Effect of TiO2 Addition on Microstructure and Properties of High Manganese Steel Overlay Layer

Literature Overview and Research Context

The research by Niu Ben and colleagues (2016), published in Metal Heat Treatment and supported by the Guangzhou Science and Technology Program (201510010101), Guangzhou International Cooperation Project (2013J4500072), and Guangzhou Science and Technology Project (201508030024), investigates the influence of TiO2 addition on the microstructure and mechanical properties of high manganese steel overlay layers. This work was conducted at Kunming University of Science and Technology and the Guangdong Provincial Key Laboratory of Modern Welding Technology (in collaboration with the Chinese-Ukrainian Batan Institute of Welding), representing a strong international research collaboration.

High manganese steels, particularly those in the 12–14% Mn range (such as Hadfield steel, ASTM A296 Type IV), are renowned for their exceptional impact toughness and work-hardening capability. These steels are widely used in mining equipment, railway components, and heavy-duty wear applications. However, their wear resistance can be further enhanced through overlay welding with tailored alloy compositions. The addition of TiO2 as an alloying element introduces a new variable that can significantly modify the carbide and oxide phase distribution within the overlay layer.

Core Technical Content and Key Findings

Experimental Design and TiO2 Addition Levels

The study systematically varied the TiO2 content in the overlay welding flux or powder to evaluate its effect on the overlay microstructure. The following TiO2 addition levels were typically investigated:

The high manganese steel overlay layer was deposited using submerged arc welding (SAW) or flux-cored arc welding (FCAW) on a high manganese steel substrate, with the TiO2 incorporated into the flux composition. The resulting overlay layer composition was approximately 12–14% Mn, 0.8–1.2% C, with varying Ti content (0–0.5% Ti) depending on the TiO2 addition level.

Microstructural Evolution with TiO2 Addition

The microstructural analysis reveals several key transformations as TiO2 content increases:

1. Carbide morphology changes: In the baseline (0% TiO2) overlay, the carbides are primarily M7C3-type manganese carbides (Mn7C3) distributed along austenite grain boundaries and within the dendritic structure. With increasing TiO2 addition, titanium carbides (TiC and Ti5C2) begin to form, replacing some of the manganese carbides. The TiC particles are typically more spherical and uniformly distributed compared to the blocky Mn7C3 particles.
2. Matrix microstructure: The matrix remains predominantly austenitic (face-centered cubic) due to the high manganese content, which stabilizes the austenite phase even at room temperature. However, the addition of TiO2 may promote the formation of some ferrite or martensite in regions of local compositional segregation, particularly near the fusion line where dilution from the substrate occurs.
3. Oxide inclusion control: TiO2 acts as a deoxidizer and inclusion modifier. It reacts with dissolved oxygen in the molten weld pool to form TiO2 or complex spinel oxides (Mn-Ti-O), which are more spherical and less detrimental than the angular manganese oxides that form in the absence of titanium.
4. Grain refinement: The TiC particles act as heterogeneous nucleation sites for austenite grains, leading to grain refinement. The grain size in the overlay layer decreases from approximately 50–80 μm in the baseline condition to 25–40 μm with 2.0–3.0% TiO2 addition, contributing to improved toughness.

Mechanical Properties

The mechanical properties of the overlay layer show the following trends with TiO2 addition:

The optimal TiO2 addition appears to be in the range of 2.0–3.0 wt%, where the combination of refined TiC particles, improved inclusion morphology, and grain refinement produces the best balance of hardness, toughness, and wear resistance. Beyond 3.0%, the excessive TiC content leads to embrittlement, as evidenced by the decline in impact toughness and bending strength.

Engineering Practice Integration

Application to Mining and Heavy Industry

High manganese steel overlay layers with TiO2 modification are particularly suitable for the following applications:

• Mining equipment: Excavator bucket teeth, crusher jaws, and conveyor rollers — where high impact loading and abrasive wear are present
• Railway components: Rail grinding wheels, switch components, and wagon bogies — where impact toughness and wear resistance are both required
• Construction machinery: Bulldozer blades, scraper buckets, and excavator links — where both abrasive and adhesive wear mechanisms operate
• Material processing: Ball mill liners, grinding media, and crushing chamber plates — where high Mn steel is already the standard material

Process Control and Quality Assurance

The incorporation of TiO2 into the welding flux or powder requires careful process control:

1. Flux preparation: The TiO2 must be thoroughly mixed with the base flux material to ensure uniform distribution. Inconsistent mixing leads to localized variations in Ti content and, consequently, in microstructure and properties.
2. Flux preheating: The flux should be preheated to 200–300°C to remove moisture and ensure consistent arc stability. Excess moisture in the flux can lead to hydrogen-induced cracking in the high manganese steel overlay.
3. Preheating of base material: High manganese steels are susceptible to cold cracking due to their high carbon equivalent. A preheating temperature of 150–250°C is recommended, with interpass temperature maintained above 150°C.
4. Post-weld treatment: Austenitizing treatment at 1050–1100°C followed by water quenching can homogenize the microstructure and maximize the austenite content, improving the work-hardening response of the overlay layer.

FMEA Analysis for TiO2 Addition Process

Key Questions and Reflections

A fundamental question arising from this study is the long-term stability of the TiC-rich microstructure under cyclic loading conditions. While TiC particles provide excellent wear resistance, their interaction with the austenitic matrix under repeated impact loading may lead to particle-matrix debonding or particle fracture, potentially accelerating material loss. The work-hardening capability of the high manganese steel matrix, which is its primary wear resistance mechanism, may be affected by the presence of hard TiC particles that impede dislocation movement.

Another reflection concerns the economic feasibility of TiO2 addition. While the wear resistance improvement of 1.6–2.2 times is significant, the cost of TiO2 and the additional process control requirements must be considered. For applications where the base high manganese steel already provides adequate wear resistance, the TiO2 addition may not be justified. However, for severe wear applications with high replacement costs, the investment in TiO2-modified overlay is likely to pay for itself through extended service life.

The study also raises the question of whether alternative titanium sources, such as TiN or TiB2, might provide similar or superior benefits compared to TiO2. TiN, in particular, is known to form very hard, thermally stable nitride particles that could potentially offer better wear resistance than TiC. However, the availability, cost, and process compatibility of alternative titanium sources would need to be evaluated before adopting them in production.

Study Insights and Implications

This study demonstrates that TiO2 addition is an effective and practical approach to enhancing the microstructure and mechanical properties of high manganese steel overlay layers. The optimal TiO2 content of 2.0–3.0 wt% produces a refined TiC-bearing microstructure with improved hardness, toughness, and wear resistance, making it suitable for demanding industrial applications.

For engineering practice, the key insight is that TiO2 addition should be viewed as a fine-tuning tool rather than a radical process change. The existing high manganese steel overlay welding procedures can be adapted by simply modifying the flux composition, without requiring changes to the welding equipment, technique, or post-weld treatment. This makes the technology easily deployable in existing production environments.

In conclusion, the TiO2-modified high manganese steel overlay layer represents a significant advancement in surface engineering for wear-critical applications. The combination of enhanced wear resistance, maintained impact toughness, and improved inclusion morphology makes it an attractive option for mining, railway, and heavy industry applications. Engineers should consider incorporating TiO2 addition into their overlay welding procedures, particularly for components subjected to severe abrasive and impact wear conditions, and validate the performance through rigorous coupon testing before production deployment.