Synergistic effect of nano-size precipitations and dislocations on the strength and fatigue behavior of bainitic rail steel

Abstract

This study examines the effect of tempering at 300–500 °C after forced-air cooling on fatigue properties of bainitic rail steel. Microstructural studies revealed that the specimens subjected to the four different heat treatments consisted of lath bainite and film-like retained austenite. The dislocation density for forced-air cooled, 300 °C, 400 °C, and 500 °C tempered conditions were 9.4 × 10¹⁴ m⁻², 7.9 × 10¹⁴ m⁻², 6.5 × 10¹⁴ m⁻², and 4.1 × 10¹⁴ m⁻², respectively. V(C,N) precipitates larger than 20 nm were primarily observed in the forced-air cooled steel, exhibited a unimodal size distribution with an average particle size of ∼ 25 nm. Tempering at 300 °C had minimal influence on the precipitation behavior. After tempering at 400 °C and 500 °C, new nano-size VC precipitates with an average particle size of 7.8 nm were observed, exhibiting a bimodal size distribution. At 400 °C, the strengthening effect of nano-size precipitates outweighs softening due to dislocation recovery, resulting in highest yield strength and fatigue limit. Fatigue fracture analysis revealed that the 500 °C tempered steel exhibited interior inclusion-induced crack initiation, while samples subjected to other heat treatments showed crack initiation from the surface or surface inclusions. Newly formed small grains were observed only at the crack initiation site induced by internal inclusions in the 500 °C tempered steel. This indicated that high-temperature tempering significantly reduced the dislocation density, which is detrimental to fatigue behavior. The optimal fatigue performance at 400 °C ascribed to the balanced synergy between moderate dislocation density and nano-precipitation. This finding provides new strategy to design bainitic rail steels with high fatigue property.