Effect of Composition and Microstructure of Hot Rolling Mill Roll Overlay Layer on Thermal Fatigue and Wear Resistance

Literature Overview

This 1992 publication in the Journal of Xi'an Jiaotong University, authored by Xue Jin, Wang Yasheng, Wang Jian, Lang Yi, and Lou Baicheng from Xi'an Jiaotong University and Ansteel Iron and Steel Research Institute, represents one of the early comprehensive studies on weld overlay technology for hot rolling mill rolls. The research addresses the critical challenge of improving the service life of hot rolling mill rolls, which are subjected to extreme thermal cycling, mechanical loading, and abrasive wear during the hot rolling process. This work was conducted at a time when hot rolling mill roll technology was undergoing significant advancement, and the findings have continued to influence overlay design philosophy for heavy industrial applications.

Core Technical Viewpoints

The study systematically investigates the relationship between overlay composition, microstructure, and performance in hot rolling mill roll applications. The key findings include:

• The overlay composition must be carefully designed to balance thermal fatigue resistance, wear resistance, and mechanical strength under the extreme conditions of hot rolling.
• The microstructure of the overlay, particularly the type, size, and distribution of carbides, is the primary determinant of thermal fatigue and wear performance.
• A multi-pass overlay approach is necessary to achieve the required composition and microstructure, with the first pass serving as a transition layer to manage dilution.
• The optimal overlay composition for hot rolling mill rolls typically contains 6–10% Cr, 2–4% C, 0.5–1.5% Mo, and 0.3–0.8% V, with optional additions of W, Co, and Nb.

Microstructural Analysis and Phase Composition

The microstructure of hot rolling mill roll overlays is characterized by a complex combination of phases that must be optimized for the specific service conditions:

The microstructure of the overlay after welding typically consists of a martensitic matrix with dispersed carbides. The carbide type and distribution depend on the cooling rate and composition. Rapid cooling produces finer, more uniformly distributed carbides, while slower cooling allows carbide coarsening and segregation.

The thermal fatigue behavior of the overlay is primarily governed by the following factors:

1. Thermal expansion coefficient: The overlay should have a thermal expansion coefficient close to that of the base roll steel to minimize thermal stresses during heating and cooling cycles.
2. Thermal conductivity: Higher thermal conductivity reduces thermal gradients and thermal stresses, but may reduce wear resistance.
3. Carbide stability: Carbides must remain stable at the operating temperature (typically 800–1100°C for hot rolling) without excessive coarsening or phase transformation.
4. Residual stress: Compressive residual stresses in the overlay surface improve thermal fatigue resistance by inhibiting crack initiation.

Wear Mechanism Analysis Under Hot Rolling Conditions

The wear mechanisms acting on hot rolling mill roll overlays are complex and involve multiple simultaneous mechanisms:

1. Abrasive wear: Caused by iron oxide scale (Fe₂O₃, Fe₃O₄) generated during hot rolling, which acts as an abrasive between the roll surface and the workpiece. The hardness and toughness of the overlay carbides determine resistance to this mechanism.
2. Adhesive wear: Caused by direct metal-to-metal contact between the roll and the hot workpiece. The chemical composition of the overlay, particularly the presence of chromium and molybdenum, influences the formation of protective oxide layers.
3. Thermal fatigue wear: Caused by cyclic thermal loading during rolling, which induces microcracks that propagate and eventually lead to material loss. The thermal fatigue resistance is determined by the thermal expansion coefficient, thermal conductivity, and microstructural stability.
4. Impact wear: Caused by the mechanical impact of the workpiece during rolling. The toughness of the overlay matrix is critical for resisting impact-induced crack initiation.

The specific wear rate of the optimized overlay composition was found to be approximately 3–5 times lower than that of conventional cast iron roll overlays, with a service life improvement of 2–3 times under comparable rolling conditions.

Process Design and Qualification

The overlay welding process for hot rolling mill rolls requires careful design and qualification:

• Process selection: Submerged arc welding (SAW) and gas metal arc welding (GMAW) are the most commonly used processes for hot rolling mill roll overlays. SAW provides high deposition rates and deep penetration, while GMAW offers better control over the microstructure and is suitable for repair welding.
• Welding parameters: The heat input should be optimized to achieve the desired microstructure. For SAW, a heat input of 20–40 kJ/mm is typical; for GMAW, 15–30 kJ/mm is recommended. The travel speed should be controlled to minimize thermal distortion of the roll.
• Preheating and interpass temperature: Preheating to 200–300°C is recommended to reduce residual stresses and prevent cracking. The interpass temperature should be maintained below 300°C to avoid excessive grain growth.
• Post-weld heat treatment: A tempering treatment at 550–650°C for 2–4 hours is typically required to relieve residual stresses and stabilize the microstructure. The tempering temperature must be carefully controlled to avoid excessive carbide coarsening.
• Inspection: The overlay should be inspected by magnetic particle testing (MT) or liquid penetrant testing (PT) after welding and after heat treatment. Ultrasonic testing (UT) may be required for thick overlays to detect internal defects.

Engineering Applications and Practical Recommendations

The overlay technology for hot rolling mill rolls has been widely adopted in the steel industry, with significant improvements in roll life and productivity. The following practical recommendations are derived from the study:

• Overlay thickness: The overlay thickness should be designed to be 5–15 mm, depending on the roll diameter and expected service life. A minimum of 3–5 mm of undiluted overlay should be maintained after grinding and dressing.
• Overlay composition selection: The overlay composition should be matched to the specific rolling conditions. For hot band mills, a higher chromium content (8–10%) is recommended for improved thermal fatigue resistance. For hot strip mills, a balanced composition with moderate chromium and molybdenum content is preferred.
• Surface preparation: The roll surface should be ground to a smooth finish (Ra < 3.2 μm) before overlay welding to ensure good bond strength. Any existing cracks or defects should be repaired before overlay application.
• Grinding and dressing: After welding and heat treatment, the overlay surface should be ground to the required dimensional accuracy and surface finish. The grinding process should be optimized to minimize thermal damage to the overlay surface.

Study Insights and Implications

This early research established a foundational understanding of the relationship between overlay composition, microstructure, and performance for hot rolling mill roll applications. The systematic approach to composition optimization, combined with detailed microstructural analysis and performance testing, provided valuable guidance for the design of subsequent overlay systems.

The key insight from this research is that the performance of hot rolling mill roll overlays is governed by a complex interplay of thermal, mechanical, and chemical factors. The overlay composition and microstructure must be optimized not for a single performance metric but for a balanced combination of thermal fatigue resistance, wear resistance, and mechanical strength. This holistic approach to overlay design has continued to influence the development of modern overlay technologies for heavy industrial applications.

For contemporary engineers, the legacy of this research is evident in the widespread adoption of weld overlay technology for hot rolling mill rolls, which has become a standard practice in the steel industry. The principles established in this study—composition optimization, microstructural control, and process qualification—remain relevant and continue to guide the design and implementation of overlay systems for demanding industrial applications.