Alloy Cladding Technology for Primary Rolling Mill Rolls

Literature Overview

This entry, catalogued as No. 2136, documents alloy cladding technology applied to primary rolling mill rolls developed at the Second Primary Rolling Mill of Anshan Iron and Steel Group (Ansteel) in 1990. The work was led by Sun Guanquan and Ge Haitao, representing a significant early contribution to the domestic application of weld overlay processes in heavy rolling mill applications in China. Primary rolling mill rolls operate under extreme conditions of thermal cycling, mechanical loading, and abrasive wear, making surface engineering through cladding a critical technology for extending roll service life and reducing production downtime.

Technical Background and Application Context

Primary rolling mill rolls are subjected to a complex combination of service stresses that include:

• Thermal stress: Contact temperatures with hot slabs typically range from 800 °C to 1200 °C, creating steep thermal gradients at the roll surface.
• Mechanical stress: Rolling forces in heavy primary mills can exceed 100 MN per stand, imposing substantial compressive and shear loads at the roll surface.
• Abrasive wear: Scale removal during rolling and contact with embedded impurities in the slab cause severe abrasive wear.
• Chemical degradation: Oxidation and scale adhesion at elevated temperatures contribute to surface degradation.

The base material of primary mill rolls is typically a high-chromium cast steel (such as AISI H13 or domestic equivalents) with a hardness of 38–45 HRC in the as-cast condition. The challenge addressed by cladding is to introduce a surface layer with superior wear resistance, thermal fatigue resistance, and oxidation resistance without compromising the substrate's toughness and structural integrity.

Cladding Process Analysis

The technology described in this literature employs arc welding-based cladding processes, most likely submerged arc welding (SAW) or gas metal arc welding (GMAW) with appropriate consumables. The following table summarizes the typical process parameters and material considerations for primary mill roll cladding:

Material Selection Considerations

The cladding alloy composition is critical to achieving the desired surface properties. For primary mill roll applications, common cladding materials include:

• High-carbon high-chromium alloys (e.g., Cr20, Cr30 type): Provide excellent abrasive wear resistance through hard carbide phases (M7C3, M23C6) but may suffer from thermal fatigue cracking if improperly processed.
• Nickel-based alloys (e.g., Stellite-type): Offer superior thermal fatigue resistance and oxidation resistance at elevated temperatures, suitable for the hottest zones of the roll.
• Multi-layer cladding systems: A transition layer (low-alloy) bonded to the substrate, followed by a functional wear-resistant layer, helps mitigate cracking at the bond line due to thermal expansion mismatch.

Key Process Challenges

The primary technical challenges in cladding primary mill rolls include:

1. Bond strength assurance: Ensuring metallurgical bonding between the cast steel substrate and the overlay layer without delamination during service.
2. Crack control: Managing hot cracking in high-carbon cladding alloys and cold cracking in the heat-affected zone of the base material.
3. Residual stress management: The differential thermal expansion between the cladding layer and the roll body can create significant residual stresses that may lead to spalling during operation.
4. Roll geometry maintenance: Cladding must be performed in a manner that allows subsequent grinding to achieve precise roll profiles and surface finish requirements (typically Ra ≤ 0.8 μm after grinding).

Engineering Practice Insights

From a practical standpoint, the Ansteel development represents an important milestone in establishing domestic capability for roll surface engineering. The approach likely involved:

• Roll preparation: Thorough cleaning, removal of loose scale, and preheating to minimize hydrogen-induced cracking.
• Multi-pass cladding: Building up the cladding layer in multiple passes to control dilution and achieve the desired composition.
• Post-weld heat treatment: Tempering the entire roll after cladding to relieve residual stresses and optimize the microstructure of both the cladding layer and the heat-affected zone.
• Quality verification: Dye penetrant testing of the cladding surface, hardness profiling across the cladding layer, and possibly ultrasonic testing of the bond line.

The service performance of clad primary mill rolls typically shows a 2–4 times improvement in life compared to unclad rolls of the same base material, depending on the specific operating conditions and cladding material selection.

Study Reflections

This early work from 1990 demonstrates that Chinese heavy industry was actively pursuing surface engineering solutions for critical rolling mill components well before the widespread adoption of advanced thermal spray and laser cladding technologies. The use of conventional arc welding methods for cladding large-diameter rolls remains practical and cost-effective for many applications today. However, modern practice would supplement these approaches with more sophisticated process monitoring, improved consumable metallurgy, and advanced non-destructive testing methods such as phased array ultrasonic testing (PAUT) for bond line assessment. The fundamental principles of thermal management, dilution control, and residual stress mitigation remain unchanged, underscoring the enduring value of the foundational work documented in this literature entry.