Study Note on Selection and Testing of Roll Overlay Welding Materials

Literature Overview

The paper by Liu Huilin and colleagues in the journal Mining and Metallurgical Engineering (2004, Vol. 24, Z1, pp. 161-164) presents a systematic evaluation of four commonly used domestic overlay welding materials for hot rolling mill rolls, specifically targeting the 650 hot rolling blooming mill application. The study evaluates the materials for heat fatigue resistance, red hardness (hot hardness), conventional mechanical properties, and microstructure, with the aim of selecting the optimal overlay material for this demanding application. The results indicate that Material 1 (H25Cr3Mo2MnVA) exhibits the best overall performance under the specified welding conditions, making it most suitable for hot rolling blooming roll overlay, while Material 4 (JL-8110) demonstrates the best wear resistance and is also suitable for composite roll manufacturing.

Core Technical Content and Material Evaluation

Hot rolling mill rolls are subjected to an extraordinarily demanding combination of service conditions: high contact temperatures (exceeding 800°C at the roll surface), severe abrasive and adhesive wear from hot steel, thermal cycling (alternating between hot and cold conditions during rolling), mechanical loading from rolling forces, and potentially corrosive atmospheres containing scale and lubricants. The overlay welding of wear-resistant materials onto roll blanks offers a cost-effective solution that extends roll life and improves rolling quality, but the selection of the appropriate overlay material requires careful consideration of multiple performance characteristics.

Material Comparison Matrix

The evaluation methodology employed in this study is particularly valuable because it considers multiple performance criteria simultaneously rather than optimizing for a single property. In overlay welding material selection, it is common to encounter trade-offs: materials with excellent wear resistance (high carbon, high hardness) may suffer from poor thermal fatigue resistance, while materials with excellent thermal fatigue resistance (lower hardness, higher toughness) may not provide adequate wear resistance. The concept of "overall performance" is therefore central to the material selection process.

Heat Fatigue Resistance and Red Hardness Analysis

Heat fatigue resistance is a critical property for hot rolling mill roll overlays, as the rolls experience repeated thermal cycling that can cause surface cracking and spalling. The thermal fatigue behavior is influenced by several factors: the thermal conductivity of the overlay material (higher conductivity reduces thermal gradient and stress), the thermal expansion coefficient (lower coefficient reduces thermal strain), the strength and toughness at elevated temperatures (red hardness), and the microstructural stability at high temperatures (resistance to phase transformations and grain growth).

Red hardness, or hot hardness, refers to the ability of a material to maintain its hardness and strength at elevated temperatures. For hot rolling applications, red hardness at 600-800°C is the most relevant temperature range. Materials with high red hardness typically contain strong carbide-forming elements such as chromium, molybdenum, tungsten, and vanadium, which form stable carbides that resist softening at high temperatures. The H25Cr3Mo2MnVA material contains chromium, molybdenum, vanadium, and aluminum, providing a balanced combination of carbide stability, solid-solution strengthening, and oxidation resistance.

The JL-8110 material, with its superior wear resistance, likely contains higher carbon content and possibly tungsten or cobalt, which contribute to extreme hardness but may reduce thermal fatigue resistance. This trade-off is a classic example of the competing requirements in overlay material design: maximizing hardness for wear resistance often comes at the expense of toughness and thermal stability.

Microstructural Analysis and Phase Composition

The microstructural analysis of the overlay materials would typically reveal the type, distribution, and morphology of hard phases, which are the primary determinants of wear resistance and red hardness. For chromium-molybdenum-vanadium bearing materials, the expected hard phases include M7C3, M2C, and MC carbides, where M represents the carbide-forming elements. The MC carbides (particularly VC and NbC) are the most stable at high temperatures and contribute significantly to red hardness, while M7C3 carbides provide high hardness at room temperature but may decompose or transform at elevated temperatures.

The distribution of carbides is equally important as their type. Uniformly distributed fine carbides provide the best combination of hardness and toughness, while coarse or segregated carbides can act as crack initiation sites and reduce thermal fatigue resistance. The welding process parameters (current, voltage, travel speed, number of passes) directly influence the carbide distribution, and the study's specification of "under the described process conditions" is important for interpreting the results.

Engineering Practice Integration

For the practical application of overlay welding to 650 hot rolling blooming mill rolls, several factors must be considered beyond the material selection itself:

• The base material of the roll blank must be compatible with the overlay material to ensure good fusion and minimize cracking at the fusion boundary.
• The preheating temperature and interpass temperature must be carefully controlled to prevent cracking in the base metal and to achieve the desired microstructure in the overlay.
• The overlay thickness must be sufficient to accommodate the expected wear rate over the planned service interval, typically 1-3 mm for hot rolling applications.
• The welding sequence must be planned to minimize distortion and residual stress, which can affect roll geometry and bearing alignment.
• Post-weld heat treatment may be necessary to relieve residual stresses and optimize the microstructure of the overlay layer.

The selection of H25Cr3Mo2MnVA as the optimal material for blooming mill rolls reflects the importance of thermal fatigue resistance in this application, where the rolls experience severe thermal cycling during the blooming process. The blooming process involves rolling large ingots or slabs at high temperatures (1100-1200°C) to reduce their cross-section, resulting in significant thermal gradients and mechanical loading on the roll surface.

Key Questions and Reflections

A significant question that arises from this study is the long-term performance of the overlay materials under actual rolling conditions. Laboratory tests for heat fatigue resistance and wear resistance, while valuable, may not fully capture the complex synergistic effects of thermal cycling, mechanical loading, and chemical interaction with scale and lubricants. Field trials over extended service periods would provide more definitive performance data, including the actual service life of the overlay, the nature and extent of wear, and any degradation mechanisms that may not be evident in laboratory tests.

Another reflection concerns the evolution of overlay welding technology since 2004. Modern welding equipment offers improved parameter control, and new overlay materials with enhanced performance characteristics have been developed. However, the fundamental principles of material selection based on the balance of wear resistance, thermal fatigue resistance, and red hardness remain unchanged. The systematic evaluation approach presented in this study remains a valid methodology for material selection, even as new materials and technologies become available.

Study Insights and Conclusion

This paper provides a practical and well-structured approach to the selection of overlay welding materials for hot rolling mill rolls, emphasizing the importance of evaluating multiple performance criteria simultaneously. The identification of H25Cr3Mo2MnVA as the optimal material for blooming mill applications and JL-8110 as a high-wear-resistance alternative demonstrates the value of systematic material comparison. For engineers involved in roll maintenance and overlay welding specification, this study reinforces the principle that material selection must be driven by the specific service conditions rather than by generic material classifications. The trade-off between wear resistance and thermal fatigue resistance is a fundamental challenge in overlay material design, and the systematic evaluation approach presented here provides a sound methodology for navigating this trade-off in practical applications.