates. The original jaw plates experienced 400–500 hours of service before replacement due to excessive wear. After applying a 8 mm thick boride overlay:

Service life extended to 1800–2200 hours (4–5× improvement)
Overlay hardness maintained at 1050–1150 HV throughout service
No cracking or spalling observed under impact loading conditions
Cost per ton of material processed reduced by 70%

The boride overlay maintained its hardness and wear resistance even after extended exposure to temperatures up to 200 °C during operation, confirming the stability of the Fe2B phase under these conditions.

Key Questions and Reflections

The long-term stability of boride phases in service is a critical consideration that is often overlooked in initial alloy development. While laboratory tests at room temperature show excellent wear resistance, service conditions involving elevated temperatures may cause FeB decomposition, leading to hardness degradation. Engineers must ensure that the operating temperature remains below 250 °C for FeB-containing overlays, or design with lower B content to ensure Fe2B-only microstructures for higher-temperature applications.

Another important consideration is the interaction between borides and carbides in multi-hardening-phase overlays. The coexistence of Fe2B and M7C3 carbides can create a synergistic strengthening effect, but the relative volume fractions and spatial distribution determine the overall performance. An optimal balance between boride and carbide volume fractions (approximately 30% boride + 20% carbide in a martensitic matrix) may provide superior wear resistance compared to boride-only or carbide-only systems.

The brittleness associated with high boride volume fractions is a significant concern for impact-loaded applications. The transition from ductile to brittle fracture behavior occurs at boride volume fractions exceeding approximately 40%. For applications involving significant impact loading (e.g., mining equipment), the boride volume fraction should be limited to 25–35% to maintain adequate toughness.

Study Insights and Implications

The research on boride-strengthened iron-based overlay alloys demonstrates that boron is an effective and cost-efficient alloying element for enhancing wear resistance through hard boride phase formation. The optimal boron content of 0.6–1.0% in the overlay deposit provides a favorable balance between wear resistance and toughness, with the understanding that process-specific dilution adjustments are necessary.

For future development, the combination of boride strengthening with other hardening mechanisms—such as carbide reinforcement, dispersion strengthening, or nanostructured matrices—could offer further performance improvements. Additionally, the development of boride-containing consumables with controlled B distribution (e.g., gradient B content in wire coating) may enable the creation of functionally graded overlays with optimized surface hardness and subsurface toughness.

In conclusion, boride-strengthened iron-based overlay alloys represent a practical and cost-effective solution for wear protection in moderate-to-severe abrasion environments. Engineers designing such overlays should target 0.6–1.0% B content in the final deposit, select welding processes that provide appropriate cooling rates for the desired boride phase distribution, and validate long-term stability under representative service conditions including temperature exposure and impact loading.