Effect of Boron Content on Fe-Cr-C Surfacing Layer Properties

Literature Overview

The study by Song Xiaobo, Li Xiaoyan, He Dingyong, and Wang Zhihui (2009), from the School of Materials Science and Engineering, Beijing University of Technology, investigates the influence of boron content on the microstructure, hardness, wear resistance, and mechanical properties of Fe-Cr-C surfacing layers. Boron is a potent carbide former and grain refiner that can significantly alter the properties of iron-based surfacing alloys. The research is particularly relevant to applications in mining, construction, and material handling equipment where high-abrasion resistance is required, and where Fe-Cr-C-based surfacing layers are commonly applied to extend component service life.

Core Technical Findings

The authors systematically varied boron content from 0% to 2.0% (wt) in a Fe-20Cr-1.5C base alloy and evaluated the resulting surfacing layers through comprehensive characterization:

The data reveals a clear trade-off between hardness/wear resistance and toughness as boron content increases. The optimal boron content for balanced performance is approximately 0.5–0.8%, where the hardness reaches 720–780 HV with acceptable ductility (elongation > 3.5%).

Microstructural Evolution

The addition of boron fundamentally alters the carbide formation mechanism in Fe-Cr-C alloys:

• At low boron levels (0–0.3%): Boron acts as a grain refiner and promotes the formation of fine, uniformly distributed M₇C₃ carbides. The boron atoms substitute for carbon in the carbide lattice, creating B₄C particles that are harder (2500 HV) than M₇C₃ (1500 HV) but more brittle.
• At moderate boron levels (0.5–0.8%): A mixed carbide structure develops, with M₇C₃ providing toughness and B₄C providing hardness. This synergistic combination yields optimal wear resistance with acceptable fracture resistance.
• At high boron levels (>1.0%): The carbon activity in the melt decreases significantly due to boron's strong affinity for carbon, leading to depletion of M₇C₃ and dominance of iron borides (FeB, Fe₂B). These phases are extremely hard but brittle, resulting in a surfacing layer that is prone to spalling under impact loading.

The authors also observed that boron addition significantly reduces the grain size of the surfacing layer. At 0.5% B, the average grain size decreased from 85 μm (0% B) to 42 μm, contributing to the Hall-Petch strengthening effect and improved toughness.

Wear Mechanism Analysis

The wear behavior of the boron-modified surfacing layers was analyzed using pin-on-disk tribotests against 1020 steel counterfaces:

• Adhesive wear was the dominant mechanism at low boron levels (0–0.3%), where the relatively soft M₇C₃ carbides could not resist material transfer.
• Abrasive wear became dominant at moderate boron levels (0.5–0.8%), where the hard B₄C particles provided effective ploughing resistance.
• Fatigue wear and spalling became prevalent at high boron levels (>1.2%), where the brittle FeB network could not accommodate cyclic plastic deformation, leading to subsurface crack initiation and delamination.

The wear rate reduction of 70% achieved at 0.5% B (compared to the unmodified alloy) represents a substantial improvement in service life for components subjected to abrasive wear, such as crusher jaws, conveyor rollers, and pump impellers.

Process Considerations for Boron-Containing Surfacing

The addition of boron introduces several process challenges that must be addressed in practice:

1. Burn-on tendency: Boron has a low melting point (2076°C) but high surface tension, which can lead to tungsten electrode contamination in GTAW processes. The authors recommended using tungsten electrodes with 5% thorium or 2% lanthanum and maintaining arc lengths below 2 mm.
2. Hydrogen pickup: Boron increases the hydrogen solubility in the liquid weld pool, raising the risk of porosity and delayed cracking. Preheating of the base material to 200–250°C and post-weld baking at 150°C for 2 hours are recommended.
3. Dilution effects: The high reactivity of boron means that even moderate dilution (20–30%) can significantly reduce the effective boron content in the overlay. Multi-pass surfacing with progressively increasing boron content in the filler is advisable.
4. Cracking sensitivity: The high thermal conductivity of boron-rich phases creates steep thermal gradients, increasing residual stress and cracking risk. Controlled cooling rates (below 100°C/hour) and stress-relief annealing are essential.

Engineering Application Guidance

For practical applications, the authors recommend the following boron content ranges based on the dominant wear mechanism:

Key Reflections

This research provides a clear demonstration of how a single alloying element—boron—can transform the performance of a conventional Fe-Cr-C surfacing alloy. The key insight is that boron does not merely increase hardness; it fundamentally restructures the carbide phase assemblage, creating a synergistic combination of hard and tough phases at optimal concentrations. This concept of "carbide engineering" through controlled boron addition is directly transferable to other iron-based surfacing systems, including Fe-Ni-Cr-B alloys used in nuclear applications and Fe-Co-Cr-B alloys used in aerospace. The practical implications for extending the service life of heavy-duty mining and construction equipment are substantial, with potential cost savings of 30–50% in component replacement and maintenance downtime.