Study Note on Thermal Fatigue Performance of Rolled Roll Cladding Metal

Literature Overview

This paper published in Welding Journal (2001, Vol. 22, No. 2, pp. 19-22) by Feng Lingzhi, Li Wushen, Song Bingzhang, and Song Qingyi from Tianjin University and Xingtai Special Rolling Mill Factory, investigates the thermal fatigue performance of cladding metals used on rolling mill rolls. The research was supported by the Hebei Provincial Major Science and Technology Project (95-98-10). The authors designed a thermal fatigue testing methodology and evaluated three self-developed flux-cored wires and one domestic solid wire for roll cladding, examining thermal fatigue resistance, oxidation resistance, and the mechanisms of thermal fatigue crack initiation and propagation.

Core Technical Content

Rolling mill rolls operate under extreme thermal cycling conditions, where the roll surface alternates between high temperatures (from contact with hot rolled stock) and cooler ambient or coolant temperatures. This thermal cycling induces thermal fatigue, which is a primary failure mode for roll cladding layers. The study addresses three critical aspects:

1. Thermal fatigue crack initiation and propagation mechanisms
2. The role of oxidation in crack formation
3. The influence of microstructure and inclusions on thermal fatigue resistance

Thermal Fatigue Crack Mechanism

The study establishes that thermal fatigue cracks form and propagate through the combined action of cyclic thermal stresses and oxidation. The key findings are:

• Crack initiation: Occurs preferentially at surface sites where oxidation is most severe, often at grain boundaries or inclusion sites
• Crack propagation: Accelerated by oxidation at the crack tip, which weakens the material and facilitates crack growth
• Oxidation resistance: Higher oxidation resistance correlates with improved crack initiation resistance

The authors developed a mechanistic model diagram illustrating the thermal fatigue crack formation and propagation process, which integrates thermal stress cycling with oxidation kinetics.

Microstructural Effects

The study emphasizes that microstructural uniformity is critical for thermal fatigue performance. Inhomogeneities such as segregation zones, unmelted flux particles, or uneven carbide distribution create stress concentration sites that accelerate crack initiation.

Inclusion Effects

Inclusions—particularly oxide inclusions from flux contamination or non-metallic inclusions from the base metal—are identified as significant crack initiation sites. The study demonstrates that reducing inclusion content and improving inclusion morphology (spherical rather than elongated) significantly improves thermal fatigue life.

Thermal Fatigue Testing Methodology

The authors designed a thermal fatigue testing method, which likely involves:

• Thermal cycling between defined high and low temperatures
• Controlled cycling frequency
• Defined number of cycles to failure
• Monitoring of crack initiation and propagation

Typical thermal fatigue test parameters for roll cladding include:

• High temperature: 800-1000°C (representing hot rolled stock temperature)
• Low temperature: 20-200°C (representing cooling or ambient conditions)
• Cycling frequency: 0.1-1 Hz
• Test environment: Air (oxidizing)

Engineering Practice Considerations

Material Selection

Based on the study's findings, engineers should prioritize:

• Welding consumables with high oxidation resistance (Ni-Cr based alloys with sufficient Cr content)
• Clean melting practices to minimize inclusions
• Consistent microstructure through controlled solidification

Process Optimization

To achieve uniform microstructure:

• Control welding parameters to maintain consistent heat input
• Use appropriate preheating to reduce thermal gradients
• Apply interpass temperature control for multi-pass cladding
• Implement proper surface preparation to minimize contamination

Quality Control

Non-destructive testing should focus on:

• Surface crack detection (MT or PT)
• Subsurface inclusion assessment (UT or macrograph examination)
• Hardness mapping to identify microstructural inhomogeneities

Study Insights and Reflections

This paper provides fundamental insights into the thermal fatigue behavior of roll cladding metals, with direct implications for roll life extension in hot rolling operations. The emphasis on oxidation resistance as a key factor in thermal fatigue performance is particularly important for engineers selecting cladding materials for hot rolling applications.

The mechanistic model developed by the authors offers a framework for predicting thermal fatigue life based on material properties and service conditions. This model can be used to screen candidate materials and optimize process parameters before full-scale testing.

The finding that inclusions significantly reduce thermal fatigue life underscores the importance of consumable cleanliness. In production environments, this translates to strict control of welding consumable storage, handling, and baking practices to minimize moisture and contamination.

The study also highlights the importance of microstructural uniformity, which can be achieved through:

• Consistent welding parameters across the cladding area
• Proper welder training and technique
• Regular process monitoring and adjustment

Reference Value and Outlook

The research provides a comprehensive understanding of thermal fatigue mechanisms in roll cladding metals, with practical guidance for material selection and process optimization. Future work should explore:

• Advanced characterization techniques (such as synchrotron X-ray diffraction) for in-situ study of thermal fatigue
• Computational modeling of thermal fatigue crack propagation
• Development of novel cladding alloys with improved thermal fatigue resistance
• Life prediction models based on field data from rolling mill operations

Engineers working on roll cladding applications should adopt a holistic approach that integrates material selection, process control, and quality assurance to maximize roll life and minimize downtime.