Zhuojin Pipe Fitting Co., LtdOverlay Welding Repair Process for Continuous Casting Rolls
Literature Overview
This paper by Miao Hailiang, published in Welding Technology in 1995, details the overlay welding repair process for continuous casting rolls, which are critical components in the steelmaking continuous casting process. The work was conducted at the Tianjin Second Metallurgical Machinery Plant Repair Center. The study demonstrates that advanced overlay welding technology can extend the service life of continuous casting rolls by 2 to 3 times compared to unrepaired new rolls, with significant improvements in high-temperature strength and wear resistance.
Background and Technical Challenge
Continuous casting rolls operate under extremely demanding conditions: they are subjected to high temperatures (up to 1200–1400°C at the contact zone), intense thermal cycling, mechanical contact pressure from the solidifying steel shell, and chemical erosion from the molten steel and slag. These conditions cause the roll surface to degrade through a combination of thermal fatigue cracking, abrasion, and oxidation. Traditional repair methods, such as simple resurfacing or replacement, are either insufficient or prohibitively expensive.
Overlay Welding Repair Process
The repair process involves the following key steps:
| Process Step | Method | Purpose |
|---|---|---|
| Surface preparation | Grinding and cleaning | Remove damaged surface, ensure good weldability |
| Preheating | Induction or furnace heating | Reduce thermal stress, prevent cracking |
| Base layer welding | Flux-cored wire + flux | Provide metallurgical compatibility with base metal |
| Overlay layer welding | Flux-cored wire + flux | Provide high-temperature wear resistance |
| Post-weld heat treatment | Controlled cooling or tempering | Relieve residual stresses, refine microstructure |
Welding Consumables Selection
The study emphasizes the use of flux-cored welding wire combined with appropriate flux, which provides several advantages over solid wire welding:
- Higher deposition rates due to the flux core providing additional alloying elements and deoxidizers.
- Better slag protection, reducing oxidation and nitrogen pickup.
- Improved arc stability and wetting characteristics.
- Ability to deposit layers with tailored microstructures through flux composition control.
Metallurgical Analysis
The base metal of continuous casting rolls is typically a high-carbon, high-chromium cast iron or a high-alloy steel. The overlay weld metal must meet the following requirements:
- High-temperature hardness: The overlay must retain hardness at operating temperatures above 800°C, which requires stable carbide phases such as M₇C₃ or M₂₃C₆.
- Thermal fatigue resistance: The overlay must withstand repeated thermal cycling without cracking, which requires adequate ductility and crack arrest capability.
- Metallurgical bonding: The interface between the base metal and the overlay must have sufficient strength to resist spalling under thermal and mechanical loading.
- Low dilution: The overlay composition must be maintained with minimal dilution from the base metal to preserve the desired properties.
Microstructural Considerations
The overlay weld metal microstructure is critical for performance. The ideal microstructure for continuous casting roll overlay includes:
- A matrix of tempered martensite or austenite for toughness.
- Fine, uniformly distributed carbides for wear resistance.
- No brittle phases such as ledeburite or untempered martensite.
- A refined grain structure to improve both strength and toughness.
Engineering Practice and Economic Analysis
The 2 to 3 times life extension reported in this study represents a substantial economic benefit. Continuous casting rolls are expensive components, and their replacement requires significant downtime. The overlay welding repair approach allows for in-situ or shop repair, minimizing production interruptions.
From a quality assurance perspective, the following inspection procedures are recommended:
- Visual inspection of the overlay surface for uniformity, porosity, and undercut.
- Penetrant testing (PT) to detect surface and near-surface cracks.
- Ultrasonic testing (UT) to assess bonding quality and subsurface defects.
- Hardness testing to verify the overlay hardness profile.
- Macrographic examination of cross-sections to evaluate dilution and microstructure.
Study Insights
This work exemplifies the practical application of overlay welding technology in heavy industry. The key insight is that overlay welding is not merely a surface treatment but a comprehensive repair strategy that requires careful consideration of metallurgical compatibility, process parameters, and quality verification. The use of flux-cored wire with flux provides the necessary process flexibility to achieve high-quality overlays on thick, high-alloy substrates.
The study also highlights the importance of systematic process development. Each parameter, from preheat temperature to wire feed speed, affects the final overlay quality. The economic case for overlay welding repair is compelling, but it must be supported by rigorous quality control to ensure long-term reliability in service. This research provides a valuable reference for metallurgical repair engineers working on continuous casting equipment maintenance.