CLADDING TECHNOLOGY SHANXI CO., LTDSurface Overlay Welding Technology for Slab Continuous Casting Guide Rollers
Literature Overview and Technical Context
The 1998 publication by researchers from Taiyuan Iron and Steel Group addresses the application of overlay welding to guide rollers in slab continuous casting machines. These rollers play a critical role in the continuous casting process by supporting and guiding the solidifying steel slab as it travels through the casting line. They are subjected to extreme conditions including temperatures up to 900 to 1200 degrees Celsius from the hot slab, mechanical contact stresses from the slab weight (which can exceed 500 kg per meter of caster width), thermal cycling, and occasional impact from slab misalignment. The overlay welding technology described in this paper aims to extend the service life of these rollers by depositing a wear-resistant and heat-resistant alloy layer on their working surfaces.
Core Technical Analysis
The overlay welding of continuous casting guide rollers presents several unique challenges compared to conventional overlay applications. The rollers are typically made of medium carbon steel or low-alloy steel (such as 45 steel or 42CrMo) with diameters ranging from 200 to 500 mm and lengths of 1000 to 3000 mm. The overlay layer must withstand:
- Thermal cycling between ambient temperature and approximately 900 degrees Celsius
- Contact pressure of 50 to 150 MPa from the moving slab
- Oxidative wear from the hot steel environment
- Abrasive wear from scale and oxide inclusions on the slab surface
The overlay materials selected in this study include high-speed steel-based alloys (such as M2 or W6Mo5Cr4V2 equivalents) and cobalt-based hardfacing alloys. The welding process employed is submerged arc welding (SAW) with a specially formulated flux, chosen for its ability to produce deep, narrow welds with good slag protection and minimal spatter.
| Parameter | Specification | Rationale |
|---|---|---|
| Base material | 45 steel or 42CrMo | Structural strength and machinability |
| Overlay material | High-speed steel alloy or Co-based alloy | Wear resistance and thermal stability |
| Overlay thickness | 3-8 mm | Balance between cost and service life |
| Welding process | SAW with covered electrode | Deep penetration, low dilution |
| Preheat temperature | 200-300°C | Reduce residual stress and HAZ cracking |
| Interpass temperature | 150-250°C | Maintain ductility and prevent cracking |
Process Design and Implementation
The overlay welding process for guide rollers involves several critical steps that must be carefully controlled:
- Surface preparation: The roller surface must be machined to remove any existing oxide scale, rust, or previous overlay material. The surface should be cleaned to a minimum of Sa 2.5 grade per ISO 8501-1. Any existing cracks or defects must be repaired before overlay application.
- Backing ring installation: For cylindrical rollers, a backing ring is essential to prevent burn-through and ensure proper fusion at the overlay-base metal interface. The backing ring is typically made of the same material as the base roller and is installed with a gap of 0.5 to 1.0 mm.
- Multi-pass overlay strategy: The overlay is applied in multiple passes, with each pass building up the required thickness. The first pass establishes the bond with the base metal, while subsequent passes build up the wear-resistant layer. The final pass is critical for surface quality and must be carefully controlled to minimize surface defects.
- Post-weld machining: After overlay welding, the roller surface is machined to the final dimensions and surface finish. This step removes any surface irregularities, slag inclusions, or weld reinforcement. The final surface roughness should be Ra 3.2 to 6.3 micrometers to ensure proper contact with the steel slab.
Defect Prevention and Quality Control
The overlay welding of guide rollers is susceptible to several characteristic defects:
| Defect Type | Cause | Detection Method | Prevention |
|---|---|---|---|
| Cracking in overlay layer | Excessive cooling rate, hydrogen embrittlement | MT, PT | Preheat, low-hydrogen flux, post-weld stress relief |
| Incomplete fusion | Poor surface preparation, insufficient heat input | UT, RT | Proper cleaning, adequate current |
| Porosity | Flux contamination, moisture | RT, UT | Flux drying, controlled welding environment |
| Excessive dilution | Too high current, too fast travel speed | Metallography, hardness testing | Parameter optimization, backing ring use |
The hardness profile of the overlay layer is a critical quality indicator. For high-speed steel-based overlays, the hardness should be in the range of 55 to 65 HRC. For cobalt-based overlays, the hardness is typically 45 to 55 HRC but with superior thermal stability. The hardness should be measured at multiple locations across the overlay width and depth to ensure uniformity.
Engineering Practice and Performance Evaluation
The implementation of overlay welding on continuous casting guide rollers has demonstrated significant improvements in service life. Based on the experience reported in the literature and subsequent industry practice:
- Without overlay: Typical service life is 1 to 3 months, with frequent roller replacements required due to excessive wear.
- With overlay: Service life extends to 8 to 18 months, representing a 3 to 6 times improvement.
The economic benefit is substantial given the cost of roller replacement, including downtime for the continuous casting machine. A typical large slab caster may have 20 to 40 guide rollers, and each roller replacement can cause several hours of production loss.
The overlay welding technology described in this 1998 publication has since been refined and expanded. Modern practices incorporate additional measures such as:
- Thermal spray pre-treatment: Applying a bond coat via thermal spray before overlay welding to improve adhesion and reduce dilution.
- Laser cladding: Using laser cladding for the final pass to achieve a finer microstructure and better surface quality.
- In-situ monitoring: Implementing real-time monitoring of welding parameters and thermal profiles to ensure consistent quality.
Study Insights and Reflections
The 1998 publication from Taiyuan Iron and Steel Group represents a practical application of overlay welding technology to a critical component in the steelmaking industry. The systematic approach to process development—starting with material selection, moving through process design, and concluding with performance evaluation—provides a model for overlay welding implementation in other industrial applications. The emphasis on post-weld machining and surface finish is particularly noteworthy, as it recognizes that the final surface quality of the overlay layer is as important as its metallurgical properties.
For engineers working in the steel industry, this literature serves as a valuable reference for understanding the specific challenges and solutions associated with overlay welding of continuous casting components. The technology has since been refined and expanded, but the fundamental principles remain the same: careful material selection, controlled process parameters, and rigorous quality control are essential for successful overlay welding applications.