CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of TiC Particle-Reinforced Composite Coatings Deposited by Submerged Arc Welding
Literature Overview
This study investigates the microstructural evolution and mechanical properties of titanium carbide (TiC) particle-reinforced composite coatings produced through submerged arc welding (SAW) overlay deposition. Metal matrix composites (MMCs) reinforced with ceramic particles offer superior wear resistance and hardness while maintaining the toughness and formability of metallic matrices, making them attractive for applications in mining equipment, mining machinery, and chemical processing components where severe abrasion and erosion are encountered.
Process Parameters and Microstructural Characteristics
The SAW process was selected for TiC composite coating deposition due to its high deposition efficiency, excellent penetration, and suitability for thick overlay layers. The study examined the effects of welding parameters on coating microstructure and properties:
| Parameter | Range Studied | Optimal Value | Effect on Coating |
|---|---|---|---|
| Current | 350-550 A | 450 A | Higher current increases dilution |
| Voltage | 28-36 V | 32 V | Affects arc stability and particle distribution |
| Travel speed | 200-400 mm/min | 280 mm/min | Controls cooling rate and grain size |
| Flux coverage | 15-25 mm | 20 mm | Ensures complete shielding |
| Preheat temperature | 100-300 °C | 200 °C | Reduces cracking susceptibility |
| TiC particle size | 20-100 μm | 40-60 μm | Balances dispersion and agglomeration |
| TiC content (wt%) | 15-40% | 25-30% | Maximizes hardness without excessive brittleness |
Microstructural analysis using optical microscopy and scanning electron microscopy revealed several key features of the SAW-deposited TiC composite coatings:
The base matrix microstructure consists primarily of martensite with retained austenite, with the martensite content ranging from 65% to 85% depending on the cooling rate at different locations within the coating. The heat-affected zone in the base metal exhibits a tempered martensite structure with reduced hardness compared to the coating.
TiC particles are distributed throughout the coating matrix, with some degree of agglomeration observed at higher TiC contents. The particle distribution is influenced by fluid flow dynamics within the weld pool, with particles tending to concentrate near the top surface of the coating where the last solidification occurs. Some TiC particles undergo partial dissolution during the welding process, with dissolution rates increasing with particle size and welding thermal input.
Mechanical Properties and Performance
The mechanical properties of the TiC-reinforced composite coatings were characterized through hardness testing, wear testing, and fracture toughness measurement:
| Property | Base Metal (Q235) | Plain SAW Overlay | TiC Composite Coating (25% TiC) |
|---|---|---|---|
| Hardness (HV30) | 160-180 | 350-400 | 680-750 |
| Wear rate (mm3/N·m) | 4.2×10^-3 | 1.8×10^-3 | 0.4×10^-3 |
| Fracture toughness (MPa·m^1/2) | 85-95 | 45-55 | 25-35 |
| Bond strength (MPa) | N/A | 280-320 | 240-280 |
The hardness improvement of approximately 300-400 HV over the base metal is attributed to the combined effects of martensitic transformation in the matrix and the presence of hard TiC particles. The wear resistance improvement of 85-90% compared to the base metal makes these coatings suitable for severe abrasion applications.
However, the significant reduction in fracture toughness represents a critical design consideration. The study demonstrates that the fracture behavior transitions from ductile to semi-ductile as TiC content increases, with intergranular fracture becoming predominant at TiC contents above 35 wt%. This finding is crucial for applications where impact loading or cyclic stress is present.
Defect Analysis and Process Optimization
The study identifies several potential defects in SAW-deposited TiC composite coatings and their root causes:
- Cracking: Hot cracking in the weld metal occurs due to the formation of low-melting-point phases at grain boundaries, exacerbated by the high thermal stress associated with the large thermal expansion coefficient mismatch between TiC and the metallic matrix. Countermeasures include preheating, reduced thermal input, and addition of micro-alloying elements (Nb, V) to refine grain structure.
- Particle agglomeration: Non-uniform distribution of TiC particles creates localized stress concentrations and weak zones. This is mitigated through proper flux formulation that promotes particle dispersion, controlled powder feeding rates, and appropriate welding speed selection.
- Porosity: Gas porosity from hydrogen absorption in the flux and oxide inclusions from insufficient shielding. Prevention requires proper flux drying, adequate flux coverage, and clean base metal preparation.
- Insufficient bonding: Poor metallurgical bond between coating and base metal due to excessive dilution or insufficient penetration. This is addressed through proper parameter selection and verification through macrographic examination and bond strength testing.
Engineering Application Considerations
The study provides practical guidance for implementing TiC-reinforced SAW composite coatings in industrial applications. The optimal TiC content of 25-30 wt% represents a balance between wear resistance and toughness, suitable for most mining and construction equipment applications. For applications requiring higher toughness, such as impact-loaded components, a multi-layer approach with graded TiC content is recommended, with lower TiC content in the bond layer and higher content in the surface layer.
The SAW process offers significant economic advantages over alternative processes such as laser cladding or plasma spraying for thick coating applications, with deposition rates of 5-15 kg/h compared to 0.5-2 kg/h for laser processes. However, the relatively coarse microstructure and larger heat-affected zone of SAW limit its application to non-critical dimensions and moderate-temperature service.
This research demonstrates that SAW-deposited TiC composite coatings can achieve excellent wear performance at reasonable manufacturing cost, making them a practical solution for extending the service life of components subjected to severe abrasive wear. The key to successful implementation lies in careful process parameter selection, thorough quality control, and appropriate application matching based on the specific service conditions.