Cladding Material Selection and Overlay Microstructure for Hot Rolling Mill Rolls

Literature Overview and Research Context

This 2001 study from the Journal of Anhui University of Technology (Natural Science Edition) addresses the critical engineering challenge of hot rolling mill roll surface restoration and enhancement through arc welding overlay. Hot rolling mill rolls endure extreme conditions including temperatures up to 1000°C from the workpiece, intense contact stresses exceeding 3000 MPa, thermal cycling fatigue, and abrasive wear from scale and oxide particles. The selection of appropriate cladding materials and the control of overlay microstructure are fundamental to extending roll life and maintaining product quality.

The research by Hong Yongchang from Anhui University of Technology represents a systematic approach to material selection for hot rolling applications, considering the interplay between hardness, toughness, thermal fatigue resistance, and spalling resistance. The study reflects the metallurgical understanding of that era while establishing principles that remain relevant in contemporary roll restoration practices.

Core Technical Findings

Material Selection Criteria for Hot Rolling Rolls

The cladding materials evaluated for hot rolling mill rolls include high-speed steel (HSS), cobalt-based alloys, and specialized martensitic or austenitic hardfacing alloys. The selection is governed by the specific rolling application:

Microstructural Analysis of the Overlay Layer

The overlay microstructure is determined by the cladding material composition, welding process parameters, and post-weld heat treatment. For high-speed steel cladding, the as-welded microstructure consists of martensite with dispersed carbides (MC, M2C, M6C). The carbide type and size directly influence the wear and thermal fatigue performance.

The welding process parameters for roll cladding typically include:

• Submerged arc welding (SAW) or gas metal arc welding (GMAW) for single-pass deposits of 3-8 mm
• Heat input: 15-40 kJ/cm
• Interpass temperature: 200-350°C
• Preheat temperature: 200-400°C (to prevent quench cracking in the HAZ)

Post-weld heat treatment is essential for high-speed steel cladding, typically involving austenitization at 1050-1100°C followed by double tempering at 560-580°C to achieve the target hardness of 60-65 HRC. The tempering treatment transforms the hard, brittle martensite into tempered martensite with secondary carbide precipitation, providing an optimal balance of hardness and toughness.

Dilution Effects and Bond Line Integrity

The dilution between the cladding material and the roll body steel (typically low-carbon or low-alloy steel such as 50Mn or 55Mn2) significantly affects the overlay properties. For high-speed steel cladding, dilution of more than 25% can reduce the hardness below acceptable levels and promote the formation of undesirable phases. Process control measures include:

1. Using a high dilution-resistant alloy in the first pass (such as a Cr-V interlayer)
2. Limiting the heat input to reduce substrate melting
3. Employing multiple thin passes rather than single thick deposits
4. Ensuring adequate overlap between adjacent weld beads to minimize gaps

Engineering Practice Integration

In industrial practice, the cladding of hot rolling rolls follows a systematic approach that includes:

1. Roll preparation: Grinding of the roll surface to remove damaged layers, ensuring a clean, flat surface for cladding
2. Preheating: Uniform preheating of the entire roll to 250-350°C using induction heating or gas flame
3. Cladding execution: Application of the selected alloy using the qualified welding procedure, with careful monitoring of temperature and bead geometry
4. Post-weld treatment: Heat treatment to achieve target properties, followed by stress-relief annealing
5. Machining and grinding: Final dimensioning of the roll surface to specified tolerances (typically Ra 0.8-1.6 μm for finishing rolls)

Quality control includes hardness mapping across the clad surface, metallographic examination of the bond line for cracks or incomplete fusion, and ultrasonic testing for subsurface defects. The typical acceptance criteria require no cracks at the bond line, uniform hardness within ±5 HV across the surface, and no porosity exceeding 1% volume fraction.

Study Insights and Reflections

The research highlights that material selection for hot rolling roll cladding cannot be approached in isolation; it must be considered within the context of the specific rolling application, thermal conditions, and mechanical loading. A material that provides excellent abrasion resistance may be unsuitable if it lacks thermal fatigue resistance, and vice versa. The evolution of roll cladding technology from the early 2000s to the present has seen improvements in alloy design (such as the development of advanced HSS compositions with refined carbide distributions) and process control (such as the adoption of automated welding systems with real-time parameter monitoring). Nevertheless, the fundamental principles established in this research—balancing hardness with toughness, controlling dilution, and ensuring bond line integrity—remain the cornerstones of successful roll cladding practice.