Zhuojin Pipe Fitting Co., LtdSelf-Shielded Surfacing Technology for Raw Material Vertical Mill Rollers
Literature Overview
This paper by Yang Wei, Zhang Haiyan, and Ni Junjie (Zhengzhou Machinery Research Institute, 2009) reports on the application of self-shielded flux-cored wire surfacing technology for the repair of vertical mill rollers in cement grinding systems. The study examines welding consumables, process parameters, and resulting microstructures, with particular emphasis on the operational flexibility of self-shielded surfacing for both offline and online repair scenarios.
Technical Background
Vertical mill rollers in cement grinding circuits operate under extreme conditions: high contact stress (up to 3000 MPa), abrasive wear from hard cement clinker, and thermal cycling. The typical failure mode is progressive surface erosion of the roller shell, leading to reduced grinding efficiency and eventual replacement. Surfacing with wear-resistant alloy layers is the standard repair approach.
Self-Shielded Surfacing Advantages
| Feature | Gas-Shielded Surfacing | Self-Shielded Surfacing |
|---|---|---|
| Shielding method | External inert/active gas (Ar, CO₂) | Flux coating on wire provides shielding |
| Wind sensitivity | High (requires windbreak) | Low (suitable for outdoor/online) |
| Equipment complexity | Requires gas cylinders, hoses | Simpler setup |
| Operational flexibility | Limited to controlled environments | Suitable for online and offline repair |
| Typical wire types | Solid wire, flux-cored (gas-shielded) | Self-shielded flux-cored (SFFC) |
| Deposition efficiency | High | Moderate to high |
| Cost per meter | Higher (gas cost) | Lower |
Process and Consumable Selection
The study highlights several critical aspects of self-shielded surfacing for mill roller repair:
- Base layer (transition layer): A low-carbon or nickel-based transition layer is applied first to prevent cracking at the fusion boundary, especially when the base material is low-alloy steel (e.g., Q345 or 40Cr).
- Wear layer: Multiple passes of high-carbon, high-chromium, or high-silicon flux-cored wire are deposited to achieve the required hardness (typically 45–55 HRC for the final surface).
- Wire diameter: Commonly 1.2–1.6 mm for self-shielded flux-cored wires in this application.
- Current and voltage: DCEN (direct current electrode negative) is preferred for self-shielded flux-cored wires, with typical parameters of 180–260 A and 22–28 V.
- Travel speed: 300–600 mm/min depending on wire diameter and desired bead profile.
Microstructure and Performance
The self-shielded surfacing deposits typically exhibit a martensitic or austenitic matrix with dispersed carbides (Cr₇C₃, Cr₂₃C₆, or SiC depending on composition). The hardness of the final wear layer can reach 50–60 HRC, providing excellent abrasive wear resistance against cement clinker. However, the high carbon content increases susceptibility to hydrogen-induced cracking, making preheating (150–200 °C) and controlled cooling essential.
Engineering Practice Insights
From a practical standpoint, the self-shielded approach offers a decisive advantage for cement plants where mill rollers are massive and difficult to remove from the mill housing. Online surfacing—performed with the roller still mounted—saves significant downtime. However, the following precautions are necessary:
- Surface preparation: Thorough grinding of the worn surface to a clean, oxide-free condition; damaged areas should be machined to a smooth transition.
- Preheating: Even for self-shielded processes, preheating to 150–200 °C reduces the cooling rate and minimizes cracking risk.
- Interpass temperature control: Maintain interpass temperature below 250 °C to avoid excessive grain growth in the weld metal.
- Post-weld treatment: For critical applications, a low-temperature stress-relief treatment at 400–500 °C is recommended.
Key Reflection
The self-shielded surfacing technology represents a pragmatic engineering solution that balances performance with operational practicality. In the cement industry, where unplanned downtime can cost thousands of dollars per hour, the ability to perform online repair with minimal equipment is highly valuable. The trade-off is slightly lower deposition quality compared to gas-shielded processes, but this is acceptable for wear-layer applications where the base metal integrity is maintained by the transition layer. This paper provides valuable empirical data for process development in similar heavy-industry applications.