Microstructure and Sliding Wear Performance of High-Chromium Bimetal Wear-Resistant Plate Overlay Layer Study Note

Literature Overview and Background

High-chromium cast iron overlay layers represent one of the most widely deployed solutions for abrasive wear protection in mining, cement, power generation, and material handling industries. The literature under review examines the relationship between microstructural features of the overlay layer and the resulting sliding friction and wear performance. This connection between microstructure and tribological behavior is fundamental to material selection and process optimization for wear-resistant applications.

The bimetal construction, typically consisting of a ductile carbon steel or low-alloy steel backing plate with a high-chromium overlay layer, combines the toughness and formability of the base material with the exceptional abrasion resistance of the overlay. The overlay composition typically contains 15-30% chromium by weight, with variations in carbon content and alloying additions to control the type, distribution, and morphology of carbide phases.

Core Technical Content

Microstructural Characteristics of High-Chromium Overlay Layers

The microstructure of high-chromium overlay layers is dominated by carbide phases embedded in a matrix that may range from martensitic to austenitic depending on composition and cooling rate. The key phases identified in the study include:

The distribution and morphology of these carbides are directly controlled by the cladding process parameters. In submerged arc welding (SAW) overlay, the relatively slow cooling rates (typically 5-50 °C/s) promote the formation of coarse M7C3 carbides with a eutectic structure. In contrast, gas tungsten arc welding (GTAW) overlay produces finer carbide distributions due to higher cooling rates (50-200 °C/s).

Sliding Wear Test Methodology and Results

The study employs a pin-on-disk or block-on-ring configuration to evaluate sliding wear behavior under controlled conditions. Typical test parameters include:

The wear rate is expressed as volumetric material loss per unit sliding distance, typically in the range of 10⁻⁶ to 10⁻⁴ mm³/(N·m). The coefficient of friction (COF) is recorded throughout the test to identify steady-state behavior and any transitions between wear mechanisms.

Wear Mechanism Analysis

The study identifies three primary wear mechanisms operating in the high-chromium overlay layer:

1. Abrasive wear: Dominant at low loads and moderate speeds. Hard carbide particles resist ploughing by the counterface material, resulting in low volumetric wear rates. The effectiveness of this mechanism depends critically on carbide volume fraction (typically 40-60% for optimal performance) and carbide hardness relative to the counterface.
2. Adhesive wear: Becomes significant at higher loads where contact pressures exceed the yield strength of the matrix phase. Matrix material transfers to the counterface, creating friction welding spots that fracture during sliding. This mechanism is mitigated by maintaining a high volume fraction of hard carbides that support the matrix under load.
3. Fatigue wear: Operates at moderate loads and high sliding distances. Repeated cyclic loading causes microcrack initiation at carbide-matrix interfaces, leading to spalling of material in plate-like fragments. The carbide-matrix bonding strength and matrix toughness are critical parameters for fatigue wear resistance.

Interpretation of Key Technical Points

The Critical Role of Carbide Distribution

The most important finding from the microstructural analysis is that carbide distribution uniformity is more influential on wear performance than carbide volume fraction alone. A layer with 50% carbide volume fraction but with localized carbide clusters and carbide-free zones will exhibit significantly worse wear performance than a layer with 45% volume fraction but uniform distribution.

This observation has direct implications for process control. In multi-pass SAW overlay, the dilution between successive passes creates compositional variation that affects carbide precipitation. The first pass, with maximum dilution from the base metal, typically produces a more austenitic matrix with fewer carbides. Subsequent passes, with reduced dilution, produce progressively more carbide-rich microstructures. The transition zone between passes can become a preferential wear path if the carbide distribution is discontinuous.

Friction Coefficient Behavior

The steady-state coefficient of friction for well-designed high-chromium overlays typically ranges from 0.3 to 0.5 against ceramic counterfaces. This is significantly higher than the theoretical value for pure carbide materials (0.1-0.2) but acceptable for most industrial applications. The COF is primarily determined by:

• The ratio of carbide surface area to matrix surface area in the contact zone.
• The presence of tribofilms or oxide layers formed during sliding.
• The roughness of the overlay surface, which affects real contact area.

The study notes that surface roughness Ra values between 0.8 and 2.5 μm provide the optimal balance between low real contact area (reducing adhesive wear) and sufficient surface interlocking (preventing material transfer).

Dilution Effects on Overlay Performance

The dilution level, defined as the percentage of base metal atoms in the overlay composition, is a critical process parameter that directly affects both microstructure and wear performance:

The practical challenge is that achieving low dilution (< 15%) requires either:

• Using a consumable with significantly higher chromium content than the target overlay composition.
• Employing low-heat-input processes (GTAW, laser cladding) that limit thermal penetration.
• Using multi-pass strategies where the first pass accepts higher dilution and subsequent passes achieve the target composition.

Connection with Engineering Practice

Process Selection for Wear-Resistant Bimetal Plates

Based on the findings of this study, the following process selection guidelines emerge for manufacturing high-chromium bimetal wear-resistant plates:

Quality Control Considerations

The following quality control measures are essential for ensuring consistent wear performance:

1. Chemical analysis of each pass to verify dilution level and confirm that the overlay composition meets specification.
2. Hardness profiling across the overlay thickness to identify composition gradients and dilution effects.
3. Metallographic examination at multiple locations to verify carbide distribution uniformity.
4. Wear coupon testing from representative locations to confirm that the achieved microstructure delivers the expected tribological performance.

Engineering Case: Mining Equipment Application

A practical example from mining equipment manufacturing illustrates the application of these principles. A conveyor belt scraper plate originally made from plain carbon steel with a 6 mm high-chromium SAW overlay was failing after 45 days of service in a wet, abrasive ore handling environment. Microstructural analysis revealed:

• Excessive dilution in the first two passes (35-45%), creating a soft, carbide-poor transition zone.
• Carbide-free zones along the pass boundaries due to compositional segregation.
• Matrix softening from insufficient post-weld heat treatment.

The redesigned solution incorporated:

• A pre-weld flux layer to reduce base metal dilution.
• Modified consumable composition with 28% Cr (vs. original 22%) to compensate for expected dilution.
• Post-weld austenitizing treatment at 950°C for 2 hours followed by air cooling.
• Result: Service life extended to 120+ days with no transition zone failures.

Study Insights and Reflections

The most valuable contribution of this literature is the explicit correlation between microstructural parameters and wear performance metrics. While the individual phases and their properties are well-documented in the metallurgy literature, the systematic quantification of how distribution uniformity, volume fraction, and morphology interact to determine tribological behavior is less commonly addressed.

The concept of a "critical carbide volume fraction" below which wear resistance degrades rapidly is particularly important for process engineers. This threshold, identified at approximately 25-30% volume fraction in the study, provides a clear acceptance criterion for overlay quality. Below this threshold, the matrix material bears the majority of the sliding load and fails through adhesive and fatigue mechanisms rather than resisting through carbide-supported abrasion resistance.

The dilution issue remains the most challenging aspect of practical implementation. The theoretical microstructures achievable with low dilution are rarely reproduced in production because of the inherent variability of arc processes. The study's emphasis on compositional compensation (using higher-alloy consumables to achieve target overlay composition despite dilution) is a pragmatic approach that I have found effective in my own engineering practice.

One area where the study could be extended is the consideration of multi-body wear scenarios. In real industrial applications, the wear counterface is rarely a smooth ceramic surface; it typically consists of abrasive particles embedded in a matrix of softer material. The interaction between three-body abrasion and the overlay microstructure introduces additional complexity that merits further investigation.

Reference Value and Outlook

This literature provides engineers with a comprehensive framework for understanding and optimizing high-chromium overlay performance. The microstructure-property relationships established here are directly applicable to material selection, process design, and quality control in wear-resistant bimetal manufacturing.

Future research should focus on computational modeling of carbide precipitation kinetics during multi-pass overlay welding, enabling predictive control of microstructure through process parameter optimization. Additionally, the development of nanostructured or gradient overlays that combine high hardness at the surface with increasing toughness at depth represents a promising direction for extending service life in severe wear environments.