Heat Fatigue Resistance of Overlay Metals for Rolling Mill Rolls

Literature Overview

The paper by Feng Lingzhi and co-authors from Tianjin University and Xingtai Special Rolling Mill Factory, published in Welding Journal (Vol. 22, No. 2, 2001, pp. 19-22), addresses a critical durability challenge in hot rolling mill operations: the heat fatigue cracking of overlay metals used to harden and protect the working surface of rolling mill rolls. Rolling mill rolls are subjected to extreme thermal cycling during hot rolling operations, where the roll surface temperature fluctuates between the high temperature of the rolled stock and the relatively lower temperature of the roll core. These thermal cycles induce cyclic thermal stresses that, when combined with the oxidizing atmosphere of the rolling mill environment, lead to thermal fatigue cracking and eventual roll surface degradation. The authors investigate three self-made flux-cored wires and one commercial solid wire for roll overlay, developing a thermal fatigue test methodology and elucidating the crack initiation and propagation mechanisms.

Thermal Fatigue Test Methodology and Experimental Design

The thermal fatigue test methodology developed by the authors involves cyclically heating and cooling the overlay surface to simulate the thermal cycling experienced during hot rolling. The test parameters—including peak temperature, cooling rate, cycle frequency, and number of cycles—are designed to accelerate the fatigue process while maintaining relevance to actual service conditions. The four overlay materials tested include three proprietary flux-cored wire alloys and one commercially available solid wire alloy, providing a comparative baseline for evaluating the influence of alloy composition and microstructure on heat fatigue resistance.

The study establishes that thermal fatigue cracking in overlay metals is the result of the synergistic action of cyclic thermal stresses and the oxidizing atmosphere. This finding is significant because it implies that improving oxidation resistance can directly enhance heat fatigue life, even without modifying the mechanical properties of the overlay. The authors develop a mechanistic model for thermal fatigue crack initiation and propagation that integrates the effects of thermal stress, oxidation, and material microstructure.

Crack Initiation and Propagation Mechanism

The mechanistic model proposed by the authors identifies three stages of thermal fatigue crack development. In the first stage, surface oxidation initiates at microstructural defects such as grain boundaries, inclusion-matrix interfaces, and micro-pores. The formation of oxide scales at these sites creates local volume changes and stress concentrations that serve as crack nucleation sites. In the second stage, the cyclic thermal stresses cause the oxide-covered micro-cracks to propagate inward, with the oxide scale acting as a crack propagation accelerator by preventing crack healing during the cooling phase. In the third stage, the cracks coalesce and propagate through the overlay thickness, leading to macroscopic surface degradation and eventual roll failure.

The role of oxidation in crack propagation is particularly important. The oxide scale formed on the crack surface is brittle and prone to spalling under cyclic loading, which exposes fresh metal to further oxidation and accelerates crack growth. This oxidation-assisted crack propagation mechanism is analogous to thermal fatigue cracking in turbine blade coatings and hot-section components in power generation, where oxidation resistance is a primary design criterion.

Microstructure and Inclusion Effects on Heat Fatigue Performance

The study demonstrates that the microstructure of the overlay metal and the presence of inclusions have a profound influence on heat fatigue resistance. A homogeneous, fine-grained microstructure is beneficial because it provides uniform thermal expansion and contraction, reducing local stress concentrations. In contrast, a heterogeneous microstructure with coarse grains or segregated phases creates regions of differential thermal strain that promote crack initiation.

The detrimental effect of inclusions is particularly pronounced. Non-metallic inclusions such as manganese sulfides, silicate oxides, and alumina particles serve as crack nucleation sites because of the elastic modulus mismatch between the inclusion and the matrix, which creates local stress concentrations under thermal cycling. Additionally, the inclusion-matrix interface is a preferential site for oxidation because the oxide scale at the interface is often discontinuous and less protective than the bulk oxide scale. The authors recommend strict control of inclusion content and morphology in overlay alloys to maximize heat fatigue life.

Engineering Practice Implications for Rolling Mill Roll Design

The findings of this study have direct implications for the design and qualification of overlay welding procedures for rolling mill rolls. The following engineering guidelines can be derived from the research:

1. Alloy selection: Overlay alloys with high oxidation resistance should be prioritized, even if they require trade-offs in hardness or wear resistance. The synergistic effect of oxidation and thermal stress means that oxidation resistance is a first-order design parameter for heat fatigue life.
2. Inclusion control: The welding procedure should include measures to minimize inclusion formation, such as using low-sulfur filler metals, ensuring adequate flux coverage, and maintaining clean wire surfaces. Post-weld heat treatment can also be used to spheroidize inclusions and reduce their stress-concentrating effect.
3. Microstructure homogeneity: Welding parameters should be selected to promote a fine, homogeneous microstructure. Lower heat input, higher travel speed, and multi-pass welding with appropriate interpass temperatures can achieve this objective.
4. Surface finish: A smooth overlay surface reduces the number of surface defects that can serve as crack initiation sites. Post-weld grinding or machining of the overlay surface is recommended to remove surface irregularities and oxide scale.
5. Thermal cycling qualification: Overlay welding procedures should be qualified through thermal fatigue testing that simulates the actual thermal cycling conditions of the intended rolling application, including peak temperature, cooling rate, and cycle frequency.

Critical Reflection and Outlook

The study provides a valuable mechanistic understanding of thermal fatigue cracking in overlay metals, but several aspects warrant further investigation. The authors do not quantify the relative contributions of thermal stress and oxidation to crack propagation, which would be essential for developing quantitative design criteria. A more rigorous approach would involve conducting thermal fatigue tests under both inert and oxidizing atmospheres to isolate the oxidation contribution. Additionally, the study does not address the effect of residual stresses from the welding process on thermal fatigue life, which is a significant practical concern because welding residual stresses can be tensile and act synergistically with thermal stresses to accelerate crack initiation.

From a broader perspective, the research highlights the importance of integrating materials science, welding engineering, and tribology in the design of roll overlay systems. The thermal fatigue problem is not solely a materials problem; it is a system-level challenge that requires understanding the interaction between the roll, the rolled stock, the mill environment, and the welding process. Future research should focus on developing predictive models for thermal fatigue life that incorporate the effects of alloy composition, microstructure, residual stress, and operating conditions, enabling data-driven optimization of overlay welding procedures for specific rolling applications.

Summary

This study establishes that thermal fatigue cracking in rolling mill roll overlay metals is driven by the synergistic action of cyclic thermal stresses and the oxidizing atmosphere, with crack initiation occurring preferentially at microstructural defects and inclusions. The authors develop a mechanistic model for crack initiation and propagation and demonstrate that a homogeneous microstructure with minimal inclusions and high oxidation resistance is essential for maximizing heat fatigue life. The engineering implications are clear: overlay alloy selection, welding procedure design, and post-weld treatment must all be optimized to minimize inclusion content, promote microstructural homogeneity, and enhance oxidation resistance. This work represents an important contribution to the understanding of roll surface durability and provides a foundation for developing more reliable and longer-lasting roll overlay systems in hot rolling operations.