CLADDING TECHNOLOGY SHANXI CO., LTDSurface Hardfacing of Aluminum Billet Continuous Casting and Rolling Mill Roll Cores
Literature Overview
This technical paper by Wang Xiangdong, published in 2000 from the Equipment Factory of Bohai Aluminum Industry Co., Ltd., addresses the application of surface hardfacing technology to roll cores used in aluminum billet continuous casting and rolling mills. The aluminum continuous casting and rolling process (also known as semi-continuous casting and rolling) is a combined process that integrates the casting of aluminum ingots with immediate hot rolling to produce sheet products. The rolls used in this process face unique challenges due to the combination of high temperatures from the freshly cast aluminum, the corrosive nature of molten aluminum, and the mechanical demands of rolling reduction.
Technical Background and Process Requirements
The continuous casting and rolling mill for aluminum billets typically operates at temperatures where the aluminum surface temperature ranges from 300 to 500 °C during the rolling stage, with the roll surface potentially reaching higher temperatures during initial contact with the hot cast slab. The roll cores are typically made of cast steel or forged steel with a hardness of 180 to 240 HB, and they require surface hardening to resist wear, thermal cracking, and aluminum adhesion.
The hardfacing material selection for aluminum rolling mill rolls is particularly challenging because aluminum has a strong tendency to wet and adhere to most steel surfaces at elevated temperatures. This phenomenon, known as aluminum pick-up or aluminum transfer, can lead to rapid roll surface degradation, surface roughness, and poor product quality. The following table presents the key material and process considerations:
| Parameter | Specification |
|---|---|
| Roll core material | Cast steel or 40Cr forged steel |
| Overlay material | High-carbon high-chromium (e.g., Cr12MoV) or tungsten carbide-based |
| Overlay hardness | HRC 58-65 |
| Overlay thickness | 4-10 mm |
| Process | SAW or FCAW |
| Preheat temperature | 250-350 °C |
| Interpass temperature | 250-350 °C |
| Post-weld treatment | Stress relief 600 °C, 2-4 h |
| Surface finish after grinding | Ra 0.8-1.6 μm |
The choice of overlay material is critical. High-carbon high-chromium steels (such as Cr12MoV or D2 equivalent) provide good wear resistance and moderate thermal fatigue resistance. Tungsten carbide-based overlays offer superior abrasion resistance but may be more susceptible to thermal cracking. In some applications, a two-layer approach is used where a transition layer of medium carbon steel is applied first, followed by the hardfacing layer, to reduce dilution and improve bond strength.
Process Details and Welding Sequence
The hardfacing process for aluminum mill roll cores follows a multi-pass welding sequence designed to minimize residual stress, control dilution, and ensure uniform overlay composition. The typical sequence involves:
- Base preparation: The roll core surface is machined to remove any scale or contamination, and a shallow groove may be prepared to improve overlay adhesion.
- Transition layer: A single pass of medium-carbon steel electrode (such as E7018 or equivalent) is applied to create a metallurgical transition between the roll core and the hardfacing material. This layer typically has a thickness of 1 to 2 mm.
- Hardfacing layers: Two to four passes of the hardfacing material are applied to achieve the target overlay thickness. The first hardfacing pass may use a slightly lower carbon grade to further reduce dilution effects, while subsequent passes use the full hardfacing composition.
- Post-weld grinding: The overlay surface is ground to the required dimensional tolerance and surface finish.
The welding parameters must be carefully controlled to minimize heat input and avoid excessive dilution. A travel speed of 150 to 250 mm/min with a current of 300 to 500 A is typical for SAW processes on roll diameters of 300 to 500 mm. The shielding flux must be of high quality and properly dried to prevent porosity and nitrogen pickup.
Defect Prevention and Quality Assurance
The following defects are commonly encountered in the hardfacing of aluminum mill roll cores, each requiring specific countermeasures:
| Defect | Root Cause | Prevention Strategy |
|---|---|---|
| Cracking in overlay | High carbon content, rapid cooling | Reduce cooling rate, use preheat |
| Aluminum contamination | Inadequate base cleaning | Thorough grinding and cleaning |
| Insufficient hardness | Excessive dilution | Control travel speed, use transition layer |
| Surface roughness | Uneven deposition | Optimize torch travel, multi-pass strategy |
| Bond failure | Poor metallurgical compatibility | Transition layer, proper preheat |
Quality assurance involves hardness testing at multiple locations across the overlay surface to verify uniformity, ultrasonic testing for subsurface defects, and a bond strength test per the relevant standard. The overlay must maintain its hardness after the post-weld stress relief treatment, which is why materials with good temper stability are preferred.
Engineering Practice and Economic Considerations
The hardfacing of aluminum mill roll cores offers significant economic benefits by extending roll life and reducing the frequency of roll changes. In the continuous casting and rolling process, roll changes are particularly disruptive because they interrupt the continuous production flow. A hardfaced roll that lasts 2 to 3 times longer than an unclad roll can result in substantial savings in both material costs and production downtime.
The paper's contribution to the field lies in demonstrating that surface hardfacing is a viable and cost-effective solution for aluminum mill roll cores, despite the challenging operating conditions. The techniques described have been widely adopted in Chinese aluminum processing plants, and subsequent improvements have incorporated more advanced overlay materials and process monitoring systems.
Key Reflections
A key insight from this study is the importance of addressing the specific metallurgical challenges of aluminum service environments. The tendency of aluminum to wet and adhere to steel surfaces at elevated temperatures is a unique challenge that requires careful material selection and process design. The use of a transition layer to reduce dilution and improve bond strength is a particularly effective strategy that should be considered in all applications involving aluminum contact.
The study also underscores the importance of post-weld grinding in achieving the required surface finish for aluminum rolling. The overlay surface must be ground to a fine finish to minimize aluminum pick-up and ensure good product surface quality. This grinding operation must be performed with appropriate cooling to avoid thermal damage to the overlay surface.
Summary
This 2000 paper by Wang Xiangdong provides valuable technical insights into the hardfacing of aluminum billet continuous casting and rolling mill roll cores. The paper demonstrates that careful selection of overlay materials, proper welding sequence design, and rigorous quality control can produce reliable hardfaced rolls that significantly extend service life in the demanding environment of aluminum processing. The fundamental principles described remain relevant today, and the engineering approach of tailoring the cladding solution to the specific service conditions continues to be the most effective strategy for roll surface engineering.