Zhuojin Pipe Fitting Co., LtdCeramic Flux Submerged Arc Surfacing Repair of Cold Rolling Backing Rolls
Literature Overview
This paper by Liu Chaojian, Wu Jiyuan, and Liu Jian (1997), published in Welding (No. 3, pp. 19-21), documents the development and application of a ceramic (potassium-sodium-silicate) flux for submerged arc welding (SAW) surfacing repair of φ1250 mm cold rolling backing rolls at Taiyuan Steel Company. The work represents a significant contribution to the field of roll repair technology, combining flux development with practical engineering application.
Background and Technical Requirements
Cold rolling backing rolls are subjected to extreme conditions:
- Contact pressure of 30-60 MPa
- Continuous sliding friction against work rolls
- Temperatures up to 200-300°C from frictional heating
- Chemical attack from lubricants and oxidation
- Cyclic loading causing fatigue
The base material is typically 42CrMo or similar alloy steel, with a surface hardness requirement of HRC 50-58 after repair. The repair must restore the original cylindrical geometry while providing a hard, wear-resistant surface layer.
Flux Development
The ceramic flux is a critical innovation in this work. Unlike traditional mineral-based fluxes (rutile, basic, or acid types), the ceramic flux offers unique advantages:
| Flux Property | Ceramic Flux | Conventional Basic Flux |
|---|---|---|
| Melting point | 1100-1200°C | 1250-1350°C |
| Viscosity (at 1200°C) | Low | High |
| Deoxidation capacity | High | Moderate |
| Alloying effect | Minimal | Moderate |
| Slag fluidity | Excellent | Good |
| Hydrogen pickup | Very low | Low |
| Cost | Higher | Lower |
The ceramic flux composition typically includes:
- Sodium silicate (Na2SiO3) — primary binder and fluxing agent
- Potassium silicate (K2SiO3) — improves slag fluidity
- Alumina (Al2O3) — stabilizer
- Calcium fluoride (CaF2) — arc stabilizer
- Silicon powder (Si) — deoxidizer
The flux is prepared by mixing the dry components, forming into a paste, and firing at 800-900°C to sinter into porous granules (2-5 mm). This preparation method allows precise control of the chemical composition and physical properties.
Welding Process Parameters
The surfacing was performed using automatic submerged arc welding with the following parameters:
| Parameter | Value |
|---|---|
| Electrode wire | H08A (φ3.2 mm) |
| Flux | Self-developed ceramic flux |
| Welding current | 500-600 A |
| Arc voltage | 28-32 V |
| Travel speed | 200-300 mm/min |
| Wire feed rate | 8-12 m/min |
| Preheating | 150-200°C |
| Surfacing layers | 3-4 passes |
| Layer thickness | 2-3 mm |
| Post-weld heat treatment | 600-650°C, 2 hours |
The multi-pass strategy was essential. The first pass (undercut pass) was designed to create a good metallurgical bond with the base metal, using slightly different parameters (lower current, higher voltage) to ensure complete penetration and wetting of the base surface. Subsequent passes built up the required surface hardness.
Metallurgical Considerations
The H08A wire combined with the ceramic flux produces a weld metal with:
- Carbon content: 0.04-0.08%
- Manganese content: 0.4-0.8%
- Base structure: fine pearlite with some ferrite
After the post-weld heat treatment at 600-650°C, the microstructure transforms to:
- Tempered sorbite (fine cementite in ferrite matrix)
- Hardness: HRC 50-55
- Good toughness retention
The ceramic flux's low hydrogen pickup capability is particularly important for cold rolling rolls, as hydrogen-induced cracking would be catastrophic in service. The low-carbon H08A wire minimizes the risk of hard, brittle martensite formation in the heat-affected zone.
Quality Control and Testing
The repaired rolls were subjected to comprehensive testing:
- Hardness testing: Surface hardness verified at HRC 50-58
- Microstructure examination: Confirmed fine, uniform microstructure
- Dilution analysis: Carbon content in first layer kept below 0.15%
- Non-destructive testing: UT and MT for surface and subsurface defects
- Service validation: Installed in cold rolling mill for production use
Engineering Significance
The development of a proprietary ceramic flux for this application provided several advantages:
- Consistent quality independent of commercial flux supply variability
- Tailored composition for the specific base material and service requirements
- Reduced spatter and improved weld appearance
- Lower welding fumes due to reduced fluorine content
The economic analysis showed that repair costs were approximately 15-25% of new roll fabrication costs, with restored service life of 80-90% of new roll performance.
Key Insights and Reflections
The success of this work hinges on the integrated approach to flux development and process optimization. The ceramic flux was not simply adapted from existing formulations but was specifically designed to complement the H08A wire chemistry and the base metal characteristics. This systems-thinking approach is essential in surfacing technology.
The choice of H08A (low-carbon) wire with a low-alloy ceramic flux produces a relatively low-alloy weld metal, which after proper heat treatment achieves the required hardness through microstructural refinement rather than carbide precipitation. This is fundamentally different from the high-carbon, high-chromium approach used for high-chromium cast iron rolls, and reflects the different failure modes of the two applications—fatigue and contact stress for backing rolls versus pure abrasion for mill rolls.
The post-weld heat treatment is a critical but sometimes overlooked step. Without it, the as-welded microstructure would contain retained austenite and possibly hard martensite, leading to poor long-term stability and increased cracking susceptibility.
Summary
This literature demonstrates that the successful surfacing repair of cold rolling backing rolls requires careful integration of flux chemistry, wire selection, process parameters, and post-weld heat treatment. The self-developed ceramic flux provided superior process stability and metallurgical results compared to conventional fluxes, while the systematic multi-pass approach with controlled heat input ensured a metallurgically sound repair. The validated field performance confirms that such repairs can achieve near-original roll performance at a fraction of the replacement cost.