Zhuojin Pipe Fitting Co., LtdOnline Hardfacing Repair of Roller Press Rollers and Process Adjustment
Literature Overview
This 2014 paper by Liu Minghong from Hunan Chenxi Huazhong Cement Co., Ltd., published in Cement (Vol. 12, pp. 36–37), documents the practical implementation of online hardfacing repair for roller press rollers in cement production. The paper is classified under TG455 and provides a case study of the CLF140/65 roller press, detailing the transition from offline to online repair methodology and the associated process adjustments.
Core Technical Content
Roller Press Background
The CLF140/65 roller press is a high-pressure grinding mill used in cement production for size reduction of raw materials and clinker. Key specifications include:
| Parameter | Value |
|---|---|
| Roller Diameter | 1400 mm |
| Roller Width | 650 mm |
| Maximum Pressure | ~800–1200 tons |
| Throughput | 120–180 t/h (depending on material) |
| Application | Raw mill or finish mill circuit |
The roller surface is subjected to extreme conditions:
- High contact pressure from the grinding force
- Abrasive wear from hard particles in the material
- Thermal loading from friction and compressed material
- Continuous operation with minimal downtime tolerance
Offline vs. Online Hardfacing
| Aspect | Offline Hardfacing | Online Hardfacing |
|---|---|---|
| Equipment Removal | Roller removed from mill | Roller remains in mill |
| Downtime | 3–7 days | 12–24 hours |
| Cost | Lower labor cost | Higher labor cost (shift work) |
| Safety | Standard workshop conditions | Confined space, elevated platform |
| Equipment Access | Full access | Limited access |
| Quality Control | Full inspection possible | Limited inspection |
| Production Impact | Complete shutdown | Partial shutdown |
Repair Process Description
The online hardfacing repair process involves the following steps:
- Surface Preparation: The worn roller surface is prepared by air gouging (gas cutting with compressed air) to remove the damaged overlay material and expose sound base metal.
- Cleaning: The prepared surface is cleaned of oxide, scale, and contaminants using wire brushing and solvent cleaning.
- Preheating: The roller is preheated to 200–300°C using portable heating equipment to prevent cold cracking.
- Transition Layer Application: A transition layer of compatible alloy is deposited to ensure metallurgical bonding between the base metal and the final wear layer.
- Wear Layer Application: The final wear-resistant overlay is deposited in multiple passes to achieve the required thickness (typically 15–30 mm).
- Stress Relief: Post-weld stress relief is performed using portable induction heating or gas heating to reduce residual stresses.
- Surface Finishing: The roller surface is machined or ground to achieve the required surface profile and roughness.
Process Adjustments for Online Repair
The paper identifies several process adjustments necessary for online repair:
| Parameter | Offline (Standard) | Online (Adjusted) |
|---|---|---|
| Welding Current | 200–300 A | 180–260 A (reduced due to access limitations) |
| Travel Speed | Normal | Reduced (to compensate for limited access) |
| Weld Bead Width | 20–30 mm | 15–25 mm (narrower due to space constraints) |
| Number of Layers | 3–4 | 4–6 (more layers, thinner beads) |
| Interpass Temperature | ≤ 350°C | ≤ 300°C (stricter control due to continuous operation) |
| Welding Position | Flat/vertical | All positions (including overhead) |
Engineering Practice Analysis
Safety Considerations for Online Repair
Online hardfacing repair presents unique safety challenges:
- Confined space entry and egress
- Working at elevated heights on roller surfaces
- Exposure to hot surfaces and molten metal
- Risk of roller rotation during welding
- Inadequate lighting and ventilation in the mill housing
A comprehensive safety management plan following the PDCA (Plan-Do-Check-Act) framework should be implemented:
- Plan: Risk assessment, permit to work, safety equipment preparation
- Do: Execute the repair plan with continuous monitoring
- Check: Inspect weld quality, monitor for defects, verify safety compliance
- Act: Address any issues, update procedures for future repairs
Quality Control Approach
Given the limitations of online repair, quality control must be adapted:
| QC Method | Offline | Online |
|---|---|---|
| Visual Inspection | Full coverage | Limited coverage |
| Magnetic Particle Testing (MT) | Full surface | Critical areas only |
| Ultrasonic Testing (UT) | Full surface | Selected areas |
| Hardness Testing | Multiple points | Limited points |
| Dimensional Check | Full profile | Critical dimensions only |
Economic Analysis
| Cost Factor | Offline Repair | Online Repair |
|---|---|---|
| Labor Cost | Lower (standard shifts) | Higher (overtime, shift premiums) |
| Equipment Cost | Lower (standard equipment) | Higher (portable equipment) |
| Production Loss | High (complete shutdown) | Low (partial shutdown) |
| Total Cost | Variable | Often comparable or lower |
The decision between offline and online repair should be based on a comprehensive cost-benefit analysis that includes production losses, which often dominate the total cost.
Study Insights and Reflections
This paper provides a practical, field-oriented perspective on roller press maintenance that is valuable for engineers working in cement production. The transition from offline to online repair represents a significant operational improvement, reducing downtime from days to hours and maintaining production continuity.
From my experience with heavy equipment maintenance, the key success factors for online hardfacing repair are:
- Preparation quality: The surface preparation step is critical, and air gouging must be performed with precision to avoid damaging the base metal or creating excessive roughness.
- Welding procedure qualification: A qualified welding procedure specification (WPS) should be developed and tested before the first online repair, and the procedure should be qualified in all welding positions.
- Welder skill: Online repair requires highly skilled welders who can perform in confined spaces, in all positions, and under time pressure.
- Post-repair monitoring: The repaired roller should be monitored closely during the first 24–48 hours of operation to detect any defects early.
The paper also highlights an important maintenance philosophy: the shift from reactive to proactive maintenance. By developing online repair capabilities, the plant can respond to wear conditions more flexibly, scheduling repairs during planned maintenance windows rather than waiting for catastrophic failure.
One area for future improvement is the development of automated or semi-automated online repair systems that can maintain consistent weld quality while reducing the physical demands on welders. Additionally, the integration of real-time monitoring systems (such as acoustic emission or thermographic monitoring) during welding could enhance quality control and defect detection.
In conclusion, this paper demonstrates the practical viability of online hardfacing repair for critical cement production equipment, providing a valuable reference for engineers seeking to minimize downtime while maintaining equipment reliability.