Cladding of Large-Diameter Process Rollers in Hot-Dip Galvanizing Lines

Literature Overview

Hot-dip galvanizing (HDG) lines process steel products by immersing them in molten zinc at temperatures between 450 and 470 °C. The process rollers, which support and guide the steel products through the galvanizing cycle, are exposed to a combination of high-temperature zinc attack, mechanical wear, and thermal cycling. The literature under review addresses the cladding of large-diameter process rollers (typically 300–1,200 mm in diameter) using weld overlay technology, with a focus on achieving long service life, resistance to zinc corrosion, and dimensional accuracy.

The core challenge in HDG roller cladding is the aggressive chemical attack by molten zinc, which can cause rapid erosion of unprotected steel surfaces. The overlay material must therefore provide both mechanical wear resistance and chemical resistance to zinc. The literature examines various overlay approaches, including electroslag welding (ESW) overlay, submerged arc welding (SAW) overlay, and plasma transferred arc (PTA) cladding, and evaluates their suitability for large-diameter rollers.

Core Technical Content

Overlay Material Selection for Zinc Resistance

The selection of overlay material is governed by the requirement to resist molten zinc attack while maintaining adequate hardness for mechanical wear resistance. The following table summarizes the material systems discussed in the literature:

The literature emphasizes that chromium is the key alloying element for zinc resistance, as it forms a stable Cr2O3 oxide layer that protects the underlying metal from zinc attack. However, excessive chromium content can reduce the weldability of the overlay material and increase the risk of cracking. The optimal chromium content is typically in the range of 12–20%, depending on the specific application and temperature.

Nickel-based overlays, such as Ni20Cr5, offer superior zinc resistance and can operate at higher temperatures than chromium-based materials. However, they are significantly more expensive and are typically reserved for the most critical zones, such as the roller surface in direct contact with molten zinc. The literature recommends a hybrid approach where nickel-based overlay is used for the outermost layer and chromium-based overlay is used for the underlying layers.

Process Selection for Large-Diameter Rollers

Large-diameter rollers present unique challenges for overlay welding, including the need for uniform coverage over a large circumference, control of distortion, and management of residual stress. The literature compares three primary overlay processes:

For large-diameter rollers, ESW is the preferred process for the bulk overlay because of its high deposition rate and ability to produce uniform, dense weld metal. The literature describes a typical ESW sequence where the roller is mounted vertically and rotated, with multiple ESW torches positioned around the circumference. The slag pool is maintained by continuous feeding of flux, and the electrode is fed into the slag pool at a controlled rate. The result is a uniform overlay layer with minimal spatter and excellent surface finish.

For the final surface layer, PTA is often used to achieve a smooth, precise surface finish. The PTA process uses a plasma torch to melt a powder stream, which is deposited on the roller surface. The resulting overlay layer has a fine, uniform microstructure with excellent surface quality, which is critical for the smooth operation of the galvanizing line. The literature reports that PTA overlay layers of 1–3 mm thickness provide a smooth surface with roughness values below Ra 3.2 μm, which is essential for preventing zinc buildup and ensuring uniform coating thickness on the processed steel products.

Process Parameters and Heat Input Control

The control of heat input is critical in large-diameter roller cladding because excessive heat input can cause distortion, while insufficient heat input can lead to lack of fusion and poor bond strength. The literature documents the following parameter ranges for ESW overlay:

The literature emphasizes that the preheat temperature must be sufficient to prevent cracking in the overlay layer, particularly for high-chromium materials. A preheat of 200–300 °C is recommended for ESW overlay, with the temperature verified at multiple points around the roller circumference. The interpass temperature must be controlled to prevent excessive heat buildup, which can cause grain coarsening and reduced mechanical properties.

Engineering Practice Integration

Roller Manufacturing Process

The manufacturing process for large-diameter HDG rollers involves several sequential steps, each of which must be carefully controlled to ensure the final product meets the required specifications:

1. Roller blank preparation: The roller blank is forged or cast from low-carbon steel or low-alloy steel, and machined to the final diameter minus the overlay thickness. The surface is ground to a smooth finish to ensure good bond strength.
2. Preheat: The roller is preheated uniformly to 200–300 °C using induction heating or gas flames. The temperature is verified with infrared thermometers at intervals of 90 degrees around the circumference.
3. Bulk overlay deposition: The ESW overlay is deposited in multiple passes, building up the required thickness. The roller is rotated at a constant speed, and the ESW torches are positioned to maintain a consistent slag pool.
4. Intermediate machining: After the bulk overlay, the roller is machined to remove any surface irregularities and to achieve the target diameter. This step also removes any defects such as porosity or slag inclusions at the surface.
5. Final surface overlay: The PTA overlay is deposited as the final layer, achieving a smooth surface finish. The PTA process is performed at a slower travel speed to ensure complete fusion and a smooth surface.
6. Post-weld heat treatment: The roller is stress-relieved at 550–600 °C for 2 hours per 25 mm of thickness, followed by controlled cooling in the furnace.
7. Final machining and inspection: The roller is machined to the final diameter and surface finish, and inspected for dimensional accuracy, surface roughness, and defects.

Field Performance Data

The literature presents field performance data from multiple HDG lines, documenting the service life of rollers with different overlay configurations:

The data clearly demonstrates that overlay cladding significantly extends the service life of HDG rollers, with the best configurations achieving 20–30 times the life of bare steel rollers. The most common failure mode for overlay rollers is surface erosion at the roller edges, where the overlay layer is thinned by mechanical wear and zinc attack. This can be mitigated by designing the overlay layer to extend slightly beyond the roller face, providing a margin of material to protect the edges.

Key Defects and Countermeasures

The following table summarizes the most common defects encountered in large-diameter roller cladding and the recommended countermeasures:

The literature emphasizes that uneven overlay thickness is the most common defect in large-diameter roller cladding, and it is primarily caused by roller eccentricity and torch positioning errors. The recommended countermeasure is to use an automated torch tracking system that maintains a constant distance from the roller surface, compensating for any eccentricity. Additionally, the roller fixture should be calibrated to ensure that the roller rotates about its true center, minimizing eccentricity to less than 0.1 mm.

Study Insights and Implications

The most significant insight from this literature is that the successful cladding of large-diameter HDG rollers requires a holistic approach that integrates material selection, process control, and quality assurance. The overlay material must be carefully selected to balance zinc resistance, mechanical wear resistance, and cost, and the process must be optimized to achieve uniform coverage over the large roller circumference.

The literature also highlights the importance of the transition layer in multi-layer overlay configurations. The transition layer serves two critical functions: it provides a gradual transition in composition from the substrate to the final overlay, reducing the risk of cracking due to compositional mismatch, and it provides additional thickness to compensate for dilution at the overlay-substrate interface. Without a properly designed transition layer, the final overlay layer may be diluted, reducing its zinc resistance and mechanical properties.

From a standards perspective, the literature references GB/T 150 for pressure vessel design and fabrication, NB/T 47014 for weld procedure qualification, and JB/T 4730 for non-destructive testing. While HDG rollers are not pressure vessels, the same quality control principles apply, and the literature recommends that overlay procedures be qualified according to NB/T 47014, with full radiographic or ultrasonic inspection of the overlay-substrate interface.

In conclusion, the cladding of large-diameter process rollers in hot-dip galvanizing lines is a well-established technology that offers significant economic benefits through extended roller life and reduced downtime. The key to successful implementation lies in careful material selection, rigorous process control, and thorough inspection. The future direction includes further optimization of overlay material compositions for specific zinc alloys, development of automated multi-torch ESW systems for even larger rollers, and integration of real-time thickness monitoring to ensure consistent quality throughout production.