Effect of Microstructure on Properties in Rolling Mill Roll Cladding

Literature Overview

This study provides an in-depth analysis of how the microstructure of the overlay layer and the heat-affected zone (HAZ) in rolling mill roll cladding influences the mechanical properties, wear resistance, and fatigue performance of the cladded rolls. Rolling mill rolls are critical components in metal processing industries, and their cladding layers must withstand extreme thermal and mechanical loading. The research focuses on the relationship between microstructural features such as carbide morphology, grain size, phase distribution, and phase transformation products, and their correlation with hardness, toughness, wear resistance, and thermal fatigue resistance. The study employs advanced characterization techniques including scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and nanoindentation to provide a comprehensive microstructure-property relationship.

Microstructural Characterization

The overlay layer was deposited using a nickel-based alloy consumable (IN718-type) by plasma transferred arc (PTA) welding on a 45CrMo steel roll blank. The overlay microstructure was characterized at multiple locations: the weld centerline, the weld edge, and the fusion zone. The weld centerline exhibited a columnar dendritic microstructure with primary carbides of NbC and TiC dispersed throughout. The weld edge showed a more equiaxed microstructure due to the different thermal gradient direction. The fusion zone exhibited a narrow transition region with martensite formation due to the rapid cooling from the base metal side.

The following table summarizes the microstructural features and their corresponding mechanical properties at different locations:

The data reveal a clear trend: regions with finer microstructures and higher carbide content exhibit higher hardness but lower fracture toughness. The fusion zone represents the weakest link in terms of toughness, with values as low as 10 MPa·m^1/2, which is a concern for fatigue resistance under thermal cycling.

Wear Resistance and Thermal Fatigue Performance

The wear resistance of the overlay layer was evaluated using a pin-on-disc tribometer under dry sliding conditions against a Si3N4 ball. The wear rate was measured at 0.3 mg/1000 m for the weld centerline and 0.4 mg/1000 m for the weld edge. The superior wear resistance at the weld centerline is attributed to the higher volume fraction of hard NbC and TiC carbides. However, the thermal fatigue resistance was evaluated by cyclic heating and cooling between 20 and 800 degrees Celsius. The number of cycles to cracking was 1200 for the weld centerline and 1800 for the weld edge, indicating that the equiaxed microstructure at the weld edge provides better resistance to thermal fatigue cracking.

This apparent contradiction between wear resistance and thermal fatigue resistance highlights a critical design challenge in rolling mill roll cladding. The wear-resistant microstructure (high carbide content, fine grains) is not necessarily the most thermally fatigue-resistant. The study recommends a graded overlay approach where the surface layer is optimized for wear resistance and the subsurface layer is designed for thermal fatigue resistance.

Heat Treatment Optimization

The study evaluated the effect of post-weld heat treatment on the microstructure and properties of the overlay layer. A solution heat treatment at 1050 degrees Celsius followed by air cooling produced a uniform austenitic microstructure with dissolved carbides, resulting in a hardness of approximately 280 HV but excellent toughness of 30 MPa·m^1/2. A precipitation hardening treatment at 720 degrees Celsius for 8 hours followed by aging at 620 degrees Celsius for 6 hours produced a microstructure with fine gamma-prime precipitates, achieving a hardness of 380 HV and a toughness of 25 MPa·m^1/2.

The optimal heat treatment for rolling mill roll cladding was identified as a two-step process: first, a solution treatment at 1080 degrees Celsius for 1 hour to homogenize the microstructure, followed by a precipitation hardening treatment at 720 degrees Celsius for 8 hours. This sequence produced a balanced combination of hardness (390 HV), toughness (24 MPa·m^1/2), and thermal fatigue resistance (1500 cycles to cracking).

Study Insights and Engineering Practice

The most valuable insight from this study is the recognition that microstructure is the fundamental determinant of overlay performance, and that microstructure can be controlled through careful selection of welding parameters, consumable composition, and post-weld heat treatment. The study provides a clear framework for engineers to optimize the overlay microstructure for specific service conditions. For hot rolling mill rolls, where both wear resistance and thermal fatigue resistance are required, a graded or multi-layer overlay approach with different microstructures in different depth regions is recommended.

The study also emphasizes the importance of non-destructive testing (NDT) in detecting microstructural defects that may not be visible on the surface. Ultrasonic testing (UT) and phased array ultrasonic testing (PAUT) are recommended for detecting internal cracking and lack of bond in the overlay layer. Magnetic particle testing (MT) and liquid penetrant testing (PT) should be used for surface and near-surface defect detection. The combination of multiple NDT methods provides the most comprehensive inspection coverage for critical cladding applications.