Multi-axial Fatigue of Head-Hardened Pearlitic and Austenitic Manganese Railway Steels: A Comparative Study
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DIFFERENT parts of the rail network are subjected to different types of stress conditions, where rail wheel contact induces very high contact stresses due to rolling and sliding. S&Cs, experiencing high contact and impact stresses, are more susceptible to damage and deformation because of their complex geometric shape compared to other parts of the rail. This leads to more frequent failures and large maintenance costs of S&Cs. The two most common types of steels used in railway crossings are head-hardened pearlitic steels and austenitic manganese steel, also known as Hadfield steel. Pearlitic steels have properties such as good tensile strength, fracture toughness and wear resistance whereas Hadfield steel possesses excellent work-hardening ability as well as
S. DHAR and H.K. DANIELSEN are with the Department of Wind Energy, Technical University of Denmark, 4000 Roskilde, Denmark. Contact e-mail: [email protected] J. AHLSTRO¨M is with the Department of Industrial and Materials Science, Chalmers University of Technology, 41296 Gothenburg, Sweden. X. ZHANG and D. JUUL JENSEN are with the Department of Mechanical Engineering, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark. Manuscript submitted February 2, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
good fracture toughness and wear resistance. The stresses encountered by the rail surface are transient, multiaxial, non-proportional and in different directions.[1] The cyclic loading induces large strains in the surface layers of the rail from where cracks initiate by rolling contact fatigue (RCF) mechanisms. Rail wheel interaction induces cyclic elastic stresses in the entire rail, which can lead to high cycle fatigue (HCF), but at the surface layer, close to the rail/wheel contact, plastic deformation and ratcheting strains accumulate, which are often evaluated using low cycle fatigue (LCF) experiments.[2] Several studies have been conducted on cyclic plasticity over the last decades. It has been shown that damage is dependent on many factors including the type of loading, the presence of stress raisers and residual stresses in the material.[1] Under the conditions of strain-controlled loading, the material can exhibit cyclic hardening, cyclic softening or cyclic saturation. This is strongly related to the material microstructure including dislocation density and arrangement as well as sub-structure formation.[3] For manganese steels, fatigue behavior is affected by both dislocation and twinning mechanisms. It has been generally accepted that the high strain hardening in manganese steel is due to deformation by slip as well as twinning.[4–6] The interaction of twins with dislocations provides additional hardening to the material. Another
theory suggests a rearrangement of C-atoms in the C-Mn cluster in the core of the dislocations imparts additional hardening.[7, 8] Fatigue studies by Kang et al.[9] suggested that the LCF process and failure of manganese steel at low strains are mainly controlled by dislocation-dominated cyclic deformation
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