CLADDING TECHNOLOGY SHANXI CO., LTDNew Method for Online Overlay Welding on MLS3726 Vertical Mill Rollers
Introduction to MLS Vertical Mill Technology
The MLS (Mitsubishi Heavy Industries / Lurgi / Siemens) series of vertical roller mills are widely used in cement grinding, coal grinding, and slag grinding applications. The MLS3726 model represents a large-capacity vertical mill with a grinding table diameter of 3700 mm and a rated capacity of approximately 260 tons per hour for cement grinding. The grinding rollers, typically 4 in number, are subjected to severe abrasion from the grinding material and require periodic overlay welding to restore surface integrity.
The "new method" for online overlay welding on MLS3726 rollers refers to an innovative approach that combines selective laser melting (SLM) or plasma transferred arc (PTA) powder cladding with conventional arc welding techniques, enabling faster repair with improved surface quality and reduced dilution.
Technical Challenges Specific to MLS3726 Rollers
The MLS3726 vertical mill rollers present unique challenges for overlay welding:
- Large diameter and weight: Each roller has a diameter of approximately 1600–1800 mm and weighs 15–25 tons, making handling and positioning during welding difficult.
- Curved surface geometry: The roller surface is not flat but has a slight curvature (typically 1–2 mm crown over the full length), which complicates the welding sequence and requires careful planning to maintain uniform overlay thickness.
- Hydraulic system integration: The rollers are connected to hydraulic cylinders that apply grinding pressure. Online welding must not damage or contaminate these hydraulic components.
- High operating temperature: During operation, the roller surface temperature can reach 80–120°C due to frictional heating, which affects the residual stress state of the base metal.
The New Overlay Welding Method
The new method integrates multiple techniques to achieve faster, higher-quality overlay welding on MLS3726 rollers:
Step 1: Advanced Surface Preparation
Instead of conventional grinding, the new method employs robotic wire brushing combined with chemical cleaning to prepare the roller surface. This reduces preparation time by 30–40% and produces a cleaner surface with better adhesion.
Step 2: Hybrid Process Application
The hybrid approach uses:
- PTA powder cladding for the first 2–3 passes to create a low-dilution transition layer. PTA offers dilution rates as low as 5–15%, compared to 30–50% for conventional arc welding.
- SAW or FCAW for the remaining overlay passes to achieve high deposition rates and full coverage.
Step 3: Real-Time Process Monitoring
The new method incorporates real-time monitoring of welding parameters, including arc voltage, current, travel speed, and wire feed rate. This allows immediate correction of process deviations and ensures consistent overlay quality.
Step 4: Post-Weld Stress Relief
A controlled cooling process is applied using insulated blankets and portable heating elements. The cooling rate is maintained at less than 50°C per hour to minimize residual stress and prevent cracking.
Process Parameters for the New Method
| Process | PTA (Transition Layer) | SAW (Overlay Layer) |
|---|---|---|
| Powder/wire feed rate | 0.5–1.5 kg/h | 15–25 kg/h |
| Current | 200–350 A | 400–700 A |
| Voltage | 20–30 V | 25–35 V |
| Travel speed | 50–150 mm/min | 200–400 mm/min |
| Dilution rate | 5–15% | 30–50% |
| Typical layer thickness | 0.5–1.0 mm per pass | 3–5 mm per pass |
| Total overlay thickness | 2–3 mm | 5–8 mm |
Material Selection for MLS3726 Rollers
The overlay material selection for MLS3726 rollers depends on the grinding material being processed:
- Cement grinding: Cr-Cr2C6 type overlay with hardness HRC 55–60, providing good abrasion resistance against clinker particles.
- Coal grinding: Ni-Cr-Mo type overlay with hardness HRC 45–55, offering good toughness to resist impact from coal chunks.
- Slag grinding: W-C type overlay with hardness HRC 60–70, for extreme abrasion resistance against hard slag particles.
The base metal of MLS3726 rollers is typically a low-alloy steel (such as Q345 or 42CrMo) with a hardness of HB 180–220. The transition layer should be selected to match the thermal expansion coefficient of both the base metal and the overlay material to minimize thermal stress during operation.
Quality Control and Inspection
The new method incorporates rigorous quality control measures:
- Pre-weld inspection: Visual examination and magnetic particle testing (MT) of the prepared surface to detect cracks or defects.
- In-process monitoring: Real-time tracking of welding parameters and visual inspection of each pass.
- Post-weld testing:
- Hardness measurement at 5 locations per roll (minimum HRC 50 for cement grinding applications)
- Bond strength test per ASTM G143 (minimum 200 MPa)
- Ultrasonic thickness measurement at 10 points per roll
- Surface profile verification using a coordinate measuring machine (CMM) or laser scanner
Comparison with Traditional Method
| Aspect | Traditional Method | New Method |
|---|---|---|
| Welding time per roll | 80–120 hours | 50–70 hours |
| Overlay thickness uniformity | ±1.0 mm | ±0.5 mm |
| Dilution rate | 30–50% | 10–25% (PTA transition layer) |
| Defect rate | 5–10% | 2–5% |
| Downtime per mill | 5–7 days | 3–4 days |
| Cost per roll | 150,000–200,000 RMB | 120,000–160,000 RMB |
Engineering Practice Considerations
The implementation of the new method requires several practical considerations:
- Equipment investment: PTA equipment is significantly more expensive than conventional welding equipment, with a typical investment of 500,000–1,000,000 RMB. However, the payback period is typically 12–18 months for a plant with multiple vertical mills.
- Operator training: PTA welding requires specialized training, typically 2–4 weeks for basic competency. The operator must understand powder feed rate control, torch angle adjustment, and parameter optimization.
- Powder supply: PTA powder must be stored in a dry environment (humidity < 40%) and used within its shelf life (typically 12–24 months). Contaminated powder can lead to porosity and reduced hardness.
- Safety considerations: PTA welding generates significant fumes and UV radiation. Adequate ventilation and personal protective equipment are essential.
Study Reflection and Key Takeaways
The study of the new overlay welding method for MLS3726 vertical mill rollers demonstrates that hybrid process approaches can significantly improve both the quality and efficiency of roller repair. The integration of PTA powder cladding for the transition layer addresses the critical issue of dilution, which is the primary cause of overlay failure in conventional arc welding. The real-time process monitoring capability enables consistent quality control, reducing the need for rework and improving first-time quality rates. Engineers adopting this method should focus on developing robust procedures for surface preparation, parameter optimization, and quality verification. The economic benefits are substantial, with reduced downtime and extended roller life providing a strong return on investment. Future developments may include the use of advanced powders with tailored microstructures for specific grinding applications, and the integration of robotic systems for fully automated roller repair.