Comparison of Microstructure and Properties after Annealing Treatment of Overlay Layers on Different Material Pinch Rolls

Background and Industrial Significance

Pinch rolls are critical components in continuous casting, rolling mills, and sheet metal processing lines, where they grip and transport hot metal strips at elevated temperatures. The overlay layers on pinch rolls are subjected to extreme thermal cycling, mechanical loading, and contact wear, making the selection of overlay material and heat treatment condition a matter of paramount importance. The literature under review compares three different overlay material systems deposited on pinch roll shells made of different base materials, followed by controlled annealing treatment to evaluate microstructural stability and mechanical performance.

Materials and Process Parameters

The study examines three overlay systems:

The annealing treatment is applied after the complete overlay deposition to relieve residual stresses, homogenize the microstructure, and improve the bond strength between the overlay and the base material. The selection of annealing temperature is based on the solution treatment temperature of the respective overlay alloy, adjusted downward by 50–100°C to avoid excessive grain growth or sensitization.

Microstructural Comparison After Annealing

The as-deposited microstructures of the three overlay systems exhibit significant differences, which are further modified by the annealing treatment:

Fe-Cr-Ni (310-type) overlay: The as-deposited microstructure consists of austenite with dendritic segregation of Cr and Ni. After annealing at 1050°C, the segregation is partially homogenized, and the austenite grain size increases from an average of 25 μm to 45 μm. The grain growth is moderate and does not compromise the mechanical properties significantly. The bond interface shows a diffusion zone of approximately 80–120 μm, indicating good metallurgical bonding.

Fe-Cr-Al (2520-type) overlay: This system exhibits a duplex microstructure of austenite and ferrite in the as-deposited condition. After annealing at 1000°C, the ferrite phase partially dissolves, resulting in a predominantly austenitic microstructure with fine carbide precipitates (Cr23C6 and Al2O3). The grain size increases to 55–65 μm, which is slightly larger than the 310-type overlay. The bond interface shows a thinner diffusion zone of 60–90 μm, which may be a concern for long-term thermal fatigue resistance.

Co-Cr-W (Stellite-type) overlay: The as-deposited microstructure features a γ-Co matrix with M6C carbides (Cr, Fe, W)6C. After annealing at 1100°C, the carbides partially dissolve, and the remaining carbides coarsen from an average size of 0.5 μm to 1.5–2.0 μm. The grain size of the Co matrix increases from 15 μm to 35 μm. The bond interface shows the thickest diffusion zone of 100–150 μm, which is attributed to the high diffusivity of Co in iron at elevated temperatures.

Mechanical Properties and Performance Comparison

The comparison reveals that each overlay system offers a distinct balance of properties. The Fe-Cr-Ni system provides excellent thermal expansion compatibility with the base material and superior oxidation resistance, making it suitable for high-temperature applications. The Fe-Cr-Al system offers a good compromise between hardness and ductility, with adequate oxidation resistance for moderate temperature service. The Co-Cr-W system delivers the highest hardness and contact fatigue resistance but exhibits lower ductility and poorer thermal expansion matching with the ferrous base material.

Defect Analysis and Quality Control

The annealing treatment, while beneficial for microstructural homogenization, can introduce or exacerbate certain defects:

• Intergranular cracking: Observed in the Fe-Cr-Al overlay at the bond interface when the annealing temperature exceeds 1050°C. The cracking is attributed to the formation of brittle δ-ferrite at the grain boundaries during the cooling phase.
• Pore formation: Minor shrinkage porosity (1–2% area fraction) was detected in the Co-Cr-W overlay after annealing, likely due to the dissolution of fine carbides and the subsequent volume change.
• Delamination: Not observed in any of the three systems when the annealing temperature was maintained within the recommended range, confirming the importance of proper thermal cycle control.

Engineering Recommendations

For pinch roll applications, the selection of overlay system should be guided by the following criteria:

1. For hot strip mills operating at temperatures above 1000°C, the Fe-Cr-Ni (310-type) overlay is preferred due to its excellent thermal expansion matching and oxidation resistance.
2. For cold rolling and temper mill applications where moderate hardness and good contact fatigue resistance are required, the Fe-Cr-Al (2520-type) overlay offers a cost-effective solution.
3. For severe contact wear applications such as cold headers and cold roll stands, the Co-Cr-W (Stellite-type) overlay provides superior surface durability, provided that the thermal expansion mismatch is managed through appropriate design considerations.

Study Reflections

This comparative study highlights the importance of matching overlay material selection with the specific service environment of pinch rolls. The annealing treatment serves as a critical process step that can either enhance or degrade the performance of the overlay layer, depending on the precise thermal parameters employed. The observation that different overlay systems require different annealing temperatures underscores the need for material-specific heat treatment protocols rather than a one-size-fits-all approach. Furthermore, the defect analysis provides valuable guidance for quality control procedures, emphasizing the need for careful temperature monitoring during the annealing process and post-heat treatment inspection for interfacial integrity.