Application of Orthogonal Experimental Method in Heat Treatment Process of Overlay Welded Rolls

Literature Overview

Li Chaohui's 2004 paper published in Angang Technology presents the application of orthogonal experimental design (L9(3^4) or similar) to optimize the heat treatment process for overlay welded backup rolls used in continuous rolling mills. The study systematically investigates the influence of multiple heat treatment parameters on the performance of overlay welded rolls, which are critical components in hot strip mills where they directly contact and support the work rolls during the rolling process.

Core Technical Approach

The orthogonal experimental design method is a statistical optimization technique that allows efficient investigation of multiple factors with a reduced number of experiments. In this context, the author applied this method to identify the optimal combination of heat treatment parameters for overlay welded backup rolls.

Factors and Levels Considered

The orthogonal array design allows each factor at each level to appear an equal number of times across the experiments, ensuring balanced data collection. The analysis of variance (ANOVA) or range analysis method is then used to determine the significance of each factor and the optimal parameter combination.

Key Technical Parameters and Performance Metrics

The performance of overlay welded backup rolls is evaluated based on several critical metrics:

• Surface hardness: Typically 45–55 HRC for the overlay layer, providing abrasion resistance against hot steel strip
• Core toughness: The base roll material must maintain adequate impact toughness (≥27 J at room temperature) to resist thermal fatigue and mechanical shock
• Overlay layer integrity: No cracking, spalling, or excessive porosity at the overlay-base interface
• Thermal fatigue resistance: The ability to withstand repeated thermal cycling during rolling operations
• Service life: Number of rolling hours before surface degradation requires re-overlay

Process Analysis and Optimization Results

The heat treatment process for overlay welded rolls must address several competing requirements simultaneously:

1. Overlay layer stability: The hardfacing alloy (typically Cr-based or Co-based) must not be over-tempered, which would reduce surface hardness below the required minimum.
2. Interface bonding: The overlay-base interface must maintain adequate bond strength, which is influenced by the thermal gradient during heat treatment.
3. Base material properties: The high-strength low-alloy (HSLA) steel base roll must achieve adequate toughness through proper tempering.
4. Dimensional stability: The roll must not develop excessive distortion during heat treatment, which would affect rolling precision.

The orthogonal experimental results typically show that the austenitizing temperature is the most significant factor affecting overall performance. Too low a temperature fails to fully transform the base material, while too high a temperature risks overheating the overlay layer and causing grain coarsening at the interface.

Integration with Engineering Practice

In continuous rolling mill operations, backup rolls with overlay welds face extreme service conditions:

• Temperatures up to 900°C on the roll surface during rolling
• Repeated thermal cycling causing thermal fatigue
• Abrasive wear from scale and oxide particles in the mill
• Mechanical loading from the rolling force transmitted through the work rolls

The optimized heat treatment process derived from the orthogonal experiment typically results in:

Key Questions and Reflections

The application of orthogonal experimental design to heat treatment optimization raises several important methodological considerations:

• The orthogonal array assumes no significant interaction between factors, which may not be valid when austenitizing temperature and holding time interact strongly in the overlay layer.
• The method identifies the best combination from the tested levels but does not explore the response surface between levels, potentially missing a truly optimal solution.
• The number of experiments required (typically 9–18 for a 4-factor 3-level design) is manageable for heat treatment trials but requires careful planning of furnace scheduling.

A more advanced approach would combine the orthogonal experiment with response surface methodology (RSM) to map the complete optimization landscape, particularly for identifying interaction effects between parameters.

Study Insights and Implications

This paper demonstrates the practical value of statistical experimental design in metallurgical process optimization. For engineers responsible for maintaining overlay welded mill rolls, the systematic approach to heat treatment development provides a repeatable methodology that can be applied whenever new overlay materials or roll geometries are introduced.

The key insight is that overlay welded components require heat treatment optimization that balances competing requirements across different material zones (overlay layer, interface, base material). The orthogonal experimental method provides an efficient framework for achieving this balance while minimizing the number of expensive and time-consuming heat treatment trials. This approach directly translates to improved reliability and reduced downtime in continuous rolling operations, where backup roll failures can result in production losses of millions of dollars per incident.