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 from Zhengzhou Machinery Research Institute, published in New Century Cement Guide (2009, Vol. 15, No. 6, pp. 51-54), presents practical experience with self-shielded flux-cored wire surfacing (FCAW-S) for the repair of vertical mill rollers in cement production. The focus is on the welding consumables, process parameters, and microstructural characteristics of the self-shielded surfacing technique, with emphasis on its flexibility for both offline and online repair operations.
Technical Analysis of Self-Shielded Surfacing
Consumable Selection and Process Characteristics
Self-shielded flux-cored wire (FCAW-S) is distinguished from gas-shielded flux-cored wire (FCAW-G) by the absence of external shielding gas. The flux core contains deoxidizers, alloying elements, and gas-generating agents that create a self-contained shielding atmosphere through decomposition during welding. This eliminates the need for shielding gas cylinders, hoses, and flow regulators, making the process highly portable and adaptable to field conditions.
For vertical mill roller surfacing, the typical consumables used include:
| Parameter | Typical Specification |
|---|---|
| Wire type | Self-shielded flux-cored wire (FCAW-S) |
| Wire diameter | 1.2 mm or 1.6 mm |
| Base metal | Low-carbon steel or medium-carbon steel roller shell |
| Surfacing alloy | High-Cr, high-C, or Cr-Mo-C composite alloy |
| Current type | DCEN (direct current electrode negative) |
| Deposition rate | 2.0-4.5 kg/h depending on wire diameter |
| Penetration | Moderate to deep, depending on flux composition |
Microstructural Characteristics
The deposited layer microstructure in self-shielded surfacing is governed by the alloy composition of the flux core and the cooling rate at the weld surface. For high-carbon, high-chromium surfacing alloys, the typical microstructure consists of martensite with dispersed carbides (Cr7C3, Cr23C6, and Fe3C). The carbide distribution and morphology are critical for wear resistance, as they provide hard phase reinforcement against abrasive and adhesive wear.
The HAZ microstructure depends on the base metal composition and the thermal cycle. For low-carbon steel rollers, the HAZ typically shows fine-grained ferrite and pearlite with minimal hardening. For medium-carbon steel rollers, some martensite formation may occur in the HAZ, which could require post-weld heat treatment to ensure adequate toughness.
Process Advantages for Vertical Mill Application
The paper highlights several practical advantages of self-shielded surfacing for vertical mill roller repair:
- Offline and online repair capability: The absence of shielding gas infrastructure allows repair crews to work in confined spaces, at elevated positions, or in windy outdoor environments without gas supply logistics.
- Positional welding flexibility: FCAW-S can be applied in all positions (flat, horizontal, vertical, overhead), which is essential for roller repair where the entire circumference must be addressed.
- Reduced equipment footprint: Only a welding power source, wire feeder, and torch are required, minimizing setup time and enabling rapid deployment.
- Lower consumable cost: Self-shielded wires are generally less expensive than gas-shielded alternatives, and there is no shielding gas cost.
Engineering Practice Considerations
The self-shielded process has inherent limitations that engineers must manage. The lack of external gas shielding means that the weld is more susceptible to atmospheric contamination, particularly nitrogen and oxygen pickup, which can lead to porosity and reduced mechanical properties. In practice, this is mitigated by:
- Maintaining wire feed speed stability to ensure consistent arc characteristics
- Using flux cores with adequate deoxidizer content (Si, Mn, Al)
- Applying surfacing in multiple passes with interpass cleaning
- Controlling ambient wind speed to below 1.5 m/s or using wind shields
The paper also notes the importance of preheating for thicker roller shells to control cooling rates and prevent HAZ cracking. For low-carbon steel rollers, preheating temperatures of 100-200 °C are typically sufficient. For higher carbon or higher alloy content rollers, preheating may need to be increased to 250-350 °C.
Study Insights
The self-shielded surfacing technology represents a pragmatic solution for the cement industry's need for rapid, cost-effective roller repair. The flexibility to perform repairs both offline (in a workshop) and online (with the mill in operation, on accessible sections) significantly reduces downtime. However, engineers should be aware that the microstructural quality of FCAW-S deposits may not match that of gas-shielded processes, particularly in terms of porosity content and carbide uniformity. For critical applications where maximum wear life is required, a hybrid approach—using FCAW-S for bulk repair and FCAW-G or plasma arc for the final finishing pass—may offer the best balance of productivity and quality. The practical experience documented in this paper provides valuable reference for field welding engineers seeking to optimize repair procedures for vertical mill rollers in cement production environments.