CLADDING TECHNOLOGY SHANXI CO., LTDDevelopment of Wear-Resistant Overlay Welding Electrodes for Hot Rolling Rolls
Background and Service Environment
Hot rolling mills represent one of the most demanding wear environments in metallurgical processing. Rolling mill rolls are subjected to extreme combinations of thermal cycling, mechanical loading, and chemical attack from scale and lubricants. The roll surface experiences temperatures up to 900–1100°C during hot rolling operations, followed by rapid cooling between passes. This thermal cycling, combined with compressive and shear stresses from the rolling force (typically 10–200 kN/cm of roll length), creates a unique degradation mechanism that conventional wear-resistant materials cannot adequately address.
The development of specialized overlay welding electrodes for hot rolling rolls requires a fundamentally different approach compared to general-purpose wear-resistant overlays. The overlay must maintain hardness at elevated temperatures, resist thermal fatigue cracking, accommodate thermal expansion mismatch with the roll body, and survive the repeated compressive loading of the rolling process.
Material Design Requirements
The following table summarizes the critical performance requirements for hot rolling roll overlays:
| Property | Requirement | Significance |
|---|---|---|
| Room temperature hardness | 500–750 HV | Adequate resistance to cold work deformation |
| Elevated temperature hardness (800°C) | >350 HV | Maintains wear resistance during hot rolling |
| Thermal fatigue resistance | >500 cycles to crack initiation | Survives thermal cycling between passes |
| Oxidation resistance | Scale thickness <50 μm at 900°C/100h | Minimizes scale buildup and roll diameter change |
| Impact toughness | >50 J at 25°C | Resists impact from scale removal and edge loading |
| Dilution tolerance | Acceptable performance at 30–40% dilution | Accommodates first-layer dilution on steel substrate |
The material design philosophy centers on a tempered martensite matrix with fine, dispersed carbide particles that retain their hardness at elevated temperatures. Unlike high carbon systems that rely on massive carbides for room temperature hardness, hot rolling roll overlays require a more refined microstructure that balances hardness with thermal stability.
Electrode Composition Development
The developed electrode composition for hot rolling rolls typically falls within the following ranges:
| Element | Content (wt%) | Function |
|---|---|---|
| C | 0.8 – 1.8 | Carbide former; matrix hardening |
| Cr | 8.0 – 15.0 | Primary carbide former; oxidation resistance |
| Mo | 2.0 – 5.0 | Red hardness; temper stability |
| V | 1.0 – 3.0 | Fine carbide former; wear resistance |
| W | 0 – 3.0 | Red hardness; solid solution strengthening |
| Mn | 1.5 – 3.0 | Matrix toughening; solidification control |
| Ni | 2.0 – 6.0 | Matrix ductility; reduces cracking susceptibility |
| Si | 0.3 – 1.0 | Deoxidizer; grain refinement |
The inclusion of vanadium is particularly significant for hot rolling applications. Vanadium carbides (VC, V₄C₃) are among the most thermally stable carbides, maintaining their hardness well above 600°C. The fine dispersion of V carbides (typically 0.2–1.0 μm) provides superior wear resistance at elevated temperatures compared to the coarser Cr carbides.
Microstructural Design for Thermal Stability
The microstructure of hot rolling roll overlays is designed to achieve the following features:
- Tempered martensite matrix: Provides a good combination of strength and toughness at both room and elevated temperatures
- Fine Cr carbides (Cr₇C₃, Cr₂₃C₆): Provide room temperature hardness and contribute to wear resistance
- Ultrafine V carbides (VC, V₄C₃): Provide thermal stability and resist coarsening during service
- Mo₂C particles: Contribute to red hardness and maintain strength above 600°C
The key to thermal stability lies in the carbide type and size distribution. Cr carbides tend to coarsen significantly above 600°C, losing their strengthening effect. In contrast, V carbides and Mo₂C remain stable up to 800–900°C, maintaining the overlay's hardness during hot rolling operations.
Thermal Fatigue Performance
Thermal fatigue is the dominant failure mode for hot rolling roll overlays. Each rolling pass subjects the roll surface to a rapid temperature rise (from ambient to 900–1100°C in seconds) followed by rapid cooling. This thermal cycling induces cyclic thermal stresses that accumulate damage over time.
The thermal fatigue life of the developed overlay was evaluated using a thermal shock test simulating hot rolling conditions:
| Test Condition | Cycles to Surface Crack | Notes |
|---|---|---|
| 25°C → 900°C → 25°C (air quench) | 350–500 | Standard thermal fatigue test |
| 25°C → 1000°C → 25°C (water quench) | 200–350 | Severe thermal shock |
| 25°C → 900°C → 25°C (air quench) with scale | 400–600 | Scale provides some thermal buffering |
| 25°C → 900°C → 25°C (air quench) with lubricant | 500–700 | Lubricant reduces peak temperature |
The results demonstrate that the overlay's thermal fatigue life is significantly influenced by the cooling rate and the presence of surface films. The combination of Ni and Cr in the matrix improves thermal fatigue resistance by reducing the thermal expansion coefficient and increasing the material's ability to accommodate thermal strain without cracking.
Welding Process Optimization
The welding of overlays onto hot rolling rolls requires careful process optimization to ensure proper bonding and minimize residual stresses:
| Parameter | Specification | Rationale |
|---|---|---|
| Preheat | 250–350°C | Reduces thermal gradient; prevents cracking |
| Interpass temperature | 250–350°C | Maintains uniform thermal state |
| Layer thickness | 1.5–2.5 mm | Controls residual stress per layer |
| Number of layers | 3–5 | Achieves total thickness of 6–12 mm |
| Post-weld heat treatment | 550–650°C for 2–4h | Tempering; stress relief; carbide spheroidization |
| Weld direction | Spiral (helical) | Distributes residual stress uniformly |
The spiral welding pattern is preferred over transverse or longitudinal patterns because it distributes the residual stress more uniformly around the roll circumference, reducing the risk of localized cracking and distortion. The post-weld heat treatment is critical for hot rolling roll overlays, as it serves to temper the martensitic structure, relieve residual stresses, and promote carbide spheroidization — all of which improve the overlay's resistance to thermal fatigue.
Field Performance and Service Evaluation
The developed hot rolling roll overlay electrode was evaluated in actual hot rolling mill service:
| Application | Roll Type | Overlay Thickness | Service Life Improvement |
|---|---|---|---|
| Hot strip mill | Backup roll | 8 mm | 2.5–3.5× original life |
| Hot strip mill | Work roll | 6 mm | 2.0–3.0× original life |
| Hot bar mill | Reducing roll | 10 mm | 3.0–4.0× original life |
| Hot strip mill | Edger roll | 8 mm | 2.0–2.5× original life |
The service life improvements are attributed to the combination of improved wear resistance at elevated temperatures, enhanced thermal fatigue resistance, and better oxidation resistance compared to the original bare roll surface. The most significant improvements were observed in reducing mill rolls, where the overlay's ability to resist both wear and thermal cracking was most beneficial.
Defect Analysis and Quality Control
Common defects encountered during hot rolling roll overlay welding include:
| Defect | Cause | Prevention |
|---|---|---|
| Thermal fatigue cracks | Excessive thermal stress; coarse carbides | Optimize PWHT; control carbide size |
| Scaling and spalling | Poor oxidation resistance; thermal shock | Increase Cr content; improve cooling rate control |
| Centerline cracking | Segregation; residual stress | Control travel speed; maintain interpass temperature |
| Poor bond strength | Insufficient preheat; contamination | Increase preheat; clean surface thoroughly |
| Excessive hardness variation | Inconsistent dilution; parameter drift | Monitor dilution; maintain process discipline |
Quality control for hot rolling roll overlays requires non-destructive testing (preferably magnetic particle testing for surface cracks and ultrasonic testing for subsurface defects) after each layer and after post-weld heat treatment. Hardness profiling across the overlay depth should be performed to verify that the dilution zone meets minimum hardness requirements.
Key Technical Insights and Reflections
The development of hot rolling roll overlay electrodes highlights the importance of understanding the specific degradation mechanisms in the target application. Unlike general-purpose wear-resistant overlays, hot rolling roll overlays must be designed for a unique combination of thermal cycling, mechanical loading, and chemical attack. The traditional approach of maximizing room temperature hardness is insufficient; instead, the design must prioritize thermal stability, fatigue resistance, and oxidation resistance.
A key insight from this work is that the optimal overlay for hot rolling is not the hardest overlay, but rather the overlay that best balances hardness, toughness, thermal stability, and oxidation resistance. This requires a holistic approach to material design that considers the entire service environment rather than optimizing a single property.
Summary and Conclusions
The development of wear-resistant overlay welding electrodes for hot rolling rolls represents a specialized application of overlay technology that demands careful attention to thermal stability, fatigue resistance, and process optimization. The successful integration of Cr, Mo, and V carbide systems into a tempered martensitic matrix provides an overlay that maintains its protective function throughout the extreme thermal and mechanical cycling of hot rolling operations. Field performance data confirms significant service life improvements, validating the material design approach and process optimization strategies. Continued research should focus on further improving thermal fatigue resistance and developing predictive models for overlay life under complex rolling conditions.