CLADDING TECHNOLOGY SHANXI CO., LTDApplication of Domestic Niobium-Containing Welding Wire in Raw Mill Overlay Welding
Literature Overview
Published in 2009 in the journal "Cement" (Shuini), this paper by Tian Dabiao from the China Coal Research Institute addresses the application of domestically developed niobium (Nb)-containing welding wire for overlay welding of raw mill components in cement manufacturing. Raw mills in cement plants are subjected to extreme abrasive wear from grinding limestone, clay, and other raw materials. The overlay welding of wear-resistant materials on mill internals is a critical maintenance activity that directly impacts production efficiency and downtime.
Core Technical Content
The Wear Environment in Raw Mills
Raw mills in cement production operate under severe conditions:
- Abrasive particles: Limestone and clay particles with hardness ranging from 3–7 Mohs
- Impact loading: Mill liners and grinding elements experience repeated impact from falling material
- Temperature variation: Ambient to elevated temperatures depending on mill design
- Continuous operation: Mills typically operate 24/7 with maintenance windows limited to planned shutdowns
The wear mechanism in raw mills is predominantly abrasive, with secondary contributions from fatigue spalling and corrosion-abrasion interaction. The overlay material must therefore exhibit high hardness, good toughness, and resistance to both mechanical and chemical degradation.
Niobium-Containing Welding Wire Composition and Properties
The domestically developed Nb-containing welding wire is designed to produce overlay deposits with enhanced wear resistance through the formation of hard carbide phases. Niobium forms extremely hard carbides (NbC, with a hardness exceeding 2000 HV) that are distributed throughout the matrix, providing excellent abrasion resistance.
| Component | Typical Composition (wt%) | Function |
|---|---|---|
| Carbon | 2.5–3.5 | Carbide former, hardness contributor |
| Niobium | 0.3–0.8 | Hard carbide formation, grain refinement |
| Chromium | 5–8 | Solid solution strengthening, carbide stability |
| Manganese | 1.5–2.5 | Austenite stabilizer, toughness improvement |
| Silicon | 0.3–0.8 | Deoxidizer, minor hardening |
| Iron | Balance | Matrix material |
The resulting overlay deposit typically exhibits:
- Hardness: 55–65 HRC (580–720 HV)
- Toughness: Adequate for impact loading in mill service
- Abrasion resistance: 2–3 times that of standard high-chromium white iron
- Dilution tolerance: Maintains wear resistance even with 20–30% base metal dilution
Process Parameters for Raw Mill Overlay
| Parameter | Specification | Rationale |
|---|---|---|
| Welding process | Submerged arc welding (SAW) | High deposition rate, good protection |
| Wire diameter | 1.6–2.0 mm | Optimized for single-pass or multi-pass overlay |
| Flux type | Rutile or basic flux | Low hydrogen, good slag fluidity |
| Welding current | 250–350 A | Controls penetration and bead profile |
| Travel speed | 300–500 mm/min | Balances deposition rate and bead quality |
| Preheat | 100–150 °C | Reduces cracking in high-carbon overlay |
| Interpass temperature | ≤250 °C | Prevents overheating and microstructural degradation |
| Overlay thickness | 8–15 mm | Provides adequate wear life while minimizing heat input |
Engineering Application and Performance
Application Locations in Raw Mills
The Nb-containing welding wire overlay is applied to several critical components within raw mills:
- Mill liners: The primary wear surface contacting the grinding material
- Grinding rollers: Surface overlay to extend roller life
- Lifter bars: Components that lift material for grinding action
- Trunnion areas: High-stress regions subject to combined wear and fatigue
Performance Comparison
| Material | Hardness (HRC) | Abrasion Index | Service Life (hours) | Cost per m² |
|---|---|---|---|---|
| Standard Mn13 cast iron | 20–25 | 1.0 (baseline) | 1000 | 1.0 |
| High-chromium white iron | 55–60 | 2.5 | 2500 | 1.8 |
| Nb-containing overlay | 55–65 | 3.5 | 3500 | 2.2 |
| Carbide-nickel overlay | 60–68 | 4.0 | 4000 | 3.5 |
The Nb-containing overlay offers a favorable balance of wear resistance, toughness, and cost, making it suitable for raw mill applications where extreme hardness is not required but good abrasion resistance and impact tolerance are essential.
Quality Control Considerations
The application of high-carbon, high-alloy overlay materials presents specific quality control challenges:
- Cracking susceptibility: The high carbon content promotes the formation of hard, brittle martensite, which is susceptible to cracking during cooling. Preheat and controlled cooling rates are essential.
- Dilution control: Excessive dilution with the carbon steel base metal reduces the hardness and wear resistance of the overlay. Multi-pass welding with controlled penetration helps limit dilution.
- Bond strength: The interface between the overlay and base metal must have adequate bond strength to prevent spalling under impact loading. Proper surface preparation and first-pass technique are critical.
- Hardness uniformity: Variations in cooling rate across the overlay can produce hardness variations that affect wear performance. Consistent process parameters and adequate preheat help minimize this variation.
Engineering Practice Insights
The development and application of domestic Nb-containing welding wire represents a significant achievement in the Chinese welding consumables industry. Prior to this development, cement plants relied on imported welding wires for overlay applications, which imposed significant costs and supply chain vulnerabilities. The domestic wire achieved comparable performance to imported products while offering advantages in availability, cost, and technical support.
From a practical standpoint, the successful application of this wire in raw mill overlay welding required:
- Procedure qualification: Systematic welding procedure qualification per NB/T 47014 or equivalent standards to establish reliable process parameters.
- Operator training: Specialized training for welders to ensure consistent technique and quality.
- Inspection protocols: Development of inspection criteria specific to high-carbon overlay applications, including hardness mapping, bond strength testing, and ultrasonic inspection for interface defects.
- Performance monitoring: Tracking of overlay service life in actual mill operation to validate material performance and optimize application parameters.
Summary
This 2009 literature documents the successful application of domestically developed niobium-containing welding wire for overlay welding of raw mill components in cement manufacturing. The Nb-containing wire produces overlay deposits with excellent abrasion resistance through the formation of hard NbC carbides, offering a favorable balance of wear life, toughness, and cost. The successful commercialization of this domestic wire reduced reliance on imported consumables and provided cement manufacturers with a reliable solution for extending the service life of critical mill internals. For engineers working in mineral processing and cement industry maintenance, this reference highlights the importance of material selection tailored to specific wear environments and the value of domestic consumable development in reducing maintenance costs and improving supply chain security.