Zhuojin Pipe Fitting Co., LtdSurfacing Technology Application on Hot-Rolled Backup Rolls
Literature Overview and Context
The technical report by Ge Baowen, published in Baosteel Technology (2004, No. 1), documents the successful application of Cr-Mo series welding consumables for surfacing hot-rolled backup rolls at Baosteel. This work addresses a critical operational challenge in steel rolling mills: the limited service life of backup rolls due to wear, thermal fatigue, and mechanical damage. The report provides practical guidance on welding material selection and surfacing process implementation for a specific industrial application, making it highly relevant to engineers in the steel industry and related heavy manufacturing sectors.
The Challenge of Backup Roll Wear
Hot-rolled backup rolls are subjected to extreme operating conditions:
- High contact pressure: The rolling force transmitted through the work rolls creates extremely high contact stresses at the backup roll surface.
- Thermal cycling: Repeated exposure to hot strip temperatures (800–1200°C) followed by cooling causes thermal fatigue cracking.
- Abrasive wear: Scale and oxide particles from the rolled stock abrade the roll surface.
- Impact loading: Occasional contact with misaligned or damaged work rolls causes localized impact damage.
The combination of these factors results in surface degradation that necessitates frequent grinding and resurfacing. Without proper surfacing treatment, backup rolls may require replacement after only a few passes, leading to significant downtime and cost.
Cr-Mo Series Welding Material Selection
The authors selected Cr-Mo series welding materials based on the following considerations:
| Property Requirement | Cr-Mo Surfacing Benefit |
|---|---|
| High hardness | Cr and Mo provide solid solution and carbide strengthening |
| Wear resistance | Fine carbide distribution resists abrasive wear |
| Thermal fatigue resistance | Alloy content improves high-temperature strength |
| Machinability | Moderate hardness allows post-weld grinding |
| Dilution resistance | Alloy content maintained despite base metal dilution |
Typical Cr-Mo Surfacing Composition
| Element | Typical Range (wt%) | Function |
|---|---|---|
| Cr | 4–8 | Solid solution strengthening, carbide formation |
| Mo | 2–5 | High-temperature strength, carbide formation |
| C | 0.3–0.8 | Carbide formation, hardness |
| Mn | 1–2 | Deoxidation, solid solution strengthening |
| Si | 0.5–1.5 | Deoxidation, fluidity control |
The Cr-Mo system offers a good balance between hardness, toughness, and thermal stability. Unlike high-chromium cast irons or cobalt-based alloys, Cr-Mo surfacing layers maintain adequate toughness to resist thermal fatigue cracking while providing sufficient hardness for wear resistance.
Process Implementation and Results
The surfacing process for backup rolls typically involves:
- Surface preparation: Grinding or machining to remove damaged material and create a clean surface for welding.
- Preheating: Moderate preheat (100–200°C) to reduce residual stresses and prevent cracking.
- Surfacing deposition: Multiple passes of Cr-Mo welding material using SMAW or SAW processes.
- Post-weld treatment: Stress relief or tempering to reduce residual stresses.
- Machining: Grinding to final dimensions and surface finish.
Production Results
The study reports that the Cr-Mo surfacing application:
- Significantly extended the service life of backup rolls.
- Improved roll surface quality and dimensional accuracy.
- Reduced the frequency of roll replacement and grinding operations.
- Provided cost-effective maintenance compared to roll replacement.
Engineering Practice Considerations
Several practical considerations are important for successful backup roll surfacing:
- Geometric constraints: The cylindrical geometry of the roll requires specialized welding fixtures and positioning.
- Thermal distortion: Large diameter rolls are susceptible to thermal distortion during surfacing. Symmetrical welding sequences and controlled heat input are essential.
- Residual stress management: The combination of surfacing residual stresses and operational stresses must be managed to prevent cracking. Post-weld heat treatment is strongly recommended.
- Surface finish requirements: After surfacing, the roll surface must be ground to meet tight tolerance specifications. The surfacing layer must have adequate machinability.
- Inspection requirements: Each surfacing pass should be inspected for cracks, porosity, and lack of fusion. Final inspection should include dimensional verification and surface hardness testing.
Study Insights and Reflections
This report demonstrates the practical value of proper welding material selection in extending the service life of critical industrial components. The Cr-Mo surfacing approach provides a cost-effective alternative to roll replacement, reducing both direct material costs and indirect downtime costs.
The study also highlights the importance of matching surfacing material properties to the specific service conditions. In this case, the balance between hardness (for wear resistance) and toughness (for thermal fatigue resistance) is critical. A harder surfacing material might provide better wear resistance but could be more susceptible to thermal fatigue cracking, resulting in shorter overall service life.
From a broader perspective, this work exemplifies the application of welding technology as a maintenance and refurbishment tool in heavy industry. The ability to restore or enhance the properties of worn components through surfacing is a cornerstone of asset management and sustainability in manufacturing operations.
In summary, the application of Cr-Mo series surfacing materials on hot-rolled backup rolls represents a successful engineering solution to a common industrial problem. The technology offers significant cost savings and operational benefits, and the principles of material selection and process optimization described in this report are applicable to similar surfacing applications across the steel and heavy manufacturing industries.