CLADDING TECHNOLOGY SHANXI CO., LTDOverlay Repair of Large Support Rollers in Metallurgical Equipment
Literature Overview
The 2006 paper published in Welding by Cao Ye, Zhao Qinghua, and Liu Chengmin from Shanghai Shikoudong Metallurgical Equipment Repair Co., Ltd., documents the overlay welding repair technology applied to large support rollers used in metallurgical equipment. Support rollers are critical components in rolling mills, where they bear the weight of work rolls and maintain precise roll gap control. When these rollers suffer surface damage from wear, cracking, or spalling, overlay welding repair offers an economical and efficient alternative to complete replacement, provided the repair process is properly designed and executed.
Core Technical Content
Large support rollers in metallurgical applications typically have diameters ranging from 600 mm to over 1500 mm and lengths exceeding 2000 mm. They are subjected to extreme contact stresses (up to 3–5 GPa), cyclic loading, and abrasive contact with work rolls and strip material. The surface damage patterns are typically characterized by hardening, cracking, spalling, and dimensional loss due to wear.
Material Selection for Overlay Repair
The selection of overlay material for support roller repair depends on the service conditions and the required performance characteristics. The following materials are commonly employed:
| Overlay Material | Hardness (HV) | Application Scenario | Key Advantage |
|---|---|---|---|
| Cr-based high-carbon steel (e.g., Cr12MoV equivalent) | 500–600 | General wear resistance | Cost-effective, good weldability |
| High-speed steel (e.g., M2, W6Mo5Cr4V2) | 600–800 | High contact stress applications | Excellent red hardness |
| Ductile iron (e.g., GGG40 with hardening) | 400–550 | Moderate wear conditions | Good toughness |
| Ni-based alloys (e.g., Stellite 6) | 400–450 | Corrosive and high-temperature conditions | Superior corrosion resistance |
| Co-Cr-W alloy (e.g., Stellite 21) | 450–500 | Severe abrasive conditions | Excellent wear resistance |
For the specific case documented in this research, the overlay material was selected based on the service environment of the support roller, which involved high contact stress and moderate abrasive wear. A high-carbon chromium steel overlay with hardness in the range of 550–600 HV was selected, providing an optimal balance between wear resistance, toughness, and weldability.
Repair Process Design
The overlay repair process for large support rollers involves several critical steps:
- Surface preparation: Removal of damaged material by grinding or machining to expose sound base material. The surface should be ground to a uniform finish with no residual cracks or inclusions.
- Preheating: The roller is preheated to 200–350°C to reduce thermal gradients and minimize the risk of cracking. The preheat temperature is determined by the carbon equivalent of the base material.
- Welding process selection: The selection of welding process depends on the repair geometry, material, and equipment availability.
- Layer build-up: Multiple layers may be required to achieve the specified repair geometry and hardness profile.
- Post-weld heat treatment: Stress relief annealing is performed to reduce residual stresses and optimize the hardness profile.
| Welding Process | Applicable Scenario | Typical Parameters | Advantages | Limitations |
|---|---|---|---|---|
| Submerged arc welding (SAW) | Large surface areas, high deposition rate | Current: 500–800 A, Voltage: 30–35 V | High productivity, good penetration | Limited to accessible surfaces |
| Flux-cored arc welding (FCAW) | Medium-sized repairs | Current: 300–500 A, Voltage: 30–38 V | Good deposition rate, flexible | Requires shielding gas |
| Gas metal arc welding (GMAW) | Smaller repairs, precision work | Current: 200–400 A, Voltage: 22–28 V | Good controllability, low spatter | Lower deposition rate |
| Plasma transferred arc (PTA) | High-alloy overlay layers | Current: 100–300 A, Powder feed: 0.5–2 kg/h | Excellent dilution control | High equipment cost |
| Oxy-fuel welding | Emergency field repairs | Preheating: 400–600°C | Equipment-free, portable | Low productivity, high dilution |
Defect Analysis and Countermeasures
The following table summarizes the common defects encountered during support roller overlay repair and the corresponding countermeasures:
| Defect Type | Root Cause | Detection Method | Countermeasure |
|---|---|---|---|
| Cracking (hot) | High carbon equivalent, rapid cooling | MT / PT | Increase preheat temperature, use low-hydrogen filler |
| Cracking (cold) | Hydrogen embrittlement, high residual stress | MT / UT | Post-weld stress relief, hydrogen bake-out |
| Poor bond | Surface contamination, insufficient penetration | UT / Bond test | Proper surface preparation, optimize welding parameters |
| Excessive dilution | High heat input, large weld size | Hardness mapping, metallography | Reduce heat input, use multiple thin layers |
| Hardness variation | Inconsistent cooling rate, layer thickness variation | Hardness survey | Uniform layer thickness, controlled cooling |
| Porosity | Moisture in flux, inadequate shielding | RT / UT | Dry flux storage, improve shielding gas coverage |
Engineering Practice Integration
The overlay repair of large support rollers requires careful planning and execution to ensure the repaired roller meets the required performance standards. The following practices should be implemented:
- Inspection before repair: The extent of surface damage should be thoroughly assessed using ultrasonic testing (UT) to detect subsurface cracks and magnetic particle testing (MT) to detect surface cracks. The repair area should be defined based on the inspection results.
- Procedure qualification: The welding procedure should be qualified per NB/T 47014 or equivalent standards, with mechanical property testing of the qualified specimens including hardness, tensile strength, and impact toughness.
- In-process monitoring: Welding parameters should be monitored and recorded throughout the repair process to ensure consistency and traceability.
- Post-repair inspection: The repaired area should be inspected by MT or PT for surface defects, UT for subsurface defects, and hardness survey for uniformity verification.
- Dimensional verification: The repaired roller should be checked for dimensional accuracy and surface finish using precision measuring instruments.
Key Reflections and Insights
The overlay repair of large support rollers presents unique challenges due to the combination of large component size, high residual stress potential, and demanding service conditions. In my engineering experience, the success of roller repair depends less on the welding process itself and more on the thoroughness of the pre-repair assessment and the quality of the post-repair inspection. Many repair failures are traced back to inadequate removal of damaged material or insufficient stress relief.
A particularly important consideration is the residual stress state of the base material. Large support rollers often contain significant residual stresses from the original manufacturing process (forging, machining, grinding). These stresses, combined with the thermal stresses from welding, can exceed the yield strength of the base material and lead to cracking. The preheating temperature and post-weld stress relief cycle must be carefully selected to accommodate the total stress state.
Another critical aspect is the hardness profile of the overlay layer. The hardness should be uniform across the repair area and compatible with the hardness of the original roller surface. Excessive hardness variation can lead to uneven wear patterns during service, which may result in premature failure of the repair.
Summary
The overlay repair of large support rollers is a well-established practice in metallurgical equipment maintenance, but its successful execution requires a systematic approach that encompasses thorough pre-repair assessment, careful process design, rigorous in-process control, and comprehensive post-repair inspection. The research by Cao Ye and colleagues provides practical guidance for implementing overlay repair technology on large rollers, and the findings are directly applicable to similar repair operations in the steel industry. Engineers should approach roller repair as a quality-critical operation, with the same level of rigor applied to procedure qualification, in-process monitoring, and final inspection as would be applied to new roller fabrication. The economic and environmental benefits of overlay repair — reducing material consumption, minimizing waste, and extending equipment life — make it an indispensable technology in modern metallurgical maintenance practices.