Surfacing Materials and Process Research for Hot Rolling Rolls

Literature Overview

This 1998 publication in Materials for Mechanical Engineering by Shen Fenggang and colleagues from Xi'an Jiaotong University presents a systematic investigation into surfacing materials and processes for hot rolling mill rolls. The study focuses on two alloy systems—Cr-W-V and Cr-Mo-V—and examines their thermal fatigue behavior, microstructural evolution, and wear resistance under simulated rolling conditions. The work addresses the dual degradation mechanisms of hot rolling rolls: thermal fatigue cracking and surface wear, which together determine roll life and surface quality of rolled products.

Core Technical Content

The authors identified the intrinsic factors governing thermal fatigue and wear, then formulated appropriate alloy systems and microstructures for the surfacing metal. Thermal fatigue testing was conducted to observe microstructural changes during repeated thermal cycling, and the effects of composition and microstructure on thermal fatigue strength and wear resistance were evaluated. Practical surfacing processes were developed for both grip rolls and backup rolls.

Key Findings on Alloy Design

Interpretation of Technical Points

The degradation of W-carbides through coarsening during thermal cycling is a critical finding. Tungsten carbides (WC) are inherently stable at high temperatures, but in the complex multicomponent alloy environment of a surfacing overlay, they can participate in coarsening reactions that reduce their dispersion strengthening effectiveness. This observation has profound implications for alloy design: while W provides initial hardness, it may not sustain that advantage under prolonged thermal cycling.

The Ni addition threshold of less than 1% is a nuanced finding. Nickel stabilizes the austenite phase and can refine the microstructure, improving thermal fatigue resistance. However, excessive Ni depresses the Ac1 temperature, which means that during thermal cycling the alloy may undergo more phase transformations, generating additional transformation stresses that accelerate cracking. This represents a classic trade-off between thermal fatigue and wear resistance, requiring careful alloy design optimization.

Microstructural Requirements for Optimal Performance

The study identifies the ideal microstructure as uniform, stable lenticular (plate-like) martensite with finely dispersed, stable vanadium carbides. This combination provides:

• Lenticular martensite offers superior thermal fatigue resistance compared to acicular or needle-like martensite due to its lower internal stress and better crack deflection capability
• Vanadium carbides (VC) are highly stable at elevated temperatures due to the strong V-C bond, maintaining dispersion strengthening throughout thermal cycling
• The uniformity of the martensite ensures consistent mechanical properties across the overlay thickness
• The stability of the carbides prevents coarsening and degradation during service

Engineering Practice Implications

For hot rolling mill operations, this research provides actionable guidance on roll surfacing specifications. The distinction between grip rolls and backup rolls is important, as these components experience different thermal and mechanical loading conditions. Grip rolls undergo more severe thermal cycling due to direct contact with hot strip, while backup rolls primarily experience mechanical loading with less thermal variation.

The developed surfacing processes should incorporate the following considerations:

• Preheating to reduce thermal stress during surfacing and minimize substrate dilution effects
• Interpass temperature control to maintain the desired microstructure in multi-pass builds
• Post-weld heat treatment to temper the martensite and relieve residual stresses without causing carbide coarsening
• Regular microstructural inspection of rolls in service to detect early signs of carbide coarsening or phase instability

Key Questions and Reflections

The study was published in 1998, and subsequent decades of research have expanded understanding of surfacing alloy design for hot rolling applications. One question that remains relevant is whether the Cr-Mo-V system with controlled Ni addition has been superseded by newer alloy designs incorporating rare earth elements or nitrogen. Modern surfacing alloys for hot rolling often include additions of Nb, Ti, or rare earths to further stabilize carbides and improve thermal fatigue performance.

Another consideration is the effect of surfacing layer thickness on performance. Thicker overlays provide more material for wear but also increase the thermal mass, potentially affecting thermal fatigue behavior. The optimal thickness must balance wear life extension against thermal fatigue susceptibility.

Study Insights and Implications

This foundational work established critical principles for surfacing alloy design in hot rolling applications, particularly the importance of microstructural stability under thermal cycling. The identification of lenticular martensite with stable V-carbides as the optimal microstructure remains a guiding principle in modern surfacing alloy development. Engineers working on roll life improvement programs should reference this work when evaluating surfacing material options, and the Cr-Mo-V system with sub-1% Ni addition remains a viable and cost-effective choice for many hot rolling applications. The systematic approach of linking microstructural requirements to thermal fatigue and wear performance provides a methodological template that can be applied to other surfacing applications involving combined thermal-mechanical degradation.