TiC-Reinforced Iron-Based Surfacing Layer Microstructure and Properties

Literature Overview

The research by Song Sili, Wang Xinhong, Zou Zengda, and Qu Shiyao from Shandong University, published in the Journal of Shandong University (Engineering Science) (2004, Vol. 34, No. 2), investigates the synthesis and characterization of TiC-reinforced iron-based surfacing layers using low-cost raw materials. The work was funded by the Ministry of Education Doctoral Fund (20020422032) and the Shandong Provincial Natural Science Foundation (Z2000F02). This study is particularly significant for its approach to in-situ synthesis of ceramic reinforcements during the welding process, offering a cost-effective alternative to externally added pre-formed particles.

In-Situ Synthesis Methodology

The study employs a welding arc metallurgy approach to synthesize TiC ultra-hard particles in situ within an iron-based deposited layer. The raw materials used are:

• Titanium iron (FeTi) as the titanium source
• Rutile (TiO2) as an additional titanium/oxide source
• Graphite (C) as the carbon source

The synthesis reaction occurs during the molten pool stage of the welding process:

Ti + C → TiC (in situ formation)

This approach eliminates the need for expensive pre-made TiC powder and leverages the reducing environment of the welding arc to drive the carbothermic reduction of TiO2 and subsequent TiC formation.

Microstructural Characterization

Scanning electron microscopy (SEM), X-ray diffraction (XRD), and wear resistance testing were employed to characterize the deposited layer. The key microstructural findings are:

The TiC particles are uniformly dispersed on the low-carbon martensite and retained austenite matrix. The combination of hard ceramic particles with a relatively ductile matrix provides an excellent balance of wear resistance and fracture toughness.

Optimal Composition Window

The study identifies a critical composition window for optimal performance:

Outside this window, the following degradation mechanisms occur:

• Excessive FeTi (>30%): Over-reduction leads to TiC aggregation and matrix embrittlement due to excessive hard phase volume fraction.
• Insufficient FeTi (<25%): Incomplete TiC formation, resulting in free titanium nitride or oxide inclusions that may act as crack initiation sites.
• Excessive graphite (>10%): Excess carbon dissolves into the matrix, forming secondary carbides and reducing TiC purity.
• Insufficient graphite (<8%): Carbon starvation prevents complete TiC formation, leaving unreacted TiO2 or FeTi phases.

Performance Analysis and Wear Mechanism

The deposited layer achieves hardness above 55 HRC with high wear resistance and good crack resistance. The wear mechanism involves:

1. TiC particles resist micro-cutting and ploughing by abrasive particles due to their extreme hardness (approximately 2800 HV).
2. The martensitic matrix provides load-bearing capacity and absorbs impact energy.
3. Retained austenite contributes to strain hardening during wear, maintaining surface integrity.
4. The uniform TiC distribution prevents localized failure and crack propagation.

Engineering Practice Considerations

For practical implementation of TiC-reinforced surfacing:

• Wire preparation: The flux core wire must be carefully formulated to ensure uniform distribution of FeTi, rutile, and graphite throughout the core.
• Welding parameters: Current density and travel speed affect the thermal input, which in turn influences TiC particle size and matrix transformation.
• Substrate compatibility: The thermal expansion mismatch between TiC particles and the iron matrix must be managed through appropriate matrix ductility.
• Multi-pass deposition: For thick surfacing layers, interpass temperature control is essential to maintain the desired martensitic structure.

Comparative Analysis with Other Reinforcement Systems

The in-situ TiC synthesis approach offers a compelling cost-performance advantage, particularly for large-scale industrial surfacing applications where wire cost is a significant factor.

Study Insights and Reflections

The elegance of this approach lies in its simplicity: using common, inexpensive raw materials and the welding process itself as the synthesis reactor. The key innovation is recognizing that the welding arc provides the necessary reducing atmosphere and thermal energy for carbothermic TiC formation. This concept can be extended to other in-situ synthesis strategies, such as TiB2, TiN, or Si3N4 formation, opening possibilities for tailored ceramic-metal composite surfacing layers.

The optimal composition window of 25–30% FeTi and 8–10% graphite is a practical result that can be directly applied in wire formulation. However, engineers should note that wire manufacturing consistency and arc stability are critical to achieving the predicted microstructure in production settings.