Thermal Fatigue Performance of Overlay Metals on Roll Cladding

Literature Overview and Background

Roll cladding is a widely employed technique in hot rolling mills to extend the service life of work rolls exposed to extreme thermal and mechanical loading conditions. The literature under review focuses on the thermal fatigue performance of overlay metals deposited on roll blanks, addressing the critical challenge of balancing wear resistance with thermal fatigue crack resistance. Hot strip mill work rolls experience cyclic temperature variations of 200-400°C per roll revolution, generating thermal stresses that initiate and propagate cracks in the overlay layer.

The study investigates how overlay composition, microstructure, and residual stress state influence thermal fatigue crack initiation and propagation, providing guidance for selecting overlay materials and welding procedures that maximize roll life in aggressive hot rolling environments.

Core Technical Content

Thermal Fatigue Mechanisms in Roll Overlay

Thermal fatigue in roll overlays arises from cyclic thermal stresses generated by the differential expansion and contraction of the overlay surface relative to the cooler roll core. The thermal stress magnitude can be estimated using:

σ_th = E × α × ΔT / (1 - ν)

where E is Young's modulus, α is the coefficient of thermal expansion, ΔT is the temperature amplitude, and ν is Poisson's ratio. For typical hot rolling conditions with ΔT = 300°C, the induced thermal stress in a high-carbon martensitic overlay can reach 300-500 MPa, approaching the yield strength of the material.

The study identifies three primary thermal fatigue crack initiation sites:

Overlay Material Comparison

The study evaluates several overlay material systems for thermal fatigue performance:

The study reveals that austenitic and nodular cast iron overlays exhibit superior thermal fatigue life due to their higher ductility and capacity for plastic deformation under cyclic thermal stress. However, these materials may sacrifice some wear resistance compared to martensitic overlays, necessitating a careful trade-off analysis based on the specific rolling mill application.

Process Analysis and Standards

Welding Procedure Requirements

The thermal fatigue performance of the overlay is significantly influenced by the welding procedure employed. The study compares electroslag welding (ESW), submerged arc welding (SAW), and gas metal arc welding (GMAW) for roll cladding:

• ESW: Produces a coarse but uniform microstructure with lower residual stresses due to the self-healing effect of the slag pool. Suitable for thick overlay layers (10-25 mm) with good thermal fatigue performance.
• SAW: Offers good productivity and moderate residual stresses. Multi-pass SAW with appropriate interpass temperature control can produce fine-grained martensitic microstructures with acceptable thermal fatigue resistance.
• GMAW: Provides excellent control over heat input and is suitable for thin overlay layers (2-5 mm) or repair applications. However, higher residual stresses may compromise thermal fatigue performance if not properly managed.

The relevant standards governing roll overlay welding include NB/T 47014 for weld procedure qualification, GB/T 150 for pressure vessel fabrication (where rolls may be classified), and industry-specific standards such as those issued by the International Roll Manufacturers Association (IRMA).

Residual Stress Management

Residual stresses from welding can either accelerate or retard thermal fatigue crack initiation depending on their sign and magnitude. Compressive residual stresses at the overlay surface are beneficial, as they oppose the tensile thermal stresses generated during heating cycles. The study recommends the following strategies for promoting beneficial residual stress states:

1. Multi-pass welding with the final pass providing surface peening effect.
2. Controlled cooling rates to minimize thermal gradients and associated residual stresses.
3. Post-weld stress relief at 550-650°C for martensitic overlays, followed by controlled cooling to avoid re-austenitization.
4. Mechanical peening or shot peening of the overlay surface to introduce surface compressive stresses.

Common Defects and Countermeasures

Thermal fatigue cracking manifests in several characteristic patterns that can be identified through surface inspection and non-destructive testing:

Engineering Practice Integration

In hot strip mill operations, roll life is typically measured in tonnage of steel processed before re-grinding or replacement is required. The study's findings have direct implications for roll selection and maintenance strategies. For example, in a 2000 mm wide hot strip mill rolling low-carbon steel at 1000-1200°C, work rolls with high-chromium martensitic overlays may achieve 30,000-50,000 tons of throughput before thermal fatigue cracking necessitates roll replacement, while austenitic overlays may extend this to 60,000-80,000 tons at the cost of increased wear rate requiring more frequent re-grinding.

The FMEA (Failure Mode and Effects Analysis) approach can be applied to roll overlay thermal fatigue by systematically identifying potential failure modes, their causes, and their effects on mill operation. A typical FMEA entry might identify "circumferential cracking at the overlay-base metal interface" as the failure mode, "excessive residual tensile stress" as the cause, "unplanned roll change and mill downtime" as the effect, with a recommended action of "implementing post-weld stress relief and residual stress measurement."

Key Questions and Reflections

The study raises important considerations regarding the scalability of thermal fatigue test results from laboratory coupons to full-size rolls. Laboratory thermal fatigue tests typically employ small specimens with simplified thermal cycling protocols, whereas full-size rolls experience complex thermal loading involving simultaneous radial and circumferential temperature gradients, mechanical contact stress from the rolled strip, and chemical interaction with scale and lubricant residues. The study acknowledges this limitation but suggests that relative comparisons between overlay materials remain valid even if absolute life predictions require correction factors.

Another significant question is the interaction between thermal fatigue and wear. In practice, rolls are periodically re-grinded to remove worn surface layers, which also removes shallow thermal fatigue cracks. However, if thermal fatigue cracks extend beyond the grinding depth, the remaining crack can propagate rapidly during subsequent service, leading to catastrophic roll failure. The study recommends that thermal fatigue crack depth be monitored during re-grinding operations using ultrasonic testing, with a maximum permissible crack depth of 0.5 mm below the final ground surface.

Study Insights and Implications

The literature provides a comprehensive framework for understanding thermal fatigue in roll overlays, emphasizing the critical role of material ductility, residual stress state, and microstructural stability. The finding that austenitic and nodular cast iron overlays outperform martensitic overlays in thermal fatigue resistance challenges the conventional assumption that higher hardness always correlates with better roll performance. This insight has significant implications for material selection strategies in hot rolling mill operations.

The study's emphasis on residual stress management as a key lever for improving thermal fatigue performance is particularly practical. Engineers can implement residual stress measurement and control procedures using established techniques such as X-ray diffraction and hole-drilling methods, without requiring fundamental changes to the welding process or overlay material selection.

Conclusion

The thermal fatigue performance of overlay metals on roll cladding is governed by a complex interplay of material properties, microstructure, residual stresses, and thermal loading conditions. This literature provides valuable guidance for engineers seeking to optimize roll overlay selection and welding procedures for maximum service life in hot rolling applications. The key insight is that thermal fatigue resistance cannot be improved by simply increasing hardness; rather, it requires a holistic approach that considers ductility, thermal conductivity, residual stress state, and microstructural stability. Engineers should adopt a systematic approach combining material selection, procedure optimization, and in-service monitoring to achieve the best possible roll performance.