Zhuojin Pipe Fitting Co., LtdHardfacing Technology for Rolling Mill Assist Rolls and Deep Bending Rolls
Literature Overview and Industrial Context
This study, authored by Ma Yaodong and colleagues from Dalian Huarui Heavy Industry and the Central Iron and Steel Group Building Research Institute, investigates the hardfacing technology applied to two critical rolling mill components: assist rolls (also known as support rolls or guide rolls) and deep bending rolls. These components are subjected to severe wear conditions during steel rolling operations, with assist rolls experiencing abrasive contact with hot steel strip and deep bending rolls undergoing intense mechanical loading and abrasion during the bending process. The research employs YJ356-S and YJ358-S flux-cored wires for hardfacing, with electromagnetic stirring applied during the welding process to improve deposit quality and performance.
Hardfacing Process and Material Selection
The study compares two specialized hardfacing wires—YJ356-S for assist rolls and YJ358-S for deep bending rolls—selected based on the specific wear mechanisms and loading conditions of each component. The hardfacing process employs flux-cored arc welding (FCAW) with electromagnetic stirring to enhance deposit uniformity and hardness distribution.
| Parameter | Assist Rolls (YJ356-S) | Deep Bending Rolls (YJ358-S) |
|---|---|---|
| Wire Type | Flux-cored | Flux-cored |
| Wire Diameter (mm) | 3.2 | 3.2 |
| Current (A) | 350–450 | 350–450 |
| Voltage (V) | 30–36 | 30–36 |
| Travel Speed (mm/min) | 200–350 | 200–350 |
| Preheat Temperature (°C) | 150–250 | 150–250 |
| Interpass Temperature (°C) | <250 | <250 |
| Post-weld Heat Treatment | Tempering at 600°C | Tempering at 600°C |
The electromagnetic stirring technique applied during hardfacing is a significant process innovation. By superimposing an alternating electromagnetic field on the welding arc, the stirring effect enhances the mixing of the molten pool, promotes uniform distribution of alloying elements and carbides, and reduces macrosegregation. This results in improved hardness uniformity and enhanced wear resistance across the entire deposit thickness.
Microstructure and Hardness Analysis
The microstructural analysis reveals distinct but complementary microstructures in the two hardfacing deposits:
| Microstructural Feature | Assist Rolls (YJ356-S) | Deep Bending Rolls (YJ358-S) |
|---|---|---|
| Primary Phase | Tempered martensite | Tempered martensite |
| Secondary Phase | Bainite + sorbite (minor) | Dispersed carbides |
| Carbide Type | High-hardness carbides on martensite matrix | Fine, uniformly dispersed carbides |
| Hardness (HRC, as-deposited) | 55–62 | 58–65 |
| Hardness (HRC, after tempering) | 59.9 | 63.1 |
| Relative Wear Resistance Coefficient | 0.105 | 0.121 |
The lower relative wear resistance coefficient indicates superior wear resistance; therefore, YJ356-S (0.105) outperforms YJ358-S (0.121) in the specific wear test conditions employed. However, the higher hardness of YJ358-S (HRC 63.1) makes it more suitable for the deep bending roll application where higher contact stresses are encountered. The wear test results should be interpreted in the context of the specific wear mechanisms—abrasive wear for assist rolls and a combination of abrasive and impact wear for deep bending rolls.
Electromagnetic Stirring Effects
The application of electromagnetic stirring during hardfacing produces several beneficial effects:
- Improved hardness uniformity: The stirring reduces the hardness gradient from the surface to the root of the deposit, minimizing soft zones that could initiate wear
- Enhanced carbide distribution: Carbides are more uniformly dispersed throughout the deposit, improving wear resistance consistency
- Reduced porosity: The stirring promotes gas escape from the molten pool, resulting in denser deposits
- Improved dilution control: Better mixing between the deposited metal and base material leads to more predictable dilution rates
The electromagnetic stirring parameters—frequency, amplitude, and duration—must be optimized for each specific application. In this study, the stirring was applied during the welding process, with parameters adjusted based on wire type and desired deposit characteristics.
Engineering Practice and Application Results
The hardfacing technology described in this study has been successfully applied in rolling mill operations, with significant improvements in component service life. Field performance data indicates:
- Assist roll service life: Increased from 3–6 months to 18–24 months
- Deep bending roll service life: Increased from 2–4 months to 12–18 months
- Reduction in unplanned downtime: Significant decrease due to fewer roll failures
- Cost savings: Reduced frequency of roll replacement and associated production losses
In my professional experience, the hardfacing of rolling mill rolls is a critical maintenance activity that directly impacts production efficiency and cost. The selection of appropriate hardfacing materials and processes must consider not only wear resistance but also impact resistance, thermal fatigue resistance, and compatibility with the base roll material. The electromagnetic stirring technique represents a valuable process enhancement that can significantly improve deposit quality and performance, particularly for thick deposits where hardness uniformity is critical.
Key Questions and Reflections
Several important considerations emerge from this research for engineering practice. First, the wear test results, while informative, may not fully represent the complex wear conditions in actual rolling mill service, which involve a combination of abrasive, adhesive, and impact wear mechanisms. Second, the long-term stability of the hardfacing layer under cyclic thermal loading—particularly for assist rolls in contact with hot steel strip—requires continued evaluation. Third, the economic viability of electromagnetic stirring equipment must be assessed against the productivity gains achieved, particularly for high-volume applications.
Study Insights and Conclusions
This research provides valuable insights into the hardfacing technology for rolling mill assist rolls and deep bending rolls, demonstrating the effectiveness of specialized flux-cored wires combined with electromagnetic stirring to achieve superior wear resistance and deposit quality. The microstructural analysis reveals the critical role of tempered martensite with dispersed high-hardness carbides in providing the required wear resistance, while the electromagnetic stirring technique enhances deposit uniformity and performance consistency. For engineers working in rolling mill maintenance and operations, the key takeaways are that material selection must be matched to the specific wear mechanism, that electromagnetic stirring can significantly improve hardfacing quality, and that comprehensive process control—including preheat, interpass temperature, and post-weld heat treatment—is essential for achieving optimal service life. The findings support the continued development of advanced hardfacing technologies to extend component life and reduce maintenance costs in heavy industrial applications.