TiC Particle Reinforced Composite Coating by Submerged Arc Surfacing

Literature Overview

This paper published in the Chinese Journal of Nonferrous Metals (2011, Vol. 21, No. 3, pp. 663-668) by Liu Junhai and colleagues from the University of Science and Technology Beijing investigates the microstructure and wear properties of TiC particle reinforced iron-based composite coatings produced by submerged arc surfacing (SAS). The research was funded by the Shandong Provincial Science and Technology Project (2007GG30003003). The work focuses on in-situ synthesis of TiC particles within the overlay deposit, offering a practical alternative to ex-situ particle addition methods.

Core Technical Approach

The researchers used a submerged arc surfacing technique with a blend of alloy powders including TiFe powder, Cr powder, Ni powder, Fe powder, and colloidal graphite as the feedstock. The key innovation is the in-situ reaction synthesis of TiC particles within the molten weld pool, rather than relying on pre-made TiC particles that may suffer from poor bonding with the iron matrix.

Reaction Mechanism

The in-situ synthesis of TiC occurs through the following reactions in the molten pool:

• Ti (from TiFe) + C (from colloidal graphite) → TiC
• The reaction is thermodynamically favorable at welding temperatures due to the high stability of the TiC phase (melting point approximately 3150°C, lattice energy approximately 666 kJ/mol).
• The presence of Fe as a diluent and solvent medium facilitates the reaction by providing a liquid environment for diffusion and nucleation.

Microstructural Analysis

The in-situ synthesized TiC particles are remarkably fine, with sizes below 2 μm, and are uniformly dispersed throughout the coating. This fine particle size and uniform distribution are critical for achieving the reported wear resistance, as they provide numerous hard points that resist abrasive wear while the surrounding matrix provides ductility and crack resistance.

Hardness and Wear Performance

The average microhardness of 601 HV0.2 represents approximately three times the hardness of the Q235 carbon steel substrate. The wear mass of the coating is approximately one-tenth that of the base metal under room-temperature dry sliding wear conditions. These results demonstrate that the TiC particle reinforcement strategy is highly effective for improving wear resistance.

Submerged Arc Surfacing Process Analysis

Submerged arc surfacing offers several advantages for this application:

• High deposition rate: The buried arc process provides high heat input and deposition rates, enabling thick coatings to be built up efficiently.
• Low dilution rate: The flux coverage protects the molten pool and reduces atmospheric contamination, while the powder feedstock can be designed to minimize substrate dilution.
• Controlled cooling: The flux layer provides thermal insulation, allowing controlled solidification rates that influence the final microstructure.
• Scalability: The process is readily scalable from laboratory to production, making it suitable for industrial applications.

However, several process challenges must be addressed:

• Powder blending: The alloy powder blend must be carefully prepared to ensure homogeneous distribution of TiFe, Cr, Ni, Fe, and colloidal graphite. Poor blending can result in localized TiC-rich or TiC-poor regions.
• Flux selection: The flux composition must be compatible with the powder feedstock and must provide adequate arc stability, slag protection, and deoxidation without introducing unwanted alloying elements.
• Multi-pass welding: For thick coatings, multiple passes are required. The interpass temperature must be controlled to avoid excessive grain growth while ensuring adequate fusion between passes.
• Coating thickness control: The geometry of the deposited coating must be controlled to ensure uniform thickness, which is critical for even wear performance.

Single-Layer vs. Two-Layer Coating Comparison

The study's comparison of single-layer and two-layer coatings reveals important metallurgical insights:

• In the two-layer coating, the martensite content decreases while the austenite and TiC content increase. This shift is likely due to the different thermal cycling history between single-pass and multi-pass deposits. The second pass experiences a preheated substrate from the first pass, which alters the solidification conditions and promotes retained austenite formation.
• The increased austenite content in the two-layer coating may contribute to improved toughness, as austenite is more ductile than martensite.
• The increased TiC content in the two-layer coating suggests that the interpass thermal cycle may promote additional TiC formation through continued reaction between dissolved Ti and C in the solidifying second layer.
• The two-layer coating exhibits better wear resistance than the single-layer coating, suggesting that the combined effect of increased TiC content and retained austenite is beneficial.

Engineering Practice Implications

For engineers considering TiC-reinforced composite coatings for wear protection:

1. Application selection: The in-situ TiC synthesis approach is particularly suitable for applications requiring thick coatings, such as mining equipment, construction machinery, and heavy industrial components where substantial material removal is expected.
2. Substrate compatibility: The Q235 substrate used in this study is a plain carbon steel. For higher-alloy substrates, the dilution chemistry will differ, and the final coating composition may require adjustment of the powder feedstock.
3. Wear regime consideration: The reported wear testing was performed under room-temperature dry sliding conditions. Engineers should verify the coating's performance under their specific wear conditions, which may include lubricated sliding, erosive wear, or high-temperature wear.
4. Cost-benefit analysis: The powder feedstock includes titanium-bearing materials (TiFe), which are more expensive than iron-based powders alone. The economic justification should be based on the extended service life relative to simpler overlay approaches.

Study Insights and Reflections

This study demonstrates a practical and effective approach to producing TiC-reinforced composite coatings through submerged arc surfacing with in-situ TiC synthesis. The fine TiC particles (< 2 μm) produced in-situ offer superior bonding with the iron matrix compared to ex-situ particles, which can suffer from interfacial reactions and poor wetting. The threefold hardness improvement and tenfold wear resistance improvement over the base metal are compelling results for industrial applications.

The comparison between single-layer and two-layer coatings provides valuable guidance for coating design. The finding that two-layer coatings exhibit better wear resistance due to increased TiC and retained austenite content suggests that multi-pass strategies can be used to optimize the coating microstructure. However, engineers should be aware that additional passes increase processing time and cost, and the optimal number of passes depends on the specific application requirements.

A limitation of this study is the absence of detailed information on the exact powder blend composition, the specific flux used, and the detailed process parameters. Additionally, the wear testing was limited to room-temperature dry sliding conditions, and the long-term durability under cyclic loading or elevated temperatures was not evaluated. Future work should incorporate more comprehensive characterization and testing to provide a complete performance profile.

In conclusion, this study demonstrates that submerged arc surfacing with in-situ TiC particle synthesis using a blend of TiFe, Cr, Ni, Fe, and colloidal graphite powders produces composite coatings with average microhardness of 601 HV0.2, approximately three times the substrate hardness, and wear resistance approximately ten times that of the base metal, with two-layer coatings offering superior performance through increased TiC and retained austenite content.