CLADDING TECHNOLOGY SHANXI CO., LTDOpen Arc Weld Overlay Repair Technology for Coal Mill Rollers
Literature Overview
This technical paper, published in Casting Technology (2009), presents an open arc welding overlay repair methodology for coal mill rollers developed by Cao Zhaoxia and Ding Zhenbo from Baotou Vocational and Technical College and Wang Dong from China Ordnance Industry Institute 52. The research addresses the widespread problem of wear and damage to coal mill rollers in power generation and cement production facilities, where coal grinding efficiency directly impacts overall plant productivity and fuel cost.
Core Technical Content
Coal mill rollers operate under uniquely demanding conditions involving simultaneous abrasive wear from coal and stone particles, impact loading from coal fragments, and thermal cycling from the hot flue gas environment. The working surface of the roller must maintain precise geometry to ensure uniform grinding pressure across the coal bed. Conventional repair methods involving replacement of entire rollers or boring and regrinding without overlay protection result in excessive material removal and shortened component life.
Open Arc Welding Process Characteristics
The open arc welding process (also referred to as open flame welding or semi-automatic arc welding in some contexts) offers several advantages for coal mill roller repair:
- Flexibility: Suitable for on-site repair without requiring specialized equipment or clean room conditions
- Adaptability: Can be applied to various roller geometries and damage patterns
- Cost-effectiveness: Lower equipment investment compared to fully automated or robotic systems
- Accessibility: Can be performed by trained welders in maintenance workshops
The process typically employs a submerged arc welding (SAW) or gas shielded metal arc welding (GMAW) configuration with the arc exposed to the atmosphere, requiring careful control of shielding gas flow to prevent contamination.
Consumable Selection and Microstructure
The overlay consumable selected for coal mill roller applications is typically a high-chromium white cast iron alloy or a high-chromium high-molybdenum martensitic stainless steel. The microstructure of the deposited layer consists of:
| Phase | Composition | Hardness (HV) | Function |
|---|---|---|---|
| Chromium carbides | Cr7C3, Cr23C6 | 1400-1800 | Primary wear resistance mechanism |
| Martensitic matrix | Fe-Cr-Mo solid solution | 600-800 | Supports carbide particles |
| Ferrite (minor) | BCC iron | 200-300 | Provides toughness |
The target hardness of the overlay layer is 55-65 HRC (approximately 600-700 HV), providing sufficient resistance to abrasive wear while maintaining adequate fracture toughness to withstand impact loading.
Process Development and Optimization
Surface Preparation Protocol
- Removal of the worn layer by machining or grinding to expose sound base material
- Cleaning of the surface by grinding to bare metal, removing all oxide scale, rust, and contaminants
- Application of a groove profile (typically a shallow V-groove or U-groove) to improve mechanical bonding
- Preheating to 150-250°C to reduce thermal gradient and prevent cracking
- Application of a transition layer (if required) to reduce dilution and improve metallurgical compatibility
Welding Sequence and Parameters
The overlay is deposited in multiple passes following a systematic sequence designed to minimize residual stress and ensure uniform layer composition:
- First pass: Low heat input, reduced current (150-200 A) to establish a thin, well-bonded interface layer
- Intermediate passes: Moderate heat input (200-280 A) to build layer thickness
- Final pass: Controlled heat input with careful arc manipulation to achieve a smooth, uniform surface profile
The welding speed is maintained at 0.15-0.30 m/min depending on the layer thickness required. The arc length is kept short (2-3 mm) to minimize atmospheric contamination and spatter. The interpass temperature is monitored and maintained below 300°C to prevent softening of previously deposited layers.
Defect Analysis and Countermeasures
| Defect Type | Cause | Detection Method | Countermeasure |
|---|---|---|---|
| Cracking (transverse) | Excessive residual stress, high carbon equivalent | MT or PT | Preheating, low heat input, post-weld stress relief |
| Cracking (longitudinal) | Hydrogen pickup, hydrogen-induced cracking | UT or MT | Low-hydrogen consumables, post-weld baking |
| Porosity | Gas pickup from atmosphere or surface contamination | RT or UT | Improved shielding, surface cleaning, dry consumables |
| Undercut | Excessive current or speed, improper electrode angle | Visual inspection | Parameter optimization, electrode angle correction |
| Excessive dilution | High heat input, insufficient preheat groove | Hardness testing | Reduced current, increased speed, groove preparation |
| Spalling | High residual stress, poor interface bonding | Impact testing | Stress relief, improved surface prep, transition layer |
Engineering Practice Integration
In power plant applications, coal mill roller repair represents a critical maintenance activity with significant economic implications. A typical coal mill roller in a 300 MW power plant can weigh 3-5 tons and cost several tens of thousands of dollars for replacement. The overlay repair technology allows the roller to be restored to service specification at a fraction of the replacement cost, typically reducing maintenance expenditure by 60-80% per repair cycle.
Field experience has demonstrated that properly applied overlay layers on coal mill rollers can extend service life from 3-6 months to 12-18 months under standard operating conditions. The key to achieving this performance is not merely the hardness of the overlay but the quality of the interface between the overlay and the base material. A well-bonded interface prevents spalling under cyclic loading, while the overlay itself resists progressive material removal through abrasion.
The open arc welding approach is particularly advantageous for plant maintenance because it can be performed in the maintenance workshop without requiring the roller to be transported to a specialized cladding facility. This reduces logistics costs and minimizes downtime for the grinding mill. Training programs for maintenance welders should include practical exercises on sample coupons to develop the skill of maintaining consistent bead geometry and surface quality.
Key Reflections and Study Insights
This research highlights the practical engineering approach to solving maintenance challenges through appropriate technology selection. The open arc welding process may not produce the most refined microstructure or the lowest dilution rates, but it delivers the required performance at a cost and complexity level appropriate for the industrial context. Engineers must resist the temptation to apply the most advanced technology to every problem and instead select the optimal solution based on the specific requirements, constraints, and economic realities of each application.
The study also reinforces the importance of consumable qualification and process standardization. Without rigorous qualification testing and well-documented procedures, the performance of overlay welding cannot be reliably predicted or reproduced. The collaboration between vocational education institutions and research institutes in developing this technology model represents a productive approach to technology transfer and workforce development in the manufacturing sector.