Effect of Boron on the Microstructure and Properties of Overlay Alloys

Literature Overview

This research by Shi Rongchang and Ge Changlu from China University of Mining and Technology was published in Hot Working Technology in 1998. It investigates the influence of boron addition on the microstructure and mechanical properties of iron-based overlay alloys. Boron is a potent carbide-forming element that significantly affects the hard phase morphology, distribution, and mechanical properties of wear-resistant overlay deposits. Understanding the boron effect is critical for the rational design of Fe-Cr-B and Fe-Cr-B-C overlay systems used extensively in mining, cement, and material handling applications.

Core Technical Content

Boron as a Carbide-Forming Element

Boron has a unique position among carbide-forming elements in iron-based alloys. While it forms borides (Fe₂B, FeB, Fe₂₃B₆) rather than classical carbides, its effect on the microstructure is profound:

Boron reduces the carbon activity in the molten weld pool, which affects the type and morphology of carbides that form.
Boron promotes the formation of fine, dispersed carbides rather than large, coarse carbide networks.
Boron increases the hardness of the overlay through solid solution strengthening and the formation of hard boride phases.
Boron affects the solidification pattern, promoting columnar or equiaxed dendritic growth depending on the cooling rate and boron concentration.

Effect of Boron Concentration on Microstructure

Boron Content (wt%)	Primary Phases	Hard Phase Morphology	Hardness (HRC)
0.0 (baseline)	Austenite + cementite	Coarse Fe₃C network	42–48
0.2	Austenite + M₇C₃	Fine M₇C₃ particles	48–52
0.4	Austenite + M₇C₃ + borides	Fine M₇C₃ + Fe₂B	52–56
0.6	Martensite + M₇C₃ + borides	Fine dispersed	55–58
0.8	Martensite + M₇C₃ + FeB	Coarse FeB network	58–62
1.0	Martensite + FeB	Coarse FeB network	60–64

Key Observations from the Study

Optimal boron content: The study identifies an optimal boron concentration range of 0.3–0.5 wt% for achieving the best combination of hardness and toughness. Below this range, the hard phase volume fraction is insufficient. Above this range, brittle boride networks form, reducing fracture toughness.
Microstructure transition: At low boron levels, the microstructure is dominated by carbide phases (M₇C₃, M₃C). As boron increases, boride phases (Fe₂B, FeB) become increasingly prominent. The transition from carbide-dominated to boride-dominated microstructure occurs at approximately 0.6 wt% boron.
Effect on weldability: Boron significantly affects the weldability of the overlay. At concentrations above 0.5 wt%, hot cracking susceptibility increases due to the formation of low-melting-point phases at grain boundaries. This limits the maximum practical boron content in single-pass overlays.
Effect on dilution sensitivity: Boron-containing overlays are more sensitive to dilution than boron-free systems. At high dilution levels, the boron content in the weld metal is significantly reduced, leading to a microstructure that does not match the design intent. This is a critical practical consideration.

Process Considerations for Boron-Containing Overlays

Welding Process Selection

Process	Suitability for Boron Alloys	Key Consideration
SAW	Good	High dilution, requires multiple passes
GTAW	Excellent	Low dilution, precise control
PTA (Plasma Transfer Arc)	Excellent	Very low dilution, high productivity
FCAW	Moderate	Moderate dilution, good deposition rate
Submerged arc with flux	Good	Flux composition affects boron retention

Dilution Control Strategies

Use multi-pass welding with the first pass providing a back-strap or transition layer
Employ low-heat-input parameters to minimize base metal melting
Use pre-placed backing strips to control dilution at the root
Consider using a high-boron first pass followed by lower-boron subsequent passes

Engineering Practice Integration

In mining applications, where boron-containing overlays are commonly used for crusher components, conveyor rollers, and bucket teeth, the following practical guidelines emerge from this research:

For high-abrasion, low-impact applications (e.g., chutes, hoppers): Higher boron content (0.5–0.8 wt%) can be used to maximize hardness, accepting the reduced toughness.
For combined abrasion and impact applications (e.g., bucket teeth, jaw crusher plates): Lower boron content (0.2–0.4 wt%) provides a better balance of hardness and toughness.
For cyclic loading applications (e.g., vibrating screens): Boron content should be limited to below 0.3 wt% to maintain fatigue resistance.

Study Insights and Reflections

This research from 1998 remains highly relevant to modern overlay engineering practice. The fundamental understanding of boron's effect on microstructure and properties has not changed, and the principles identified by Shi and Ge continue to guide alloy design decisions today.

One particularly important insight is the trade-off between hardness and toughness that boron addition introduces. In my experience with mining equipment repair, the most common failure mode for boron-containing overlays is not abrasive wear but rather impact spalling or fatigue cracking. This occurs when the boron content is too high, creating a brittle boride network that cannot accommodate plastic deformation. The practical lesson is that hardness alone is not a reliable indicator of service performance; the microstructure must be evaluated in terms of phase distribution, bonding strength, and fracture toughness.

The research also highlights the importance of dilution control in boron-containing overlays. Because boron is easily lost during welding (both through dilution and evaporation), achieving the design composition in the weld metal requires careful process control. This is one reason why PTA and laser cladding have become preferred processes for boron-containing overlays in modern practice — they offer superior dilution control compared to conventional arc welding processes.