Research Progress on Cladding Technology for Cold Rolling Backup Rolls

Literature Overview

This 2013 review paper by Cheng Zhonggeng, Cai Yangchuan, and Liu Renpei from Nanjing University of Aeronautics and Astronautics provides a comprehensive overview of the cladding technologies applied to cold rolling backup rolls. Backup rolls serve as the load-bearing support for work rolls in cold rolling mills, experiencing extreme compressive contact stresses, thermal cycling, and potential fretting damage at the journal and raceway areas. The selection of cladding technology directly determines the service life, reliability, and economic viability of the backup roll.

Cladding Technologies for Backup Rolls

The review examines multiple cladding approaches including weld overlay cladding, laser cladding, plasma arc cladding, and electro-slag welding overlay, evaluating each against the specific demands of backup roll applications. Backup rolls typically have a carbon steel or low-alloy steel core with a surface layer designed to provide wear resistance, fatigue resistance, and sometimes corrosion resistance.

Microstructural Requirements for Backup Roll Cladding

The overlay material must satisfy several demanding requirements. Hardness must be sufficient to resist fretting wear at the bearing contact interface, typically requiring surface hardness of 40–55 HRC for raceway areas. Fatigue resistance is critical because backup rolls experience cyclic contact stresses exceeding 2500 MPa during operation. The overlay-substrate bond must withstand repeated thermal cycling without delamination, requiring a metallurgical bond with adequate toughness. Residual stresses should be compressive or at least neutral to prevent fretting crack propagation.

The review highlights that the optimal cladding material system depends on the specific service environment. For hot-rolled steel mills, nickel-based alloys such as Stellite 6 or Inconel 625 provide excellent wear and corrosion resistance. For cold-rolled steel applications, martensitic stainless steels or high-speed steel overlays offer superior hardness and fatigue resistance. The selection must also consider the compatibility with the bearing material and lubrication system.

Process Development and Quality Control

The review emphasizes the importance of process development and qualification for each specific application. A typical qualification program includes:

1. Base material characterization to establish substrate properties and identify potential cracking susceptibility.
2. Process parameter optimization through trial welds to achieve target dilution, hardness, and microstructure.
3. Mechanical property testing including hardness profiles, tensile tests, and fatigue tests on coupon specimens.
4. Non-destructive testing including ultrasonic testing for lack of fusion, magnetic particle testing for surface cracks, and dye penetrant testing for fine surface defects.
5. Full-scale repair trials on actual rolls with post-repair dimensional and surface finish verification.

Common Defects and Countermeasures

The review identifies several recurring defects in backup roll cladding operations:

• Fretting cracks at the overlay-substrate interface, caused by insufficient bond strength or excessive residual tensile stress. Countermeasure: optimize heat input to achieve full fusion, apply post-weld stress relief at 550–650 degrees Celsius.
• Spalling or delamination of the overlay layer, caused by thermal mismatch between overlay and substrate during service. Countermeasure: select overlay materials with thermal expansion coefficient matching the substrate, use gradual transition layers.
• Surface porosity, caused by gas entrapment during deposition. Countermeasure: ensure proper shielding gas coverage, use dry and clean consumables, control travel speed to allow adequate gas escape.
• Excessive dilution leading to hardness below specification. Countermeasure: increase powder feed rate relative to heat input, use multi-pass strategies with the final pass optimized for surface composition.

Engineering Practice Implications

For engineers managing backup roll cladding programs, the review offers several practical recommendations. First, process qualification should be performed for each specific roll geometry and material combination, as results from one application cannot be directly extrapolated to another. Second, a combination of NDT methods is essential for ensuring overlay quality; no single method can detect all relevant defect types. Third, in-service monitoring of cladded surfaces through periodic visual inspection and dimensional measurement can provide early warning of fretting damage or overlay degradation.

The review also notes the trend toward hybrid cladding approaches, where different technologies are combined to optimize performance. For example, electroslag welding may be used to deposit a thick base layer, followed by laser cladding for the final wear-resistant surface layer. This approach leverages the high deposition rate of ESW for bulk material and the low dilution of laser cladding for surface quality.

Study Insights and Reflections

This review serves as an excellent reference for engineers entering the field of backup roll cladding. The breadth of coverage, spanning multiple technologies and their comparative evaluation, provides a comprehensive understanding of the available options and their trade-offs. The emphasis on process qualification and quality control reflects the practical reality that cladding success depends as much on process discipline as on technology selection. For engineers involved in bimetal pressure vessel fabrication, the principles of overlay qualification and quality control described here are directly transferable, as both applications require reliable metallurgical bonding and controlled microstructure in the overlay layer.