CLADDING TECHNOLOGY SHANXI CO., LTDApplication of Weld Overlay Technology on Hot Rolling Cast Rollers
Literature Overview and Background
The hot rolling cast roller (hot rolling cladding roller) is a critical consumable component in steel strip and plate rolling mills. Its surface is subjected to extreme thermal cycling, mechanical abrasion, and chemical interaction with the hot metal being rolled. Conventional monolithic rollers, whether made of high-chrome cast iron or tool steel, face a fundamental trade-off between core toughness and surface hardness. Weld overlay technology offers a strategic solution by depositing a hard, wear-resistant surface layer onto a tough base material, thereby decoupling the performance requirements of the core and the working surface.
The literature under review focuses on the practical application of weld overlay processes—primarily submerged arc welding (SAW), gas metal arc welding (GMAW), and plasma transferred arc (PTA) welding—to extend the service life of hot rolling cast rollers in the hot strip finishing mill and slab reheating furnace environments. The study examines overlay compositions, process parameter optimization, and field performance data collected from multiple rolling mill operators in China.
Core Technical Analysis
Overlay Material Selection
The selection of overlay alloy is governed by the specific service conditions of the roller. For hot rolling applications, the overlay must withstand temperatures ranging from 900°C to 1250°C (depending on the steel grade being rolled), resist thermal cracking during cooling, and exhibit high hardness retention at elevated temperatures.
| Overlay System | Typical Composition | Hardness (HV) | Service Temperature | Application |
|---|---|---|---|---|
| High-chrome cast iron (Cr15-Cr20) | Cr 15-20%, C 2.5-3.5%, Mo 1-2% | 600-750 | ≤900°C | Finishing mill work rolls |
| Ni-Cr-Mo alloy | Ni 15-25%, Cr 8-12%, Mo 3-5% | 450-550 | ≤1000°C | Slab reheating furnace guides |
| High-speed steel type | W 6%, Cr 4%, V 3%, Mo 5% | 650-780 | ≤950°C | High-performance finishing rolls |
| Stellite 6 | Co 60%, Cr 21%, W 7% | 400-450 | ≤1100°C | Special high-temp applications |
The literature emphasizes that the Cr-C-Mo system offers the best balance of cost and performance for most hot rolling applications. The formation of M7C3 and M23C6 carbides in the overlay microstructure provides the primary wear resistance mechanism. However, excessive carbon content leads to carbide coarsening during thermal cycling, which accelerates wear.
Welding Process Parameters
The welding process parameters are critical to achieving a defect-free overlay with proper metallurgical bonding to the cast roller base material. The base material is typically a high-chrome white iron or a ductile iron with a carbon equivalent (CE) of 2.5-3.5%, which poses significant hot cracking and cracking risks during welding.
| Process | Preheat Temperature | Interpass Temperature | Overlay Thickness | Travel Speed | Key Consideration |
|---|---|---|---|---|---|
| SAW | 250-350°C | 250-300°C | 1.5-3.0 mm/pass | 150-250 mm/min | Flux composition critical |
| GMAW | 200-300°C | 200-250°C | 1.0-2.5 mm/pass | 200-350 mm/min | Wire feed rate stability |
| PTA | 150-250°C | 150-200°C | 0.5-1.5 mm/pass | 80-150 mm/min | Powder feed precision |
A key finding from the literature is that the preheat temperature must be carefully controlled. Insufficient preheating leads to high cooling rates at the weld interface, promoting the formation of martensite and increasing residual stress. Excessive preheating (>400°C) can soften the base material and reduce the hardness gradient between the overlay and the substrate. The recommended practice is to apply a local induction preheat to achieve a temperature of 250-350°C within a 100 mm radius of the weld line, followed by immediate welding to minimize heat loss.
Microstructure and Performance
Metallographic examination of the overlay-substrate interface reveals a gradient transition zone approximately 0.5-2.0 mm in depth. In this zone, the microstructure transitions from the ledeburitic structure of the cast iron base through a mixed zone containing both austenite and martensite, to the fully austenitic or martensitic structure of the overlay. This transition zone is the most critical region for crack initiation.
The literature reports that a properly designed multi-pass overlay with a compatible transition layer (such as a low-carbon nickel-based filler) can significantly reduce the hardness mismatch at the interface. A typical hardness profile shows:
- Base material: 200-350 HV
- Transition layer: 350-500 HV
- Overlay layer: 600-750 HV
This graded hardness distribution minimizes stress concentration at the interface and improves the fatigue resistance of the overlay.
Engineering Practice and Case Studies
Case 1: 2000 mm Hot Strip Finishing Mill
A major steel producer in East China reported the application of Cr15-Mo2 overlay on 2000 mm wide hot strip finishing mill work rollers. The overlay was applied using SAW with a specialized low-hydrogen flux and a wire composition of Cr15-C3.0-Mo2. The overlay thickness was 2.5 mm, applied in three passes with a preheat of 300°C.
The results showed a 2.3-fold improvement in roller life compared to the uncoated cast iron rollers. The overlay maintained surface hardness above 600 HV after 6 months of continuous operation at 1050°C rolling temperature. The primary failure mode observed was surface spalling caused by thermal fatigue cracking at the overlay-substrate interface, which was mitigated in subsequent trials by introducing a 0.5 mm nickel-based transition layer.
Case 2: Slab Reheating Furnace Guide Rollers
Guide rollers in the slab reheating furnace operate under lower mechanical stress but higher thermal cycling conditions. The literature describes the use of Ni-Cr-Mo overlay applied by GMAW on carbon steel guide rollers. The overlay composition was Ni20-Cr10-Mo4, providing good oxidation resistance and moderate hardness. The overlay thickness of 2.0 mm extended roller life from 3 months to 14 months under identical operating conditions.
Key Defects and Countermeasures
| Defect Type | Root Cause | Detection Method | Countermeasure |
|---|---|---|---|
| Hot cracking at interface | High carbon equivalent of base material | MT, RT | Increase preheat, use low-C transition layer |
| Porosity in overlay | Flux contamination, high travel speed | UT, RT | Improve flux drying, reduce speed |
| Incomplete fusion | Insufficient heat input | MT, UT | Increase current, reduce travel speed |
| Cracking during cooling | Residual stress from thermal mismatch | Visual, MT | Post-weld stress relief at 600-650°C |
| Overlay spalling | Poor metallurgical bonding | UT, impact test | Introduce compatible transition layer |
Study Insights and Reflections
The most valuable insight from this literature is the recognition that weld overlay technology for hot rolling rollers is not merely a matter of selecting a hard alloy and welding it on. The metallurgical compatibility between the overlay and the cast iron base material is the governing factor for long-term performance. The thermal cycling environment of the hot rolling mill creates a unique fatigue regime that is not well captured by laboratory wear tests. Engineers must consider the combined effects of thermal stress, mechanical loading, and chemical interaction when designing overlay systems.
Another important observation is the role of the transition layer. In many industrial applications, the transition layer is omitted to save cost and welding time, but the literature clearly demonstrates that even a thin (0.3-0.5 mm) compatible transition layer can dramatically improve the durability of the overlay. This is particularly important for cast iron base materials with high carbon equivalents.
The economic analysis presented in the literature is also noteworthy. While the initial cost of overlaying a roller is 30-50% higher than purchasing a monolithic roller, the extended service life results in a total cost of ownership reduction of 40-60% over the roller's operational cycle. This economic argument is compelling for plant managers and should be communicated clearly in technical proposals.
In conclusion, the application of weld overlay technology on hot rolling cast rollers represents a mature and well-validated engineering practice. The key to successful implementation lies in the careful selection of overlay composition, precise control of welding parameters, and the strategic use of transition layers to manage the metallurgical interface. Future developments should focus on advanced overlay materials with improved thermal fatigue resistance and on the development of automated overlay systems that can ensure consistent quality across large roller diameters.