Heat-Resistant Fatigue Performance of Roll Overlay Metals

Literature Overview

This 2001 study from Tianjin University and Xingtai Special Roll Company, supported by the Hebei Provincial Major Science and Technology Project (Grant No. 95-98-10), investigates the heat-resistant fatigue performance of overlay metals used for hot rolling mill rolls. Published in the Welding Journal (Chinese edition, Hanyang Xuebao), this research addresses a critical durability issue in the steel and non-ferrous metal rolling industry, where rolls are subjected to severe thermal cycling, mechanical loading, and oxidative degradation during the hot rolling process. The overlay metals, typically deposited on the roll body by welding, serve as the working surface that directly contacts the hot metal strip and must maintain their structural integrity over thousands of thermal cycles.

Engineering Background and Service Conditions

Hot rolling mill rolls operate under extreme and combined loading conditions. The roll surface is subjected to temperatures ranging from 800 °C to over 1100 °C, depending on the material being rolled and the rolling stage. The thermal gradient between the roll surface and the roll core can exceed 500 °C, generating significant thermal stresses. In addition to thermal loading, the roll surface experiences high contact stresses from the rolling force, abrasive wear from scale and inclusions on the strip surface, and oxidative degradation from the hot atmosphere. The combination of these factors leads to a complex degradation mechanism involving thermal fatigue cracking, abrasive wear, oxidative spalling, and impact damage.

Overlay welding is the primary method for providing the working surface of modern hot rolling mill rolls. The overlay layer, typically 5–30 mm thick, is deposited on a steel roll body (usually a low-alloy steel or a cast steel roll core) using processes such as submerged arc welding (SAW), flux-cored arc welding (FCAW), or electroslag welding (ESW). The overlay material must be selected to provide an optimal combination of heat resistance, thermal fatigue resistance, wear resistance, and spalling resistance for the specific rolling application.

Overlay Material Selection and Composition

The selection of overlay material for hot rolling mill rolls depends on the type of rolling mill, the material being rolled, and the rolling conditions. The following table summarizes common overlay materials and their typical applications:

The study likely focused on one or more of these material systems, investigating their thermal fatigue behavior under conditions that simulate actual rolling service. The thermal fatigue testing would involve cyclic heating and cooling of the overlay surface, with the temperature range and cycle frequency selected to represent the thermal loading experienced during rolling.

Thermal Fatigue Mechanism and Crack Initiation

Thermal fatigue in roll overlay metals is driven by the mismatch in thermal expansion between the overlay layer and the roll body, as well as by the thermal gradients within the overlay layer itself. During each thermal cycle, the overlay surface expands when heated and contracts when cooled. Because the overlay is constrained by the roll body and by the cooler layers beneath, this thermal expansion and contraction generates tensile and compressive stresses that alternate in sign. After a certain number of cycles, these cyclic stresses lead to crack initiation, typically at the surface or at subsurface defects such as inclusions or micro-porosity.

The crack initiation and propagation behavior depends on several factors:

1. Thermal stress amplitude: Determined by the temperature range and the thermal expansion coefficient of the overlay material.
2. Cycle frequency: Higher frequencies allow less time for heat to diffuse into the roll body, resulting in higher thermal gradients and higher thermal stresses.
3. Overlay material properties: Young's modulus, thermal conductivity, thermal expansion coefficient, and creep resistance all influence the thermal fatigue behavior.
4. Microstructure: Grain size, phase composition, and the presence of brittle phases affect crack initiation and propagation.
5. Surface condition: Surface roughness, residual stresses, and pre-existing defects serve as crack initiation sites.

The study likely employed a thermal fatigue testing rig that subjects the overlay specimen to cyclic thermal loading using induction heating, gas flame heating, or electric resistance heating, with rapid cooling by water spray or air blast. The number of cycles to crack initiation and the crack propagation rate are measured as functions of the thermal stress amplitude and the temperature range.

Microstructural Evolution During Thermal Fatigue

During thermal fatigue cycling, the microstructure of the overlay metal undergoes significant evolution. The following table summarizes the key microstructural changes:

The thermal fatigue life of the overlay is often limited by the formation and growth of surface cracks that eventually lead to spalling of the overlay material. The spalling process begins with crack initiation at the surface, followed by crack propagation into the overlay depth. When the crack reaches a critical length, the overlay material above the crack separates from the underlying material, resulting in material loss and exposure of the fresh overlay surface. This process repeats, leading to progressive degradation of the roll surface.

Engineering Practice and Process Optimization

Based on the thermal fatigue performance data, several process and material optimization strategies can be identified:

1. Overlay material selection: Selecting an overlay material with a lower thermal expansion coefficient and higher thermal conductivity can reduce the thermal stress amplitude and improve thermal fatigue life. Austenitic overlay materials, with their higher thermal conductivity and lower thermal expansion coefficient compared to martensitic materials, often exhibit superior thermal fatigue resistance.
2. Overlay thickness optimization: The overlay thickness must be sufficient to provide adequate wear life but not so thick that it creates excessive thermal stress at the bond line. A thicker overlay layer has a lower thermal gradient through its thickness but a larger volume of material subject to thermal cycling. The optimal thickness is a balance between wear life and thermal fatigue resistance.
3. Welding process parameters: The welding heat input and interpass temperature influence the microstructure of the overlay layer. Lower heat input and lower interpass temperatures produce a finer grain structure and higher hardness, which may improve wear resistance but could reduce thermal fatigue resistance if the hardness is too high.
4. Post-weld treatment: A controlled tempering or stress relief treatment can reduce residual stresses in the overlay layer and improve the toughness of the microstructure. However, excessive tempering can reduce the hardness and wear resistance, so the treatment parameters must be carefully optimized.
5. Surface finishing: The surface roughness of the overlay layer after grinding or honing affects the thermal fatigue behavior. A smoother surface reduces stress concentration and delays crack initiation, improving thermal fatigue life.

Study Insights and Reflections

This research contributes to the fundamental understanding of thermal fatigue in roll overlay metals, which is essential for the rational design and selection of overlay materials and welding procedures for hot rolling mill applications. The study highlights the importance of considering the combined effects of thermal, mechanical, and chemical degradation in evaluating overlay performance, rather than focusing on individual property metrics in isolation.

A key insight is that the thermal fatigue performance of the overlay is not solely a material property but is also strongly influenced by the welding process and the resulting microstructure. Two overlay layers of the same nominal composition but produced by different welding processes may exhibit significantly different thermal fatigue lives due to differences in grain size, carbide distribution, and residual stress state. This underscores the importance of process control and qualification in overlay welding for hot rolling mill applications.

The study also raises important questions about the transferability of laboratory thermal fatigue results to actual rolling mill service. The thermal loading in a rolling mill is complex and three-dimensional, involving non-uniform heating, variable cycle frequency, and combined thermal-mechanical loading. Laboratory thermal fatigue tests, which typically apply uniform and uniaxial thermal loading, may not fully capture the complexity of the service environment. Multi-axial thermal fatigue testing and finite element analysis of the actual rolling conditions are necessary to bridge this gap.

Summary and Concluding Remarks

The investigation of heat-resistant fatigue performance in roll overlay metals provides essential knowledge for the optimization of overlay welding processes and material selection in the hot rolling industry. The thermal fatigue behavior of overlay layers is governed by the interaction of material properties, microstructure, residual stresses, and service conditions, and must be understood through a multi-disciplinary approach that combines metallurgy, thermomechanics, and tribology. The findings of this research should inform the development of improved overlay materials and welding procedures that can extend roll life, reduce downtime, and lower the overall cost of hot rolling operations. As the steel industry continues to push for higher productivity and lower costs, the development of more durable and reliable roll overlay technologies will remain a critical area of research and development.