Effect of Rare Earth on TiC-Based Cermet Overlay Microstructure and Properties

Literature Overview

Wang Xinhong and Zou Zengda of Shandong University's School of Materials Science and Engineering published this study in 2001 in the Chinese Journal of Rare Earths, supported by the Shandong Provincial Natural Science Foundation (Grant Z2000F02). The research investigates the addition of rare earth (RE) elements to TiC-based cermet (ceramic-metal composite) overlay materials, examining how RE modifies microstructure, mechanical properties, and wear resistance. TiC-based cermets represent a high-performance category of overlay materials achieving hardness levels exceeding 1200 HV, making them candidates for severe abrasion environments in mining, cement, and slurry handling applications.

Core Technical Findings

Rare Earth Addition and Microstructure Modification

The authors added rare earth elements (primarily Ce and La) at levels of 0.1–0.5 wt% to TiC-based cermet matrices. The key microstructural observations include:

Parameter	Without RE	With 0.3% RE	Improvement
Matrix hardness (HV30)	850–950	950–1050	+10–12%
TiC particle size (μm)	8–15	3–8	Reduced 50–60%
TiC distribution uniformity	Non-uniform	Uniform	Significant
Crack density in overlay	Moderate	Low	Substantially reduced
Abrasive wear life (ASTM G65)	Baseline	+35–50%	Significant

The primary mechanism identified is the "purity refinement" effect of rare earth elements. RE atoms segregate to the liquid-solid interface during solidification, inhibiting TiC nucleation and growth, resulting in finer and more uniformly distributed TiC particles. Additionally, RE reacts with residual oxygen and sulfur in the melt to form RE2O3 and RES compounds, effectively scavenging impurities that would otherwise promote intergranular cracking.

Crack Suppression Mechanism

A particularly important finding for engineering application is the crack suppression effect. TiC-based cermets are inherently prone to cracking due to the large thermal expansion coefficient mismatch between the hard TiC phase and the metallic binder. The authors demonstrate that RE addition reduces crack density by:

Reducing residual stress through impurity scavenging and grain refinement.
Promoting a more ductile binder phase through modified solidification sequence.
Forming RE-rich intergranular films that improve interfacial bonding.

Wear Resistance Mechanism

The improved abrasive wear resistance (35–50% improvement) is attributed to the combined effect of refined TiC particles providing more uniform load-bearing points and the reduced crack density preventing early spalling of the overlay surface under abrasion.

Engineering Practice Implications

For overlay applications on pressure vessel internals and heat exchanger tubes exposed to slurry erosion, TiC-based cermets with RE modification offer a compelling solution. However, several practical considerations must be addressed:

Process compatibility – TiC-based cermets typically require high-energy-density processes such as plasma transferred arc (PTA) cladding or laser cladding to maintain TiC integrity, as conventional arc welding causes excessive TiC dissolution.
Bond layer requirement – A transition layer (e.g., NiCrSiB or Ni-based alloy) is essential between the carbon steel substrate and the cermet face layer to manage thermal expansion mismatch.
RE cost and supply – While the performance improvement is significant, RE element cost and supply chain considerations must be evaluated for large-scale pressure vessel applications.
Standard coverage – Current standards (ASME IX, NB/T 47014) do not explicitly address RE-modified cermet overlays, requiring qualified welding procedure specifications (WPS) with extended qualification testing.

Key Reflections

This 2001 study was ahead of its time in recognizing the potential of rare earth modification for cermet overlays. The concept of using RE as a microstructure refiner and crack suppressor in hard-facing materials has since been validated in numerous subsequent studies. The engineering relevance is particularly acute for pressure vessel applications involving slurry service, where conventional hard-facing materials (Cr-C-Mo high-speed steel types) often fail within months due to abrasive wear, while TiC-based RE-modified cermets can extend service life by a factor of 3–5. The study's limitation lies in the relatively small-scale laboratory testing; full-scale qualification on thick-section pressure vessel components remains a challenge that requires additional engineering development.