CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Materials and Processes for Hot Rolling Mills
Literature Overview
Hot rolling mill rolls are subjected to extreme conditions during operation: contact temperatures exceeding 800–1000°C, high contact stresses from the rolling force, severe thermal cycling, and abrasive wear from scale and oxide particles. Weld overlay (also called weld hardfacing) is the primary technology used to restore and enhance the surface properties of hot rolling mill rolls. This study note examines the overlay materials, welding processes, and metallurgical considerations for hot rolling mill roll overlay.
Overlay Material Systems
The selection of overlay material depends on the roll type and the specific rolling operation:
| Roll Type | Typical Base Material | Recommended Overlay | Hardness (HV) | Key Property |
|---|---|---|---|---|
| Roughing mill | Medium carbon steel | High-Cr (Cr12–20) | 500–650 | Thermal fatigue resistance |
| Finish rolling mill | High-Cr steel | High-Cr (Cr18–25) | 550–700 | Abrasive wear resistance |
| Wire rod rolling | High-Cr steel | Ni-Cr-Mo alloy | 400–500 | Thermal shock resistance |
| Strip finishing | High-Cr steel | Cr-Mo-V alloy | 500–600 | Combined wear and fatigue resistance |
High-chromium iron-based alloys (Cr12–25) are the most widely used overlay materials for hot rolling mill rolls. The high chromium content promotes the formation of hard chromium carbides (M7C3, M23C6, and Cr7C3) that provide excellent abrasive wear resistance. The addition of molybdenum and vanadium further refines the carbide morphology and improves thermal stability.
Welding Process Selection
The welding process must be selected based on the roll geometry, the required overlay thickness, and the production volume. The following processes are commonly employed:
- Submerged Arc Welding (SAW): The most widely used process for large roll overlay due to its high deposition rate (5–10 kg/h), low spatter, and consistent weld quality. Suitable for overlay thicknesses of 3–15 mm.
- Flux-Cored Arc Welding (FCAW): Offers good deposition rates and the ability to weld in all positions. Suitable for medium-sized rolls and repair work.
- Gas Metal Arc Welding (GMAW): Flexible and suitable for smaller rolls and localized repairs. Lower deposition rate than SAW.
- Laser Cladding: Emerging technology offering low heat input, minimal dilution, and high-quality overlay. Suitable for precision overlay of small rolls.
| Process | Deposition Rate (kg/h) | Dilution (%) | Heat Input (kJ/mm) | Cost |
|---|---|---|---|---|
| SAW | 5–10 | 10–20 | 5–15 | Low |
| FCAW | 3–6 | 15–25 | 4–12 | Moderate |
| GMAW | 2–4 | 20–30 | 3–10 | Moderate |
| Laser Cladding | 0.5–2 | 2–8 | 1–3 | High |
Metallurgical Considerations and Defect Analysis
The overlay microstructure is typically a eutectic structure of austenite and carbides, with the carbide morphology being the most critical factor for wear resistance. Spheroidal carbides provide better toughness than network or skeletal carbides, which can initiate cracks under thermal cycling. The carbide morphology is controlled by the cooling rate and the alloy composition.
Common defects in roll overlay include:
- Cracking at the overlay-base interface: Caused by excessive residual stress and thermal mismatch. Mitigated by preheating to 200–300°C and using a ductile transition layer.
- Cracking within the overlay layer: Caused by high carbon equivalent and rapid cooling. Mitigated by using a slightly lower carbon content and applying a post-weld stress relief at 600–650°C.
- Dilution: Excessive dilution from the base metal reduces the hardness and wear resistance of the overlay. Mitigated by using a larger diameter wire and optimizing the travel speed.
Engineering Practice and Recommendations
In my experience, the key to successful roll overlay is not just the material selection but the careful control of the welding process parameters and the post-weld treatment. A systematic approach using the PDCA cycle — planning the overlay procedure, carrying out the welding with strict parameter control, checking the overlay quality through hardness profiling and metallographic examination, and acting on any deficiencies — is essential for consistent quality. The literature confirms that a well-designed overlay system can extend roll life by 2–3 times compared to the base material, significantly reducing production costs in hot rolling operations.