CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Microstructure and Composition on Properties of Hot-Rolled Roll Overlay Layer
Literature Overview
This 1991 study published in Ansteel Technology, authored by researchers from Xi'an Jiaotong University and the Ansteel Iron and Steel Research Institute, addresses the relationship between microstructure, composition, and the resulting mechanical and tribological properties of overlay layers on hot-rolled mill rolls. Hot-rolled roll overlay is a critical technology in the steel industry, where rolls undergo severe thermal and mechanical loading during the rolling process, leading to rapid wear and degradation.
Core Technical Content
Hot-rolled mill rolls are subjected to extreme conditions during operation: temperatures exceeding 900°C at the roll surface, high contact pressures, and continuous sliding friction. The overlay layer must therefore exhibit excellent wear resistance, thermal stability, and sufficient toughness to resist cracking under thermal cycling. This study systematically investigates how variations in microstructure and chemical composition influence these critical properties.
Microstructural Analysis
The overlay layer on hot-rolled rolls typically consists of a hardened matrix with dispersed carbide phases. The microstructure is primarily governed by the cooling rate from the welding process and the alloying element composition. Key features include:
- Matrix structure: Martensitic or austenitic matrix depending on composition and cooling rate
- Carbide morphology: Spherical, worm-like, or network carbides depending on the type of carbide-forming elements
- Grain size: Typically 10-50 μm in the overlay layer, with coarser grains near the substrate interface
- Phase distribution: Uniformity of carbide distribution significantly affects wear resistance
Composition-Property Relationships
The chemical composition of the overlay layer is the primary lever for controlling its performance characteristics. The following table summarizes the key compositional parameters and their effects:
| Element | Typical Content | Effect on Properties |
|---|---|---|
| Carbon (C) | 2.0-4.0% | Increases hardness, promotes carbide formation |
| Chromium (Cr) | 8-12% | Improves wear resistance, forms stable carbides |
| Molybdenum (Mo) | 2-4% | Enhances thermal stability, refines carbides |
| Vanadium (V) | 0.5-1.5% | Forms hard MC carbides, improves red hardness |
| Titanium (Ti) | 0.3-1.0% | Forms TiC carbides, resists coarsening at high temperature |
| Nickel (Ni) | 1-3% | Stabilizes austenite, improves toughness |
| Silicon (Si) | 0.5-2.0% | Deoxidizer, improves thermal stability |
Process-Structure-Property Chain
Understanding the process-structure-property relationship is essential for optimizing hot-rolled roll overlay layers. The following analysis traces the causal chain from welding process parameters through microstructure to final performance:
Process Parameters
The overlay process for hot-rolled rolls commonly employs electroslag welding, submerged arc welding, or arc surfacing. Key process parameters include:
- Heat input: 2-6 kJ/mm, influencing grain size and phase composition
- Cooling rate: Determined by roll diameter, ambient conditions, and quenching
- Deposition rate: Affects dilution and compositional uniformity
- Number of passes: Multi-pass deposition can improve compositional homogeneity
Microstructural Evolution
The cooling rate after overlay deposition determines the final microstructure. For high-carbon, high-chromium overlay compositions:
- At slow cooling rates (below 5°C/s), coarse pearlite or martensite with carbide precipitation forms, resulting in moderate hardness but lower thermal stability.
- At moderate cooling rates (5-20°C/s), fine martensite with dispersed carbides develops, providing an optimal balance of hardness and toughness.
- At rapid cooling rates (above 20°C/s), retained austenite may form, which can transform during service, potentially causing dimensional instability.
Performance Metrics
The key performance indicators for hot-rolled roll overlay layers include:
| Property | Target Value | Test Method |
|---|---|---|
| Hardness (as-deposited) | 55-65 HRC | Rockwell hardness test |
| Hardness (after thermal cycling) | 50-60 HRC | Simulated rolling conditions |
| Wear resistance | >5 g weight loss | Pin-on-disk test |
| Thermal fatigue life | >500 cycles | Thermal cycling test |
| Bond strength | >350 MPa | Peel test or bond strength test |
| Impact toughness | >20 J | Charpy impact test |
Engineering Practice Considerations
In my experience with roll overlay applications, the most common failure modes are thermal fatigue cracking, abrasive wear, and delamination from the base material. Each of these failures can be traced back to specific microstructural and compositional deficiencies identified in this type of research.
Thermal Fatigue Resistance
Thermal fatigue is the dominant failure mode for hot-rolled roll overlay layers. The repeated thermal cycling during rolling creates alternating tensile and compressive stresses at the roll surface. The overlay layer must have sufficient toughness to resist crack initiation and propagation. Key factors influencing thermal fatigue resistance include:
- Adequate toughness in the matrix to resist crack propagation
- Uniform carbide distribution to prevent stress concentration
- Sufficient ductility to accommodate thermal strain
- Proper bond strength to prevent delamination
Wear Resistance Optimization
Abrasive wear from scale and oxide particles during rolling is the second most common failure mode. Wear resistance is primarily determined by the hardness and morphology of the carbide phase. The following strategies improve wear resistance:
- Increasing the volume fraction of hard carbides (TiC, NbC, Cr7C3)
- Optimizing carbide size distribution for maximum resistance to abrasive particles
- Ensuring uniform carbide distribution throughout the overlay layer
- Maintaining a hard, wear-resistant matrix to support the carbide phase
Key Questions and Reflections
This study raises several important questions for engineering practice. First, what is the optimal balance between hardness and toughness for a given rolling condition? Second, how does the overlay composition interact with the base material composition to influence the dilution zone properties? Third, what are the long-term stability characteristics of the overlay microstructure under sustained thermal cycling?
The research also highlights the importance of considering the entire process-structure-property chain when designing overlay layers for hot-rolled rolls. A composition that provides excellent hardness may sacrifice toughness, leading to premature thermal fatigue failure. Conversely, a composition optimized for toughness may not provide sufficient wear resistance. The art of overlay design lies in finding the optimal compromise for the specific service conditions.
For engineers involved in roll maintenance and overlay technology, this work provides a valuable framework for understanding how microstructural features and compositional variables interact to determine the final performance of the overlay layer. The systematic approach to composition optimization, combined with careful process control, is essential for achieving reliable and long-lasting overlay layers in the demanding environment of hot rolling mills.