Submerged Arc Overlay TiC Particle Reinforced Composite Coatings on Carbon Steel

Literature Overview

This paper by Liu Junhai, Huang Jihua, Liu Junbo, and Song Guixiang, published in the Chinese Journal of Nonferrous Metals in 2011 (Vol. 21, No. 3, pp. 663-668), investigates the microstructure and tribological properties of TiC particle-reinforced Fe-based composite coatings prepared by in-situ synthesis using alloy powder submerged arc overlay welding on Q235 steel. Funded by the Shandong Provincial Science and Technology Program (2007GG30003003), this work demonstrates a cost-effective approach to producing wear-resistant coatings through in-situ reaction synthesis during the welding process.

Core Technical Innovation

The key innovation of this research is the in-situ synthesis of TiC particles during the submerged arc welding (SAW) process using a carefully designed alloy powder mixture. Rather than adding pre-formed TiC particles to the welding consumable, the authors use TiFe powder, Cr powder, Ni powder, Fe powder, and colloidal graphite as raw materials. During the welding process, these elements react in the molten pool to form TiC particles in situ. This approach offers several advantages over ex-situ methods:

Microstructural Characterization

The coatings were characterized using SEM, XRD, and EDS, revealing a complex but well-organized microstructure:

Coating Microstructure Composition

The average microhardness of the coating reached 601 HV0.2, approximately three times that of the Q235 carbon steel substrate. This significant hardness enhancement is attributed to the combined effect of the hard TiC particles and the martensitic matrix. The wear resistance improvement is even more dramatic, with the coating exhibiting approximately one-tenth the wear mass loss of the base metal under room temperature dry sliding conditions.

Single-Layer vs. Double-Layer Coating Comparison

The observation that the double-layer coating exhibits better wear resistance than the single-layer coating is particularly interesting. The increased austenite content in the double-layer coating likely contributes to improved toughness, while the increased TiC content provides additional hard phase reinforcement. The reduction in martensite content is beneficial because high martensite volumes can be susceptible to cracking, particularly under thermal cycling conditions.

In-Situ Reaction Mechanism

The in-situ synthesis of TiC during submerged arc welding involves several sequential and concurrent reactions:

1. Melting and mixing: The alloy powders melt and mix in the molten pool under the flux cover
2. Oxidation reduction: Colloidal graphite acts as a carbon source and reducing agent
3. Carbide formation: Ti reacts with C to form TiC: Ti + C → TiC
4. Alloying: Cr and Ni dissolve into the matrix, modifying its properties
5. Solidification: The molten pool solidifies with TiC particles distributed throughout

The role of colloidal graphite is multifunctional: it serves as the carbon source for TiC formation, acts as a deoxidizer to prevent excessive oxide formation, and influences the solidification behavior of the molten pool. The fine particle size of colloidal graphite ensures rapid and uniform carbon availability for TiC nucleation.

The submerged arc welding process is particularly well-suited for this application because:

• The flux cover provides excellent protection against atmospheric contamination
• The deep penetration and high deposition rate enable efficient multi-layer builds
• The relatively slow cooling rate promotes complete reaction and coarse precipitate formation
• The process is amenable to automation for consistent production

Wear Testing and Performance

The wear testing was conducted under standardized conditions at room temperature with dry sliding friction. While the specific test parameters (load, speed, counterface material) are not explicitly detailed in the abstract, the results demonstrate a clear hierarchy of wear resistance:

The tenfold reduction in wear mass loss is a remarkable achievement for a coating produced by a relatively simple and cost-effective process. This level of performance improvement is comparable to more expensive overlay techniques such as plasma spray or laser cladding, while offering advantages in terms of equipment cost and process scalability.

Engineering Implementation Considerations

For practical application of TiC-reinforced composite coatings via submerged arc overlay welding, the following considerations are important:

Process Parameters Optimization

Quality Control Measures

1. Pre-weld preparation: Clean substrate surface, remove rust and coatings, ensure proper fit-up
2. Powder quality control: Verify powder composition, particle size distribution, and moisture content
3. Welding parameter monitoring: Real-time monitoring of current, voltage, and travel speed
4. Post-weld inspection: Visual inspection, dimensional measurement, and hardness testing
5. Destructive testing: Cross-sectional microstructural examination, wear testing on coupon samples
6. Service validation: Field trials to confirm performance under actual operating conditions

Application Suitability

TiC-reinforced Fe-based composite coatings are particularly suitable for:

• Mining equipment components (excavator buckets, conveyor rollers, crusher plates)
• Cement industry equipment (kiln liners, mill liners, grinding media)
• Agricultural machinery (plowshares, seed drills, harvester components)
• Construction equipment (hydraulic cylinder rods, bucket teeth)
• Power generation equipment (fan blades, boiler tubes)

The Fe-based matrix ensures good compatibility with carbon and low-alloy steel substrates, minimizing dilution-related property degradation. The in-situ TiC formation provides a self-replenishing wear mechanism where the hard carbide phase continues to protect the coating even as the surface is worn away.

Study Insights and Critical Reflection

This research demonstrates the power of in-situ synthesis as a strategy for producing high-performance surface composite materials. The ability to form TiC particles during the welding process, using inexpensive elemental powders, represents a significant advancement in cost-effective surface engineering. The resulting coatings, with their fine TiC particle distribution and excellent wear resistance, are competitive with more expensive coating technologies.

One limitation of the study is the relatively narrow range of wear testing conditions. Room temperature dry sliding friction, while providing a standardized and repeatable measurement, does not capture the full spectrum of service conditions encountered in industrial applications. Future work should investigate wear performance under:

• Elevated temperature conditions (thermal fatigue and oxidation)
• Wet or corrosive environments (corrosion-wear synergy)
• Impact loading conditions (cyclic stress and fatigue)
• Multi-directional abrasion (simulating real-world contact patterns)

The double-layer coating approach, with its improved wear resistance and enhanced toughness, suggests a pathway toward multi-layer coating designs that can be tailored for specific service requirements. By varying the powder composition between layers, engineers can create gradient coatings that transition from a tough, wear-resistant surface layer to a ductile, high-adhesion bond layer. This gradient approach can effectively address the common challenge of balancing hardness and toughness in overlay coatings.

The economic viability of this technology is particularly attractive for large-scale industrial applications where coating area is extensive. The submerged arc welding process is highly productive, capable of depositing large volumes of material at high rates, making it suitable for coating large components such as mill liners, kiln shells, and structural steel members. The relatively low cost of the raw materials (elemental powders and colloidal graphite) compared to pre-formed TiC or specialized welding consumables further enhances the economic case.

This work represents a paradigm shift in surface engineering from ex-situ addition of pre-formed reinforcement particles to in-situ synthesis during the welding process. The implications extend beyond TiC formation to other carbide and intermetallic systems that can be synthesized in situ during welding, opening new possibilities for tailored surface compositions and properties. Engineers should consider this approach as a viable alternative to traditional overlay welding methods when designing wear-resistant coatings for industrial applications.