Interface Microstructure and Properties of TiC Steel-Cemented Carbide and Surfacing Transition Layer

Literature Overview

This paper by Wu Shufei, Wei Wei, Ma Hexiang, Chen Yong, Liu Shengxin, and Huang Zhiquan from Zhengzhou University School of Materials Science and Engineering and Zhengzhou Mechanical Research Institute Co., Ltd., published in the journal "Thermal Processing Technology" (2024, Vol. 53, No. 11, pp. 37-41), investigates the interface microstructure and properties between TiC steel-cemented carbide and ERNiCrMo-3 surfacing metal deposited under different heat input conditions. The research was funded by the Henan Province Science and Technology Key Project (222102230038). The study provides valuable insights into the metallurgical bonding mechanisms and mechanical performance of the interface, with direct implications for the design and optimization of surfacing processes for cemented carbide components.

Research Background and Motivation

TiC steel-cemented carbide is a high-performance material used in applications requiring exceptional wear resistance and hardness, such as cutting tools, mining equipment, and industrial wear parts. However, the extremely high hardness and brittleness of cemented carbide make it difficult to weld or bond to other materials using conventional methods. The surfacing of a transition layer between the cemented carbide substrate and the final functional layer is a common strategy to achieve a sound metallurgical bond and manage the significant property mismatch between the two materials.

The ERNiCrMo-3 alloy (equivalent to ENiCrMo-3 per AWS A5.14) is a nickel-based welding alloy containing chromium and molybdenum. It is widely used for surfacing applications requiring corrosion resistance, wear resistance, and high-temperature strength. The selection of ERNiCrMo-3 as the transition layer material is based on its compatibility with cemented carbide substrates, its ability to form a sound metallurgical bond, and its mechanical properties that provide a gradual transition from the hard cemented carbide to the softer functional overlay.

Experimental Methodology

The study employed a systematic experimental approach to investigate the effect of welding heat input on the interface microstructure and properties. The key experimental parameters and characterization methods are summarized below:

Characterization methods included:

• Scanning Electron Microscopy (SEM): Microstructure observation at various magnifications
• Energy Dispersive X-ray Spectroscopy (EDS): Elemental composition analysis at the interface
• Microhardness testing: Hardness profile across the interface region
• Universal testing machine: Shear strength measurement of the interface

Interface Microstructure Analysis

The study revealed that a fusion zone with a thickness of 60-150 μm exists between the TiC steel-cemented carbide substrate and the ERNiCrMo-3 deposited metal. This fusion zone represents the region where the substrate material has melted and mixed with the filler metal, creating a metallurgical bond. The thickness of this fusion zone is directly influenced by the welding heat input, with higher heat input resulting in thicker fusion zones due to greater substrate melting.

Effect of Heat Input on Fusion Zone Thickness

The relationship between welding heat input and fusion zone thickness is critical for optimizing the surfacing process. Higher heat input increases the amount of substrate material that melts and dilutes into the weld metal, which can affect the composition and properties of the deposited layer. However, excessive heat input can also lead to adverse effects such as grain coarsening, carbide dissolution, and potential cracking.

The study found that at a heat input of 3.5 kJ/cm, the interface microstructure exhibited uniform distribution without porosity, cracking, or abnormal grain growth. This represents an optimal heat input level that provides sufficient melting for a sound metallurgical bond while avoiding the adverse effects of excessive heat input.

Elemental Diffusion at the Interface

The EDS analysis revealed elemental diffusion between the substrate and the deposited metal at the interface. This diffusion is a key mechanism for achieving metallurgical bonding and property gradation across the interface. The diffusion of elements such as nickel, chromium, molybdenum, titanium, and carbon across the interface creates a gradual transition in composition, which helps to reduce property mismatch and stress concentration.

The diffusion zone extends beyond the fusion zone into the heat-affected zone (HAZ) of the substrate and into the deposited metal near the interface. The extent of diffusion is influenced by the welding heat input, the cooling rate, and the holding time at elevated temperatures. Higher heat input and slower cooling rates promote greater diffusion, resulting in a wider transition zone.

Mechanical Properties of the Interface

The microhardness profile across the interface shows a gradual transition from the high hardness of the TiC steel-cemented carbide substrate to the lower hardness of the ERNiCrMo-3 deposited metal. This gradual hardness transition creates a buffer zone that helps to accommodate the property mismatch between the two materials and reduces the risk of cracking during service.

The shear strength testing revealed that the interface shear strength reaches a maximum of 443 MPa at the optimal heat input of 3.5 kJ/cm. This shear strength value is sufficient to withstand the mechanical loading conditions encountered in wear-resistant applications, providing confidence in the durability of the surfaced components.

Key Reflections and Technical Insights

The study provides important insights into the metallurgical bonding mechanisms at the interface between cemented carbide and nickel-based surfacing alloys. The identification of an optimal heat input of 3.5 kJ/cm for achieving a defect-free interface with maximum shear strength is a valuable practical result that can guide process parameter selection in industrial applications.

From a materials science perspective, the elemental diffusion observed at the interface is a fundamental mechanism for achieving metallurgical bonding between dissimilar materials. The diffusion of alloying elements across the interface creates a composition gradient that helps to reduce property mismatch and thermal stress. Understanding the factors that influence diffusion, including temperature, time, and composition, is essential for optimizing the surfacing process.

The gradual hardness transition from the substrate to the deposited metal is another important feature of the interface. This transition zone acts as a buffer that accommodates the significant hardness difference between the cemented carbide (typically >1500 HV) and the nickel-based alloy (typically 300-400 HV). Without this transition zone, the abrupt property change could lead to stress concentration and cracking during service.

The maximum shear strength of 443 MPa achieved at the interface is a critical performance metric for wear-resistant applications. This value indicates that the interface can withstand significant shear loading without failure, which is essential for components subjected to impact and abrasion. The achievement of this shear strength at a moderate heat input of 3.5 kJ/cm demonstrates the effectiveness of the ERNiCrMo-3 transition layer in providing a sound metallurgical bond.

Summary

This study provides valuable insights into the interface microstructure and properties of TiC steel-cemented carbide bonded to ERNiCrMo-3 surfacing metal. The identification of an optimal heat input of 3.5 kJ/cm for achieving a defect-free interface with maximum shear strength of 443 MPa is a significant practical result. The understanding of elemental diffusion mechanisms and hardness transition zones at the interface provides a scientific basis for optimizing surfacing processes for cemented carbide components. Engineers working on wear-resistant component manufacturing should apply these findings to improve the reliability and performance of surfaced cemented carbide components in demanding industrial applications.