Selection and Testing of Overlay Materials for Hot Rolling Mill Rolls

Literature Overview

This paper by Liu Huilin, Zheng Boping, Liao Zhi, and Cheng Xiaojun from Liangang Electromechanical Co., Ltd., published in Mining and Metallurgical Engineering (2004, Vol. 24, Z1, pp. 161-164), evaluates four commonly used domestic overlay welding materials for hot rolling mill rolls. The study focuses on the 650 mm hot rolling blooming mill application and compares the materials based on hot hardness, thermal fatigue resistance, conventional mechanical properties, and microstructure. The results identify material 1# (H25Cr3Mo2MnVA) as the optimal choice for the blooming mill rolls, while material 4# (JL-8110) demonstrates superior wear resistance suitable for composite roll manufacturing.

Technical Requirements for Rolling Mill Roll Overlay

Hot rolling mill rolls operate under extreme conditions combining high contact pressure, sliding friction, elevated temperatures (up to 1000°C at the roll surface), and cyclic thermal loading from alternating contact with hot and cool workpieces. The overlay material must simultaneously provide:

• Hot hardness (red hardness) sufficient to resist plastic deformation at operating temperatures
• Thermal fatigue resistance to survive repeated heating and cooling cycles
• Adequate toughness to resist cracking from thermal shock and mechanical impact
• Wear resistance against abrasive contact with scale and oxide layers on the workpiece

Material Evaluation Results

The four materials were evaluated under identical welding process conditions and testing protocols. Material 1# (H25Cr3Mo2MnVA) demonstrated the best comprehensive performance, combining excellent hot hardness retention at 800°C, superior thermal fatigue life, and adequate room temperature mechanical properties. The high chromium, molybdenum, manganese, vanadium, and aluminum content provides multiple strengthening mechanisms including solid solution hardening, precipitation hardening, and secondary phase reinforcement.

Material 4# (JL-8110) exhibited the highest room temperature hardness and wear resistance, making it suitable for applications where abrasive wear is the dominant failure mechanism. However, its thermal fatigue performance was inferior to material 1#, limiting its application to composite rolls where a harder overlay layer can be combined with a tougher base material.

The microstructural analysis revealed that material 1# maintains an austenitic matrix with fine carbide precipitates at elevated temperatures, which provides both strength and ductility. The thermal fatigue testing involved cyclic heating and cooling of test specimens, with crack initiation and propagation monitored through optical microscopy and scanning electron microscopy.

Process Considerations and Quality Control

The welding process parameters for roll overlay are critical to achieving the desired material properties. Key process variables include:

1. Preheat temperature: 200-300°C to minimize thermal gradients and reduce residual stress.
2. Interpass temperature: Maintained below 300°C to avoid excessive grain growth and ensure proper heat treatment response.
3. Welding current and voltage: Optimized for adequate penetration without excessive dilution of the base material.
4. Number of passes: Multiple thin passes preferred over single thick deposits to ensure uniform microstructure and minimize residual stress.
5. Post-weld heat treatment: Normalization or tempering to relieve residual stresses and optimize the precipitation state.

Quality control measures for roll overlay welding include hardness profiling across the overlay thickness, microstructural examination for cracking or porosity, and thermal fatigue testing on representative coupons. The overlay thickness is typically 3-10 mm depending on the roll diameter and expected service life.

Engineering Practice and Material Selection Strategy

For the 650 mm hot rolling blooming mill, the selection of H25Cr3Mo2MnVA as the overlay material represents an optimal balance of hot hardness, thermal fatigue resistance, and toughness. The blooming mill experiences the highest temperatures and most severe thermal cycling of any mill in the rolling train, making thermal fatigue resistance the primary selection criterion. The material's ability to maintain adequate hardness at 800°C ensures that the roll surface resists plastic deformation during contact with hot billets, while the thermal fatigue life determines the frequency of roll regrinding or replacement.

For engineers involved in roll maintenance and overlay welding operations, this study provides a clear material selection framework. The decision matrix should weigh the relative importance of wear resistance versus thermal fatigue resistance based on the specific application. High-speed finishing mills may prioritize wear resistance (favoring JL-8110 type materials), while blooming and roughing mills should prioritize thermal fatigue resistance (favoring H25Cr3Mo2MnVA type materials). The study also highlights the importance of process control in achieving the full potential of the selected material, as even the best alloy composition will underperform if welding parameters are not properly optimized.

Study Insights and Implications

This paper provides a practical, application-oriented approach to overlay material selection for rolling mill rolls. The comparative testing methodology, while straightforward, produces actionable data that directly informs engineering decisions. The identification of H25Cr3Mo2MnVA as the optimal material for blooming mill applications is supported by comprehensive testing across multiple performance criteria. For organizations managing rolling mill operations, this type of material evaluation study should be conducted periodically as new materials become available and operating conditions evolve. The emphasis on thermal fatigue as a primary selection criterion reflects the real-world operating experience where thermal cycling, rather than pure wear, is often the dominant failure mechanism for hot rolling mill rolls.