Experimental characterization of pearlitic rail steel after thermomechanical straining
Examensarbete för masterexamen
Advanced engineering materials, MSc
Rails are subjected to very high contact loads during service. The high contact loads cause the surface layer of the rails to be heavily deformed and aligned. The anisotropic nature of the deformed surface layer is prone to crack initiation. The deformed surface layer is also very thin and has a large gradient of accumulated strain. This large gradient makes it difficult to examine the material behavior with conventional testing methods because they requires a fairly uniform microstructure. A predeformation method developed by CHARMEC researchers have proven to be able to produce a material with a fairly uniform microstructure which is consistent with rail field samples with high accumulated shear strain. The aim with the Master Thesis was to expand the knowledge of the material behaviour of pearlitic rail steels (grade R260) under combined thermal and cyclic mechanical loading. The goal was to produce a microstructure with higher accumulated strains compared to previous work. It was achieved by adding a heat treatment to the predeformation method. An axial-torsion test rig with an induction coil has been used to deform and heat treat solid cylindrical test bars. This was done to obtain a microstructure that was similar to the one found in the field. The material was compared with field samples in terms of microstructure and hardness. The results of this thesis describes the mechanical behavior of a pearlitic rail steel during simultaneous axial compression and torsion with different compression loads at elevated temperature. The microstructures have been characterized and accumulated strain and hardness have been measured. The highest amount of accumulated strain was obtained with constant heating at 350 °C with an axial compression of 350 MPa and twist rate of 1.5 °/s. The amount of twisting was 3.5 times higher compared to previous work. Heating in between the twisting cycles resulted in the least amount of accumulated strain.
Materialvetenskap , Produktion , Maskinteknik , Materialteknik , Materials Science , Production , Mechanical Engineering , Materials Engineering