Zhuojin Pipe Fitting Co., LtdAnalysis of Quality Decline After Overlay Welding on Roller Press Machines
Literature Overview
This paper by Li Mingxu, Wu Jun, and Wang Dong, published in Cement (No. 4, 2019), investigates the phenomenon of declining production quality and throughput after overlay welding or replacement of rollers in a roller press machine used in cement production. The authors from Lu'an Conch Cement Co., Ltd. analyzed the working mechanism of the roller press, identified the root causes of the quality decline, and implemented corrective measures that partially resolved the problem. This case study is relevant to engineers working in mineral processing, cement, and other industries where roller presses are used for size reduction.
Core Technical Points
The roller press is a size reduction machine used in cement grinding circuits to crush clinker and other materials. It consists of two counter-rotating rollers that squeeze the material between them. The roller surfaces are periodically overlay welded to restore the original diameter and surface profile, or the rollers are replaced entirely. After overlay welding or replacement, the production quality and throughput often decline, which is a common but poorly understood problem in the industry.
Working Mechanism and Force Analysis
The roller press operates on the principle of high-pressure compression. The material is fed between the two rollers, which are pressed together by hydraulic or mechanical force. The key operating parameters include:
| Parameter | Typical Value | Effect on Quality |
|---|---|---|
| Roll pressure | 100–300 MPa | Higher pressure improves fineness but increases power consumption |
| Roll gap (exit) | 10–50 mm | Determines product size distribution |
| Roll speed | 0.5–2.0 m/s | Affects throughput and product size |
| Feed rate | 200–800 t/h | Must match roll pressure and speed |
| Roller diameter | 1000–2000 mm | Larger diameter improves efficiency |
After overlay welding, the roller surface profile changes due to the deposition geometry, the welding-induced distortion, and the machining tolerance. These changes affect the roll gap distribution along the roller length, which in turn affects the compression force distribution and the product size distribution.
Root Cause Analysis
The authors identified several factors contributing to the quality decline:
- Roll gap asymmetry: The overlay welding process introduces local dimensional variations along the roller length, causing the roll gap to vary from the center to the edges. This results in uneven compression and a wider product size distribution.
- Surface roughness: The overlay welded surface, even after machining, has a different surface roughness and texture than the original roller surface. This affects the friction coefficient between the roller and the material, which in turn affects the grip and the compression efficiency.
- Roller hardness distribution: The overlay material may have a different hardness profile than the original roller material, leading to uneven wear and premature degradation of the roller surface.
- Roller alignment: After overlay welding or replacement, the roller alignment may be slightly off, causing uneven loading and vibration.
Process and Standards Analysis
The overlay welding process for roller press rollers typically uses either arc welding (SMAW or GMAW) with a hardfacing alloy or thermal spraying. The key process parameters are:
| Process Parameter | Arc Welding | Thermal Spraying |
|---|---|---|
| Heat input | 20–50 kJ/cm | 10–30 kJ/cm |
| Deposition rate | 1.0–3.0 kg/h | 2.0–5.0 kg/h |
| Surface hardness | 50–65 HRC | 55–70 HRC |
| Dilution rate | 10–30% | 5–15% |
| Post-weld treatment | Stress relief at 550–600 °C | Not required |
The machining after overlay welding is critical. The roller surface must be machined to a surface roughness of Ra 1.6–3.2 μm and a dimensional tolerance of ±0.05 mm per meter of roller length. Any deviation from these tolerances will affect the roll gap and the product quality.
The relevant standards for roller press operation include ISO 14644 (cleanliness of air for roller press operation) and industry-specific guidelines from the cement manufacturing association. The quality of the roller press product is typically measured by the size distribution, the specific energy consumption, and the throughput.
Integration with Engineering Practice
In my experience with roller press operations in cement plants, the quality decline after overlay welding is a well-known problem that is often attributed to the welding process itself. However, the root cause is usually a combination of factors, including the welding process, the machining process, and the operational parameters.
The corrective measures implemented by the authors included:
- Roller profile optimization: The roller surface profile was adjusted to compensate for the expected wear pattern, using a slightly crowned profile that maintains a more uniform roll gap during operation.
- Operational parameter adjustment: The roll pressure, feed rate, and roll speed were optimized to match the new roller surface characteristics. This involved a systematic trial-and-error approach, adjusting one parameter at a time and monitoring the product quality.
- Roller alignment verification: The roller alignment was checked and corrected using laser alignment tools to ensure that the roll gap is uniform along the roller length.
The results showed that the product quality and throughput were partially restored, but not to the original level. This suggests that there are additional factors, such as the material properties of the overlay layer and the interaction between the roller surface and the material, that are not fully understood.
Key Questions and Reflections
One key question is whether the quality decline is inherent to the overlay welding process or whether it can be eliminated through better process control. The authors' results suggest that the decline is partially due to the welding and machining process, but also due to the operational parameters that were not adjusted accordingly. A systematic approach to roller press operation, including regular monitoring of the roll gap, surface roughness, and product quality, could help identify the root causes and implement corrective measures more effectively.
Another consideration is the economic trade-off between overlay welding and roller replacement. Overlay welding is significantly cheaper than replacing the rollers, but the quality decline may result in higher energy consumption and lower throughput, which may offset the savings. A life-cycle cost analysis should be conducted to determine the optimal maintenance strategy.
Summary
This paper provides a valuable case study of the quality decline phenomenon after overlay welding on roller press machines, identifying the root causes and implementing corrective measures. The key takeaway is that the quality decline is a multifactorial problem that requires a systematic approach to diagnose and correct. Engineers working with roller press operations should adopt a proactive approach to monitoring and adjusting the operational parameters after any maintenance activity, rather than assuming that the previous settings are still optimal.