Improvement of Heat Treatment Process for Hardfaced Backup Rolls Using Orthogonal Experimental Design

Literature Overview

This paper by Zhao Hui, published in Heat Processing Technology in 2004 (Vol. 33, No. 4, p. 59), addresses the critical heat treatment process for hardfaced backup rolls in rolling mill applications. The study focuses on a φ1550 backup roll repaired using submerged arc automatic welding (SAW), where improper heat treatment directly causes weld layer spalling, surface delamination, and ultimately roll scrapping. The author employs orthogonal experimental design methodology to optimize heat treatment parameters and improve furnace design, resulting in improved hardfacing quality and significantly reduced energy consumption.

Technical Background and Problem Analysis

Application Context

Backup rolls in hot rolling mills are subjected to extreme conditions:

• Operating temperatures: 200-600°C depending on the rolling process
• Contact pressures: 100-300 MPa at the roll-to-roll contact zone
• Continuous cyclic loading from the rolling process
• Exposure to scale, water, and chemical environments

The hardfacing layer on backup rolls provides the wear-resistant surface that maintains dimensional accuracy and surface quality of the rolled product. Failure of the hardfacing layer — through spalling, cracking, or delamination — results in immediate production stoppage and potentially costly roll replacement.

Root Cause Analysis of Heat Treatment Failures

The paper identifies several factors contributing to hardfacing layer failure:

1. Thermal stress mismatch: Differences in thermal expansion coefficients between the hardfacing layer and the roll body create residual stresses during cooling from heat treatment temperatures.
2. Excessive cooling rates: Rapid cooling can induce martensitic transformation in the hardfacing deposit, creating high residual stresses and reducing toughness.
3. Insufficient stress relief: Inadequate stress relief treatment leaves high residual stresses from the welding process, promoting delayed cracking and spalling.
4. Temperature gradients: Non-uniform heating and cooling across the roll thickness create differential stresses that can initiate delamination at the weld interface.

Orthogonal Experimental Design Methodology

Experimental Design Parameters

The orthogonal experimental design (Taguchi method) was applied to optimize the following heat treatment parameters:

The orthogonal array selected was L9(3^4), requiring only 9 experimental runs to evaluate the effects of 4 factors at 3 levels each. This represents a significant efficiency improvement over full factorial testing, which would require 81 runs.

Key Results

The optimized heat treatment parameters identified were:

• Heating temperature: 600°C
• Holding time: 4 h
• Heating rate: 75°C/h
• Cooling method: Controlled furnace cooling to below 100°C before air cooling

These parameters minimize thermal stress while ensuring adequate stress relief. The furnace cooling approach eliminates the thermal gradients that cause delamination during air cooling.

Furnace Design Improvements

The study also addresses furnace modifications to support the optimized process:

• Improved thermal uniformity through redesigned burner arrangement
• Enhanced temperature control with multi-zone thermocouple monitoring
• Modified loading pattern to ensure even heat distribution across the roll length
• Improved insulation to reduce energy consumption

Engineering Practice Integration

Process Implementation Protocol

For practical implementation on φ1550 backup rolls:

1. Pre-treatment preparation: Clean the hardfaced surface to remove scale and contaminants. Verify the integrity of the hardfacing layer through visual inspection and magnetic particle testing.
2. Heating phase: Ramp temperature at 75°C/h from room temperature to 600°C. Monitor temperature at multiple locations on the roll to verify uniformity (±20°C tolerance).
3. Soak phase: Hold at 600°C for 4 hours. Ensure temperature stability within ±10°C throughout the soak period.
4. Cooling phase: Furnace cool at a rate of 50-75°C/h to below 200°C, then allow air cooling to room temperature. The slower cooling rate during the critical temperature range (400-200°C) prevents re-introduction of thermal stresses.
5. Post-treatment inspection: Verify hardness profile across the deposit thickness, check for surface cracks using MT, and measure dimensional accuracy of the roll.

Quality Assurance Criteria

Energy Efficiency Improvements

The optimized process and furnace design achieved significant energy savings:

• Reduced furnace heating time through improved insulation
• Eliminated excessive temperature overshoot through better control
• Reduced cooling time through optimized cooling curves
• Overall energy consumption reduction of approximately 30-40% compared to the previous process

Key Reflections and Study Insights

This paper exemplifies the power of systematic experimental design in solving practical manufacturing problems. The use of orthogonal experimental design not only identifies optimal parameters efficiently but also reveals the relative importance of each factor, providing deeper process understanding.

The finding that controlled furnace cooling is critical — rather than simply air cooling — highlights a common misconception in industrial practice. Many shops default to air cooling due to its simplicity, not recognizing the thermal stress implications for thick-section hardfaced components.

The integration of process optimization with equipment improvement (furnace redesign) demonstrates a holistic approach to quality improvement. Engineers should recognize that process parameters and equipment capability are interdependent — optimizing one without the other often yields suboptimal results.

Reference Value and Outlook

This paper provides a practical methodology for heat treatment optimization that can be adapted to other hardfaced components, including work rolls, guide rolls, and other rolling mill equipment. The orthogonal experimental design approach is universally applicable to any multi-parameter process optimization problem.

The energy efficiency gains achieved through furnace improvement are particularly relevant in today's context of increasing energy costs and environmental regulations. Manufacturers should consider similar systematic approaches to reducing energy consumption in their heat treatment operations.

Future work should investigate the long-term service performance of rolls treated with the optimized process, including tracking of hardfacing life and failure modes in actual rolling mill service.