Preliminary Study on Roll Cladding Process

Literature Overview and Research Background

Roll cladding is a critical technology in the steel rolling industry, where work rolls and backup rolls experience extreme conditions including high temperatures (up to 900 degrees Celsius at the roll surface), intense contact pressures, and severe abrasive and adhesive wear. The traditional approach of manufacturing entire rolls from expensive alloy materials is economically impractical for large-diameter rolls. Weld overlay cladding of the roll surface with specialized wear-resistant alloys provides a cost-effective solution that combines the toughness and strength of a steel substrate with the surface durability of a specialized alloy. This literature presents a preliminary investigation into roll cladding process development, focusing on process parameter optimization, microstructural control, and performance evaluation for hot rolling mill applications.

Roll Cladding Requirements and Material Selection

Hot rolling mill work rolls must withstand combined thermal fatigue, abrasive wear from oxide scale, and adhesive wear from the rolled stock. The cladding material must also exhibit good thermal conductivity to facilitate heat dissipation and adequate thermal fatigue resistance to withstand repeated heating and cooling cycles.

For hot rolling applications, the Ni-Cr-Mo alloy was selected as the primary cladding material due to its superior thermal fatigue resistance and acceptable wear rate, despite lower hardness compared to high-speed steel alternatives.

Process Development and Parameter Optimization

Roll cladding presents unique process challenges due to the large diameter of the roll, the requirement for circumferential uniformity, and the need to maintain dimensional accuracy within tight tolerances. The following processes were evaluated:

Submerged arc welding with roll rotation was selected as the primary process for the bulk cladding layer, supplemented by GMAW for surface finishing and repair. The roll is mounted on a rotating fixture with controlled rotation speed synchronized to the welding travel speed to achieve uniform circumferential deposition.

Welding Sequence and Thermal Management

The welding sequence is designed to minimize thermal distortion and ensure uniform properties:

1. Surface preparation: grind the roll surface to Ra 3.2-6.3 um, remove all contaminants
2. First pass: low-heat-input SAW with dilution-reducing alloy at 15-20 kJ/mm
3. Intermediate passes: standard SAW at 20-28 kJ/mm for bulk build-up
4. Final pass: GMAW finishing pass at 10-14 kJ/mm for surface quality
5. Post-weld stress relief: furnace treatment at 600-650 degrees Celsius for 4 hours per 25 mm thickness

The total cladding thickness for hot rolling work rolls typically ranges from 8-15 mm, with the first 2-3 mm serving as a transition layer to manage dilution effects.

Microstructural Characteristics and Performance

The cladding layer microstructure consists of a fine-grained austenitic-ferritic matrix with dispersed M7C3 and M23C6 carbides. The grain size at the surface is approximately 15-25 micrometers, finer than in the bulk due to the higher cooling rate at the free surface. The hardness distribution shows a gradient from 320 HV at the interface to 380-400 HV at the surface.

Thermal fatigue testing conducted at 800 degrees Celsius with cyclic heating and cooling at a rate of 50 degrees C per minute demonstrates that the cladding layer withstands over 60,000 cycles before the first crack initiation, compared to approximately 20,000 cycles for the uncladded roll substrate. The improved thermal fatigue performance is attributed to the lower thermal expansion coefficient mismatch between the cladding alloy and the roll substrate, combined with the compressive residual stress state at the surface.

Common Defects and Quality Control

Study Insights and Reflections

This preliminary study establishes a solid foundation for roll cladding process development in hot rolling applications. The selection of SAW as the primary process for bulk deposition is justified by its excellent deposition rate and interface quality, while the GMAW finishing pass addresses the surface quality requirements. The emphasis on circumferential uniformity is particularly important, as non-uniform cladding thickness leads to roll imbalance during operation, which can cause vibration, premature bearing failure, and reduced product quality. The thermal fatigue performance improvement of three-fold over the uncladded substrate is a compelling economic argument for implementing roll cladding in hot rolling operations. However, the literature acknowledges that this is a preliminary study, and further investigation is needed into the long-term performance of cladded rolls under actual mill conditions, including the effects of roll cooling water impingement, scale adhesion, and the interaction between the cladding layer and the rolled product surface. The development of predictive models for cladding life based on rolling schedule parameters would be a valuable extension of this work.