CLADDING TECHNOLOGY SHANXI CO., LTDCladding Repair of High Chromium Iron Grinding Rolls
Literature Overview
This 1996 paper by Yu Fuxiang, Zhou Haifeng, and Lu Jianqing from the Shanghai Electric Power Construction Research Institute and Wuxi Roll Repair Factory, published in Welding Technology, addresses the critical challenge of repairing high chromium iron grinding rolls used in coal milling systems at power plants. These rolls are subjected to severe abrasive wear from coal particles and must be periodically restored to maintain proper grinding efficiency. The study presents a comprehensive repair methodology including surface preparation, cladding material selection, process parameters, and quality verification.
Service Conditions and Failure Analysis
High chromium iron grinding rolls in coal mills operate under the following conditions:
| Parameter | Typical Value | Impact on Wear |
|---|---|---|
| Coal particle size | 0-50 mm | Abrasive severity |
| Roll surface speed | 10-15 m/s | High sliding velocity |
| Contact pressure | 50-100 MPa | Subsurface fatigue |
| Temperature | 80-200°C | Moderate thermal load |
| Environment | Dry with fine dust | Secondary abrasion |
| Service life (unrepaired) | 8,000-15,000 hours | Economic replacement threshold |
| Original roll hardness | 58-65 HRC | High chromium iron |
| Wear rate | 0.02-0.05 mm/1000 hours | Progressive diameter loss |
The primary failure mechanism was identified as three-body abrasion, where hard coal particles (primarily quartz and pyrite) are pressed into the roll surface and plough grooves through the material. The original high chromium iron roll material (typically 14-18% Cr, 2.5-3.5% C) provided adequate initial hardness but could not be economically replaced at the required frequency, necessitating cladding repair.
Cladding Material Selection and Design
The authors evaluated multiple overlay material systems for the repair application, considering the requirement to match or exceed the original roll hardness while ensuring adequate bond strength to the worn base material.
Overlay Material Comparison
| Material | Composition | Hardness (HRC) | Bond Strength | Application Suitability |
|---|---|---|---|---|
| High Cr iron (D207) | Cr 26-30, C 2.5-3.5 | 58-62 | Good | General repair |
| High Cr high C (D407) | Cr 20-25, C 3.0-4.0 | 60-65 | Moderate | Heavy wear areas |
| Co-based (Stellite 6) | Co 60, Cr 28, W 7 | 42-46 | Excellent | Critical applications |
| Ni-Cr-B-Si | Ni 55, Cr 25, B 6, Si 5 | 40-44 | Excellent | High temperature |
| Layered Cr/Co | Alternating layers | 50-62 | Good | Premium repair |
The recommended approach for routine repair was a two-layer system: a D207 transition layer (1-2 mm) followed by a D407 hard facing layer (3-5 mm). This layered approach combined the good weldability of the D207 material with the superior hardness of the D407, while the transition layer ensured adequate bond strength despite the high dilution potential of the base material.
Welding Process Parameters (SAW with Ceramic Flux)
- Flux type: Ceramic-type (HJ431 or equivalent)
- Filler wire: H08Cr28SiMn or H10Cr30 (for D207 layer)
- Current: 450-600 A (DC, electrode negative)
- Voltage: 30-35 V
- Travel speed: 300-500 mm/min
- Wire diameter: 2.0-2.5 mm
- Preheating: 150-200°C (uniform around circumference)
- Inter-pass temperature: 200-300°C
- Post-weld cooling: controlled (no water quenching)
- Number of passes: 3-5 total
Surface Preparation and Quality Control
Surface preparation was identified as the most critical factor in repair success. The worn roll surface required thorough cleaning to remove loose material, oxidation, and embedded coal particles. The recommended sequence was:
- Shot blasting to remove loose material and create a surface profile of 3-5 micrometers Ra
- Visual inspection for cracks, porosity, or delamination in the remaining base material
- Magnetic particle testing of the original roll surface to detect subsurface cracks
- Grinding of severely worn areas to establish a sound base surface
- Preheating to 150-200°C uniformly around the entire roll circumference
Post-cladding quality verification included:
| Test Method | Acceptance Criteria | Frequency |
|---|---|---|
| Hardness (Rockwell C) | 58-65 HRC uniform | Every 500 mm along roll |
| UT (contact, 2.5 MHz) | No cracks or lack of fusion | Full coverage |
| Dimensional check | Roundness < 0.05 mm | Full length |
| Visual inspection | No spatter, undercut, or porosity | Full surface |
| Peel test (witness coupon) | Bond strength > 350 MPa | Per repair batch |
Study Insights and Reflections
This paper provides an excellent example of systematic repair engineering methodology. The authors' approach of combining transition and hard facing layers demonstrates sophisticated understanding of metallurgical compatibility requirements. The emphasis on surface preparation as the primary determinant of repair success is particularly valuable, as it addresses a common source of field repair failures.
The selection of ceramic-type flux for submerged arc cladding of high chromium iron is noteworthy. Ceramic fluxes provide superior slag protection and promote a more controlled solidification rate compared to granular fluxes, resulting in finer microstructure and reduced cracking tendency in high-carbon, high-chromium deposits. This represents practical metallurgical knowledge that has direct applicability to similar repair operations in other industries.
The economic analysis implicit in the paper is compelling: the cost of cladding repair (approximately 15-25% of new roll cost) combined with the ability to restore full service life makes this approach overwhelmingly more economical than replacement, particularly for large-diameter grinding rolls where new roll costs can exceed hundreds of thousands of dollars.