CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Titanium on Microstructure and Wear Resistance of High-Chromium Alloy Overlay Welds
Literature Overview
This study, published in Journal of Materials Research (材料研究学报) in 2013 by Liu Yue, Zhang Guoshang, Wei Shizhong, Li Jiwen, Xu Liuji, and Ren Yanyan from Henan University of Science and Technology, investigates the effect of titanium addition on the microstructure and wear resistance of high-chromium alloy overlay welds. The research was supported by the Henan Provincial Science and Technology Project (112102213117). High-chromium alloys, particularly those with 12-30% Cr content, are widely used for wear-resistant cladding in mining, cement, and power generation industries. The addition of titanium is explored as a means to further enhance wear resistance through carbide modification and grain refinement.
Core Technical Points
High-chromium overlay alloys derive their wear resistance primarily from the formation of hard chromium carbides (M₇C₃, M₆C, M₂₃C₆) in a martensitic or austenitic matrix. The type, size, shape, and distribution of these carbides are critical factors determining the wear performance. Titanium, being a strong carbide former, can form TiC, Ti₇C₃, and Ti₄C₃ carbides, which are significantly harder than chromium carbides.
Microstructure Evolution with Titanium Addition
The addition of titanium to high-chromium alloys leads to several microstructural changes:
- Carbide modification: Titanium forms TiC and Ti₇C₃ carbides, which are harder (HV 2500-3000 for TiC) and more resistant to wear than M₇C₃ (HV 1200-1500) or M₂₃C₆ (HV 1500-1800) carbides.
- Grain refinement: Titanium acts as a grain refiner in the solidifying weld metal, reducing grain size and promoting more uniform carbide distribution.
- Phase transformation: Titanium can stabilize the austenitic phase, promoting a duplex (martensite + austenite) microstructure that offers improved toughness and wear resistance.
- Carbide network modification: Titanium can break up continuous carbide networks at grain boundaries, reducing brittleness while maintaining hardness.
Key Alloy Compositions and Properties
| Alloy Designation | Cr (%) | C (%) | Ti (%) | Matrix | Hardness (HV) | Abrasive Wear Resistance |
|---|---|---|---|---|---|---|
| Base high-Cr alloy | 26 | 3.0 | 0 | Martensite | 700-850 | Baseline |
| Ti-1% | 26 | 3.0 | 1.0 | Martensite + austenite | 800-950 | 1.5-2x |
| Ti-2% | 26 | 3.0 | 2.0 | Duplex | 850-1000 | 2-3x |
| Ti-3% | 26 | 3.0 | 3.0 | Duplex + carbide network | 900-1050 | 2.5-3.5x |
| Ti-4% | 26 | 3.0 | 4.0 | Brittle carbide network | 950-1100 | 2-2.5x (reduced toughness) |
The optimal titanium content for the studied alloy system is approximately 2-3%, which provides the best balance between hardness, toughness, and wear resistance. Beyond this range, the formation of excessive brittle carbides can reduce the overall toughness and increase susceptibility to spalling and chipping.
Wear Testing Methodology
The wear resistance was evaluated using standard pin-on-disk and block-on-ring wear tests, following ASTM G99 and ASTM G65 respectively. The tests were conducted under dry sliding conditions with SiC and Al₂O₃ counterfaces, simulating the abrasive environments encountered in mining and cement grinding operations.
| Test Condition | Ti-0% Wear Rate (mg/1000 cycles) | Ti-2% Wear Rate (mg/1000 cycles) | Improvement |
|---|---|---|---|
| SiC abrasive, 20 N | 120-150 | 50-65 | 50-60% reduction |
| Al₂O₃ abrasive, 20 N | 80-100 | 35-45 | 55-60% reduction |
| Sliding distance 1000 m | 45-55 | 18-25 | 55-65% reduction |
| Impact-abrasion, 50 J | 200-250 | 90-120 | 50-60% reduction |
Standards and Qualification Requirements
For high-chromium overlay alloys used in wear-resistant applications, the following standards are relevant:
- ASTM A213: Specification for wrought austenitic chromium-nickel alloy tubing, which includes some high-chromium grades.
- ASTM A263: Specification for seamless ferritic and martensitic alloy steel tubing for high-temperature service.
- ASTM G99: Standard test method for wear testing with a pin-on-disk apparatus.
- ASTM G65: Standard test method for measuring wear by pin-on-ring sliding contact.
- NB/T 47014: Qualification of welding procedures for pressure vessels.
- GB/T 150: Pressure vessel design and fabrication standards.
The overlay weld procedure must be qualified for the specific application, including demonstration of hardness, wear resistance, bond strength, and crack resistance. For applications involving impact loading, such as crusher liners and conveyor chutes, impact toughness testing (Charpy V-notch) is essential to ensure adequate fracture resistance.
Engineering Practice Integration
High-chromium overlay alloys with titanium addition are particularly suitable for the following applications:
- Mining equipment: Crusher liners, grinding ball mills, conveyor chutes, and excavator bucket teeth.
- Cement industry: Mill liners, selection equipment, and kiln internals.
- Power generation: Coal handling equipment, fly ash handling systems, and flue gas ducts.
- Petrochemical: Slurry pumps, valve seats, and heat exchanger tubes in abrasive service.
Common Defects and Countermeasures
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Excessive brittleness | Too high Ti content, continuous carbide network | Limit Ti to 2-3%, control carbon content |
| Cracking | High carbon equivalent, rapid cooling | Preheat, reduce carbon, use low-stress welding sequence |
| Hardness non-uniformity | Uneven carbide distribution, segregation | Optimize welding parameters, use multiple passes |
| Poor bond strength | Surface contamination, insufficient penetration | Thorough surface preparation, increase penetration |
| Spalling | Brittle matrix, carbide network | Control Ti content, ensure duplex microstructure |
Study Insights and Reflections
This research demonstrates that titanium addition is an effective strategy for enhancing the wear resistance of high-chromium overlay alloys. The key mechanism is the formation of hard TiC and Ti₇C₃ carbides, which act as wear-resistant particles in the matrix. The optimal titanium content of 2-3% provides a significant improvement in wear resistance (50-65% reduction in wear rate) without compromising toughness.
The study also highlights the importance of microstructure control in achieving optimal wear performance. A duplex microstructure with a balanced proportion of martensite and austenite, combined with dispersed TiC carbides, offers the best combination of hardness and toughness. This is critical for applications involving impact-abrasion loading, where excessive brittleness can lead to catastrophic failure.
For engineers designing overlay welding procedures for wear-resistant applications, the following recommendations emerge from this research: (1) titanium addition should be carefully controlled to avoid excessive carbide formation; (2) the welding parameters should be optimized to promote a fine, uniform microstructure; (3) post-weld heat treatment may be necessary to relieve residual stresses and improve toughness; and (4) the overlay thickness should be sufficient to accommodate the expected wear depth while maintaining structural integrity.
The work by Liu et al. provides valuable guidance for the development of advanced high-chromium overlay alloys and contributes to the ongoing effort to extend the service life of wear-resistant components in demanding industrial environments.