Comparison of the fatigue and impact fracture behaviour of five different steel grades used in the frog of a turnout

Abstract

The material requirements for the frog of a turnout are quite complex. The crossing nose region of the frog represents a discontinuity in the track as the wheel needs to change from one rail to the other. This causes increased dynamic contact forces and slip between wheel and frog. The high contact pressures and slip can cause damage such as wear, rolling contact fatigue and severe plastic deformation in the frog. The material from which the frog is constructed plays a crucial role in the development of damage. On the one hand, a plastic material response can reduce the contact pressures by an adaption of the frog’s geometry. On the other hand, materials have a different resistance to wear, crack initiation and growth. To clarify the requirements for materials for the manufacture of frogs, five different steel grades used to create frogs that are in service in various railway turnouts were investigated. The chosen materials were ‘Hadfield steel’ (an Mn-13, abrasion-resistant grain-refined steel with 400 HV), bainitic railway steel, tempered steel 51CrV4 (DIN 1.8159) and maraging steel. They were investigated in the same conditions as they would experience in service, covering a variety of steel microstructures and macroscopic hardness values. To compare the materials’ responses to the high impact forces, their impact fracture toughness was measured and selected fracture surfaces were investigated with scanning electron microscopy. To compare their response to the cyclic loading, the materials were tested with respect to their monotonic tensile strength as well as fatigue strength by means of low cycle fatigue tests. Measurements of the fracture toughness KIC were performed on all materials to compare their fracture resistance. The achieved results can be used as input data for finite-element-based calculations of the local loading conditions in railway turnouts. Additionally, the comparison can help the choice of the best material for specific loading conditions, i.e. specific crossing geometries and track loading conditions.