Abrasion and Erosion Behavior of CrMnB Weld Overlay Alloys

Literature Overview and Research Context

CrMnB-type weld overlay alloys have emerged as a prominent class of high-hardness wear-resistant coatings, achieving as-welded hardness values of 58–65 HRC due to the formation of boride phases (M2B and M23B6) and martensitic matrix. These alloys are widely used in mining equipment, cement mills, hydraulic machinery, and petrochemical applications where severe abrasive and erosive wear conditions prevail. This study systematically investigates the abrasion and erosion behavior of CrMnB weld overlay alloys with varying Cr, Mn, and B compositions, produced by submerged arc welding and gas metal arc welding processes. The research addresses a critical gap in the existing literature: while the room-temperature abrasion resistance of CrMnB alloys is well documented, their performance under erosive conditions involving solid particle impact at high velocities remains poorly understood, despite the prevalence of erosion-abrasion combined wear in industrial applications.

Compositional Design and Microstructural Characteristics

The study examines five CrMnB alloy compositions with varying Cr (8–14%), Mn (2–6%), and B (0.8–2.0%) contents. The microstructure consists of a martensitic matrix with a volume fraction of 15–35% boride phases, depending on the boron content. The primary boride phases are M2B (Fe, Cr, Mn)2B and M23B6, with M2B being the harder phase (approximately 1500–1800 HV) and M23B6 being slightly softer (approximately 1200–1400 HV). Increasing boron content from 0.8% to 2.0% increases the boride volume fraction from 18% to 35%, but also increases the brittleness of the deposit due to the formation of continuous boride networks at grain boundaries. The optimal boron content for balanced hardness and toughness is 1.2–1.5%, which produces a boride volume fraction of 25–28% with a discontinuous distribution.

Abrasion Test Results and Mechanism Analysis

Dry sliding abrasion tests using ASTM G65 methodology with alumina abrasives (SiC paper, 60 grit and 120 grit) reveal that the wear rate decreases with increasing hardness, but the relationship is not linear. The CrMnB-3 alloy (63 HRC) exhibits the lowest specific wear rate of 2.1 × 10⁻⁶ mm³/N·m, which is 45% lower than CrMnB-1 (3.8 × 10⁻⁶ mm³/N·m). Post-wear surface analysis shows that the primary wear mechanism for CrMnB alloys is microploughing and microcutting, where the hard boride particles resist penetration by the abrasive particles, and material removal occurs primarily from the softer martensitic matrix between the borides. When the boride volume fraction exceeds 30%, the wear mechanism shifts to microfracture, as the continuous boride network creates brittle zones that crack under the abrasive load. The transition from microcutting to microfracture is the key factor explaining the non-linear wear rate-hardness relationship.

Erosion Test Results and Impact Velocity Effects

Erosion tests conducted using a sandblast erosion apparatus with 100 μm alumina particles at impact velocities of 15, 25, and 35 m/s reveal a complex behavior. At low impact velocities (15 m/s), the wear rate increases monotonically with hardness, as expected from the abrasive wear mechanism. However, at higher impact velocities (25–35 m/s), the CrMnB-5 alloy with the highest boride content (35 vol%) exhibits a significantly higher erosion rate than CrMnB-3, despite its higher hardness. This counterintuitive result is attributed to the brittle fracture of the continuous boride network under high-velocity particle impact, which creates large fragments that are easily removed from the surface. The erosion rate at 35 m/s for CrMnB-5 is 3.2 × 10⁻⁶ mm³/mg, compared to 1.8 × 10⁻⁶ mm³/mg for CrMnB-3, representing a 78% increase. This finding has significant implications for the selection of CrMnB alloys for erosive service conditions, as the commonly used hardness-based selection criteria are inadequate for high-velocity erosion environments.

Engineering Recommendations and Selection Guidelines

Based on the combined abrasion and erosion test results, the study recommends the following selection guidelines for CrMnB weld overlay alloys. For pure abrasive wear conditions (mining shovels, conveyor rollers, cement mill liners), alloys with CrMnB-3 or CrMnB-4 composition (12–14% Cr, 4–5% Mn, 1.5–1.8% B) provide the best wear resistance. For combined abrasion-erosion conditions (hydraulic pump nozzles, fan blades, sand slurry pumps), alloys with CrMnB-2 or CrMnB-3 composition (10–12% Cr, 3–4% Mn, 1.2–1.5% B) are recommended, as they offer the best balance between hardness and toughness. The welding process should be SAW with a flux-cored wire for thick deposits (5–10 mm), as this method provides the best dilution control and consistent composition. The interpass temperature should be maintained below 200°C to ensure full martensitic transformation and maximum hardness. Post-weld heat treatment is generally not recommended for CrMnB alloys, as tempering reduces the hardness and eliminates the beneficial boride phase stability.

Study Insights and Practical Implications

This study provides critical insights into the erosion behavior of CrMnB weld overlay alloys that are not captured by conventional hardness-based selection methods. The finding that excessive boron content degrades erosion resistance at high impact velocities is particularly important for engineers specifying coatings for erosive service, as it challenges the common assumption that higher hardness always correlates with better erosion resistance. The practical implication is that coating selection for erosive applications should consider not only the hardness but also the microstructural characteristics, particularly the boride distribution and volume fraction. Future research should investigate the effect of microalloying additions such as titanium, niobium, or vanadium on the boride morphology and erosion resistance of CrMnB alloys, as these elements may promote the formation of more thermally stable and fracture-resistant boride phases. Engineers should also consider the processing history of the coating, as the welding process and parameters significantly influence the boride distribution and, consequently, the erosion resistance. A comprehensive coating selection strategy should integrate hardness, microstructure, and service condition data to achieve optimal performance and service life.