CLADDING TECHNOLOGY SHANXI CO., LTDSurface Weld Overlay Technology for Slab Continuous Casting Machine Guide Rollers
Literature Overview
This 1998 study by Tang Qiong and Ma Jinglan from Taiyuan Iron and Steel (Group) Company examines the application of surface weld overlay technology to guide rollers in slab continuous casting machines. Continuous casting is a critical process in modern steelmaking where molten steel is cast into semi-finished slabs, billets, or blooms. Guide rollers direct the solidifying strand as it passes through the mold, secondary cooling zone, and final shaping sections. These rollers operate under severe conditions involving contact with hot steel (temperatures up to 1200 degrees Celsius in the mold exit region), mechanical loading from strand weight, and exposure to cooling water and scale. Surface wear, thermal cracking, and scale adhesion are the primary degradation mechanisms that necessitate periodic overlay repair or replacement.
Operating Conditions and Wear Mechanisms
Guide rollers in continuous casting machines experience a complex combination of wear mechanisms that vary by location along the casting line:
| Location | Temperature | Primary Wear Mechanism | Typical Life |
|---|---|---|---|
| Mold exit guide | 1000 to 1200 °C | Thermal fatigue, scale adhesion | 3 to 6 months |
| Secondary cooling zone | 800 to 1000 °C | Abrasive wear, scale erosion | 6 to 12 months |
| Final shaping section | 600 to 800 °C | Mechanical wear, oxidation | 12 to 24 months |
| Run-out table | 400 to 600 °C | Abrasive wear | 24 to 36 months |
The base material of guide rollers is typically a ductile iron or medium-carbon steel with a hardness of 180 to 250 HV. This base material provides adequate toughness for the cyclic mechanical loading but offers insufficient wear resistance for the hot, abrasive casting environment. Weld overlay with a hardfacing alloy creates a surface layer with hardness of 500 to 700 HV, significantly extending service life.
Overlay Process Selection and Parameters
The study likely evaluated multiple overlay processes for guide roller application, with submerged arc welding (SAW) and gas shielded arc welding (GMAW) being the most practical for cylindrical roller surfaces. For large-diameter guide rollers (typically 300 to 600 mm), SAW with a flux-cored wire provides high deposition rates and consistent overlay quality. For smaller rollers or complex geometries, GMAW with a solid or flux-cored wire offers better positional flexibility.
The overlay material selection is governed by the specific service conditions at each roller location. For high-temperature zones, alloys with high chromium content (12 to 20 percent) provide oxidation resistance and thermal stability. For abrasive wear zones, alloys with high carbide content (Cr7C3, Cr3C2 type) provide superior hardness and abrasion resistance. A common approach is to use a multi-layer strategy:
- Bond coat layer: A transition alloy (such as a Ni-Cr type) deposited directly on the prepared base surface to ensure metallurgical compatibility and reduce cracking susceptibility.
- Build-up layer: One or more passes of a medium-hardness alloy to achieve the required dimensional build-up.
- Wear-resistant surface layer: A hardfacing alloy deposited as the final layer to provide the required hardness and wear resistance.
| Process Parameter | SAW Specification | GMAW Specification |
|---|---|---|
| Current | 400 to 600 A | 200 to 350 A |
| Voltage | 28 to 35 V | 22 to 28 V |
| Travel speed | 200 to 400 mm/min | 300 to 600 mm/min |
| Wire diameter | 1.6 to 2.4 mm | 1.2 to 1.6 mm |
| Shielding gas (GMAW) | Ar + CO2 (80:20) | Ar + CO2 (80:20) |
| Preheat | 100 to 200 °C | 100 to 200 °C |
| Interpass temperature | ≤ 250 °C | ≤ 250 °C |
Surface Preparation and Defect Control
Surface preparation is critical for achieving a sound bond between the overlay layer and the roller base. The existing worn surface must be ground to a smooth finish, and any cracks, scale, or oxidation must be completely removed. A V-groove preparation with an included angle of 60 to 90 degrees is typically machined to provide mechanical interlock and ensure adequate weld penetration. The groove depth should be at least 1.5 times the expected wear depth to provide sufficient overlay material reserve.
Common defects in guide roller overlay include:
- Cracking at the bond line: Caused by high carbon equivalent of the base material, inadequate preheat, or excessive cooling rates. Prevention requires preheating to 150 to 200 degrees Celsius and using low-hydrogen consumables.
- Spalling of the overlay layer: Results from poor bond strength due to contamination or insufficient penetration. Prevention requires thorough surface cleaning and verification of weld toe fusion.
- Delamination during service: Caused by thermal fatigue cycling or residual stresses. Prevention requires post-weld stress relief treatment and selection of overlay alloys with good thermal fatigue resistance.
Engineering Practice and Cost-Benefit Analysis
From an economic perspective, weld overlay repair of guide rollers is significantly more cost-effective than replacement. A typical guide roller for a slab caster costs 15,000 to 40,000 RMB for replacement, while overlay repair costs 2,000 to 5,000 RMB including consumables and labor. However, the overlay must be performed with sufficient quality to ensure the repaired roller achieves at least 70 to 80 percent of the original service life. Poorly executed overlay repairs that fail prematurely can be more costly than replacement due to unplanned casting interruptions.
The study likely emphasized the importance of establishing a systematic roller maintenance schedule that includes periodic measurement of overlay thickness, hardness verification, and NDT inspection. When the overlay thickness is reduced to less than 1.5 mm remaining above the original surface, the roller should be removed for re-overlay before in-service failure occurs.
Study Insights
This work represents an important application of weld overlay technology in the steelmaking industry, where the operating conditions are among the most severe encountered in industrial welding. The integration of metallurgical knowledge with practical maintenance strategies demonstrates how overlay technology can be used not merely for repair but as a planned maintenance strategy that optimizes equipment availability and total cost of ownership. The systematic approach to material selection, process parameter control, and quality verification provides a template applicable to similar overlay applications in other high-wear industrial environments.