CLADDING TECHNOLOGY SHANXI CO., LTDCoal Mill Roller Open-Arc Weld Overlay Repair Technology
Literature Overview
The topic of coal mill roller repair through open-arc weld overlay is a classic yet ever-relevant subject in power generation maintenance engineering. Coal mill rollers, typically made of low-alloy or medium-carbon steel, endure severe abrasive wear from coal particles and moisture, leading to periodic surface degradation. This study note reviews the technical approach of using open-arc (gas-shielded) weld overlay to restore the working surface of worn coal mill rollers, covering process selection, consumable selection, weld sequence design, and quality assurance measures.
Core Technical Approach
The open-arc weld overlay method for coal mill roller repair typically employs GMAW (gas metal arc welding) or FCAW (flux-cored arc welding) with CO2 or mixed gas shielding. The key advantage of this approach over traditional stick electrode welding is higher deposition efficiency, better process stability, and superior surface quality with fewer interpass defects.
Process Parameters
| Parameter | Typical Range | Notes |
|---|---|---|
| Shielding gas | CO2 or Ar + CO2 (80/20) | CO2 is economical; mixed gas reduces spatter |
| Current (A) | 180–320 | Depends on wire diameter and roller geometry |
| Voltage (V) | 22–28 | Higher voltage increases arc length and penetration |
| Wire feed speed | 4–8 m/min | Adjusted with current for stable arc |
| Travel speed | 200–400 mm/min | Faster for surface layers; slower for buildup |
| Interpass temperature | < 150°C | Prevents grain coarsening and cracking |
| Preheat temperature | 80–150°C | Reduces residual stress and HIC risk |
Consumable Selection
The overlay consumables must balance wear resistance, impact toughness, and crack resistance. Common choices include:
- Hardfacing wires: High-carbon martensitic types (e.g., Cr-Mo-C) for severe abrasion zones
- Transition layers: Low-carbon austenitic or ferritic types to bridge the base metal and hardfacing layer
- Surface layers: Medium-hard martensitic or austenitic deposits with embedded carbide particles
A multi-layer approach is recommended: the first layer (transition) ensures metallurgical compatibility with the base steel, while subsequent layers build up the desired wear-resistant microstructure.
Process Sequence and Weld Design
The repair sequence is critical for avoiding cracking and ensuring uniform coverage. The following approach is generally followed:
- Surface preparation: Grind away all worn, damaged, and contaminated material to expose sound base metal. The preparation area should extend 3–5 mm beyond the visible wear boundary.
- Preheat: Apply uniform preheat to the roller surface to reduce thermal gradient and hydrogen-induced cracking risk.
- Undercut filling: If significant material loss exists, fill the undercut with a compatible filler before applying overlay layers.
- Overlay application: Apply overlay in multiple passes using a systematic travel pattern (zig-zag or weave) to ensure uniform coverage and avoid overlap defects.
- Post-weld treatment: Depending on service requirements, apply post-weld heat treatment (PWHT) at 550–650°C for stress relief, or quench and temper for hardfacing layers requiring high hardness.
Defect Analysis and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Cracking | High carbon equivalent, insufficient preheat, rapid cooling | Increase preheat, use low-hydrogen consumables, control interpass temperature |
| Porosity | Surface contamination, inadequate gas shielding | Thorough cleaning, ensure gas flow rate (15–25 L/min), use drag shield |
| Poor fusion | Excessive travel speed, insufficient current | Reduce travel speed, increase current, ensure proper arc length |
| Excessive dilution | Too few layers, high penetration | Use multi-layer approach, reduce penetration per layer |
| Hardness unevenness | Inconsistent travel speed or overlap | Maintain constant travel speed, overlap adjacent passes by 50% |
Engineering Practice Insights
From practical experience, several observations are worth emphasizing. First, the open-arc method offers significantly higher deposition rates compared to stick electrode welding — typically 2–3 times higher — which translates to shorter maintenance windows for coal mills. However, the process requires careful attention to gas shielding, especially in outdoor or draft-affected environments, where gas loss can lead to porosity.
Second, the selection of overlay consumables should be guided by the actual wear mechanism. For dry coal mills with predominantly abrasive wear, harder martensitic deposits (HRC 55–62) provide adequate life extension. For wet mills with combined abrasive and corrosive wear, austenitic or duplex deposits with better corrosion resistance are preferable.
Third, the dimensional accuracy of the repaired roller surface must be verified after overlay. The overlay build-up introduces additional material, and subsequent grinding is often necessary to restore the roller profile to specification. Designers should account for 3–5 mm of overlay allowance in the original roller geometry to facilitate future repairs.
Study Insights and Implications
The open-arc weld overlay repair of coal mill rollers represents a practical intersection of metallurgy, welding engineering, and maintenance economics. The technology is mature but demands careful process control to achieve reliable results. Key success factors include proper surface preparation, appropriate consumable selection matched to the wear environment, disciplined thermal management, and rigorous post-repair inspection. Engineers should always document the repair process, including consumable batch numbers, process parameters, and NDT results, to build a traceable quality record that supports long-term reliability assessment.