Cladding Repair of 2800 Rolling Mill Blooming Mill Rolls

Literature Overview

This 2004 paper from Wuhan Iron and Steel Company (WISCO), authored by Chen Jianming, Chen Yonghui, Gu Bin, Gao Naiyong, Cheng Qunwei, and Wu Binghuo, and published in Metal World, documents the successful cladding repair of blooming mill rolls on a 2800 rolling mill. The work represents a significant engineering achievement in the field of roll repair, demonstrating the application of weld overlay technology to restore worn or damaged heavy-duty rolling mill rolls to serviceable condition.

Blooming mill rolls are among the most heavily loaded components in a steel mill. They are subjected to extreme contact stresses, thermal cycling, and abrasive wear during the hot rolling of steel slabs. The 2800 rolling mill refers to a mill with a 2800 mm reduction capability, and the rolls used in such mills can weigh several tons each. When these rolls wear beyond acceptable limits or develop surface defects such as cracks, spalling, or thermal fatigue, replacement with new rolls is extremely costly. Weld overlay repair provides an economic alternative that can extend the service life of rolls significantly.

Roll Materials and Wear Mechanisms

Blooming mill rolls are typically made from high-chromium cast iron or forged steel with surface hardening. The common materials include:

The primary wear mechanisms affecting blooming mill rolls include:

• Abrasive wear: Caused by contact with hot steel slabs containing scale and oxide particles.
• Thermal fatigue: Resulting from repeated heating and cooling cycles during rolling.
• Adhesive wear: Due to material transfer between the roll surface and the rolled stock.
• Roll surface spalling: Caused by subsurface crack initiation and propagation under cyclic loading.

When these wear mechanisms reduce the roll diameter below the minimum acceptable limit or create surface defects that compromise rolling quality, the roll must be either replaced or repaired. Weld overlay repair involves removing the damaged surface layer by grinding or machining and then applying a new overlay of wear-resistant material.

Cladding Process Selection and Parameters

The paper describes the use of submerged arc welding (SAW) with appropriate filler materials for the cladding repair of the blooming mill rolls. The selection of the welding process and filler material is critical and depends on the roll material, the service conditions, and the required overlay properties.

For high-chromium white iron rolls, the filler material must be compatible with the high carbon and chromium content of the base metal. Common filler materials for this application include:

• High-carbon, high-chromium cast iron filler: Provides a hard, wear-resistant overlay but may be susceptible to cracking during welding due to the high carbon equivalent.
• Nickel-based alloy filler: Provides excellent weldability and crack resistance, with good wear resistance in the as-welded condition.
• Ductile iron filler: Offers a balance of wear resistance and toughness, with lower susceptibility to cracking.

The welding parameters for roll repair are typically as follows:

Repair Procedure and Quality Control

The repair procedure for blooming mill rolls involves several critical steps:

1. Inspection and assessment: The damaged roll is inspected using magnetic particle testing (MT) to identify cracks, and the extent of wear is measured to determine the required repair volume.
2. Surface preparation: The worn or damaged surface is ground or machined to remove all defects and create a clean, uniform surface for overlay application. Any cracks identified by MT must be fully removed by grinding or machining.
3. Preheating: The roll is preheated to the specified temperature using induction heating or gas heating. Uniform preheating is essential to prevent thermal shock and cracking.
4. Welding: The overlay is applied in multiple passes, with each pass overlapping the previous one. The welding sequence is planned to minimize residual stress and distortion.
5. Post-weld heat treatment: After welding, the roll is heated to 500–600°C and held for a sufficient time to relieve residual stresses and improve the microstructure of the overlay.
6. Machining: The overlay surface is machined to the required dimensional tolerance and surface finish.
7. Final inspection: The repaired roll is inspected by MT and dimensional measurement to verify compliance with specifications.

Performance Evaluation and Results

The paper reports on the performance of the repaired rolls in actual service conditions. Key performance indicators include:

• Overlay hardness: The hardness of the overlay layer should be sufficient to resist wear, typically 45–55 HRC for high-chromium overlays.
• Bond strength: The bond between the overlay and the base metal must be strong enough to withstand the contact stresses in service. Shear bond tests typically require a minimum strength of 200–300 MPa.
• Service life: The repaired rolls should achieve a service life comparable to new rolls, typically measured in terms of tonnage rolled or number of rolling passes.
• Defect rate: The frequency of failures such as spalling, cracking, or delamination should be comparable to or better than new rolls.

The results reported in the paper indicate that the cladding repair procedure was successful, with repaired rolls achieving service lives comparable to new rolls. The key to success was the careful control of preheat temperature, welding parameters, and post-weld heat treatment, which together minimized the risk of cracking and ensured a sound bond between the overlay and the base metal.

Engineering Practice Implications

The cladding repair of heavy-duty rolling mill rolls presents unique challenges that distinguish it from more conventional cladding applications such as pressure vessel fabrication:

• Large component size: Blooming mill rolls can have diameters of 1000–1500 mm and lengths of 2000–3000 mm, requiring specialized welding equipment and procedures.
• Thick base metal: The high mass of the rolls creates a large heat sink effect, which can lead to excessive cooling rates and brittle microstructures if not properly managed through preheating and interpass temperature control.
• High carbon base metal: The high carbon equivalent of cast iron and forged steel roll materials makes them highly susceptible to cracking during welding, requiring careful control of heat input and preheat temperature.
• Service conditions: The repaired rolls must withstand extreme contact stresses (up to 3000–4000 MPa), thermal cycling (from room temperature to 1200°C), and abrasive wear, which places demanding requirements on the overlay material and bond quality.

For engineers involved in roll repair operations, the following recommendations emerge from this study:

1. Always perform thorough pre-repair inspection using MT and ultrasonic testing (UT) to identify all defects and ensure complete removal of damaged material.
2. Use high preheat temperatures (300–400°C) for high-carbon materials and maintain interpass temperatures throughout the welding sequence.
3. Select filler materials that are specifically designed for welding to cast iron or high-carbon steels, with low carbon equivalent and good crack resistance.
4. Implement post-weld heat treatment as a mandatory step to relieve residual stresses and improve the microstructure of the overlay.
5. Conduct regular in-service monitoring of repaired rolls to track wear rates and identify early signs of failure.

Study Insights and Implications

This paper demonstrates the successful application of weld overlay technology to a demanding industrial repair application. The cladding repair of blooming mill rolls is not merely a cost-saving exercise but a critical technology that ensures the continuity of steel production operations. The ability to repair expensive, large rolls in-house or at a nearby workshop significantly reduces downtime and maintenance costs.

The paper also highlights the importance of process discipline in roll repair. Unlike pressure vessel fabrication, where quality is governed by stringent codes and standards, roll repair operations often lack formal quality systems. The paper's emphasis on systematic inspection, controlled welding parameters, and post-weld heat treatment provides a framework for establishing quality assurance procedures in roll repair operations.

One of the most valuable aspects of this work is the documentation of the repair procedure in sufficient detail to allow replication by other engineers. The specific parameters for preheat temperature, welding current, travel speed, and post-weld heat treatment provide practical guidance that can be adapted to different roll materials and service conditions. This type of detailed technical documentation is essential for the transfer of knowledge and the continuous improvement of repair practices in the steel industry.