Microstructure and Wear Resistance of Fe-Cr-B-C Overlay Alloys

Literature Overview and Composition Design Philosophy

This study examines the microstructural evolution and wear resistance characteristics of Fe-Cr-B-C system weld overlay alloys, which represent an important class of hardfacing materials used in industrial applications involving severe abrasion. The addition of boron to the traditional Fe-Cr-C system introduces new carbide phases and modifies the eutectic reaction, creating microstructures with enhanced hardness and wear resistance. The research investigates compositions ranging from 5-20 wt% Cr, 1-5 wt% B, and 1-4 wt% C, deposited using submerged arc welding and plasma transferred arc processes onto carbon steel substrates.

Microstructural Characterization and Phase Evolution

The Fe-Cr-B-C system produces a complex array of carbide phases including WC7B3, CrB, Cr2B, and Fe3B, depending on the relative concentrations of boron and chromium. The study identifies four distinct microstructural regimes based on boron content:

A critical observation is that boron content above 3 wt% promotes the formation of a continuous boride network, which significantly increases hardness but severely compromises fracture toughness. The optimal boron content for balanced performance is identified at 2.0-3.0 wt%, where a discontinuous distribution of CrB and Fe3B particles within a tough matrix provides excellent wear resistance without excessive brittleness.

The chromium content interacts with boron to determine the relative stability of different carbide phases. At chromium levels above 12 wt%, CrB becomes the dominant boride phase, which is more thermodynamically stable and harder than Fe3B. The study demonstrates that the combination of 15 wt% Cr and 2.5 wt% B produces a microstructure with approximately 25 vol% CrB particles dispersed in an austenitic matrix, achieving a hardness of 1850 HV with acceptable fracture toughness.

Wear Performance and Mechanism Analysis

Wear testing under dry sliding conditions against alumina (Al2O3) and silicon carbide (SiC) counterfaces reveals that the Fe-Cr-B-C overlays exhibit wear rates 3-8 times lower than conventional Fe-Cr-C overlays of comparable hardness. The enhanced wear resistance is attributed to the higher intrinsic hardness of boride phases and their resistance to plastic deformation. However, the study also identifies a critical wear threshold: when boron content exceeds 3.5 wt%, the continuous boride network becomes a preferential crack propagation path, leading to catastrophic delamination failure under impact loading.

The wear mechanism transitions from abrasive wear at low sliding distances to a mixed abrasive-adhesive mechanism at moderate distances, and finally to delamination wear at extended sliding distances for high-boron compositions. For compositions with 2.0-2.5 wt% B, the wear surface shows evidence of material removal by microcutting and ploughing without significant delamination, indicating a stable wear behavior suitable for long-term service.

Engineering Applications and Specification Guidance

For engineering applications, the study provides specific composition recommendations based on service conditions. For severe abrasion without impact loading, such as conveyor rollers and slurry pumps, compositions with 3.0-3.5 wt% B and 15-18 wt% Cr are recommended to maximize hardness. For applications involving both abrasion and impact, such as crusher hammers and excavator buckets, compositions with 2.0-2.5 wt% B and 10-15 wt% Cr provide the optimal balance of hardness and toughness. For applications requiring resistance to both wear and corrosion, such as chemical processing equipment, compositions with 18-20 wt% Cr and 1.5-2.0 wt% B offer adequate wear resistance with improved corrosion resistance.

The study also addresses welding process considerations, noting that submerged arc welding produces coarser microstructures with larger carbide particles compared to plasma transferred arc welding. For thin overlays below 3 mm, PTA is preferred for microstructural refinement, while SAW remains practical for thick overlays above 5 mm where deposition rate is critical.

Study Insights and Practical Implications

The most important finding from this study is the identification of a critical boron content threshold of approximately 3.0-3.5 wt% beyond which the microstructural transition from discontinuous to continuous boride networks leads to unacceptable brittleness. This finding has direct implications for alloy specification and welding procedure qualification, as it defines a clear compositional boundary that must be respected in production. Engineers should also note that boride phases are sensitive to heat treatment, and post-weld heat treatment above 900°C can cause significant coarsening and softening of the boride phases, negating the benefits of the as-welded microstructure.