Ti-Ceramic Particle Reinforced Iron-Based Overlay Alloy: Microstructure and Wear Resistance

Literature Overview

Published in Ordnance Materials and Science Engineering, Volume 49, Issue 4, pages 74–79, this study by Zhu Shuzhi, Jia Hua, Xu Ling, and Yu Jinghang from Dalian Ocean University investigates the effect of titanium addition on the microstructure and wear resistance of iron-based overlay alloy layers applied to Q235 steel by open arc surfacing. The work is supported by multiple provincial and institutional research grants, reflecting its recognition as a meaningful contribution to the field of surface engineering.

The research addresses a fundamental challenge in wear-resistant overlay welding: how to enhance hardness and wear resistance without compromising the metallurgical integrity of the overlay layer. The addition of titanium, which forms hard ceramic phases such as TiC, TiB₂, and TiN during solidification, represents a well-established strategy, but the specific effects on microstructure and wear behavior in the Fe-Cr-C-B-N system warrant detailed investigation.

Core Technical Findings

The study compares two overlay compositions: a baseline Fe-Cr-C-B-N alloy and a modified Fe-Cr-C-B-N-Ti alloy with 4 wt% titanium addition. Both were applied to Q235 steel substrates by open arc surfacing (a form of submerged arc or shielded metal arc welding adapted for overlay applications).

Microstructural Characterization

The key observation is that titanium addition does not merely add hard phases — it fundamentally alters the microstructural architecture of the overlay. The gray-black titanium-containing ceramic particles (TiC, TiB₂, TiN) appear as fine, irregularly shaped particles distributed uniformly throughout the matrix. The primary austenite grains are refined, with lath martensite forming within them during cooling. The eutectic network, which typically contains the bulk of the hard phases in unmodified compositions, shows a reduced quantity of M₂₃(C,B)₆, M₃(C,B), and M₂B phases, but with a more uniform distribution.

Wear Performance Comparison

The 20.8% reduction in wear mass loss represents a substantial improvement in wear resistance, translating directly to extended service life in abrasive applications. The 5.3% increase in hardness, while seemingly modest, is significant because hardness is a primary contributor to wear resistance in abrasive wear regimes.

Interpretation of Technical Points

The mechanism of wear resistance enhancement is multifaceted and reflects the complex interaction between the ceramic particles and the matrix:

Role of TiC, TiB₂, and TiN

These titanium-containing ceramics possess extremely high hardness values — TiB₂ in particular reaches hardness values exceeding 4500 HV, making it one of the hardest known ceramics. Their function in the overlay is threefold:

1. Direct load-bearing: The hard particles bear a significant portion of the contact load during sliding, reducing the plastic deformation of the softer matrix.
2. Abrasive resistance: The particles resist ploughing and cutting by the counterface, reducing the volume of material removed per unit sliding distance.
3. Matrix strengthening: The particles act as obstacles to dislocation motion, contributing to solid solution and precipitation strengthening of the matrix.

Microstructural Refinement Effect

The observation that primary austenite grains are refined in the titanium-modified composition is noteworthy. Grain refinement generally improves both strength and toughness according to the Hall-Petch relationship. In the context of wear resistance, finer grains mean that the hard phases are distributed at shorter intervals, creating a more effective barrier to abrasive attack. The formation of lath martensite within the refined austenite grains also contributes to a high-strength, high-toughness matrix that supports the ceramic particles without cracking.

Reduction of Eutectic Network Phases

The reduction in the quantity of M₂₃(C,B)₆, M₃(C,B), and M₂B phases in the eutectic network is a subtle but important observation. These phases, while hard, are brittle and their presence in a continuous network can create crack initiation sites under cyclic loading. By reducing their quantity and improving their distribution, the titanium addition effectively improves the toughness of the overlay without sacrificing hardness. This is consistent with the well-known principle in wear-resistant alloy design that an optimal balance between hardness and toughness is required for maximum wear life.

Process Analysis

The open arc surfacing process used in this study is a variant of arc welding adapted for overlay applications. The key process parameters that influence the microstructure include:

The in-situ formation of TiC, TiB₂, and TiN during solidification is governed by thermodynamic stability and kinetic factors. TiC forms preferentially at higher temperatures due to its lower formation enthalpy relative to TiB₂. TiN forms readily in the presence of nitrogen, which is available from the welding atmosphere or from nitrogen-containing fluxes. The uniform distribution of these particles suggests that the nucleation and growth kinetics are favorable under the process conditions employed.

Integration with Engineering Practice

The findings of this study have direct relevance to several industrial applications:

Application Scenarios

• Mining equipment: Crusher liners, conveyor rollers, and chutes subject to abrasive wear from ore and rock
• Cement industry: Mill liners, grinding media, and kiln wear plates exposed to abrasive cement clinker
• Power generation: Coal handling equipment, boiler wear plates, and ash handling components
• Marine applications: Pump impellers, propeller surfaces, and seabed mining equipment

Process Recommendations

For engineers implementing titanium-modified iron-based overlay welds in practice, the following recommendations emerge from this study:

1. Titanium content optimization: The 4 wt% Ti level used in this study represents a starting point. Higher titanium contents may further improve wear resistance but could compromise weldability and increase cracking susceptibility due to the formation of brittle intermetallic phases.
2. Controlled cooling rate: The formation of fine, uniformly distributed Ti-containing particles requires a controlled solidification rate. Excessively rapid cooling may lead to segregation and uneven particle distribution, while excessively slow cooling may promote coarsening.
3. Nitrogen management: Since TiN is one of the key hard phases, ensuring adequate nitrogen availability during welding is important. This can be achieved through nitrogen-containing shielding gases or fluxes, but must be balanced against the risk of porosity.
4. Post-weld treatment: While not investigated in this study, controlled tempering of the overlay could potentially improve toughness without significantly reducing hardness, extending the service life in applications subject to impact loading.

Key Questions and Reflections

Several aspects of this research merit further consideration:

1. Long-term wear behavior: The wear test results are based on laboratory sliding wear conditions. In actual service, the wear mechanisms may include a combination of abrasive, adhesive, and erosive wear, and the relative contribution of the Ti-containing phases to each mechanism may differ.
2. Effect of dilution: The actual Ti concentration in the overlay layer depends on the dilution ratio, which varies with process parameters and substrate composition. A systematic study of dilution effects on Ti phase formation would be valuable for process optimization.
3. Multi-pass overlay considerations: In thick overlay applications, the microstructure of subsequent passes is affected by the thermal history of previously deposited layers. The interaction between pass geometry and Ti phase distribution in multi-pass overlays warrants investigation.
4. Cost-benefit analysis: Titanium-containing consumables are more expensive than conventional iron-based overlay alloys. The 20.8% improvement in wear resistance must be evaluated against the increased material cost and any additional processing requirements.

Study Insights and Implications

This study demonstrates that the strategic addition of titanium to iron-based overlay alloys can significantly enhance wear resistance through the in-situ formation of hard ceramic particles. The improvement is not merely a result of adding harder phases but involves a fundamental restructuring of the microarchitecture — grain refinement, reduced eutectic network continuity, and more uniform hard phase distribution.

From a broader perspective, this work exemplifies the philosophy of in-situ composite synthesis in welding, where the welding process itself serves as the synthesis route for reinforcing phases. This approach offers advantages over post-weld infiltration or coating methods because the reinforcing phases are metallurgically bonded to the matrix, ensuring good interfacial integrity and resistance to delamination under wear loading.

For engineers designing wear-resistant overlay systems, the key takeaway is that alloy design must consider not only the hardness of individual phases but also their distribution, morphology, and interaction with the matrix. A composition that produces a high volume fraction of hard phases in a continuous network may actually be inferior to one that produces fewer but more uniformly distributed hard particles in a refined matrix.