Microstructure and Properties of TiC-Reinforced Wear-Resistant Overlay Welds Using Dual-Deposit Electrode Arc Welding

Literature Overview

This paper by Li Lingyu, Zhao Wei, Zhang Hui, and Gao Liying from Qilu University of Technology (Shandong Academy of Sciences), published in the Journal of Qilu University of Technology in 2020, investigates the microstructure and mechanical properties of TiC-reinforced wear-resistant overlay welds produced using a dual-deposit electrode arc welding process. The study compares the performance of the dual-deposit electrode process with that of the conventional single-deposit electrode arc welding process, with particular emphasis on the effect of welding current on the microstructure and microhardness of the overlay layer. The research was supported by the National Natural Science Foundation of China and the Shandong Provincial Key R&D Program.

Technical Background and Process Description

The dual-deposit electrode arc welding process is a novel welding technique that uses two electrodes simultaneously to deposit weld metal onto the base material. The two electrodes are positioned at a slight angle to each other, and the welding arc is formed between both electrodes and the workpiece. This process offers several advantages over the conventional single-electrode process, including higher deposition rate, lower dilution ratio, and finer microstructure. The lower dilution ratio is achieved because the two electrodes share the heat input, resulting in a smaller weld pool and less base metal dilution.

The TiC-reinforced overlay welds were produced using an in-situ self-reaction method, where titanium and carbon-containing compounds react within the molten weld pool to form TiC particles. The TiC particles serve as hard reinforcing phases that improve the wear resistance of the overlay layer. The TiC particles are typically in the range of 1-5 micrometers in size and are uniformly distributed throughout the overlay layer. The hardness of TiC is approximately 2500-3000 HV, which is significantly higher than the matrix material, providing excellent resistance to abrasive wear.

Welding Current Effects on Microstructure

The authors conducted welding experiments at three current levels: 150 A, 180 A, and 210 A, using both the dual-deposit electrode process and the conventional single-deposit electrode process. The microstructure of the overlay layers was examined using optical metallography and scanning electron microscopy, and the microhardness was measured using a microhardness tester with a 100 gf load.

The results clearly demonstrate that the dual-deposit electrode process produces overlay layers with finer microstructures and higher hardness compared to the conventional process at the same welding current. This improvement is attributed to the lower dilution ratio and the more uniform heat distribution achieved by the dual-electrode configuration. The finer microstructure results in a higher density of grain boundaries, which act as barriers to dislocation motion and contribute to the higher hardness.

Hardness-Temperature Relationship

The microhardness of the overlay layers exhibits a non-monotonic relationship with welding current. At low currents (150 A), the hardness is relatively low because the heat input is insufficient to fully dissolve the Ti and C-containing compounds, resulting in incomplete formation of TiC particles. As the current increases to 180 A, the heat input is sufficient to promote the in-situ reaction, leading to a higher volume fraction of TiC particles and a corresponding increase in hardness. However, at higher currents (210 A), the excessive heat input leads to grain coarsening and the dissolution of some TiC particles, resulting in a decrease in hardness.

This non-monotonic behavior is a common phenomenon in overlay welding and is related to the competition between the formation of reinforcing phases and the coarsening of the microstructure. The optimal welding current is the one that maximizes the volume fraction of TiC particles while minimizing the grain size of the matrix. In this study, the optimal current was found to be 180 A, which produced a peak hardness of 680-720 HV for the dual-deposit electrode process.

Microstructure Analysis

The microstructure of the overlay layers was analyzed in detail using scanning electron microscopy and energy-dispersive X-ray spectroscopy. The overlay layers consisted of a matrix of martensite and retained austenite, with TiC particles dispersed throughout. The TiC particles were identified by their characteristic hexagonal morphology and high carbon and titanium content. The distribution of TiC particles was more uniform in the dual-deposit electrode welds compared to the conventional welds, which is attributed to the more uniform heat distribution and the lower dilution ratio.

The fusion boundary of the overlay welds was also examined. The fusion boundary exhibited a fine microstructure with a narrow heat-affected zone, which is beneficial for the mechanical properties of the weld. The residual stress at the fusion boundary was measured using the X-ray diffraction method, and the results showed that the dual-deposit electrode process produced lower residual stresses compared to the conventional process. This is attributed to the lower heat input per unit length and the more uniform cooling rate achieved by the dual-electrode configuration.

Engineering Practice Integration

The dual-deposit electrode arc welding process offers several practical advantages for wear-resistant overlay welding applications. The higher deposition rate reduces the welding time and increases productivity. The lower dilution ratio ensures that the overlay layer retains its high hardness and wear resistance. The finer microstructure improves the fatigue resistance of the overlay layer, which is important for applications involving cyclic loading. The lower residual stresses reduce the risk of cracking and improve the dimensional stability of the welded component.

In practical applications, the dual-deposit electrode process can be used to repair or upgrade the wear resistance of components such as crusher hammers, excavator buckets, and mining equipment. The process is particularly suitable for components that require a hard surface layer but also need to maintain adequate toughness to resist impact loading. The ability to control the welding current to achieve the optimal microstructure and hardness is a significant advantage of this process.

Key Questions and Reflections

One question that arises from this study is the scalability of the dual-deposit electrode process to larger components. The process was demonstrated on laboratory-scale samples, but its application to large industrial components may require modifications to the electrode holder design and the welding equipment. The positioning of the two electrodes must be precise to ensure uniform deposition and consistent properties across the entire overlay area. In practice, this may require the use of a robotic welding system with precise control over the electrode positioning.

Another reflection is that the study focuses on the microstructure and hardness of the overlay layer but does not address the wear resistance directly. While hardness is a good indicator of wear resistance, the actual wear resistance depends on several factors, including the microstructure, the distribution of reinforcing phases, and the residual stress state. Future studies should include wear testing under conditions that simulate the actual service environment to validate the wear resistance of the overlay layers.

Study Insights and Implications

This paper represents a significant contribution to the field of wear-resistant overlay welding, demonstrating the advantages of the dual-deposit electrode process over the conventional single-electrode process. The systematic investigation of the welding current effects on microstructure and hardness provides valuable guidance for the optimization of the welding parameters. The findings suggest that the dual-deposit electrode process is a promising technology for the production of high-performance overlay welds, with potential applications in a wide range of industries. The key insight is that the process parameters must be carefully optimized to achieve the desired balance between the formation of reinforcing phases and the control of the microstructure, and that the dual-electrode configuration offers superior control over these parameters compared to the conventional process. The paper underscores the importance of microstructural analysis in understanding and improving the performance of overlay welds.