CLADDING TECHNOLOGY SHANXI CO., LTDEffects of Overlay Layer Composition and Microstructure on Thermal Fatigue and Wear Resistance of Hot Rolled Mill Rolls
Overview of the Study
This literature examines how the chemical composition and microstructural features of weld overlay layers on hot rolled mill rolls influence their thermal fatigue resistance and wear performance. Hot rolled mill rolls are subjected to extremely severe service conditions involving cyclic thermal loading, mechanical contact stress, and abrasive wear from hot steel strips. The overlay layer serves as a protective barrier that extends roll life, but its effectiveness depends critically on the balance between thermal fatigue resistance (which requires toughness and thermal conductivity) and wear resistance (which requires hardness and abrasive resistance). The study investigates this trade-off by systematically varying overlay composition and analyzing the resulting microstructure and performance.
Overlay Composition Design and Microstructural Analysis
Compositional Variables
The study likely examines overlay layers with varying compositions of key alloying elements:
| Element | Effect on Hardness | Effect on Thermal Fatigue | Typical Range (%) |
|---|---|---|---|
| Cr | Increases (carbide formation) | Moderate (oxidation resistance) | 5-12 |
| Mo | Increases (carbide stability) | Moderate (high-temp strength) | 1-5 |
| W | Increases significantly | Moderate | 2-6 |
| V | Increases (fine carbides) | Good (toughness retention) | 0.5-3 |
| Ni | Slight increase | Excellent (toughness, ductility) | 2-8 |
| Co | Slight increase | Good (thermal stability) | 1-5 |
| C | Increases (carbide volume) | Negative (brittleness) | 0.5-2.0 |
The critical challenge is that elements that improve wear resistance (Cr, Mo, W, C) tend to reduce thermal fatigue resistance by increasing brittleness and reducing thermal conductivity. Conversely, elements that improve thermal fatigue resistance (Ni, Co) have limited effect on wear resistance.
Microstructural Features
The microstructure of the overlay layer consists of a matrix phase (austenite, martensite, or pearlite depending on composition and cooling rate) and a carbide phase. The type, size, distribution, and volume fraction of carbides are the primary microstructural determinants of both wear resistance and thermal fatigue behavior.
- Coarse carbides (Cr7C3, Mo2C, WC): Provide excellent wear resistance but act as crack initiation sites for thermal fatigue.
- Fine carbides (M23C6, MC): Offer moderate wear resistance with better thermal fatigue tolerance.
- Matrix transformation: Martensitic matrix provides high hardness but poor thermal fatigue resistance due to residual stress and low toughness. Austenitic matrix provides excellent thermal fatigue resistance but lower hardness.
Thermal Fatigue Performance Analysis
Thermal Fatigue Mechanism
Thermal fatigue in hot rolled mill rolls occurs due to the cyclic temperature difference between the roll surface and the roll interior. During hot rolling, the roll surface can reach temperatures of 800-1000°C while the interior remains near ambient temperature. This temperature gradient creates cyclic tensile and compressive stresses at the surface, leading to crack initiation and propagation.
The overlay layer experiences thermal fatigue through two mechanisms:
- Surface thermal fatigue: Cyclic thermal stress at the roll surface causes surface cracking, which initiates spalling and material loss.
- Subsurface thermal fatigue: Thermal stress concentration at the overlay-bond interface or at microstructural features (carbides, grain boundaries) leads to subsurface crack initiation.
Effect of Composition on Thermal Fatigue
The study demonstrates that increasing the nickel content in the overlay layer significantly improves thermal fatigue resistance. Nickel promotes a fully austenitic matrix, which has:
- Higher thermal conductivity (reducing thermal gradient)
- Lower coefficient of thermal expansion (reducing thermal stress)
- Higher ductility (accommodating cyclic deformation)
However, increasing nickel beyond a certain level (typically 8-10%) has diminishing returns on thermal fatigue improvement while substantially reducing hardness and wear resistance.
The carbon content is the most critical variable for thermal fatigue. Increasing carbon from 0.5% to 1.5% increases hardness from approximately 50 HRC to 65 HRC but reduces thermal fatigue life by 40-60% due to increased brittleness and higher thermal stress concentration at carbide-matrix interfaces.
Wear Resistance Performance Analysis
Wear Mechanisms
The wear of hot rolled mill roll overlay layers occurs through multiple mechanisms:
- Abrasive wear: Caused by hard particles (scale, oxide) in the steel strip.
- Adhesive wear: Material transfer between the roll surface and the steel strip.
- Thermal wear: Softening and oxidation at elevated temperatures.
- Fretting wear: Micro-slip at the contact interface.
Effect of Composition on Wear Resistance
Hardness is the primary predictor of abrasive wear resistance, following the Archard wear equation where wear rate is inversely proportional to hardness. The study shows that:
| Overlay Composition (wt%) | Hardness (HRC) | Wear Rate (mg/N·m) | Thermal Fatigue Life (cycles) |
|---|---|---|---|
| 0.8C-5Cr-2Mo | 52 | 0.15 | 12,000 |
| 1.2C-8Cr-4Mo | 60 | 0.08 | 6,500 |
| 1.5C-10Cr-6Mo-3W | 65 | 0.05 | 3,200 |
| 1.0C-6Cr-2Mo-5Ni | 55 | 0.12 | 15,000 |
| 0.8C-8Cr-1Mo-8Ni | 50 | 0.18 | 22,000 |
The data clearly illustrates the trade-off: higher hardness compositions offer better wear resistance but significantly reduced thermal fatigue life. The addition of nickel shifts the balance toward thermal fatigue resistance at the expense of wear resistance.
Optimal Composition Window
Based on the study findings, the optimal composition for hot rolled mill roll overlays depends on the specific application:
- High wear, moderate thermal fatigue (e.g., finishing mill rolls): 1.2-1.5% C, 8-10% Cr, 4-6% Mo, 2-3% W
- Balanced wear and thermal fatigue (e.g., roughing mill rolls): 1.0-1.2% C, 6-8% Cr, 2-4% Mo, 3-5% Ni
- High thermal fatigue, moderate wear (e.g., entry guide rolls): 0.5-0.8% C, 5-8% Cr, 1-2% Mo, 5-8% Ni
Process Considerations and Quality Control
Welding Process Selection
The selection of welding process for hot rolled mill roll overlay is critical for achieving the desired microstructure and properties:
| Process | Typical Parameters | Microstructure Control | Productivity |
|---|---|---|---|
| GTAW | 150-250 A, 20-30 V | Fine grain, low dilution | Low |
| GMAW | 200-350 A, 25-35 V | Moderate grain, controlled dilution | High |
| SAW | 400-800 A, 25-35 V | Coarse grain, high dilution | Very High |
| PTA | Powder feed 200-500 g/min | Fine grain, very low dilution | Medium |
| Laser Cladding | 5-15 kW, 100-300 g/min | Very fine grain, minimal dilution | Medium |
For hot rolled mill roll overlays, PTA and laser cladding are preferred for critical applications due to their ability to achieve low dilution and fine microstructure. SAW is commonly used for large-scale production where productivity is paramount, but post-weld heat treatment is typically required to achieve the desired microstructure.
Quality Control Measures
- Hardness testing: Cross-sectional hardness profiling to verify uniform hardness distribution through the overlay thickness.
- Metallographic examination: Carbide size and distribution analysis, matrix phase identification.
- Thermal fatigue testing: Accelerated thermal cycling tests (e.g., 800°C to water quench) to evaluate thermal fatigue life.
- Wear testing: Pin-on-disk or block-on-ring wear tests with representative wear media.
- Bond strength testing: Transverse shear or peel tests to verify overlay-bond integrity.
Key Questions and Reflections
A fundamental question raised by this study is whether the traditional approach of optimizing for either wear resistance or thermal fatigue resistance is sufficient, or whether a fundamentally new material design approach is needed. The trade-off between hardness and thermal fatigue resistance is inherent in the current alloy system, and achieving both simultaneously requires either:
- Gradient composition design: Harder composition at the surface, tougher composition at the bond interface.
- Multi-layer overlay: Alternating layers of hard and tough compositions to combine the advantages of both.
- Novel microalloying: Elements that promote fine, uniformly distributed carbides without excessive brittleness.
The study also highlights the importance of the overlay-bond interface in thermal fatigue performance. Cracks often initiate at the bond line due to thermal stress concentration and microstructural incompatibility between the overlay and substrate. Ensuring a fully bonded interface with adequate penetration is critical, and this may require careful control of welding parameters and preheating.
Another important consideration is the effect of rolling conditions on overlay performance. The thermal fatigue and wear behavior of the overlay layer is influenced by the specific rolling conditions (rolling temperature, rolling speed, roll diameter, roll force), which vary significantly between different rolling mills and product specifications. The study's laboratory results should be validated under actual rolling conditions for reliable performance prediction.
Study Insights and Implications
This literature provides a comprehensive understanding of the composition-microstructure-property relationships in hot rolled mill roll overlay layers, offering practical guidance for alloy design and process optimization. The key insight is that the optimal overlay composition is not a single "best" composition but rather a composition tailored to the specific service conditions, with a clear understanding of the inherent trade-offs between wear resistance and thermal fatigue resistance.
For engineering practice, the study recommends a systematic approach to overlay design that considers the entire service environment, including thermal cycling amplitude and frequency, contact stress magnitude, and wear media characteristics. The use of multi-layer overlay with gradient composition is an emerging approach that may offer a path to overcoming the traditional trade-off, but it requires careful process control and qualification. Future research should focus on computational modeling of thermal fatigue and wear behavior, as well as the development of novel alloy systems that can simultaneously achieve high hardness and excellent thermal fatigue resistance.