Characteristics of Different Carbide Particles in Overlay Composite Materials

Literature Overview

This paper by Gao Feng from the Beijing Institute of Mining Research, China Coal Science and Technology Group Corporation, published in Journal of the China Coal Society (1996, Vol. 21, No. 6, pp. 644–649), investigates the characteristics of different types of carbide particles—sintered WC, cast WC, and sintered TiC—when used as hardfacing alloys in overlay composite materials. The research is funded by the Coal Science Fund and employs hardness testing, metallographic analysis, X-ray diffraction (XRD), and wear testing to compare the performance of these carbide-containing overlay alloys. The study is particularly relevant to mining and coal industry applications where components are subjected to severe abrasive wear.

Core Technical Findings

Types of Carbide Particles Evaluated

The study evaluates three types of carbide particles used in hardfacing alloys:

Carbide Type	Manufacturing Method	Key Characteristics
Sintered WC	Powder metallurgy sintering	High purity, uniform particle size, good bonding
Cast WC	Casting process	Larger particles, irregular shape, lower purity
Sintered TiC	Powder metallurgy sintering	Lower hardness than WC, better weldability

Wear Resistance Comparison

The wear resistance results reveal a clear hierarchy:

Carbide Type	Relative Wear Resistance	Wear Mechanism
Cast WC	Highest	Abrasion resistance due to high hardness and large particle size
Sintered WC	Moderate	Abrasion resistance with uniform particle distribution
Sintered TiC	Moderate (comparable to sintered WC)	Abrasion resistance with lower hardness particles

The superior wear resistance of cast WC is attributed to its larger particle size and irregular shape, which provide greater resistance to abrasive particles through a ploughing and cutting resistance mechanism. However, this comes at the cost of impact toughness.

Impact Toughness and Weldability

Carbide Type	Impact Toughness	Weldability
Cast WC	Poor	Moderate
Sintered WC	Moderate	Good
Sintered TiC	Good	Poor

Cast WC exhibits the poorest impact toughness due to the large, irregular carbide particles that act as stress concentrators and crack initiation sites. The sintered WC provides a better balance between wear resistance and impact toughness, while sintered TiC offers the best impact toughness but with lower wear resistance.

The weldability of sintered TiC is reported as poor, likely due to the chemical reactivity of TiC with the molten metal and the tendency to form brittle intermetallic compounds at the carbide-matrix interface. This poor weldability limits the application of sintered TiC in arc welding processes.

Microstructural Analysis

Metallographic and XRD analysis reveal the following microstructural features:

Cast WC overlay: Large, irregular WC particles embedded in a metallic matrix. The particle-matrix interface may show signs of partial dissolution or intermetallic formation, which can affect bonding strength.
Sintered WC overlay: Uniformly sized WC particles with good bonding to the matrix. The particle distribution is more homogeneous, leading to more consistent wear performance.
Sintered TiC overlay: Smaller TiC particles with potentially weaker bonding to the matrix. The TiC particles may undergo partial dissolution during welding, affecting the final microstructure.

Engineering Practice and Application Considerations

Selection Criteria for Carbide Type

The selection of carbide type for overlay composite materials depends on the specific application requirements:

Application Requirement	Recommended Carbide Type	Rationale
Maximum wear resistance	Cast WC	Highest abrasion resistance despite poor toughness
Balanced wear and toughness	Sintered WC	Good wear resistance with acceptable toughness
High impact loading	Sintered TiC	Best impact toughness despite lower wear resistance
Arc welding process	Sintered WC or Cast WC	Better weldability than sintered TiC
Mining equipment	Cast WC or Sintered WC	High abrasion resistance for severe wear conditions
Pump impellers	Sintered WC	Balanced properties for erosion-corrosion service

Comparison with Conventional Hardfacing Alloys

Alloy Type	Hardness (HV)	Wear Resistance	Impact Toughness	Cost
Cast WC composite	1000–1500	Excellent	Poor	High
Sintered WC composite	800–1200	Very good	Moderate	Moderate
Sintered TiC composite	600–900	Good	Good	Moderate
Stellite 6	400–500	Good	Good	Moderate
Cr-C-N hardfacing	800–1000	Very good	Moderate	Low

Process Considerations for Carbide Composite Overlay Welding

The welding of carbide-containing overlay alloys presents specific challenges:

Particle dissolution: During welding, the carbide particles may partially dissolve into the molten metal, altering the composition and microstructure of the deposited layer.
Particle redistribution: The fluid flow in the molten pool can redistribute the carbide particles, leading to non-uniform distribution in the final deposit.
Intermetallic formation: Carbide particles may react with the molten metal to form brittle intermetallic compounds at the particle-matrix interface.
Cracking susceptibility: The presence of hard, brittle carbide particles can increase the susceptibility to cracking, especially in high-restraint welding conditions.

To mitigate these challenges, the following process measures are recommended:

Low heat input: Minimize the thermal cycle to reduce carbide dissolution and particle redistribution.
Multi-pass welding: Use multiple passes with controlled interpass temperature to maintain carbide integrity.
Preheating: Moderate preheating (150–250°C) to reduce thermal gradients and cracking susceptibility.
Post-weld stress relief: Stress relief at 600–700°C to reduce residual stresses without degrading carbide properties.

Key Questions and Reflections

A critical question raised by this study is the long-term stability of carbide particles in service. Under abrasive wear conditions, the carbide particles may be gradually removed, exposing the softer matrix and accelerating wear. The rate of carbide removal depends on the particle size, bonding strength, and the abrasive severity of the service environment. Engineers should consider the service life of the overlay and plan for re-overlay or replacement accordingly.

Another consideration is the effect of thermal cycling on the carbide-matrix interface. In applications involving thermal cycling (e.g., mining equipment exposed to hot and cold environments), the thermal expansion mismatch between the carbide particles and the metallic matrix can induce thermal stresses at the interface, potentially leading to debonding or cracking. This is particularly relevant for cast WC, which has larger particles and thus greater thermal stress at the interface.

The poor weldability of sintered TiC is a significant limitation for its application in arc welding processes. Alternative processes such as thermal spraying (HVOF, plasma spray) or brazing may be more suitable for depositing TiC-containing overlays. Engineers should evaluate the process-material compatibility before selecting a carbide type for a specific application.

Summary

This study provides valuable comparative data on the performance of sintered WC, cast WC, and sintered TiC carbide particles in overlay composite materials. The results demonstrate that cast WC offers the highest wear resistance but with poor impact toughness, while sintered WC provides a balanced performance, and sintered TiC offers the best impact toughness but with lower wear resistance and poor weldability. Engineers selecting carbide types for overlay applications should carefully consider the service conditions, including wear severity, impact loading, thermal cycling, and welding process constraints, to achieve the optimal balance between wear resistance, toughness, and processability.