Microstructure and Properties of TiC Particle Reinforced Composite Coating Produced by Submerged Arc Cladding

Literature Overview

This study investigates the microstructure and mechanical properties of a TiC particle reinforced composite coating produced by submerged arc welding (SAW) cladding. Submerged arc cladding is a well-established industrial process for depositing thick wear-resistant layers, but the incorporation of ceramic particles such as TiC presents unique challenges related to particle retention, distribution, and interfacial bonding. The research addresses the fundamental metallurgical interactions between the TiC particles and the molten weld metal, as well as the resulting effects on hardness, wear resistance, and bonding strength. This topic is of considerable practical interest given the widespread use of SAW cladding in heavy industry and the growing demand for high-performance composite coatings.

Core Technical Points

Submerged Arc Cladding Process Parameters

The SAW cladding process was conducted using a flux-cored wire electrode with TiC particles pre-mixed into the flux or embedded in the wire composition. The welding parameters were carefully selected to balance deposition rate, penetration depth, and particle retention. The study examined welding currents from 300 to 500 A, arc voltages from 28 to 38 V, and travel speeds from 100 to 200 mm/min. A multi-pass approach was employed, with a single-pass deposition thickness of 3–5 mm, achieving total cladding thicknesses of 10–15 mm.

Microstructural Evolution

The microstructure of the SAW-cladded TiC composite coating exhibits a complex morphology that reflects the interaction between the ceramic particles and the solidifying weld metal. The TiC particles, with an initial size distribution of 50–150 μm, undergo partial dissolution and redistribution during the welding thermal cycle. The resulting microstructure consists of a martensitic matrix with retained austenite (8–15%) and a dispersed distribution of TiC particles in the range of 20–100 μm.

A key observation is the formation of a reaction layer at the TiC-matrix interface. During solidification, the molten weld metal reacts with the TiC particles, forming a thin layer of Fe₃C and Fe₇C₃ carbides at the interface. This reaction layer serves as a mechanical bond between the ceramic particle and the metallic matrix, but it also introduces a potential weakness due to the lower hardness and ductility of the iron carbide phases. The thickness of the reaction layer varies from 2 to 8 μm, depending on the local cooling rate and the TiC particle size.

The matrix microstructure shows a clear dependence on the cooling rate, which is directly related to the welding parameters. At lower heat inputs (12–15 kJ/mm), the cooling rate is higher, resulting in a finer martensitic structure with more retained austenite. At higher heat inputs (20–25 kJ/mm), the cooling rate is lower, leading to a coarser martensitic structure with less retained austenite and a tendency toward tempered martensite due to the self-tempering effect of the exothermic carbide formation reaction.

Hardness and Wear Resistance

The hardness of the TiC-reinforced composite coating was measured across the cross-section using Vickers indentation. The surface hardness ranges from 650 to 780 HV, with an average value of approximately 720 HV. The hardness profile shows a gradual decrease from the surface to the substrate, with the transition zone exhibiting a hardness of 350–500 HV. The substrate hardness (low-carbon steel) is approximately 180–220 HV.

The wear resistance was evaluated using a dry sliding wear test against a SiC counterface. The composite coating exhibited a wear rate of 1.2 × 10⁻⁶ mm³/N·m, which is approximately 55% lower than that of a conventional un-reinforced SAW cladding layer (2.7 × 10⁻⁶ mm³/N·m). The wear mechanism transitioned from adhesive wear in the un-reinforced coating to abrasive wear in the TiC-reinforced composite, indicating that the hard TiC particles effectively resist material removal under sliding contact.

The volume fraction of TiC particles in the final coating was determined to be approximately 25–30%, which is lower than the initial TiC content in the flux (35–40%) due to partial dissolution during the welding process. The particle size distribution in the final coating shows a bimodal distribution, with a population of large particles (50–100 μm) that survived the welding thermal cycle and a population of smaller particles (20–50 μm) that formed during solidification through the reaction of dissolved Ti and C with the matrix.

Bonding Strength and Interface Quality

The bonding strength between the cladding layer and the substrate was evaluated using the micro-shear test method in accordance with ASTM G119. The average shear strength was measured at 320 MPa, with a standard deviation of ±25 MPa. This value is comparable to the shear strength of the substrate material, indicating a strong and reliable bond. The fracture mode was predominantly transgranular within the cladding layer, far from the interface, which is a positive indicator of interface integrity.

However, the study also identified instances of micro-cracking at the TiC-matrix interface, particularly around the largest TiC particles (exceeding 80 μm). These micro-cracks are attributed to the thermal mismatch between the ceramic particles and the metallic matrix, which generates localized tensile stresses during cooling. The thermal expansion coefficient of TiC (7.9 × 10⁻⁶ /K) is significantly lower than that of the steel matrix (12–14 × 10⁻⁶ /K), resulting in a differential contraction that can initiate cracking at the interface.

Engineering Practice and Process Optimization

The SAW cladding process with TiC particle reinforcement offers several advantages for industrial applications, including high deposition rates (2–3 kg/h), thick layer deposition capability, and good process stability. However, the process also presents challenges that must be addressed to achieve consistent performance.

The particle retention rate is a critical factor that directly affects the wear resistance of the final coating. The study found that the particle retention rate was approximately 65–70%, meaning that 30–35% of the TiC particles were dissolved during the welding process. To improve particle retention, the study recommends using larger initial particle sizes (100–200 μm) and reducing the heat input to minimize the time spent at elevated temperatures. Additionally, the use of a pre-coated wire with TiC particles embedded in the sheath material, rather than mixing TiC into the flux, can improve particle retention by reducing the contact time between the particles and the molten metal.

The interpass temperature control is another critical process parameter. Maintaining an interpass temperature of 200–250°C is essential to prevent cracking in the heat-affected zone while ensuring adequate fusion between passes. Exceeding 300°C can lead to excessive grain growth in the previously deposited layers, reducing hardness and wear resistance. Below 150°C, the risk of cold cracking increases due to the high hydrogen pickup associated with the flux-cored welding process.

Key Questions and Reflections

The study raises several important questions regarding the long-term reliability of TiC-reinforced SAW cladding layers. First, the micro-cracking observed at the TiC-matrix interface may propagate under cyclic loading conditions, leading to a gradual degradation of wear resistance. This is a concern for applications involving fatigue and impact loading, such as mining equipment and construction machinery. Second, the self-tempering effect observed at higher heat inputs suggests that the as-deposited microstructure may not be the most thermodynamically stable configuration. A post-weld tempering treatment at 500–550°C for 1–2 hours could stabilize the microstructure and improve toughness, although this would result in a slight reduction in hardness.

The dilution behavior of the SAW process also warrants careful consideration. The high heat input associated with SAW cladding results in significant dilution of the cladding layer by the substrate material, which can reduce the effective TiC content and alter the matrix composition. For thick cladding layers, the dilution ratio in the first pass can exceed 30%, leading to a microstructure that is more representative of the substrate than the intended cladding alloy. Engineers should consider applying a transition layer with lower dilution before the final TiC-reinforced cladding pass, or use a lower heat input for the first pass to minimize dilution.

The particle size distribution in the final coating is another factor that significantly affects performance. Large TiC particles (>80 μm) can act as stress concentrators and initiate cracking, while very fine particles (<20 μm) may not provide sufficient wear resistance. An optimal particle size distribution with a peak around 40–60 μm is recommended for the best balance between wear resistance and toughness. This can be achieved by controlling the initial particle size and the welding parameters to minimize particle dissolution and fragmentation.

Study Insights and Implications

This literature provides a comprehensive understanding of the microstructural evolution and mechanical performance of TiC particle reinforced composite coatings produced by submerged arc cladding. The key finding is that the SAW process can effectively produce TiC-reinforced coatings with hardness exceeding 700 HV and wear rates approximately 55% lower than conventional cladding layers. However, the process requires careful optimization of welding parameters, interpass temperature control, and particle size selection to achieve consistent and reliable performance. Engineers should view the SAW TiC composite cladding as a viable option for thick-layer wear protection applications, particularly where high deposition rates are required and moderate to high wear resistance is needed. The technology is particularly suitable for applications such as excavator buckets, crusher liners, and conveyor components, where the combination of thick cladding layers and high wear resistance is essential for extending component life.