Effect of Final Rolling Temperature on Microstructure and Properties of Stainless Steel Carbon Steel Clad Plate

Literature Overview

The paper under review investigates how the final rolling temperature (FRT) during the roll-bonding process of austenitic stainless steel and carbon steel clad plates influences the interfacial bonding quality, microstructural evolution, and mechanical performance of the resulting bimetal product. This topic is of fundamental importance in the manufacture of clad plate pressure vessels, hydrogenation reactors, and heat exchangers where corrosion-resistant overlay layers are bonded to structural carbon steel substrates through thermomechanical processing. The study adopts a systematic experimental approach, varying the FRT across a range of approximately 850°C to 1100°C while maintaining other rolling parameters such as total reduction ratio, rolling speed, and interfacial cleanliness constant.

Core Technical Points and Microstructural Analysis

The central finding of the study is that the FRT exerts a decisive influence on the bonding mechanism at the stainless steel-carbon steel interface. At lower FRT values near 850°C, the interface is characterized by incomplete metallurgical bonding, with residual oxide films and unmelted interfacial regions creating weak zones susceptible to delamination. As the FRT increases toward 1000°C to 1050°C, the interface undergoes a transition from mechanical interlocking to true metallurgical bonding, with the formation of a diffusion bond zone containing intermetallic phases such as Fe-Ni compounds and chromium carbides.

Microstructural Zones Identified

The study demonstrates that an FRT in the range of 980°C to 1050°C yields optimal bonding strength, with shear test results showing bond strengths exceeding 250 MPa, well above the minimum requirement of 200 MPa specified in ASTM A264 for weld-overlay clad plate and comparable to the bond strength requirements in EN 10028-7 for explosion-clad plate.

Mechanical Property Correlation

The mechanical properties of the clad plate are directly correlated with the FRT. At optimal FRT values, the clad plate exhibits good ductility in the carbon steel layer with a uniform distribution of elongation values, while the stainless steel layer retains its austenitic microstructure with elongation exceeding 40 percent. However, the study identifies a critical upper limit: when FRT exceeds 1080°C, excessive grain coarsening in both layers leads to a significant reduction in yield strength, particularly in the carbon steel layer where the yield strength drops below the minimum requirement of 245 MPa specified in GB/T 150 for Q345R grade steel.

Mechanical Property Summary

Engineering Practice Integration

From a pressure vessel fabrication standpoint, the findings of this study have direct implications for the manufacturing of clad plate pressure vessels governed by GB/T 150 and ASME VIII Div.1. The FRT window of 980°C to 1050°C must be carefully controlled during the rolling process, and online temperature monitoring systems should be deployed to ensure compliance. Furthermore, the study highlights the importance of post-rolling heat treatment: a stress-relief annealing at 620°C to 650°C for carbon steel clad plates and at 1050°C followed by water quenching for austenitic stainless steel clad plates is essential to mitigate residual stresses and stabilize the microstructure.

Process Control Recommendations

• Deploy infrared pyrometers at the exit of the rolling mill to monitor FRT in real time, with a control tolerance of ±15°C
• Implement a cooling rate control strategy post-rolling to prevent martensitic transformation in the carbon steel layer, targeting a cooling rate below 10°C per second
• Conduct 100 percent ultrasonic testing (UT) of the interface per NB/T 47014 and ASME V Section Article 4, with acceptance criteria per ASTM A264 Section 7
• Perform intergranular corrosion testing per ASTM A263 Section 9 on the stainless steel layer to verify sensitization resistance

Key Questions and Reflections

A critical question that emerges from this study is the interaction between FRT and the subsequent welding process during pressure vessel fabrication. When clad plates are cut and welded into vessel components, the heat input during welding can locally modify the interface microstructure. The study does not address this aspect, which represents an important gap for pressure vessel engineers who must ensure that the welding process does not compromise the interfacial bond established during rolling. Additionally, the study focuses on austenitic stainless steel clad plates, but the FRT sensitivity may differ significantly for ferritic, duplex, or nickel-based alloy clad plates, warranting further investigation.

The study also raises questions regarding the long-term performance of clad plates under cyclic thermal loading conditions, such as those experienced in hydrogenation reactors operating at elevated temperatures. The stability of the interfacial diffusion zone under prolonged thermal exposure at 350°C to 450°C remains uncertain, and accelerated aging tests would provide valuable data for design life assessment.

Study Insights and Implications

The most significant insight from this literature is the identification of a relatively narrow optimal FRT window that balances bonding quality against grain coarsening. This narrow window demands sophisticated process control capabilities in modern steel mills, and it underscores the importance of process qualification and documentation in compliance with ASME IX and NB/T 47014 requirements for clad plate production. For pressure vessel engineers, the study reinforces the principle that the quality of the clad plate interface is established during the rolling process and cannot be fully remediated by subsequent welding or heat treatment. Therefore, rigorous incoming inspection of clad plates, including interface shear testing, is not merely a regulatory requirement but a fundamental engineering necessity for ensuring the long-term integrity of bimetal pressure vessels.