CLADDING TECHNOLOGY SHANXI CO., LTDWear-Resistant Overlay Welding Technology for Roller and Grinding Disc Surfaces of Vertical Mills
Literature Overview
This publication, authored by Zhang Kunmou, Zheng Guoliang, and Peng Xinqiao from Kunshan Huifeng Wear-Resistant Industry Co., Ltd., was published in New Century Cement Report in 2005. The study addresses the application of wear-resistant overlay welding technology for the surface renewal and protection of vertical mill rollers and grinding discs. Vertical mills are widely used in the cement industry for grinding raw materials and cement clinker. The rollers and grinding discs are subjected to severe abrasive wear from the grinding of hard, abrasive materials, and their service life is limited by the wear of the working surfaces. Wear-resistant overlay welding provides an economical and effective method for extending the service life of these components.
Core Technical Content
Service Conditions of Vertical Mill Rollers and Grinding Discs
The vertical mill rollers and grinding discs operate under extreme conditions:
| Parameter | Value / Description |
|---|---|
| Operating temperature | 80–150 °C |
| Grinding pressure | 20–50 MPa |
| Abrasive material | Cement clinker, limestone, gypsum |
| Abrasive particle size | 0.1–5 mm |
| Service life (as-welded) | 3–6 months |
| Service life (with overlay) | 12–24 months |
| Overlay thickness | 15–30 mm |
| Overlay material | High-Cr high-C alloy |
| Base material | Cast steel / forged steel |
Wear-Resistant Overlay Materials
The selection of overlay material is critical for achieving the desired wear resistance and service life. Common overlay materials for vertical mill rollers and grinding discs include:
| Material Type | Composition (wt%) | Hardness (HRC) | Wear Resistance | Cost |
|---|---|---|---|---|
| High-Cr high-C alloy | Cr 25–30, C 3–5 | 55–65 | Excellent | Medium |
| Medium-Cr alloy | Cr 10–15, C 2–3 | 45–55 | Good | Low |
| High-silicon alloy | Si 15–20, C 1–2 | 40–50 | Good | Low |
| Nickel-based alloy | Ni 60–70, Cr 10–15 | 30–40 | Moderate | High |
| Composite alloy | Cr 20–25, C 3–4, Mo 2–3 | 55–65 | Excellent | Medium-High |
Welding Process Selection
The selection of welding process depends on the component geometry, overlay thickness, and production requirements:
| Process | Advantage | Limitation | Application |
|---|---|---|---|
| SMAW (stick welding) | Simple, portable, low cost | Low deposition rate, high labor cost | Field repair, small components |
| SAW (submerged arc) | High deposition rate, low cost | Limited to flat/large surfaces | Large grinding discs |
| GMAW (MIG/MAG) | Moderate deposition rate, good control | Moderate cost | Medium-sized components |
| TIG (GTAW) | Precise control, low heat input | Low deposition rate, high labor cost | Thin overlays, repair |
| PTA (plasma transfer arc) | High deposition rate, uniform composition | High equipment cost | Large-scale production |
| Laser cladding | High dilution control, high quality | High equipment cost, limited thickness | High-value components |
Overlay Welding Procedure
The typical overlay welding procedure for vertical mill rollers and grinding discs includes:
- Surface preparation: The worn surface is ground or machined to remove all damaged material and provide a clean, flat surface for welding.
- Preheating: The component is preheated to 200–300 °C to reduce residual stress and prevent cracking.
- Overlay welding: The overlay is applied in multiple layers (typically 3–5 layers) using the selected welding process. The first layer is a transition layer with a composition between the base metal and the overlay material to reduce dilution and improve bonding.
- Post-weld heat treatment: The component is heat treated at 550–650 °C for 2–4 hours to relieve residual stress and improve toughness.
- Machining and finishing: The overlay surface is machined to the required dimensions and surface finish.
- Inspection: The overlay is inspected using PT, MT, and/or UT to detect any cracks or defects.
Microstructure and Hardness of the Overlay
The microstructure of the overlay weld is critical for its wear resistance. The high-Cr high-C alloy overlay typically exhibits a microstructure consisting of:
- Matrix: Martensite or tempered martensite
- Carbides: M₇C₃, M₂₃C₆, and M₆C type carbides
- Retained austenite: 5–15% (depending on composition and cooling rate)
The hardness of the overlay is primarily determined by the volume fraction and size of the carbides. The typical hardness distribution across the overlay is:
| Depth from Surface (mm) | Hardness (HRC) |
|---|---|
| 0–2 | 55–65 |
| 2–5 | 50–60 |
| 5–10 | 45–55 |
| 10–15 | 40–50 |
| 15–20 | 35–45 |
The hardness decreases with depth due to the increasing dilution with the base metal. The transition layer at the bottom of the overlay has a lower hardness but provides adequate bonding strength and toughness.
Common Defects and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Cracking | Excessive residual stress, high carbon equivalent | Preheating, low heat input, post-weld heat treatment |
| Porosity | Moisture in flux, contaminated surface | Dry flux, surface cleaning, controlled welding environment |
| Incomplete fusion | Insufficient heat input, wrong technique | Increase current, optimize travel speed, proper technique |
| Excessive dilution | Too much base metal melted | Use transition layer, reduce heat input, multiple layers |
| Poor bonding | Contaminated surface, insufficient heat | Surface preparation, proper preheating, adequate heat input |
Engineering Practice Cases
Case 1: Vertical Mill Roller Renewal
A vertical mill roller with a worn surface was renewed using wear-resistant overlay welding. The roller was preheated to 250 °C, and a 20 mm thick high-Cr high-C alloy overlay was applied using SMAW welding in 5 layers. The overlay was post-weld heat treated at 600 °C for 3 hours. The renewed roller achieved a service life of 18 months, compared to 4 months for the original roller.
Case 2: Grinding Disc Overlay
A vertical mill grinding disc was overlaid with a 15 mm thick high-Cr high-C alloy using SAW welding. The overlay was applied in 4 layers with a transition layer at the bottom. The grinding disc was post-weld heat treated at 550 °C for 2 hours. The overlay achieved a hardness of 55–60 HRC and a service life of 15 months.
Case 3: Field Repair of Roller
A vertical mill roller was damaged during operation and required field repair. The damaged area was ground to remove all cracked material, and a 10 mm thick overlay was applied using SMAW welding. The repair was completed in 8 hours, and the roller returned to service within 24 hours.
Study Insights and Implications
The study demonstrates that wear-resistant overlay welding is a highly effective and economical method for extending the service life of vertical mill rollers and grinding discs. The key factors for successful overlay welding include:
- Proper material selection: The overlay material should be selected based on the service conditions, required hardness, and cost considerations.
- Adequate surface preparation: The worn surface must be cleaned and prepared to ensure good bonding and avoid defects.
- Controlled welding parameters: The welding parameters should be optimized to achieve the desired overlay thickness, composition, and microstructure.
- Post-weld heat treatment: PWHT is essential for relieving residual stress and improving the toughness of the overlay weld.
- Quality inspection: The overlay should be inspected for cracks and defects before returning the component to service.
The study also highlights the economic benefits of overlay welding compared to component replacement. The cost of overlay welding is typically 30–50% of the cost of a new component, and the service life extension is 3–5 times that of the original component. This makes overlay welding a highly cost-effective solution for vertical mill maintenance.
Summary
This study provides a comprehensive overview of wear-resistant overlay welding technology for vertical mill rollers and grinding discs. The application of high-Cr high-C alloy overlays using SMAW, SAW, or GMAW welding processes can extend the service life of these components by 3–5 times, significantly reducing maintenance costs and downtime. The key to successful overlay welding lies in proper material selection, surface preparation, welding parameter optimization, and post-weld heat treatment. Engineers should consider overlay welding as a preferred maintenance strategy for vertical mill components, and should develop qualified welding procedures and inspection protocols to ensure consistent quality and performance.