Microstructure and Properties of Transfer Roll Overlay Layers After Annealing - Technical Study Note

Literature Overview

This paper by Teng Hongyin, Wang Yinjun, and Wu Suotuan, published in Metal Heat Treatment (2025, Vol. 50, No. 1, pp. 155-162), presents a comprehensive comparative study of three overlay welding wire compositions used for hot rolling transfer roll applications. The research, funded by Meishan Steel Co., Ltd. (Project RH2200002225), investigates the microstructural evolution and mechanical property changes following annealing treatment at 500°C and 540°C. The study evaluates Delstain-442 (current standard), Multipass-249, and Multipass-224HC as potential alternatives, addressing the critical need for cost-effective replacement materials in heavy steel rolling operations.

Background and Application Context

Transfer rolls in hot rolling mills endure extreme operating conditions including:

• Temperatures ranging from 600°C to 900°C during steel strip transfer
• Repeated thermal cycling causing thermal fatigue
• Abrasive contact with scale-covered hot steel
• Mechanical impact from steel strip handling

The overlay layer on transfer rolls serves as the primary wear-resistant surface, and its performance directly impacts mill availability, strip surface quality, and overall production economics. The selection of appropriate overlay material and post-weld heat treatment is therefore critical to maximizing roll service life.

Comparative Material Characteristics

Annealing Treatment Effects on Microstructure

Residual Austenite Transformation

The most significant metallurgical finding is the transformation of residual austenite to martensite during annealing:

1. Delstain-442: Shows the most complete transformation of residual austenite to martensite during annealing, indicating that the carbon and alloy composition promotes austenite instability at these temperatures.
2. Multipass-249: Retains some austenite even after annealing, attributed to higher Cr and Mo content stabilizing the austenite phase.
3. Multipass-224HC: Has minimal residual austenite in the as-welded condition due to high carbon content, so the transformation effect is less pronounced.

The residual austenite-to-martensite transformation during annealing is driven by the carbon redistribution and the thermodynamic instability of austenite at elevated temperatures, particularly when cooling is slow enough to allow martensitic transformation during subsequent cooling from the annealing temperature.

Phase Composition Analysis

After annealing treatment:

• Delstain-442: Predominantly tempered martensite with fine Cr7C3 and Mo2C carbide precipitates
• Multipass-249: Mixture of martensite and retained austenite with Cr23C6 and Mo-rich carbides
• Multipass-224HC: High-carbon tempered martensite with abundant M7C3 and M23C6 carbides

Mechanical Properties Comparison

Hardness and Wear Resistance

The 540°C annealed Multipass-249 overlay demonstrates superior high-temperature wear resistance due to the combined effect of retained austenite providing thermal stability and the duplex structure offering resistance to thermal fatigue.

Toughness and Brittleness Assessment

Multipass-224HC exhibits significantly higher brittleness compared to the other two materials, which is a critical concern for transfer roll applications where impact loading is inevitable. The high carbon content responsible for its excellent hardness simultaneously degrades ductility and impact resistance.

Performance Trade-off Analysis

Wear Rate Comparison

The wear rate reduction of Multipass-224HC relative to Delstain-442 is reported to be approximately one order of magnitude (10x improvement), which is remarkable from a pure wear resistance perspective. However, this improvement must be weighed against the significantly reduced toughness:

• Wear resistance ranking: Multipass-224HC > Delstain-442 > Multipass-249 (at room temperature)
• High-temperature wear ranking: Multipass-249 (540°C) ≥ Delstain-442 (540°C) > Multipass-224HC
• Toughness ranking: Multipass-249 > Delstain-442 > Multipass-224HC
• Overall service life prediction: Depends heavily on the specific loading conditions

Temperature-Dependent Performance

A critical insight from this study is the temperature-dependent nature of overlay performance:

1. Below 500°C: High-carbon martensitic overlays (Multipass-224HC) dominate in wear resistance
2. 500-700°C: Duplex stainless overlays (Multipass-249) become competitive due to retained austenite stability
3. Above 700°C: All materials experience significant softening, but Multipass-249 retains relatively better properties

This temperature-dependent behavior is crucial for transfer roll applications where operating temperatures can fluctuate significantly.

Engineering Practice Implications

Material Selection Decision Framework

Based on the study results, the following decision framework can be applied:

1. If maximum wear life is the priority and impact loading is minimal: Multipass-224HC with 540°C annealing offers the best wear resistance, but brittleness risk must be managed through careful weld procedure design.
2. If balanced performance with good toughness is required: Multipass-249 with 540°C annealing provides the best compromise, though the impact toughness data warrants further verification.
3. If proven reliability is essential: Delstain-442 with 540°C annealing remains the conservative choice, with well-documented performance and predictable behavior.

Annealing Temperature Optimization

The comparison between 500°C and 540°C annealing reveals important optimization opportunities:

• 500°C annealing: Preserves higher hardness but may leave residual stresses from welding
• 540°C annealing: Better stress relief, more complete austenite transformation (for Delstain-442), improved high-temperature stability
• Recommendation: 540°C appears optimal for all three materials in transfer roll applications, providing the best balance of hardness retention, stress relief, and microstructural stability

Key Questions and Further Investigation Needs

The authors appropriately note that the impact toughness of 540°C annealed Multipass-249 raises concerns that require further validation before recommending it as a replacement for Delstain-442. Specific questions that need resolution include:

1. Is the observed toughness sufficient for the actual impact loading conditions in the specific mill application?
2. How does the material perform under thermal cycling (1000+ cycles between room temperature and 800°C)?
3. What is the long-term stability of retained austenite in Multipass-249 under sustained high-temperature exposure?
4. How do the overlay materials perform when combined with different substrate materials (e.g., 42CrMo vs. carbon steel roll cores)?

Study Insights and Implications

This study exemplifies the rigorous approach needed when evaluating alternative materials for critical industrial applications. The systematic comparison of three commercially available overlay wires, combined with controlled annealing experiments and comprehensive property characterization, provides actionable data for material selection decisions.

The finding that 540°C annealing optimally transforms residual austenite in Delstain-442 while providing the best high-temperature wear performance for Multipass-249 is particularly valuable for process optimization. It suggests that the current practice of using Delstain-442 without post-weld annealing may be leaving significant performance on the table.

The cautionary finding about Multipass-224HC's brittleness, despite its excellent wear resistance, reinforces a fundamental principle in wear part engineering: the optimal material is not always the one with the highest hardness, but rather the one that provides the best balance of wear resistance, toughness, and service life under actual operating conditions.

From a broader perspective, this study demonstrates the value of post-weld heat treatment as a tool for optimizing overlay performance. The same overlay material can exhibit dramatically different properties depending on the annealing temperature, highlighting the importance of integrating welding and heat treatment as a unified process rather than treating them as separate operations.

In conclusion, this paper provides excellent guidance for transfer roll overlay material selection, demonstrating that Multipass-249 with 540°C annealing shows great promise as a potential replacement for Delstain-442, while emphasizing the need for comprehensive validation of impact toughness and thermal cycling resistance before any material change is implemented in production.