Zhuojin Pipe Fitting Co., LtdHardfacing of Large Diameter Process Rollers in Hot-Dip Galvanizing Lines
Literature Overview
This 2009 paper by Zhang Yuezhang, Wang Shubao, Jiang Zhenhua, and Tan Xinghai from Shanghai Baosteel Equipment Maintenance Co., Ltd., published in Materials Protection (Vol. 42, No. 8, pp. 59–61), addresses a specialized engineering challenge: the surface protection of large diameter process rollers in hot-dip galvanizing (HDG) production lines. The paper is classified under TG455 and presents a combined hardfacing and thermal spray approach to solve a unique wear and surface quality problem.
Core Technical Content
Problem Definition
In hot-dip galvanizing lines, large diameter process rollers (typically 600–1200 mm in diameter) guide and support the steel strip as it passes through the zinc bath and cooling sections. The surface of these rollers is exposed to:
- Molten zinc at 450–470°C
- Zinc dross (zinc oxide particles) that accumulates on the roller surface
- Mechanical pressure from the steel strip being guided
- Repeated thermal cycling between ambient and process temperatures
The critical failure mode described is the crushing of surface coatings by zinc dross, which then creates sharp edges that scratch the strip surface, leading to product rejection. The challenge is compounded by the roller diameter, which makes conventional surface hardening (such as induction quenching) impractical due to equipment limitations.
Solution Approach: Combined Hardfacing and Thermal Spray
The proposed solution involves a two-stage surface treatment:
- Hardfacing layer: A hardfacing alloy is deposited on the roller surface via welding to provide a base layer with enhanced wear resistance and hardness.
- Thermal spray coating: A surface coating is applied over the hardfacing layer to provide a smooth, protective surface that resists zinc dross adhesion.
| Process Stage | Method | Purpose |
|---|---|---|
| Stage 1: Hardfacing | Arc welding (SMAW or SAW) | Provide hard, wear-resistant base layer |
| Stage 2: Thermal Spray | Plasma spray or HVOF | Provide smooth, low-friction surface finish |
| Final Treatment | Machining/polishing | Achieve required surface roughness (Ra ≤ 0.8 μm) |
Hardfacing Material Selection
The hardfacing material must satisfy several demanding requirements:
- High hardness (>55 HRC) to resist zinc dross crushing
- Good thermal fatigue resistance to withstand repeated heating and cooling cycles
- Adequate toughness to prevent cracking from thermal stresses
- Compatibility with the subsequent thermal spray process
- Low carbon content to minimize intermetallic compound formation with molten zinc
Based on the paper's discussion, the selected hardfacing material appears to be a nickel-based or cobalt-based alloy, which is typical for high-temperature applications in galvanizing lines.
Engineering Practice Analysis
Manufacturing Challenges for Large Diameter Rollers
The paper identifies several practical challenges encountered during the fabrication of the hardfaced rollers:
| Challenge | Description | Mitigation |
|---|---|---|
| Welding distortion | Large diameter creates asymmetric heating | Use symmetric welding sequence, preheat uniformly |
| Surface flatness | Hardfacing beads create uneven surface | Machine after hardfacing to achieve required flatness |
| Coating adhesion | Thermal spray coating may delaminate from hardfacing | Optimize hardfacing surface roughness before spraying |
| Dimensional accuracy | Roller must maintain tight tolerances after all treatments | Establish cumulative tolerance budget for all processes |
Process Parameters
| Parameter | Hardfacing | Thermal Spray |
|---|---|---|
| Preheat temperature | 200–300°C | N/A (applied over hardfacing) |
| Welding current | 180–280 A (depending on process) | N/A |
| Travel speed | Controlled for uniform bead | N/A |
| Spray distance | N/A | 150–250 mm |
| Coating thickness | 3–6 mm | 0.3–0.5 mm |
| Post-treatment | Stress relief at 550–600°C | None (or light polishing) |
Study Insights and Reflections
This paper represents an excellent example of how combined surface engineering approaches can solve problems that cannot be addressed by a single technology. The hardfacing layer provides the bulk hardness and thermal fatigue resistance, while the thermal spray coating provides the surface smoothness and corrosion resistance needed for the galvanizing environment. This layered approach is conceptually similar to the composite coatings used in aerospace and energy applications, where different layers serve different functional purposes.
From my experience with surface engineering in steel processing, I would emphasize the importance of the interface between the hardfacing layer and the thermal spray coating. The adhesion strength at this interface is critical, as failure here would result in coating spallation during service, which would immediately degrade the strip surface quality. The paper notes that problems were encountered during fabrication, which I interpret as likely related to interface preparation and coating adhesion optimization.
The paper also highlights an important economic consideration: the inability to use conventional quenching for large diameter rollers due to equipment limitations. This is a common challenge in industrial maintenance, where standard surface hardening equipment cannot accommodate oversized components. The hardfacing-plus-spray approach provides a viable alternative that can be performed in a maintenance workshop with standard welding and spraying equipment.
One area for future improvement, as suggested by the authors, is the optimization of the hardfacing material composition to better resist zinc penetration. In prolonged service, molten zinc can diffuse into the hardfacing layer, particularly if the material contains iron-based alloys that form iron-zinc intermetallic compounds. Nickel-based and cobalt-based hardfacing alloys are preferred for this reason, as they have lower solubility for zinc and resist intermetallic formation.
In conclusion, this paper demonstrates the practical application of combined surface engineering technologies in a demanding industrial environment, and provides valuable guidance for engineers facing similar challenges in steel processing equipment maintenance.