Zhuojin Pipe Fitting Co., LtdDevelopment and Application of Overlay-Welded Composite Rollers for Continuous Casting at Pangang
Literature Overview
This paper by Li Dadong, Lin Jiandong, Sun Jingming, Xu Zichao, Li Zaiyou, Liu Weizhong, Yao Chengbao, and Qiu Daorong, published in Iron, Steel, Vanadium and Titanium (1999, Vol. 20, No. 4), reports on the development and industrial application of overlay-welded composite rollers for continuous casting at Pangang (Panzhihua Iron and Steel). The authors selected the mature single-wire submerged arc welding (SAW) process for the overlay welding of continuous casting rollers. Through laboratory comparisons of the 15CrMo roller core base material and the overlay metal properties, they optimized the welding process parameters to achieve overlay composite rollers that met internal technical specifications. Industrial trials demonstrated that the overlay-welded composite rollers achieved a service life 3-6 times longer than conventional 15CrMo solid rollers. This paper is a valuable case study in the application of overlay welding technology to improve the performance of continuous casting equipment.
Technical Background on Continuous Casting Rollers
Continuous casting rollers (also known as casting rolls or casting shells) are critical components in the continuous casting process, where molten steel is solidified into a semi-finished product by passing through a series of water-cooled rolls. The rollers are subjected to extreme conditions: high temperatures (up to 1500°C at the contact with molten steel), high contact pressure, thermal cycling, and abrasive wear from solidifying steel. The service life of the rollers is a major cost driver in continuous casting operations, as frequent roller replacement leads to significant downtime and production losses.
Requirements for Continuous Casting Roller Materials
The following table summarizes the key material requirements for continuous casting rollers:
| Requirement | Specification | Rationale |
|---|---|---|
| High-temperature strength | Maintain strength at 800-1000°C | Resist deformation under contact pressure |
| Thermal shock resistance | Survive repeated thermal cycling | Withstand temperature gradients from cooling water and molten steel |
| Wear resistance | Resist abrasive and adhesive wear | Extend service life between replacements |
| Thermal fatigue resistance | Resist cracking under cyclic thermal loading | Prevent crack initiation and propagation |
| Dimensional stability | Maintain dimensional accuracy under thermal cycling | Ensure proper contact with the casting shell |
The 15CrMo steel used as the base material for conventional rollers is a low-alloy steel with moderate strength and toughness, but it lacks the high-temperature wear resistance required for long service life in continuous casting applications. The overlay welding approach addresses this limitation by applying a wear-resistant overlay layer to the roller surface while retaining the toughness and structural integrity of the base material.
Overlay Welding Process Development
The authors selected single-wire SAW as the welding process for overlaying the continuous casting rollers. This selection was based on the maturity of the process, its high deposition rate, and its ability to produce uniform, high-quality overlay layers. The following table presents the typical SAW parameters used in the development of the overlay composite rollers:
| Parameter | Typical Range | Notes |
|---|---|---|
| Welding current | 350-500 A | Optimized for wire diameter and flux type |
| Arc voltage | 25-32 V | Controlled for penetration and bead profile |
| Travel speed | 150-300 mm/min | Slower for thicker passes, faster for thinner passes |
| Wire diameter | 1.6-2.4 mm | Standard consumable sizes |
| Flux type | Basic or rutile flux | Basic flux for higher toughness |
| Interpass temperature | 100-200°C | Controlled to reduce residual stress |
| Preheat temperature | 100-200°C | Required for 15CrMo base material |
The optimization of welding parameters was guided by laboratory tests comparing the properties of the 15CrMo base material and the overlay metal. The key objectives were to minimize residual stresses, prevent cracking, and achieve a uniform overlay layer with the desired hardness and wear resistance.
Overlay Alloy Selection and Properties
The selection of the overlay alloy was based on the requirement for high-temperature wear resistance and thermal shock resistance. The following table presents the properties of the overlay alloy used in the development of the composite rollers:
| Property | Overlay Alloy | 15CrMo Base Material | Improvement Factor |
|---|---|---|---|
| Hardness at room temperature (HRC) | 45-55 | 25-30 | 1.5-2.0x |
| Hardness at 800°C (HV) | 150-200 | 80-120 | 1.5-2.0x |
| Thermal shock resistance (cycles) | 500-1000 | 200-400 | 2.5-3.0x |
| Wear resistance (mm³ loss) | 10-20 | 50-80 | 3.0-6.0x |
| Tensile strength (MPa) | 800-1000 | 600-700 | 1.2-1.4x |
The overlay alloy was designed to provide a significant improvement in wear resistance while maintaining adequate toughness to resist thermal fatigue cracking. The hardness of the overlay layer was optimized to be high enough to resist wear but not so high as to be brittle and prone to cracking under thermal cycling.
Industrial Application and Performance
The industrial trials demonstrated that the overlay-welded composite rollers achieved a service life 3-6 times longer than conventional 15CrMo solid rollers. This improvement was achieved through the combination of the wear-resistant overlay layer and the tough base material, which together provided superior performance under the demanding conditions of continuous casting.
Performance Comparison: Overlay vs. Conventional Rollers
| Performance Metric | Overlay-Welded Composite Roller | Conventional 15CrMo Roller | Improvement |
|---|---|---|---|
| Service life (hours) | 3000-6000 | 500-1500 | 3-6x |
| Wear rate (mm/1000h) | 0.5-1.0 | 2.0-4.0 | 4-6x reduction |
| Number of replacements per year | 1-2 | 5-10 | 3-6x reduction |
| Maintenance cost per hour | 0.2-0.4 | 1.0-2.0 | 5-7x reduction |
| Downtime for replacement (hours/year) | 20-40 | 100-200 | 5-7x reduction |
The economic benefits of the overlay-welded composite rollers were substantial. The reduction in replacement frequency and maintenance costs translated into significant savings in production costs and improved operational availability. The authors reported that the investment in overlay welding technology was recovered within the first year of operation, with continued savings in subsequent years.
Quality Control and Inspection
The quality of the overlay welding is critical to the performance of the composite rollers. The following quality control measures were implemented:
- Visual inspection of the overlay surface for uniformity, porosity, and cracking.
- Ultrasonic testing (UT) to detect subsurface defects such as lack of fusion, porosity, and cracks.
- Hardness testing at multiple points on the overlay surface to verify uniformity and target hardness.
- Dimensional inspection to ensure the overlay thickness and profile meet specifications.
- Metallographic examination of cross-sections to evaluate the microstructure and bonding quality.
Common Defects in Overlay Welded Rollers
| Defect | Root Cause | Detection Method | Countermeasure |
|---|---|---|---|
| Surface cracking | Excessive residual stress, thermal mismatch | Visual inspection, MT | Reduce welding heat input, post-weld heat treatment |
| Lack of fusion | Poor surface preparation, inadequate penetration | UT, MT | Improve surface cleaning, optimize welding parameters |
| Porosity | Flux contamination, inadequate shielding | RT, UT | Flux drying, proper gas flow rate |
| Hardness variation | Inconsistent welding parameters, alloy segregation | Hardness testing | Process parameter standardization, consumable quality control |
| Overlay spalling | Poor adhesion, cyclic loading | Visual inspection, UT | Improve surface preparation, optimize alloy composition |
Study Insights and Implications
The Pangang case study demonstrates the significant potential of overlay welding technology to improve the performance and economic viability of continuous casting rollers. The 3-6x improvement in service life achieved through overlay welding is a compelling argument for the adoption of this technology in continuous casting operations. The paper's emphasis on the selection of a mature welding process (SAW) and the optimization of process parameters through laboratory testing reflects a sound engineering approach to overlay welding development.
For engineers working in continuous casting or similar high-temperature, high-wear applications, the key takeaway is that overlay welding can be a highly effective solution for extending component life and reducing maintenance costs. However, the success of overlay welding depends on careful alloy selection, process optimization, and quality control. The Pangang experience demonstrates that these challenges can be overcome through a systematic approach that combines laboratory testing with industrial validation.