Interface Microstructure and Properties of TiC Cemented Carbide and Weld Overlay Transition Layer

Literature Overview

This 2024 publication by Wu Shufei, Wei Wei, Ma Hexiang, Chen Yong, Liu Shengxin, and Huang Zhiquan from Zhengzhou University and Zhengzhou Mechanical Research Institute Co., Ltd. investigates the interface microstructure and properties at the junction between TiC cemented carbide and a weld overlay transition layer. This research was supported by the Henan Provincial Science and Technology Key R&D Project (222102230038). The study addresses a critical engineering challenge in the manufacturing of composite cutting tools and wear-resistant components where TiC-based cemented carbide is joined to a steel substrate through a weld overlay transition layer.

Technical Background and Motivation

TiC cemented carbide (typically 60-80% TiC with Co or Ni binder) offers exceptional hardness (HV 1800-2200) and wear resistance, making it ideal for cutting tools, mining bits, and severe wear applications. However, the direct joining of TiC cemented carbide to steel substrates presents significant challenges due to:

• Huge thermal expansion mismatch: TiC (~7.5 × 10⁻⁶ /K) vs. steel (~12 × 10⁻⁶ /K)
• Chemical incompatibility: TiC is chemically stable but can react with Fe-C alloys at elevated temperatures
• Brittleness: Both TiC and cemented carbide are inherently brittle, making them susceptible to crack initiation at the interface
• Thermal conductivity differences: TiC (~22 W/m·K) vs. steel (~25-50 W/m·K)

The weld overlay transition layer serves as an intermediary that accommodates these mismatches through:

• Gradient composition design
• Microstructural engineering
• Residual stress management

Interface Microstructure Analysis

The study examines the interface region through comprehensive metallographic and microstructural characterization:

Characterization Techniques Employed

Interface Zone Characterization

The interface region typically exhibits several distinct microstructural zones:

1. TiC cemented carbide zone: Retains original microstructure with TiC grains in Co/Ni binder
2. Reaction zone: Thin layer (5-50 μm) where chemical reactions between TiC and the molten transition layer occur
3. Transition layer proper: Multi-pass weld overlay with controlled composition gradient
4. Heat-affected zone (HAZ): In the steel substrate, exhibiting microstructural changes due to thermal cycling

Phase Formation at Interface

The critical phases that form at the TiC/transition layer interface include:

• TiFe₂: Iron-titanium intermetallic compound, formed by Ti diffusion into the molten steel
• Ti₅Fe₈: Another iron-titanium intermetallic, indicating further reaction
• Fe₃C: Cementite, formed in the transition layer
• TiC (retained): Unreacted TiC particles that remain intact

The formation of these intermetallic phases is both beneficial and detrimental:

• Beneficial: Provides metallurgical bonding between dissimilar materials
• Detrimental: Intermetallics are typically brittle and can serve as crack initiation sites

Mechanical Properties and Performance

The study evaluates the mechanical properties across the interface:

Hardness Distribution

Bond Strength

The bond strength between TiC cemented carbide and the transition layer is characterized through:

• Peel tests: Measuring the force required to separate the carbide from the transition layer
• Shear tests: Evaluating interfacial shear strength
• Indentation tests: Assessing local bonding quality

Typical bond strength values for well-executed TiC/transition layer joints range from 80-150 MPa in shear, depending on the transition layer composition and welding parameters.

Fracture Behavior

Fracture analysis reveals characteristic failure modes:

1. Interfacial fracture: Complete separation along the TiC/transition layer interface (unacceptable)
2. Transitional fracture: Fracture propagating through the reaction zone with some TiC pull-out (acceptable for many applications)
3. Cohesive fracture: Fracture propagating through the transition layer material itself (indicates strong bonding)

The optimal design aims for transitional or cohesive fracture, indicating that the interface strength exceeds the bulk strength of the transition layer.

Process Optimization and Design Considerations

The study provides guidance on optimizing the weld overlay process to achieve desirable interface characteristics:

Transition Layer Design Principles

1. Composition gradient: Multi-pass welding with gradually changing filler metal composition to create a diffusion barrier and reduce thermal stresses
2. Microstructure control: Heat treatment cycles to refine grain structure and relieve residual stresses
3. Residual stress management: Stress-relief annealing to reduce interfacial tensile stresses

Welding Process Parameters

Filler Metal Selection

The transition layer filler metal composition is critical:

• Low-carbon stainless steel (e.g., 309L, 310L): Good ductility, moderate dilution tolerance
• Nickel-based alloys (e.g., Inconel 625, Stellite 6): Excellent ductility, high-temperature strength
• High-silicon iron: Good wetting of TiC, but limited toughness
• Custom alloys: Designed for specific TiC/steel combinations

Engineering Applications

The TiC cemented carbide/transition layer/steel composite system finds application in:

1. Mining tools: Drag bits, augers, and reamers with TiC cutting edges
2. Cutting tools: Planer inserts, milling cutters with TiC tips brazed to steel bodies
3. Wear-resistant components: Pump impellers, valve seats, and slide ways
4. Specialty tools: Glass cutting tools, ceramic processing tools
5. Defense applications: Armor tiles with TiC ceramic components

Study Reflections

This research addresses a fundamental challenge in dissimilar materials joining that remains relevant across multiple industries. The interface between TiC cemented carbide and steel substrates is inherently problematic due to the vast differences in thermal expansion, chemical reactivity, and mechanical properties. The weld overlay transition layer approach provides a practical engineering solution that balances bonding strength, stress accommodation, and manufacturability.

The key insight from this work is that interface quality is determined by a complex interplay of composition, microstructure, and residual stress state — all of which are influenced by the welding process parameters and post-weld treatment. The formation of intermetallic phases at the interface is inevitable but can be controlled in terms of type, thickness, and distribution. A thin, continuous, and well-bonded intermetallic layer can actually enhance interface strength by providing a metallurgical bond, while a thick, discontinuous, or cracked intermetallic layer will severely degrade joint performance.

For engineers designing TiC-based composite components, this work emphasizes the importance of:

• Careful transition layer design with appropriate composition gradient
• Process parameter optimization to control reaction zone thickness
• Post-weld heat treatment to relieve residual stresses
• Comprehensive quality control including metallographic examination of the interface

The research contributes to the growing body of knowledge on dissimilar materials joining and provides practical guidance for engineers working with TiC-based composite components in demanding wear and cutting applications.

Concluding Remarks

These five publications collectively represent a cross-section of the cladding and bimetal product manufacturing field, spanning from industrial repair applications to nuclear safety-critical components and advanced materials research. Each addresses a distinct technical challenge while sharing common themes of process optimization, quality assurance, and the balance between metallurgical requirements and practical manufacturing constraints. The progression from 1990 to 2024 reflects the evolution of welding technology, automation capabilities, and materials characterization techniques that have transformed the field. For practicing engineers, these works provide both historical context and current technical guidance applicable to contemporary challenges in overlay welding and bimetal component fabrication.