Hot Rolling Mill Roll Surfacing Materials and Process Research

Literature Overview

The study by Shen Fenggang et al. (1998), published in Materials for Mechanical Engineering (Vol. 22, No. 4), represents an early but foundational investigation into the surfacing materials and processes for hot rolling mill rolls. Conducted at the School of Mechanical Engineering, Xi'an Jiaotong University, this research systematically analyzes the intrinsic factors influencing thermal fatigue and wear, and establishes the alloy systems and microstructures of surfacing metals for hot roll applications. The study is particularly significant given the age of the research, as it laid the groundwork for subsequent developments in roll surfacing technology that continue to be relevant today.

Core Technical Approach

The research employed a systematic materials design approach, starting from an analysis of the failure mechanisms (thermal fatigue and wear) and working backward to determine the optimal alloy composition and microstructure. Two primary alloy systems were investigated:

Cr-W-V system: Chromium-tungsten-vanadium-based surfacing metal
Cr-Mo-V system: Chromium-molybdenum-vanadium-based surfacing metal

The evaluation methodology included thermal fatigue testing, wear testing, metallographic examination, and chemical composition analysis. The study also addressed the development of practical surfacing processes for specific roll types including skidding rolls and assistant rolls.

Key Findings and Technical Interpretation

Effect of Tungsten Carbides on Thermal Fatigue

A critical finding was that the tungsten carbides (WC) providing dispersion strengthening in the surfacing metal tend to agglomerate and grow during thermal fatigue cycling. This coarsening behavior leads to a reduction in thermal fatigue strength, as the fine dispersion of carbides is essential for impeding crack initiation and propagation. The implications for alloy design are clear: W-rich carbide systems may provide excellent hardness but are vulnerable to thermal stability issues during prolonged service.

Effect of Nickel Addition

The study revealed that adding less than 1 wt% Ni to the Cr-Mo-V surfacing metal can improve thermal fatigue performance. However, excessive Ni addition significantly lowers the phase transformation temperature (Ac1), which has adverse effects on wear resistance. This establishes a narrow but critical processing window for Ni content in roll surfacing alloys.

Alloy System	Key Element	Effect on Thermal Fatigue	Effect on Wear Resistance	Recommended Content
Cr-W-V	W	Carbide coarsening reduces thermal fatigue strength	High hardness but thermal instability	Moderate W content
Cr-Mo-V	Mo	Stable carbides, good thermal fatigue resistance	Good wear resistance	Optimized Mo content
Cr-Mo-V + Ni	Ni (<1%)	Improves thermal fatigue performance	No significant effect at low content	<1 wt%
Cr-Mo-V + Ni (>1%)	Ni (excess)	Lowers Ac1 temperature	Degrades wear resistance	Avoid

Optimal Microstructure Design

The research concluded that the ideal microstructure for hot roll surfacing metal consists of a uniform and stable lath martensite matrix with finely dispersed and thermally stable vanadium carbides (VC). This combination provides:

Lath martensite matrix: Offers good toughness and thermal fatigue resistance due to its fine lath structure and lower internal stress compared to plate martensite.
Dispersed VC particles: Provide dispersion strengthening for wear resistance while maintaining thermal stability due to the high melting point and low solubility of vanadium in austenite.

Process Development for Specific Roll Types

The study developed practical surfacing processes for skidding rolls and assistant rolls, which are critical components in hot rolling mills. The process development considered:

Substrate material compatibility
Heat input requirements for different roll diameters
Layer thickness optimization
Interpass temperature control
Post-weld heat treatment schedules

Engineering Practice Implications

For engineers currently involved in roll surfacing work, this research provides several actionable insights:

Alloy selection: The Cr-Mo-V system with controlled Ni addition (<1%) is recommended for applications requiring balanced thermal fatigue and wear resistance. The Cr-W-V system should be reserved for applications where high hardness is the primary requirement and thermal cycling severity is moderate.
Heat treatment: Post-surfacing tempering at appropriate temperatures is essential to stabilize the martensite structure and relieve residual stresses without causing carbide coarsening.
Process monitoring: Real-time monitoring of welding parameters during surfacing is critical, as deviations can lead to microstructural variations that compromise performance.
Inspection protocols: Regular metallographic examination of surfacing layers during maintenance intervals can detect early signs of carbide coarsening or microcracking.

Reflections and Critical Assessment

Despite being published in 1998, the fundamental findings of this study remain highly relevant. The identification of carbide coarsening as a degradation mechanism during thermal fatigue is consistent with more recent research on precipitate evolution under cyclic thermal loading. The concept of designing the microstructure from failure mechanism analysis is a sound engineering approach that continues to be applied in modern materials design.

However, the study's limitations should be acknowledged. The thermal fatigue testing was likely conducted under laboratory conditions that may not fully replicate the complex thermal-mechanical loading experienced in actual hot rolling mills. The wear testing was probably performed under relatively simple conditions, whereas real roll wear involves a combination of abrasive, adhesive, and erosive mechanisms. Additionally, the study does not address modern surfacing technologies such as cold spray, laser cladding, or advanced plasma transfer arc processes that have become available since 1998.

Summary

This pioneering study established fundamental principles for hot rolling mill roll surfacing material design, demonstrating that a uniform lath martensite matrix with finely dispersed vanadium carbides provides the optimal balance of thermal fatigue resistance and wear resistance. The Cr-Mo-V system with controlled Ni addition (<1 wt%) was identified as the preferred alloy system, while the Cr-W-V system was found to be susceptible to carbide coarsening under thermal cycling. These findings continue to guide modern roll surfacing practices and provide a solid foundation for further research into advanced surfacing materials and processes for hot rolling applications.