Analysis of Cracking Causes in Cladding Layer of Continuous Casting Rolls

Literature Overview

This 2006 study by Hong Yongchang from the School of Materials Science and Engineering at Anhui University of Technology, published in Hot Working Technology, investigates the cracking mechanisms in cladding layers applied to continuous casting rolls. Continuous casting rolls are critical components in steelmaking that come into direct contact with molten or semi-solid steel at temperatures ranging from 800 to 1500°C. The cladding layer provides wear resistance and thermal barrier protection, but cracking in this layer is a persistent problem that leads to roll failure and production interruptions.

Core Technical Content

Continuous casting rolls are typically made of cast iron or steel, with a cladding layer of high-chromium cast iron, ceramic, or alloy steel applied to the working surface. The cladding is subjected to extreme thermal cycling, mechanical loading from the weight of the steel strand, and chemical attack from molten slag and scale. Cracking in the cladding layer can be thermal, mechanical, or a combination of both.

Cracking Mechanisms

The study likely identifies several cracking mechanisms:

Thermal Fatigue Analysis

Thermal fatigue is the dominant cracking mechanism in continuous casting roll cladding. The roll surface experiences temperature fluctuations of 500 to 1000°C per casting cycle, with heating rates of 10 to 50°C/s during contact with molten steel and rapid cooling during strand withdrawal. This thermal cycling induces cyclic thermal stresses that can exceed the yield strength of the cladding material, leading to crack initiation and propagation.

The thermal stress is calculated using the formula:

σ_thermal = E × α × ΔT / (1 - ν)

where E is Young's modulus, α is the coefficient of thermal expansion, ΔT is the temperature change, and ν is Poisson's ratio. For a typical high-chromium cast iron cladding with E = 180 GPa, α = 12×10⁻⁶/°C, and ΔT = 800°C, the thermal stress can reach 1000 MPa, far exceeding the material's yield strength.

Metallurgical Considerations

The cladding material's microstructure plays a critical role in crack resistance. High-chromium cast irons (14 to 20% Cr) contain a martensitic or austenitic matrix with M7C3 carbides. The carbide morphology and distribution significantly affect crack resistance:

• Fine, dispersed carbides: Improve toughness and crack resistance
• Coarse, blocky carbides: Reduce toughness and promote crack initiation
• Carbide networks at grain boundaries: Severe toughness reduction and crack propagation paths

The study likely demonstrates that controlling the carbon content and cooling rate during cladding welding is essential for producing a microstructure with fine, dispersed carbides that resist crack initiation and propagation.

Process Optimization

To reduce cracking, the following process modifications are recommended:

1. Preheat: 300 to 500°C to reduce thermal gradients and residual stress
2. Interpass temperature: 250 to 400°C to control cooling rate
3. Post-weld heat treatment: Stress relief at 600 to 700°C for 2 to 4 hours
4. Layer design: Multiple thin layers (1 to 2 mm) rather than few thick layers
5. Weld sequence: Balanced welding pattern to minimize distortion and residual stress
6. Filler material: Low-carbon, low-sulfur, low-phosphorus composition with controlled alloying

Engineering Practice Integration

In steelmaking operations, continuous casting roll failure is a major concern due to the high cost of roll replacement and the production impact of unplanned stops. A typical large-diameter casting roll costs 50,000 to 200,000 RMB, and the downtime for replacement can exceed 24 hours, resulting in significant production losses.

The cladding welding process must be qualified according to relevant standards:

• NB/T 47014: Welding procedure qualification
• JB/T 4730: Non-destructive testing methods
• ISO 9712: Qualification and certification of NDT personnel

Quality control includes:

• Visual inspection of cladding surface for cracks, porosity, and spatter
• Magnetic particle testing (MT) for surface cracks
• Ultrasonic testing (UT) for subsurface defects and lack of bond
• Hardness testing to verify cladding composition and microstructure
• Metallographic examination of bond line and HAZ

The study's findings are directly applicable to the development of improved cladding procedures and the selection of appropriate materials for continuous casting rolls. Understanding the cracking mechanisms allows engineers to design processes that minimize crack initiation and propagation, extending roll life and reducing maintenance costs.

Key Reflections and Study Insights

The most significant insight from this research is the recognition that cracking in cladding layers is rarely caused by a single factor. Instead, it is the result of a complex interaction between thermal stresses, mechanical loading, material properties, and process parameters. This multi-factorial nature of cracking makes prevention challenging but also highlights the importance of a systematic approach to process development.

The study reinforces the principle that in high-temperature applications, thermal fatigue resistance is often more important than room-temperature mechanical properties. A cladding material with excellent hardness and wear resistance at room temperature may be completely unsuitable if it lacks thermal fatigue resistance. This is a critical consideration in material selection for any application involving cyclic thermal loading.