Achievement of high strength-ductility combination in railway wheel steel with thin pearlite and spherical cementite via composition and undercooling design

Abstract

To achieve high strength and ductility simultaneously, an improved railway wheel steel with high content of Si, i.e., HiSi steel was designed. For comparison, traditional wheel steel of ER8 was also machined from a commercial wheel rim. Then HiSi and ER8 steels were subjected to complete austenitizing followed by water quenching and spray cooling. The stop-cooling temperature during water quenching was about 580, 550 and 520 °C to achieve various degrees of undercooling. Specimens cooled to 520 °C were defined as ER8-520 and HiSi-520. Microstructure characteristics in terms of volume fraction proeutectoid ferrite (VFPF), interlamellar spacing of pearlite (ISP), cementite morphology were examined and mechanical properties including Brinell hardness, tensile properties, Charpy impact properties were characterized. The results indicated that spherical cementite of about 120 nm and submicron plate cementite were achieved simultaneously in HiSi-520 steel. ISP and VFPF in HiSi-520 were 120 nm and 8.9%, respectively. HiSi-520 steel exhibited higher hardness, tensile strength, Charpy impact energy and similar elongation compared to ER8-520 steel. Nanoscale spherical cementite, submicron plate cementite, thin ISP and some proeutectoid ferrite as well as solid solution strengthening from Si and Ni led to the excellent mechanical properties in HiSi-520 steel simultaneously.