Corrosion Behavior of High-Strength Bainitic Rail Steels
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INTRODUCTION
DEVELOPMENT of high-strength and tough bainitic steels with excellent wear and corrosion resistance is considered to be the current research trend in modern rail applications.[1–3] Conventional C-Mn rail steels are based on fully pearlitic microstructure.[2] These steels are normally close to eutectoid composition consisting of 0.70 to 0.80 wt pct carbon.[2,3] The presence of high amount of cementite in pearlite microstructure renders the structure prone to enhanced corrosion.[3–5] Recently, a microalloyed Cr-Cu-Ni (NCC) rail steel with pearlite microstructure has been developed with improved atmospheric and crevice corrosion resistance.[2,3] Moreover, there has been development of improved rail steels with bainitic microstructure for heavy load applications.[6–8] It is known that bainitic microstructure has some significant benefits in terms of mechanical properties over the existing pearlitic rail steels.[8] Strength of pearlitic rail steels originates from lamellar microstructure consisting of ferrite and cementite. In contrast, bainitic steel, which forms at lower temperatures below pearlitic region, has finer non-lamellar carbide distribution. Higher mechanical strength of bainite derives from ultrafine microstructures with high dislocation densities.[9] Good ductility of bainitic steel is attributed to the film of retained austenite present in between bainitic lathes. It is also known that negative effects of carbides can also be overcome by discouraging its precipitation with the addition of silicon.[10] Further, Jia et al.[11] have reported that the bainitic steel exhibits A.P. MOON, PhD Student, S. SANGAL, Professor, and KALLOL MONDAL, Associate Professor, Faculty, are with the Department of Materials Science and Engineering, Indian Institute of Technology, Kanpur 208 016, India. Contact e-mail: [email protected] S. LAYEK, PhD Student, is with the Department of Physics, Indian Institute of Technology. S. GIRIBASKAR, Post Doctorate Student, is with the Advanced Forming Research Centre, Department of Design, Manufacture and Engineering Management, University of Strathclyde, Glasgow, U.K. Manuscript submitted August 31, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
higher corrosion resistance than that of the pearlitic steel at higher carbon level. It has also been reported that the ferrite phase has higher activity and dissolution rate than the bainitic phase.[12] Wang et al.[13] have also reported improved corrosion resistance of bainitic steel, and related this improved corrosion resistance to the enrichment of Cu and Cr at the interface between rust layer and substrate. Enrichment of Cu and Cr leads to compactness and adhesion of rust layer to the substrate. Kazum et al.[14] have reported higher polarization resistance for nanostructured bainitic steel than that of martensitic steel. However, understanding on corrosion behavior of bainitic rail steel is still not fully clear, and few reports exist on the corrosion behavior of bainitic microstructures.[12–16] The present study examines the aque
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