Wear-Resistant Alloy Cladding for Large Hot Rolling Blooming Mill Rolls

Literature Overview

This entry, catalogued as No. 2137, documents wear-resistant alloy cladding technology for large hot rolling blooming mill rolls developed at the North Machinery Factory of Anshan Iron and Steel Group in 1992. The work was led by Zhang Zhongtao and represents a continuation and extension of the surface engineering efforts initiated at Ansteel for primary rolling mill rolls. Blooming mill rolls present unique challenges due to the combination of high rolling forces, severe abrasive wear from slab scale, and thermal shock during the blooming operation.

Application Environment and Wear Mechanisms

Large hot rolling blooming mills process reheated slabs at temperatures of 1000–1250 °C, reducing them to blooms or slabs of smaller cross-section. The rolls in blooming mills experience a distinctive combination of wear mechanisms:

• Adhesive-abrasive wear: Scale adhesion followed by tearing creates surface damage.
• Thermal fatigue: Repeated heating and cooling cycles cause surface cracking.
• Indentation and deformation: Hard inclusions in the slab indent the roll surface.
• Oxidative wear: High-temperature oxidation of the roll surface accelerates material loss.

The blooming mill environment is generally more severe than secondary rolling mills due to the higher temperatures and the presence of thicker, harder scale on the incoming slab.

Cladding Technology Details

Consumable Selection

For blooming mill roll cladding, the following alloy systems are commonly employed:

Process Parameters

The cladding process for large blooming mill rolls typically involves:

• Substrate: Cast steel rolls (e.g., AISI H13 equivalent) with diameter of 600–1200 mm and barrel length of 1000–2000 mm.
• Cladding thickness: 4–10 mm depending on wear severity and roll life requirements.
• Welding method: Submerged arc welding (SAW) or flux-cored arc welding (FCAW) for high deposition rates on large rolls.
• Preheating: 300–400 °C to prevent cracking in the thick section of the roll.
• Cooling control: Slow cooling in insulated blankets or furnace annealing to minimize residual stresses.

Microstructural Considerations

The effectiveness of the cladding layer depends on the microstructure of the deposited alloy:

• Carbide distribution: Fine, uniformly distributed carbides (M7C3, M23C6, MC) provide optimal wear resistance. Coarse carbide networks at grain boundaries can lead to spalling.
• Matrix hardness: The binder matrix should have sufficient hardness (400–550 HV) to support the carbide particles without deformation.
• Bond line quality: A sound metallurgical bond without oxide inclusions, porosity, or cracks is essential for service life.

Quality Control and Inspection

The following quality control measures are critical for cladding quality assurance:

1. Visual inspection: Check for surface defects, undercut, and excessive spatter.
2. Dye penetrant testing (PT): Detect surface cracks in the cladding layer.
3. Ultrasonic testing (UT): Assess bond line quality and detect subsurface defects.
4. Hardness survey: Measure hardness across the cladding thickness to verify proper microstructure.
5. Metallographic examination: Evaluate the bond line, carbide morphology, and any cracking.

Engineering Practice and Service Performance

The practical implementation of blooming mill roll cladding at Ansteel's North Machinery Factory involved several key considerations:

• Roll geometry: The cladding must be deposited in a manner that allows grinding to the final roll profile, typically requiring 2–3 mm of excess material to be ground off.
• Thermal distortion control: Large rolls are susceptible to distortion during welding; this must be minimized through symmetric welding sequences and controlled preheating.
• Service life improvement: Clad blooming mill rolls typically achieve 2–3 times the service life of unclad rolls, with some applications reporting up to 4 times improvement.
• Cost-effectiveness: Despite the additional cladding and heat treatment costs, the extended roll life and reduced downtime make the technology economically advantageous.

Study Reflections

This 1992 work represents a maturation of the roll cladding technology initiated at Ansteel. The progression from primary mill roll cladding (1990) to blooming mill roll cladding (1992) demonstrates a systematic approach to addressing wear problems across different sections of the steel rolling process. The technology described here relies on well-established arc welding methods, which remain cost-effective for large rolls. However, contemporary practice would benefit from the integration of advanced techniques such as hot-wire TIG cladding for improved dilution control, laser cladding for localized repair, and plasma transferred arc (PTA) powder cladding for superior microstructural control. The fundamental principles of alloy selection, thermal management, and quality verification remain highly relevant to current engineering practice.