CLADDING TECHNOLOGY SHANXI CO., LTDSelection and Testing of Overlay Materials for Rolling Mill Rolls
Literature Overview
This 2004 study published in Mining and Metallurgical Engineering (矿冶工程), authored by Liu Huilin, Zheng Boping, Liao Zhi, and Cheng Xiaojun from Liangang Electromechanical Co., Ltd., addresses the practical selection and testing of overlay welding materials for hot rolling mill rolls. The study reflects the industrial reality of roll repair operations where overlay welding is used to restore worn roll surfaces and extend roll life, and provides valuable insights into material selection criteria and performance evaluation methods for this demanding application.
Core Technical Content
Hot rolling mill rolls experience extreme service conditions: high temperatures (up to 1000–1200°C for hot strip mills), heavy contact pressure (100–300 MPa), sliding friction with the workpiece, and thermal cycling during each roll pass. Overlay welding is used to repair worn rolls or to apply wear-resistant surface layers to new rolls, and the selection of overlay material is critical to achieving acceptable roll life and surface quality.
Material Selection Criteria
The selection of overlay materials for rolling mill rolls must consider multiple competing requirements:
| Requirement | Priority | Typical Specification |
|---|---|---|
| Abrasion resistance | High | Hardness ≥ 45 HRC at operating temperature |
| Thermal fatigue resistance | High | Crack resistance at 800–1100°C |
| Contact fatigue resistance | Medium-High | Resistance to surface cracking under cyclic loading |
| Thermal conductivity matching | Medium | Minimize thermal stress at interface |
| Hot hardness retention | High | Hardness retention at 800°C > 30 HRC |
| Weldability | Medium | Low cracking susceptibility |
| Surface finish quality | Medium | Smooth surface for product quality |
| Cost-effectiveness | Medium | Acceptable cost per unit roll life |
Common Overlay Material Categories
Several categories of overlay materials are commonly used for rolling mill roll repair:
Category 1: High-chromium cast iron type
- Composition: 15–25% Cr, 3–5% C, 1–3% Mo
- Hardness: 50–60 HRC
- Characteristics: Excellent abrasion resistance, moderate thermal fatigue resistance
- Applications: Cold rolling, light hot rolling
Category 2: Nickel-based austenitic type
- Composition: 20–30% Ni, 5–10% Cr, 0.5–1.5% C
- Hardness: 35–45 HRC
- Characteristics: Excellent thermal fatigue resistance, good contact fatigue resistance
- Applications: Heavy hot rolling, slab mills
Category 3: Hardfacing alloy type
- Composition: 8–15% Cr, 2–4% C, 5–15% Ni, with optional Mo, V, W
- Hardness: 45–60 HRC
- Characteristics: Balanced abrasion and thermal fatigue resistance
- Applications: General hot rolling, medium severity
Category 4: Co-based alloy type
- Composition: 50–60% Co, 20–30% Cr, 5–10% W, 0.5–2% C
- Hardness: 45–55 HRC (retained at 800°C)
- Characteristics: Excellent hot hardness, superior thermal stability
- Applications: Severe hot rolling, high-temperature applications
Testing and Evaluation Methods
The study emphasizes the importance of systematic testing to evaluate overlay material performance under simulated service conditions:
- Dry sand abrasion testing: Standardized tests (e.g., ASTM G65) using SiC paper or sand abrasion to quantify wear volume loss
- Thermal fatigue testing: Cyclic heating and cooling of overlay specimens to simulate roll thermal cycling
- Contact fatigue testing: Rolling contact tests using ball-on-disc or cylinder-on-disc configurations
- Hardness testing at elevated temperature: Measurement of hardness retention at service temperatures (400°C, 600°C, 800°C)
- Metallographic examination: Analysis of carbide morphology, distribution, and matrix structure
- Rolling mill field trials: Actual performance evaluation on production rolls
Engineering Practice and Process Considerations
The welding process used for roll overlay is typically multi-pass submerged arc welding (SAW) or flux-cored arc welding (FCAW), as these processes provide high deposition rates and good surface quality. Key process parameters include:
| Parameter | Typical Value | Rationale |
|---|---|---|
| Heat input | 1.5–3.0 kJ/mm | Balance between dilution and residual stress |
| Interpass temperature | 150–300°C | Prevent cold cracking, control grain size |
| Number of passes | 3–6 | Achieve required overlay thickness (5–15 mm) |
| Preheat temperature | 200–350°C | Reduce residual stress, prevent cracking |
| Post-weld heat treatment | 500–650°C temper | Reduce hardness, improve toughness |
The post-weld heat treatment is particularly important for roll overlay applications. The as-welded overlay layer typically has excessive hardness (60–68 HRC) that is unsuitable for rolling service, as it leads to poor contact fatigue performance and excessive roll wear. Tempering to 500–650°C reduces hardness to the target range (40–50 HRC) while improving toughness and thermal fatigue resistance.
Study Insights and Reflections
The practical challenge of roll overlay material selection lies in the multi-objective optimization problem: no single material provides optimal performance for all service conditions. The selection must be tailored to the specific mill configuration, product grade, rolling temperature, and severity of wear conditions.
A key insight from this research is that the transition zone between the overlay and the roll body is often the critical failure location. Cracking typically initiates at or near this interface due to thermal mismatch, residual stress concentration, and microstructural incompatibility. The use of a compatible transition layer or careful control of the first pass composition can significantly improve overlay integrity.
The study also highlights the importance of process discipline in roll overlay operations. Even with optimal material selection, poor welding technique, inadequate surface preparation, or improper heat treatment can result in premature overlay failure. A comprehensive quality management system incorporating process parameter monitoring, in-process inspection, and post-weld testing is essential for consistent performance.
The economic analysis of roll overlay must consider not only material and labor costs but also the cost of downtime during roll changes, the productivity impact of roll life, and the quality implications of roll surface condition. In many cases, the investment in premium overlay materials and careful process control pays for itself within the first roll change cycle through reduced downtime and improved product quality.