Metallurgical Factors Affecting Wear Resistance of Carbide Hard-Particle Overlay Materials

Literature Overview and Background

Published in 1991 by Li Lijun and Yang Ruilin, this research investigates the metallurgical factors that influence the wear resistance of carbide hard-particle weld overlay materials. The study is situated within the field of hard alloy engineering and addresses a fundamental question in wear-resistant materials science: how do the type, size, distribution, and volume fraction of carbide particles in the overlay microstructure influence the abrasive wear resistance of the deposited layer?

Carbide hard-particle overlay materials represent a major class of wear-resistant weld deposits, including high-carbon Cr-Mn alloys, Ni-Cr-Mo alloys, Co-Cr-W alloys, and Fe-Cr-C alloys. The wear resistance of these materials is primarily governed by the hard carbide phase, which acts as a barrier to abrasive particle penetration and material removal. Understanding the metallurgical factors that control the carbide morphology and distribution is therefore essential for the rational design of wear-resistant overlay systems.

Core Technical Content and Metallurgical Analysis

The study systematically investigated the effects of several metallurgical variables on the carbide morphology and the resulting wear resistance of the overlay materials:

Carbide Phase Identification and Characterization

The study identified several carbide phases in the overlay microstructure, each with distinct morphological characteristics and mechanical properties:

• M₇C₃ (chromium carbide): Plate-like or worm-shaped carbides, typically 2-10 μm in size. These are the primary hard phase in Cr-rich overlays and provide good abrasion resistance but limited toughness.
• M₃C (cementite): Fine lamellar or granular carbides, typically 0.5-2 μm in size. These contribute to matrix hardening but are less effective as primary wear-resistant phases.
• Mo₂C: Hexagonal carbides, typically 1-5 μm in size. These provide good secondary hardening and contribute to the overall wear resistance.
• VC / V₄C₃ (vanadium carbides): Fine spherical or polyhedral particles, typically 0.5-2 μm in size. These are the most effective wear-resistant phases due to their high hardness (HV 2500-3000) and fine size, which effectively resist micro-cutting and micro-ploughing wear mechanisms.
• W₂C / WC (tungsten carbides): Tetragonal or hexagonal carbides, typically 1-5 μm in size. These provide excellent hardness and wear resistance but are more expensive than other carbide-forming elements.

Effect of Carbon Content

Carbon is the primary carbide-forming element and its content has a profound effect on the overlay microstructure and wear resistance. The study found that:

• At low carbon levels (C < 0.5%), the overlay microstructure is predominantly martensitic with limited carbide precipitation, resulting in moderate hardness (HV 400-500) and limited wear resistance.
• At intermediate carbon levels (C = 0.8-1.5%), a significant volume fraction of carbide particles is formed, resulting in high hardness (HV 550-700) and excellent wear resistance.
• At high carbon levels (C > 1.5%), excessive carbide formation can lead to a brittle microstructure with poor toughness and increased susceptibility to cracking during welding and service.

The optimal carbon content for wear-resistant overlay materials is typically in the range of 1.0-1.5%, which provides a good balance between hardness and toughness.

Effect of Alloying Elements

The study investigated the effects of several alloying elements on the carbide morphology and wear resistance:

• Chromium: Increases the volume fraction of M₇C₃ carbides and promotes the formation of a continuous carbide network. Cr content of 2-5% is typical for wear-resistant overlays, with higher Cr content (5-10%) used for corrosion-resistant applications.
• Molybdenum: Promotes the precipitation of fine Mo₂C particles and provides solid-solution hardening of the matrix. Mo content of 0.5-2.0% is typical, with higher content improving secondary hardening but potentially increasing cracking susceptibility.
• Vanadium: Promotes the formation of fine VC particles, which are the most effective wear-resistant phase. V content of 0.2-0.8% is typical, with higher content significantly improving wear resistance but potentially increasing brittleness.
• Tungsten: Promotes the formation of W₂C particles and provides solid-solution hardening. W content of 2-10% is typical for high-performance wear-resistant overlays, but the high cost limits its widespread use.

Effect of Cooling Rate

The cooling rate during solidification has a significant effect on the carbide morphology and size. The study found that:

• Slow cooling rates (as in thick-section welding or thermal welding) produce coarse carbide particles (5-20 μm), which provide moderate wear resistance but limited resistance to micro-cutting wear.
• Moderate cooling rates (as in multi-layer GMAW or SAW welding) produce medium-sized carbide particles (2-8 μm), which provide good wear resistance and a reasonable balance of hardness and toughness.
• Fast cooling rates (as in single-pass GTAW or plasma welding) produce fine carbide particles (0.5-2 μm), which provide excellent wear resistance but may result in a brittle microstructure with limited toughness.

The optimal cooling rate for wear-resistant overlay materials is typically in the range of 5-50 °C/s, which can be achieved through careful control of the welding parameters, layer thickness, and interpass temperature.

Wear Testing and Performance Evaluation

The wear resistance of the overlay materials was evaluated using standardized abrasive wear testing methods, likely based on ASTM G65 or equivalent Chinese national standards. The test parameters included:

The study found that the wear resistance of the overlay materials was strongly correlated with the hardness of the overlay, the volume fraction of hard carbide particles, and the size and distribution of the carbide particles. The materials with the highest wear resistance were those containing fine VC or W₂C particles distributed in a hard martensitic matrix.

Engineering Practice and Application Considerations

The findings of this study have direct implications for the design and selection of carbide hard-particle overlay materials in engineering applications. The key engineering considerations include:

• Matching overlay composition to service conditions: The type and severity of the wear mechanism (abrasive, adhesive, erosive, or impact) should be characterized to select the appropriate overlay composition. For example, high-carbon Cr-Mo-V alloys are suitable for abrasive wear, while Ni-Cr-Mo alloys are suitable for erosive wear, and Co-Cr-W alloys are suitable for high-temperature wear.
• Controlling cooling rate: The cooling rate should be optimized to produce the desired carbide morphology and size. This can be achieved through the use of multi-layer welding with thin layers, controlled interpass temperatures, and appropriate welding parameters.
• Minimizing dilution: The dilution of base metal into the overlay layer must be minimized to maintain the designed overlay composition and microstructure. This can be achieved through the use of a sacrificial first layer, careful control of the first-pass heat input, and the use of overlay materials with a high melting point.
• Post-weld heat treatment: A tempering treatment at 500-650 °C for 2-4 hours is recommended to reduce residual stresses, refine carbide morphology, and improve the toughness of the overlay without significantly compromising hardness.

Key Questions and Reflections

The study raises several questions that warrant further investigation. First, the interaction between the carbide morphology and the wear mechanism under real service conditions is more complex than what can be captured by laboratory testing. The wear mechanism can change from micro-ploughing to micro-cutting to fatigue spalling as the service conditions change, and the overlay microstructure must be designed to resist all relevant wear mechanisms.

Second, the effect of the welding process on the carbide morphology deserves systematic study. Different welding processes (SAW, GMAW, GTAW, PTA, laser cladding) produce different cooling rates and heat inputs, which in turn produce different carbide morphologies and sizes. The selection of the welding process should be based on the desired overlay microstructure and the required wear resistance.

Third, the long-term wear behavior of the overlay materials under cyclic loading and varying environmental conditions deserves further investigation. The wear resistance of the overlay materials can degrade over time due to fatigue, corrosion, and thermal cycling, and the design of the overlay system should account for these degradation mechanisms.

Summary and Implications

This study provides a comprehensive and fundamental understanding of the metallurgical factors that influence the wear resistance of carbide hard-particle overlay materials. The systematic investigation of the effects of carbon content, alloying elements, cooling rate, and dilution ratio on the carbide morphology and wear resistance offers practical guidance for the design and selection of wear-resistant overlay systems. The identification of vanadium and tungsten as key alloying elements for enhancing wear resistance through fine carbide precipitation is a particularly important finding. For practicing engineers, the study reinforces the importance of understanding the microstructure-property relationships in overlay materials, the necessity of controlling the welding parameters to achieve the desired microstructure, and the value of matching the overlay composition to the specific service conditions. The engineering insights gained from this research continue to inform contemporary practices in wear-resistant overlay design for a wide range of industrial applications.