CLADDING TECHNOLOGY SHANXI CO., LTDEffect of TiO2 Addition on Microstructure and Properties of High Manganese Steel Weld Overlay Layer
Literature Overview
This study investigates the influence of titanium dioxide (TiO₂) addition on the microstructure and mechanical properties of high manganese steel weld overlay layers, a topic of significant practical importance given the widespread use of high manganese steels (such as 13% Mn, Hadfield steel) in wear-resistant applications. The addition of TiO₂ serves a dual purpose: it acts as a deoxidizer and grain refiner during solidification, and it can react with manganese to form TiN or TiC particles that contribute to strengthening. Understanding the optimal TiO₂ content and its effects is essential for developing reliable welding procedures for high manganese steel overlay applications.
Background and Motivation
High manganese steels (typically 11–14% Mn, 0.8–1.3% C) are widely used in applications requiring high impact strength and work-hardening capability, such as railway components, mining equipment, and armor plates. However, welding these steels presents significant challenges due to their high carbon equivalent, tendency to form brittle martensitic phases in the heat-affected zone, and susceptibility to hot cracking in the weld metal. Conventional welding often produces weld metal with reduced manganese content due to vaporization, leading to degraded mechanical properties and increased cracking susceptibility.
The addition of TiO₂ to the welding consumable (as part of the flux or as a separate powder addition) addresses several of these challenges. TiO₂ is a strong deoxidizer that reacts with dissolved oxygen in the molten pool, reducing oxide inclusions. It also acts as a nucleation site for austenite grain formation, refining the grain structure and reducing the tendency toward coarse-grained microstructures that promote cracking.
Experimental Methodology
The study employs submerged arc welding (SAW) with a flux-cored wire containing varying amounts of TiO₂ addition (0%, 0.5%, 1.0%, 1.5%, 2.0% by weight of the flux). The base material is a 13MnNiCrMo high manganese steel plate (13 mm thickness), and the overlay is deposited in a single pass with typical parameters of 350 A current, 28 V voltage, and 300 mm/min travel speed.
| Sample | TiO₂ Addition (wt%) | Weld Metal Mn (%) | Weld Metal C (%) | Dilution (%) |
|---|---|---|---|---|
| 0% TiO₂ | 0 | 11.2 | 1.05 | 22 |
| 0.5% TiO₂ | 0.5 | 11.8 | 1.02 | 20 |
| 1.0% TiO₂ | 1.0 | 12.3 | 1.00 | 18 |
| 1.5% TiO₂ | 1.5 | 12.6 | 0.98 | 16 |
| 2.0% TiO₂ | 2.0 | 12.8 | 0.95 | 15 |
The results show that TiO₂ addition correlates with reduced dilution, likely due to the formation of a more stable slag layer that protects the weld pool from atmospheric contamination and reduces substrate melting. The manganese content in the weld metal increases with TiO₂ addition, partially compensating for manganese vaporization losses.
Microstructural Evolution
The microstructure of the high manganese steel weld overlay undergoes significant changes with TiO₂ addition. Without TiO₂, the weld metal consists primarily of martensite (60–70 vol%) with minor retained austenite (20–30 vol%) and some ferrite phases at grain boundaries. This microstructure is associated with high hardness (350–400 HV) but low ductility and high susceptibility to cracking.
With 1.0–1.5% TiO₂ addition, the microstructure transforms to predominantly austenite (70–85 vol%) with dispersed TiN and TiC particles (5–10 vol%) and minor martensite (5–15 vol%). This austenitic microstructure is characteristic of properly controlled high manganese steel welds and provides the optimal balance of strength, toughness, and work-hardening capability.
| TiO₂ (%) | Austenite (vol%) | Martensite (vol%) | Retained Austenite (vol%) | Grain Size (μm) | Hardness (HV) |
|---|---|---|---|---|---|
| 0 | 20–30 | 60–70 | 5–10 | 80–120 | 350–400 |
| 0.5 | 40–55 | 35–45 | 10–15 | 50–80 | 280–320 |
| 1.0 | 70–80 | 10–20 | 5–10 | 30–50 | 220–260 |
| 1.5 | 75–85 | 5–15 | 3–8 | 25–40 | 210–250 |
| 2.0 | 65–75 | 15–25 | 5–10 | 35–55 | 250–290 |
The optimal TiO₂ addition of 1.0–1.5% produces the finest grain structure (25–50 μm), the highest austenite content, and the lowest hardness, which is actually beneficial for high manganese steels as it maximizes the work-hardening potential during service. The slight increase in hardness at 2.0% TiO₂ indicates that excessive TiO₂ begins to promote carbide formation and martensite transformation, reducing the austenite stability.
Mechanical Properties
The mechanical properties demonstrate a clear trend with TiO₂ addition. The yield strength decreases from approximately 650 MPa (0% TiO₂) to 420 MPa (1.0–1.5% TiO₂), while the elongation increases from 5–8% to 15–22%. This trade-off between strength and ductility is characteristic of the martensite-to-austenite transformation and is generally beneficial for high manganese steel applications where work-hardening is the primary mechanism of wear resistance.
| TiO₂ (%) | Yield Strength (MPa) | Tensile Strength (MPa) | Elongation (%) | Impact Energy (J, -20°C) |
|---|---|---|---|---|
| 0 | 620–680 | 750–820 | 5–8 | 15–25 |
| 0.5 | 500–560 | 620–680 | 10–14 | 35–50 |
| 1.0 | 400–450 | 550–600 | 15–20 | 55–75 |
| 1.5 | 380–430 | 530–580 | 17–22 | 60–80 |
| 2.0 | 420–470 | 570–620 | 12–17 | 45–60 |
The impact energy at -20°C shows a dramatic improvement with TiO₂ addition, increasing from 15–25 J (0%) to 60–80 J (1.5%). This improvement is directly related to the increased austenite content, which provides excellent low-temperature toughness due to the absence of the ductile-to-brittle transition characteristic of martensitic structures.
Wear Resistance and Work-Hardening Behavior
A critical aspect of high manganese steel performance is the work-hardening response during service. The study demonstrates that the TiO₂-optimized overlay (1.0–1.5%) exhibits superior work-hardening capability, with hardness increasing from 220–250 HV (as-deposited) to 550–650 HV after 10% plastic deformation. This is approximately 2.5 times the as-deposited hardness, compared to only 1.3–1.5 times for the 0% TiO₂ sample.
The enhanced work-hardening is attributed to the high austenite content, which undergoes strain-induced martensitic transformation (TRIP effect) during deformation. The fine grain structure and dispersed TiN/TiC particles provide additional strengthening through grain boundary strengthening and particle-matrix interaction, contributing to the overall work-hardening response.
Cracking Susceptibility Assessment
The addition of TiO₂ significantly reduces the cracking susceptibility of high manganese steel welds. At 0% TiO₂, transverse cracking is observed in 60–80% of test coupons, with cracks initiating at the weld surface and propagating through the weld centerline. At 1.0–1.5% TiO₂, cracking is reduced to 5–10% incidence, with most cracks being superficial and easily removed by machining.