CLADDING TECHNOLOGY SHANXI CO., LTDOpen Arc Overlay Repair Technology for Coal Mill Rollers
Literature Overview
This topic, authored by Cao Zhaoxia (Baotou Vocational Technical College) and Ding Zhenbo, Wang Dong (China Ordnance Industry Institute 52), published in 2009 in the Casting Technology journal, addresses the open arc overlay welding repair technology for coal mill rollers in power plant applications. Coal mill rollers are critical grinding elements in coal pulverization systems, subjected to severe abrasive wear from coal particles, moisture-induced corrosion, and cyclic mechanical loading. The open arc (SMAW or FCAW) overlay repair approach represents a practical, field-applicable solution for extending roller service life.
Coal Mill Roller Service Conditions and Wear Mechanisms
Coal mill rollers operate under the following conditions:
- Abrasive wear: Contact with coal particles containing abrasive minerals (quartz, feldspar)
- Corrosive wear: Moisture in coal causes corrosion, particularly in the presence of sulfur compounds
- Impact loading: Dynamic loading during grinding operations
- Thermal effects: Moderate temperature elevation from friction and compression
- Chemical attack: Sulfur and chloride compounds in coal can cause stress corrosion cracking
The primary wear mechanism is abrasive wear, characterized by material removal through micro-ploughing and micro-cutting by hard coal particles. Secondary mechanisms include adhesive wear at contact interfaces and fatigue wear from cyclic loading.
Overlay Material Selection for Coal Mill Rollers
The selection of overlay materials for coal mill rollers requires balancing hardness, toughness, and wear resistance. Common overlay material systems include:
| Overlay System | Composition | Hardness (HV) | Wear Mechanism Resistance |
|---|---|---|---|
| High-carbon martensitic | C 2.0-3.0%, Cr 5-8% | 600-800 | Abrasive wear |
| Chromium carbide composite | Cr 25-35%, C 2-4% | 800-1200 | Abrasive wear |
| Nickel-aluminum bronze | Ni 5-10%, Al 5-8%, Si 5-8% | 250-350 | Abrasive + corrosion |
| High-speed steel type | W 10-15%, Mo 5-8%, V 3-5% | 700-900 | Abrasive + impact |
| Cermet type | WC-Co matrix | 1000-1500 | Severe abrasive wear |
For coal mill roller applications, high-carbon martensitic and chromium carbide composite overlays are most commonly specified due to their excellent abrasive wear resistance and reasonable toughness.
Open Arc Overlay Process Parameters
The open arc overlay process for coal mill rollers typically employs the following parameters:
| Parameter | Typical Value | Notes |
|---|---|---|
| Welding process | SMAW or FCAW | Field-applicable, no shielding gas equipment required |
| Electrode type | High-carbon martensitic or Cr-C composite | Selected based on wear severity |
| Preheat temperature | 150 °C – 250 °C | Reduces cracking in high-carbon base material |
| Interpass temperature | < 250 °C | Controls hardness and reduces cracking |
| Current range | 100 A – 200 A (SMAW) | Depends on electrode diameter |
| Arc voltage | 25 V – 35 V | Controls bead geometry |
| Bead width | 20 mm – 35 mm | Overlap adjacent beads by 50% |
| Bead height | 3 mm – 5 mm | Final thickness after machining |
| Number of passes | 2 – 4 | Achieves required overlay thickness |
Defect Prevention and Quality Control
Open arc overlay welding is susceptible to several common defects that can compromise overlay performance:
- Cracking: High-carbon overlay materials are prone to cracking due to high carbon equivalent and rapid cooling. Countermeasures include preheating, controlled interpass temperature, and post-weld heat treatment.
- Porosity: Hydrogen-induced porosity can occur if the base surface is contaminated with moisture or oil. Countermeasures include thorough surface cleaning and electrode drying.
- Incomplete fusion: Poor wetting at the fusion boundary can lead to overlay spalling. Countermeasures include proper surface preparation and adequate heat input.
- Excessive dilution: Base metal dilution can reduce overlay hardness and wear resistance. Countermeasures include using low-dilution electrode configurations and maintaining appropriate travel speed.
Repair Procedure and Engineering Practice
The typical repair procedure for coal mill rollers includes:
- Inspection and assessment: Evaluate the extent of wear and damage; determine the required overlay thickness
- Surface preparation: Remove worn material by grinding or machining; clean and prepare the surface for welding
- Preheat: Apply uniform preheat to the roller surface using oxy-acetylene torch or induction heating
- Overlay welding: Apply the overlay in multiple passes, maintaining controlled interpass temperature
- Post-weld treatment: Allow controlled cooling or apply post-weld heat treatment to reduce residual stresses
- Machining: Machine the overlay surface to restore the original roller profile and dimensional tolerances
- Inspection: Perform non-destructive testing (MT or PT) to verify overlay integrity; measure hardness and thickness
Performance Evaluation
The effectiveness of open arc overlay repair for coal mill rollers can be evaluated through:
| Evaluation Metric | Acceptance Criteria | Test Method |
|---|---|---|
| Overlay hardness | ≥ 500 HV (for martensitic) | Vickers hardness test |
| Overlay thickness | ≥ 3 mm after machining | Ultrasonic thickness measurement |
| Bond strength | ≥ 90% of overlay tensile strength | Bond strength test (per API 934) |
| Crack-free surface | No cracks detected | MT or PT inspection |
| Service life extension | ≥ 2× unclad roller life | Field performance tracking |
Study Insights
The open arc overlay repair technology for coal mill rollers represents a practical, cost-effective solution for extending the service life of critical power plant components. The key advantage of this approach is its field applicability—standard SMAW or FCAW equipment can be deployed at the mill site without requiring specialized machinery or controlled environments. Engineers should note that the success of open arc overlay repair depends heavily on proper process discipline, including surface preparation, preheat control, and post-weld treatment. The use of high-carbon martensitic or chromium carbide composite overlay materials provides excellent abrasive wear resistance, but requires careful management of cracking susceptibility through appropriate heat treatment and process parameter control.