Microstructure and mechanical property relationship characterization of high strength rail steels by using nanoindentation and SEM

High strength rail steels are used in heavy haul railway in Australia. However, there is still lack of understanding on the microstructures of these rail steels. Three high strength rail steels were investigated in this research. This study has three major purposes: (1) to determine microstructure quantitatively and qualitatively using SEM; (2) to investigate mechanical properties such as Young’s modulus and hardness using nanoindentation and microhardness test; (3) to correlate microstructure with mechanical properties for the high strength rail steels. In the first part, microstructure characterized by interlamellar spacing was determined quantitatively along the depth from gauge corner (GC) surface of a rail head. The results show that the interlamellar spacing increases with the depth from GC surface. This study then investigated the mechanical properties of these rail steels using nanoindentation. The Young’s modulus obtained from nanoindentation is in good agreement with monotonic tensile tests for the same grade of steels which obtained from literature. Pileups were observed in this study and this issue has been corrected using a method suggested in literature. Furthermore, the correlation between the interlamellar spacing with the mechanical properties obtained from nanoindentation has been examined. It shows that the microstructure-mechanical relationship in nanoscale cannot be described by the Hall-Petch relation, while the Hall-Petch relation can be used to describe the relationship between the interlamellar spacing and the hardness at microscale. The Hall-Petch relation at microscale obtained in this study was HV0.1=155+2185λt−1/2 HV, while the Hall-Petch relation at nanoscale was HN=19.8+4.8λt−1/2.