CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Titanium on Microstructure and Wear Resistance of High-Chromium Alloy Overlay
Literature Overview
This study note examines the influence of titanium additions on the microstructure, hardness, and wear resistance of high-chromium alloy overlay layers. High-chromium alloys, particularly those in the Cr20 and Cr28 range, are widely used for wear protection in mining, cement, and power generation industries. The addition of titanium as a micro-alloying element is investigated as a means to further enhance wear resistance through carbide modification and grain refinement mechanisms.
Core Technical Points
Metallurgical Role of Titanium in High-Chromium Alloys
Titanium is a strong carbide former with a higher affinity for carbon than chromium. In high-chromium overlay alloys, titanium preferentially forms TiC and Ti₇C₃ carbides, which are harder and more thermally stable than the Cr7C3 and Cr23C6 carbides that dominate in conventional high-chromium compositions. This carbide modification is the primary mechanism by which titanium enhances wear resistance.
The literature presents the following compositional ranges for titanium-modified high-chromium overlays:
| Alloy Designation | Cr (%) | C (%) | Ti (%) | Base Matrix | Primary Carbides |
|---|---|---|---|---|---|
| Conventional Cr20 | 19–21 | 2.5–3.5 | 0.05 (residual) | Ferritic | Cr7C3 |
| Ti-modified Cr20 | 19–21 | 2.5–3.5 | 1.5–2.5 | Ferritic | TiC + Cr7C3 |
| Conventional Cr28 | 27–29 | 3.0–4.0 | 0.05 (residual) | Ferritic | Cr23C6 |
| Ti-modified Cr28 | 27–29 | 3.0–4.0 | 1.5–2.5 | Ferritic | TiC + Cr23C6 |
Microstructural Evolution
The addition of titanium fundamentally alters the microstructure of the overlay layer. In conventional high-chromium alloys, the microstructure consists of a ferritic matrix with a network of coarse, blocky chromium carbides. These carbides provide good abrasion resistance but are susceptible to fracture under impact loading.
When titanium is added at levels of 1.5 to 2.5 percent, the microstructure transforms significantly. The TiC carbides form as fine, dispersed particles within the ferritic matrix, with sizes typically in the range of 0.5 to 3 micrometers. These fine carbides are more resistant to fracture and provide a more uniform wear surface. Additionally, the presence of titanium carbides acts as nucleation sites for grain refinement, resulting in a finer ferritic matrix grain structure.
The literature also notes that titanium additions can influence the solidification sequence. In Ti-modified Cr20 alloys, the solidification path typically proceeds as liquid → austenite → ferrite + TiC, with the TiC precipitating during the austenite-to-ferrite transformation. This precipitation hardening contributes additional strength to the matrix.
Wear Resistance Performance
The wear resistance improvement from titanium addition is quantified in the literature through standard wear testing methods, including pin-on-disc, ball-on-disc, and taber abrasion tests. The following table summarizes typical results:
| Test Method | Conventional Cr20 (mm³/g) | Ti-modified Cr20 (mm³/g) | Improvement (%) |
|---|---|---|---|
| Pin-on-disc (SiC) | 0.45–0.60 | 0.25–0.35 | 40–55 |
| Taber abrasion (CS-10) | 25–35 | 15–22 | 35–50 |
| Ball-on-disc (Al2O3) | 0.80–1.10 | 0.45–0.65 | 40–55 |
The improvement is attributed to the combined effect of increased hardness (from 600–700 HV to 750–900 HV) and improved fracture toughness of the carbide phase. The TiC carbides resist fracture and pull-out during abrasion, maintaining a smooth wear surface and reducing the material removal rate.
Process Considerations
The addition of titanium introduces several process challenges that must be addressed in overlay welding practice:
- Titanium burn-off: Titanium is highly reactive with oxygen and nitrogen. During welding, titanium burn-off can be significant, especially with gas-shielded processes. The literature recommends using a titanium-containing flux or a pre-alloyed wire with titanium content 20–30 percent higher than the target composition to compensate for burn-off.
- Nitride formation: If nitrogen is present in the welding atmosphere, titanium can form TiN, which is even harder than TiC but can reduce toughness. Argon shielding with oxygen-free gas supply is essential to minimize nitrogen pickup.
- Wetting and fluidity: Titanium additions can reduce the fluidity of the weld pool, potentially leading to poor bead shape and increased porosity risk. The literature suggests slightly increasing the heat input (by 10–15 percent) to compensate for reduced fluidity.
- Cracking susceptibility: The combination of titanium carbides and the ferritic matrix can increase susceptibility to hot cracking. The literature recommends maintaining a carbon content above 2.5 percent and ensuring adequate nickel or manganese addition to promote austenite formation during solidification, which acts as a crack-arresting phase.
Integration with Engineering Practice
The practical application of titanium-modified high-chromium overlays requires careful consideration of the service environment. The literature identifies three primary application categories where titanium addition provides the greatest benefit:
- Abrasive wear with impact: In mining applications (bucket liners, conveyor chutes), the combination of fine TiC carbides and improved toughness provides superior performance compared to conventional white iron overlays.
- Slurry erosion: In cement mill rollers and kiln liners, the fine dispersed carbides resist erosion by abrasive slurry, extending service life by 40–60 percent.
- High-temperature wear: In furnace components and hot metal handling equipment, the thermal stability of TiC (melting point 3140°C) provides better performance at elevated temperatures where conventional chromium carbides may spheroidize.
The literature also discusses the cost-benefit analysis of titanium addition. While titanium-containing consumables are 30–50 percent more expensive than conventional high-chromium consumables, the extended service life (typically 40–60 percent improvement) and reduced downtime often justify the additional material cost in critical applications.
Study Insights and Implications
The most important insight from this literature is that titanium addition is not merely a linear improvement but a transformation of the wear mechanism. In conventional high-chromium alloys, wear occurs primarily through carbide fracture and pull-out, followed by matrix plastic deformation. In titanium-modified alloys, the fine TiC carbides resist fracture, shifting the wear mechanism to a more uniform matrix deformation with periodic carbide refreshment through micro-ploughing.
This mechanistic understanding has important implications for process development. The optimal titanium content is not simply "more is better"—excessive titanium (above 3 percent) can lead to a coarse, interconnected TiC network that reduces toughness and increases brittleness. The literature identifies 1.5 to 2.5 percent as the optimal range, balancing carbide volume fraction, dispersion, and matrix integrity.
In conclusion, titanium modification of high-chromium overlay alloys represents a significant metallurgical advancement that provides measurable improvements in wear resistance through carbide refinement and matrix strengthening. The practical implementation requires careful attention to process parameters to minimize titanium burn-off and maintain the intended microstructure, but the resulting performance gains justify the additional complexity in applications where wear life is a critical economic factor.