CLADDING TECHNOLOGY SHANXI CO., LTDOnline Overlay Welding Technology for ATOX Raw Material Vertical Mill Rollers
Literature Overview and Technical Context
The 2017 publication by researchers from China United Cement Group Nanyang Branch and Sichuan University addresses the application of online overlay welding technology for repairing and enhancing the performance of rollers in ATOX raw material vertical mills. ATOX vertical mills are a type of cement grinding equipment that has gained widespread adoption in the cement industry due to their high grinding efficiency, low energy consumption, and ability to handle a wide range of raw materials. The rollers in these mills are subjected to severe abrasive wear from the grinding of raw materials such as limestone, clay, and shale, which contain abrasive minerals such as quartz and feldspar.
The "online" aspect of the overlay welding technology is particularly significant. Unlike conventional offline repair methods that require the roller to be removed from the mill, transported to a workshop, and then reinstalled, online overlay welding allows the repair to be performed in situ while the roller remains in its installed position. This approach dramatically reduces downtime and associated production losses, which can be substantial in a cement plant where a single mill may produce several hundred tons of raw meal per hour.
Core Technical Analysis
The ATOX vertical mill rollers are typically made of medium carbon steel or low-alloy steel with a diameter ranging from 1000 to 2500 mm and a length of 2000 to 4000 mm. The working surface of the roller is equipped with a wear-resistant alloy layer, which is periodically worn away by the grinding action and must be restored. The overlay welding technology described in this paper uses a combination of flux-cored arc welding (FCAW) and submerged arc welding (SAW) to deposit a wear-resistant alloy layer on the roller surface.
The key technical challenges in online overlay welding include:
- Access and positioning: The roller is located inside the vertical mill housing, which limits access for welding equipment and operator positioning. Specialized welding rigs and positioning fixtures must be designed to accommodate the confined space.
- Thermal management: The roller may still be at an elevated temperature from recent operation, and the surrounding environment may contain dust and debris. These conditions can affect the welding process and the quality of the overlay layer.
- Surface preparation: The worn roller surface must be cleaned and prepared for overlay welding. This typically involves grinding or machining to remove the worn layer and any surface contamination. The surface must be clean and free of oil, grease, and dust to ensure proper fusion.
- Dilution control: The overlay layer must have sufficient hardness and wear resistance, but excessive dilution from the base metal can reduce these properties. Backing rings and multi-pass strategies are used to control dilution and ensure a high-quality overlay.
| Parameter | Specification | Rationale |
|---|---|---|
| Base material | 45 steel or 42CrMo | Structural strength |
| Overlay material | High-chromium cast iron or high-speed steel alloy | Wear resistance |
| Overlay thickness | 5-15 mm | Restore worn surface and provide wear reserve |
| Welding process | FCAW or SAW | Deep penetration, good slag protection |
| Preheat temperature | 150-250°C | Reduce residual stress |
| Interpass temperature | 100-200°C | Maintain ductility |
| Post-weld treatment | Stress relief at 550-650°C | Relieve residual stresses |
Process Design and Implementation
The online overlay welding process for ATOX vertical mill rollers involves several critical steps:
- Pre-inspection and assessment: Before overlay welding, the roller surface must be inspected to assess the extent of wear and identify any cracks or other defects. Ultrasonic testing (UT) and visual inspection are used to evaluate the condition of the roller. If cracks are found, they must be repaired before overlay welding.
- Surface preparation: The worn surface is ground or machined to a depth of 2 to 5 mm to remove the worn layer and any surface contamination. The surface is then cleaned with a wire brush or grit blasting to ensure proper fusion.
- Backing ring installation: A backing ring is installed around the roller circumference to prevent burn-through and ensure proper fusion at the overlay-base metal interface. The backing ring is typically made of the same material as the base roller and is installed with a gap of 0.5 to 1.0 mm.
- Overlay welding: The overlay layer is applied in multiple passes, with each pass building up the required thickness. The welding process is carefully controlled to minimize heat input and prevent distortion of the roller.
- Post-weld machining: After overlay welding, the roller surface is machined to the final dimensions and surface finish. The surface roughness should be Ra 6.3 to 12.5 micrometers to ensure proper grinding action in the mill.
Defect Prevention and Quality Control
The online overlay welding of ATOX vertical mill rollers is susceptible to several characteristic defects:
| Defect Type | Cause | Detection Method | Prevention |
|---|---|---|---|
| Cracking | Excessive cooling rate, hydrogen embrittlement | MT, PT | Preheat, low-hydrogen flux, post-weld stress relief |
| Incomplete fusion | Poor surface preparation, insufficient heat input | UT, RT | Proper cleaning, adequate current |
| Porosity | Flux contamination, moisture | RT, UT | Flux drying, controlled welding environment |
| Excessive dilution | Too high current, too fast travel speed | Metallography, hardness testing | Parameter optimization, backing ring use |
| Distortion | Excessive heat input, asymmetric welding | Dimensional inspection | Multi-pass strategy, balanced welding sequence |
The hardness profile of the overlay layer is a critical quality indicator. For high-chromium cast iron overlays, the hardness should be in the range of 55 to 65 HRC. For high-speed steel-based overlays, the hardness is typically 60 to 65 HRC. The hardness should be measured at multiple locations across the overlay width and depth to ensure uniformity.
Engineering Practice and Performance Evaluation
The implementation of online overlay welding on ATOX vertical mill rollers has demonstrated significant improvements in service life and cost savings. Based on the experience reported in the literature and subsequent industry practice:
- Without overlay: Typical service life of roller surface is 6 to 12 months, with frequent roller replacements required due to excessive wear.
- With overlay: Service life extends to 18 to 36 months, representing a 2 to 3 times improvement.
The economic benefit is substantial given the cost of roller replacement, including downtime for the vertical mill. A typical cement plant may have 3 to 6 vertical mills, and each mill may have 2 to 4 rollers. The cost of roller replacement, including downtime, can exceed several hundred thousand yuan per event.
The online overlay welding technology described in this 2017 publication has since been refined and expanded. Modern practices incorporate additional measures such as:
- Robotic welding: Using robotic welding systems to ensure consistent weld quality and reduce operator fatigue in confined spaces.
- Laser cladding: Using laser cladding for the final pass to achieve a finer microstructure and better surface quality.
- In-situ monitoring: Implementing real-time monitoring of welding parameters and thermal profiles to ensure consistent quality.
Study Insights and Reflections
The 2017 publication from China United Cement Group and Sichuan University represents a practical application of overlay welding technology to a critical component in the cement industry. The "online" aspect of the technology is particularly innovative, as it addresses the economic and operational challenges of roller repair in a way that minimizes downtime and maximizes production continuity. The systematic approach to process development—starting with material selection, moving through process design, and concluding with performance evaluation—provides a model for overlay welding implementation in other industrial applications.
For engineers working in the cement industry, this literature serves as a valuable reference for understanding the specific challenges and solutions associated with overlay welding of vertical mill rollers. The technology has since been refined and expanded, but the fundamental principles remain the same: careful material selection, controlled process parameters, and rigorous quality control are essential for successful overlay welding applications. The online approach demonstrated in this paper is particularly noteworthy for its potential to be applied to other large industrial components where downtime is a critical concern.