CLADDING TECHNOLOGY SHANXI CO., LTDCladding Repair Technology for Narrow-Strip Stainless Steel Cold Rolling Backup Rolls
Literature Overview
This 2007 study by Sui Xiangrong, Shen Fenggang, Zhang Di, Wang Qingbao, Tang Chunyang, and Liao Qiuqin, published in the Chinese Journal of Surface Engineering, addresses a highly specialized and practically important topic: the cladding repair of narrow-strip stainless steel cold rolling backup rolls. The research was conducted at the Welding Research Institute of the China Metallurgical Group Corporation Building Research Institute, reflecting the direct industrial relevance of the work. Cold rolling backup rolls are critical components in the production of narrow-strip stainless steel, and their surface integrity directly affects product quality and production efficiency.
Technical Background and Challenges
Cold rolling backup rolls are large-diameter cylindrical rolls that support the work rolls during the cold rolling process. In the production of narrow-strip stainless steel, the backup rolls are subjected to extreme conditions: high rolling forces, continuous contact with work rolls and strip material, and exposure to rolling oils and coolants. The surface of the backup roll is susceptible to wear, galling, and surface damage, which can lead to defects in the rolled strip product.
The repair of damaged backup rolls is a challenging task due to several factors. First, the rolls are large and heavy, making them difficult to handle and machine. Second, the surface geometry is cylindrical, requiring specialized cladding techniques that can maintain uniform coverage. Third, the cladding layer must be compatible with the base roll material (typically case-hardened steel or high-carbon chrome bearing steel) and must withstand the high contact stresses and sliding friction of the rolling process. Fourth, the dimensional accuracy of the roll must be maintained to within tight tolerances, typically within 10 to 20 micrometers, to ensure proper rolling performance.
Cladding Process Selection and Optimization
The study evaluated several cladding processes for the repair of cold rolling backup rolls, including submerged arc welding (SAW), gas tungsten arc welding (GTAW), and plasma transferred arc (PTA) welding. Each process was assessed based on its ability to deposit a uniform, high-quality cladding layer on the cylindrical roll surface.
Submerged arc welding was identified as the primary cladding process for the bulk of the roll surface repair. SAW offers high deposition rates, deep penetration, and excellent slag protection, making it suitable for depositing thick cladding layers efficiently. However, SAW has limited flexibility for complex geometries and requires careful parameter control to avoid excessive dilution and distortion.
GTAW was used for the repair of localized damage and for the application of a transition layer between the base metal and the cladding layer. GTAW provides excellent control over heat input and penetration, making it suitable for delicate repairs on thin sections or near roll journals.
PTA was evaluated as a potential alternative for high-quality cladding deposits. PTA offers superior microstructure control and lower dilution compared to SAW, but the higher equipment cost and lower deposition rate limited its application to critical areas of the roll surface.
| Process | Deposition Rate | Dilution Rate | Surface Quality | Equipment Cost |
|---|---|---|---|---|
| SAW | High (5-15 kg/h) | 15-30% | Moderate | Low |
| GTAW | Low (0.5-2 kg/h) | 5-15% | High | Moderate |
| PTA | Moderate (2-5 kg/h) | 5-10% | Excellent | High |
Cladding Material Selection
The selection of the cladding material was guided by the requirements of the cold rolling process. The cladding layer needed to provide high hardness and wear resistance to resist galling and adhesive wear from the work rolls, while also maintaining sufficient toughness to resist contact fatigue and surface cracking.
The study investigated several cladding material compositions, including:
- High-speed steel compositions (M2, M35) for general wear resistance and hot hardness.
- Cemented carbide-containing compositions (WC-Co, TiC-Co) for enhanced abrasion resistance.
- Stainless steel compositions (310, 309) for corrosion resistance in the presence of rolling oils and coolants.
The final selection was a multi-layer approach: a transition layer of 309 stainless steel to ensure compatibility with the base roll material, followed by a functional layer of high-speed steel composition for wear resistance, and optionally a surface layer of WC-Co composite for enhanced abrasion resistance in high-wear zones.
Microstructural Characterization and Performance
The microstructure of the cladding layer was characterized using optical microscopy, SEM, and EDS. The transition layer exhibited a ferrite-austenite microstructure with good ductility, ensuring a strong metallurgical bond with the base metal. The functional layer showed a tempered martensite matrix with dispersed carbide particles, providing a hardness of 55 to 65 HRC. The surface layer, when applied, exhibited a matrix with dispersed WC particles, achieving hardness values exceeding 80 HRC.
The dilution rate was a critical parameter that affected the cladding performance. The study found that the dilution rate varied across the roll circumference due to differences in heat input distribution. The top of the roll, where the welding current was directed downward, showed lower dilution (10 to 15 percent), while the bottom of the roll, where the welding current was directed upward, showed higher dilution (20 to 30 percent). This variation was addressed by adjusting the welding parameters and the roll rotation speed to achieve a more uniform dilution profile.
The hardness profile across the cladding cross-section showed a gradual transition from the base metal hardness (around 40 HRC) to the cladding surface hardness (55 to 80 HRC), with the transition zone spanning 1 to 3 millimeters. This gradual transition was beneficial because it reduced the risk of stress concentration and cracking at the interface.
Engineering Practice and Quality Control
The practical application of the cladding repair technology required careful attention to several quality control aspects. The roll surface was prepared by grinding to remove the damaged layer and expose clean base metal. The surface roughness after grinding was controlled to Ra 3.2 micrometers or better to ensure good cladding adhesion.
The welding procedure was qualified through a series of trials on test coupons, with parameters optimized for minimum dilution, maximum hardness, and minimum defect content. The qualified procedure was then applied to the production roll, with in-process monitoring of welding parameters and periodic visual and magnetic particle inspection (MT) of the cladding layer.
Post-weld machining was performed to restore the roll geometry to the required dimensional accuracy. The machining allowance was typically 2 to 5 millimeters, depending on the cladding thickness and the initial surface condition. The final surface finish after machining was Ra 0.8 micrometers or better, meeting the requirements for cold rolling backup roll surfaces.
Non-destructive testing was performed on the cladded roll to detect any internal defects. Magnetic particle inspection was used to detect surface and near-surface cracks, while ultrasonic testing (UT) was used to detect subsurface defects such as lack of fusion and porosity. The acceptance criteria were based on industry standards for critical components, with zero tolerance for cracks and limited acceptance for porosity and slag inclusions.
Key Technical Insights and Reflections
This 2007 study demonstrates the practical application of multi-layer cladding technology to the repair of critical industrial components. The multi-layer approach, combining a transition layer, a functional layer, and an optional surface layer, is a powerful strategy for addressing the complex performance requirements of cold rolling backup rolls.
One important insight from this work is the recognition that the cladding repair of large cylindrical components requires specialized process considerations that differ from flat plate cladding. The variation in heat input distribution around the roll circumference, the challenges of maintaining dimensional accuracy, and the need for post-weld machining all require careful planning and execution.
The study also highlights the importance of material compatibility in cladding applications. The transition layer of 309 stainless steel serves a critical function by providing a metallurgical bridge between the high-carbon base roll material and the high-alloy cladding material. Without this transition layer, the risk of cracking at the interface would be significantly higher due to the large difference in thermal expansion coefficients and carbon activity between the two materials.
The quality control aspects emphasized in the study, including in-process monitoring, NDT, and post-weld machining, are essential for ensuring the reliability of the repaired roll. In modern practice, these quality control measures would be supplemented by advanced techniques such as phased array ultrasonic testing (PAUT), digital image correlation (DIC) for residual stress analysis, and computer-aided process planning for optimal welding sequence design.
Summary
The cladding repair technology for narrow-strip stainless steel cold rolling backup rolls, as investigated in this 2007 study, represents a practical and effective solution to a significant industrial maintenance challenge. The multi-layer cladding approach, combining transition, functional, and surface layers, provides the necessary combination of bond strength, wear resistance, and corrosion resistance required for the demanding cold rolling environment. The key technical contributions include the optimization of welding parameters for cylindrical components, the selection and qualification of cladding materials, and the establishment of comprehensive quality control procedures. For engineers working in the field of industrial component repair, this study provides a clear demonstration of how advanced cladding technology can extend the service life of critical components while maintaining or improving performance, offering a compelling alternative to complete component replacement.