Preliminary Investigation of Rolling Mill Roll Surfacing Processes

Literature Overview

This 1999 paper by Zhang Wen, published in Special Steel Technology (Vol. 6, Issue 1, pp. 37–39), presents a preliminary investigation of surfacing processes for rolling mill rolls. Produced by the Electromechanical Factory of Sichuan Chuantou Changte Group, this work addresses the practical challenge of restoring or enhancing the surface properties of worn or damaged rolling mill rolls through welding-based surface engineering. Rolling mill rolls are critical components in metal forming operations, and their surface condition directly affects product quality, roll life, and production efficiency.

Technical Context

Rolling mill rolls experience severe operating conditions that demand specialized surface properties:

• High contact stress: Contact pressures between the roll and the workpiece can exceed 1 GPa
• Severe abrasion: Continuous sliding contact with the metal workpiece causes progressive wear
• Thermal cycling: Heat generated during rolling can raise roll surface temperatures significantly
• Impact loading: Sudden changes in roll speed or workpiece thickness produce impact loads
• Thermal fatigue: Repeated heating and cooling cycles during rolling and cooling operations

The base material of rolling mill rolls is typically high-chromium cast iron (such as 45Cr2, 65Cr2, or equivalent), which provides good hardness and wear resistance but limited toughness. Surfacing with a harder alloy can extend roll life, but introduces challenges related to thermal shock resistance, spalling resistance, and compatibility with the base material.

Process Parameters and Their Effects

The paper emphasizes the importance of controlling process parameters and maintaining strict operational discipline to achieve satisfactory surfacing results. While the abstract is concise, the following parameters are critical in roll surfacing:

Surfacing Materials for Rolling Mill Rolls

Several surfacing materials are commonly used for rolling mill rolls, each with distinct characteristics:

• High-carbon high-chromium alloys: Provide excellent wear resistance through hard carbides (Cr₇C₃, Cr₂₃C₆) but are susceptible to thermal cracking
• Nickel-based alloys: Offer good thermal shock resistance and moderate wear resistance, suitable for hot rolling applications
• Cobalt-based alloys (Stellite): Provide excellent hot hardness and wear resistance but at higher cost
• Iron-based alloys: Moderate cost with adequate performance for many applications

The selection of surfacing material depends on the specific rolling operation:

Heat Treatment Considerations

Post-weld heat treatment is essential for roll surfacing to:

1. Relieve residual stresses: Welding-induced tensile stresses can cause spalling during rolling
2. Stabilize the microstructure: Prevent unwanted phase transformations during service
3. Optimize hardness and toughness balance: Ensure the deposit can withstand both wear and thermal shock

A typical heat treatment cycle for high-chromium surfacing on cast iron rolls would involve:

• Annealing at 800–900°C for 2–4 hours to relieve stresses and promote carbide rounding
• Controlled cooling in the furnace to prevent thermal cracking
• Optional tempering at 500–600°C for 1–2 hours to further reduce stresses

Machining After Surfacing

The paper mentions machining as part of the process, which is an important practical consideration. After surfacing and heat treatment, the deposit must be machined or ground to the final roll dimensions. This requires:

• Adequate machining allowance (typically 1–2 mm of extra deposit thickness)
• Use of appropriate cutting tools (CBN or PCD for hard surfacing materials)
• Control of grinding burns that could compromise the surface integrity
• Verification of final dimensions and surface finish

Engineering Challenges and Solutions

The primary challenges in roll surfacing include:

1. Cracking during welding: Addressed through adequate preheat, controlled cooling, and stress relief
2. Spalling during service: Prevented by ensuring adequate toughness in the deposit and proper heat treatment
3. Poor bond strength: Mitigated by controlling dilution and ensuring clean base metal preparation
4. Inconsistent hardness: Controlled through uniform welding parameters and consistent heat treatment
5. Dimensional accuracy: Achieved through adequate machining allowance and post-weld grinding

Reflections on Process Development

As a preliminary investigation, this paper acknowledges the complexity of roll surfacing while providing a foundation for further development. The emphasis on strict process control reflects the reality that roll surfacing is highly sensitive to parameter variations, and consistent results require disciplined execution. The paper's brevity suggests that more detailed process development work is needed, particularly regarding:

• Systematic parameter optimization through design of experiments
• Quantitative evaluation of wear performance under actual rolling conditions
• Long-term durability assessment including thermal fatigue and spalling resistance
• Economic analysis comparing surfacing versus roll replacement

Summary

This paper establishes the fundamental principles of rolling mill roll surfacing, emphasizing the critical importance of process parameter control, strict operational discipline, and appropriate heat treatment. While the study is preliminary in nature, it provides a practical framework for approaching roll surfacing problems in industrial settings. The key message is that satisfactory surfacing results are achievable only through comprehensive process control spanning preheat, welding parameters, interpass temperature, post-weld heat treatment, and final machining. For engineers tasked with extending rolling mill roll life, this paper serves as a starting point for developing a qualified surfacing procedure tailored to specific rolling operations and roll materials.