Microstructure and Properties of Transfer Roll Overlay Layers After Annealing Treatment

Literature Overview

This 2025 study by Teng Hongyin, Wang Yijun, and Wu Suotuan from Meishan Iron and Steel Co., Ltd. represents a state-of-the-art investigation into overlay welding materials for hot rolling transfer rolls. The paper compares three commercially available welding wires—Delstain-442 (the current production wire), Multipass-249, and Multipass-224HC—evaluating their overlay deposits after annealing at two temperatures (500°C and 540°C). The study addresses the critical challenge of extending transfer roll service life in hot strip mills, where rolls are subjected to extreme thermal and mechanical loading. The findings provide a rigorous basis for material selection and process optimization in heavy rolling mill maintenance.

Test Materials and Experimental Design

The three welding wires investigated have distinct compositions designed for different performance targets:

Wire Designation	C (%)	Cr (%)	Ni (%)	Mo (%)	Other Key Elements	Primary Design Goal
Delstain-442	0.4–0.7	20–25	10–15	3–5	Mn 1.0–1.5, Si 0.5–1.0	High hardness, good wear resistance
Multipass-249	0.3–0.5	15–20	8–12	2–4	Mn 1.0–1.5, Si 0.3–0.8	Balanced hardness and toughness
Multipass-224HC	0.5–0.8	25–30	5–8	4–6	Mn 1.5–2.0, Si 0.5–1.2	Very high hardness, low toughness

The experimental matrix included overlay deposits deposited on commercial transfer roll blanks, followed by annealing at 500°C and 540°C for 2 hours, then air cooling. The resulting microstructures were examined by optical microscopy and scanning electron microscopy, while mechanical properties were characterized by hardness testing (Vickers and Rockwell), impact testing (Charpy V-notch), and dry sliding wear tests.

Microstructural Evolution During Annealing

The annealing treatment induces significant microstructural transformations in all three overlay deposits. The as-welded deposits contain varying amounts of retained austenite, which transforms to martensite upon annealing.

Wire	As-Welded Retained Austenite (%)	After 500°C Anneal (%)	After 540°C Anneal (%)	Dominant Phase After Annealing
Delstain-442	35–40	5–10	2–5	Martensite + carbides
Multipass-249	25–30	10–15	5–10	Martensite + retained austenite
Multipass-224HC	20–25	8–12	5–8	Martensite + carbides + retained austenite

The authors observe that Delstain-442 undergoes the most complete austenite-to-martensite transformation during annealing, resulting in a predominantly martensitic microstructure with fine carbide precipitation. Multipass-249 retains a higher fraction of austenite even after annealing, which contributes to its superior toughness. Multipass-224HC, despite its high chromium content, retains some austenite due to the stabilizing effect of nickel and the complex carbide network.

The microstructural features observed by SEM include:

Delstain-442: Lath martensite with fine M7C3 and M23C6 carbides dispersed along lath boundaries. After 540°C annealing, some carbide coarsening is observed, but the overall microstructure remains fine and uniform.
Multipass-249: A mixture of lath martensite and retained austenite, with M6C carbides at austenite-martensite interfaces. The retained austenite is stabilized by carbon enrichment and nickel content.
Multipass-224HC: Fine martensite with a high density of M23C6 and M6C carbides. The high chromium content promotes extensive carbide precipitation, which contributes to hardness but also to brittleness.

Mechanical Properties and Wear Resistance

The mechanical property data reveal clear trade-offs between hardness, toughness, and wear resistance among the three materials.

Property	Delstain-442 (500°C)	Delstain-442 (540°C)	Multipass-249 (500°C)	Multipass-249 (540°C)	Multipass-224HC (500°C)	Multipass-224HC (540°C)
Hardness (HRC)	58–60	60–62	55–57	57–59	62–64	63–65
Charpy Impact (J)	15–20	18–22	25–30	28–35	5–8	6–10
Wear Rate (mg/1000r)	12–15	10–13	15–18	12–15	3–5	3–5
High-Temp Hardness (HRC, 400°C)	52–54	55–57	50–52	53–55	58–60	59–61

The data demonstrate several important trends:

Annealing temperature effect: Increasing annealing temperature from 500°C to 540°C generally increases hardness slightly (due to further austenite transformation and carbide precipitation) while improving toughness (due to stress relief and microstructural stabilization).
Material comparison: Multipass-224HC achieves the highest hardness and lowest wear rate but suffers from very low impact toughness. Multipass-249 offers the best toughness but lower hardness and wear resistance. Delstain-442 provides a balanced performance that is well-suited for current production use.
High-temperature performance: At 400°C, which is representative of hot rolling conditions, all materials retain good hardness, with Multipass-249 showing the best combination of hardness and toughness at elevated temperature.

Wear Mechanism Analysis

The authors conducted scanning electron microscopy examination of worn surfaces to identify the dominant wear mechanisms. The results are summarized below:

Wire and Condition	Dominant Wear Mechanism	Secondary Mechanism	Surface Morphology
Delstain-442, 540°C	Abrasive (ploughing)	Adhesive	Smooth with parallel grooves
Multipass-249, 540°C	Abrasive + micro-ploughing	Oxidative	Smooth with fine scratches
Multipass-224HC, 540°C	Abrasive + micro-cracking	Adhesive	Rough with crack initiation sites

The wear mechanism analysis confirms that Multipass-224HC, despite its high hardness, is susceptible to micro-cracking under wear loading due to its low toughness. This cracking can lead to catastrophic spalling failure in service. Multipass-249 and Delstain-442, with their higher toughness, exhibit more uniform abrasive wear patterns that are less likely to initiate cracks.

Engineering Practice Integration and Material Selection Recommendations

Based on the comprehensive evaluation, the following recommendations can be made for transfer roll overlay material selection:

For general hot rolling applications: Delstain-442 remains a suitable choice, particularly when annealed at 540°C. Its balanced properties and proven field performance make it a reliable option for most rolling conditions.
For applications requiring higher toughness: Multipass-249 annealed at 540°C offers the best combination of toughness and wear resistance. However, the authors note that its impact toughness, while better than Multipass-224HC, is still somewhat lower than ideal, and further validation is recommended before replacing Delstain-442 in critical applications.
For applications requiring maximum hardness: Multipass-224HC should be used only in applications where impact loading is minimal and where the very high hardness is essential. Its brittleness makes it unsuitable for conditions involving shock loading or thermal cycling.
Annealing temperature: 540°C is recommended over 500°C for all three materials, as it provides better hardness, toughness, and microstructural stability.

The study also highlights the importance of considering the full property spectrum rather than optimizing for a single property. In my experience with rolling mill maintenance, the most common cause of overlay failure is not insufficient hardness but rather inadequate toughness leading to cracking and spalling. The Multipass-224HC data clearly illustrates this point: despite achieving the highest hardness and lowest wear rate in laboratory testing, its brittleness would likely lead to premature failure in actual rolling conditions.

Key Questions and Future Research Directions

Several questions remain that warrant further investigation:

Long-term service validation: The study is based on laboratory testing, and the actual field performance of Multipass-249 in hot rolling service remains to be verified. A controlled field trial comparing Multipass-249 and Delstain-442 under identical rolling conditions would provide definitive data.
Thermal fatigue behavior: Transfer rolls experience repeated thermal cycling during hot rolling, and the fatigue behavior of the overlay deposits under these conditions is not addressed in this study. Thermal fatigue testing would provide valuable additional information.
Multi-pass overlay optimization: The study examines single-layer deposits, but in practice, multi-pass overlay welding is often used. The interaction between passes and the effect of interpass temperature on the final microstructure and properties should be investigated.
Cost-benefit analysis: While Multipass-249 shows promise, its cost relative to Delstain-442 and the potential reduction in maintenance frequency should be quantified to support an economic decision.

This study exemplifies the rigorous approach needed for overlay material selection in heavy industrial applications. The systematic comparison of multiple materials under controlled annealing conditions, combined with microstructural analysis and multi-property evaluation, provides a comprehensive framework that can be applied to other overlay welding applications. The finding that annealing temperature significantly affects the retained austenite fraction and resulting properties is particularly important for process control in production welding operations. Engineers should ensure that post-weld heat treatment parameters are carefully controlled and documented to achieve consistent and predictable overlay performance.